ARTICLE MADE OF ALLOY CONSISTING OF TWO OR THREE OF TI, ZR AND HF, AND HAVING A MARTENSITIC CRYSTAL STRUCTURE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]The present application claims priority to European Patent Application No. 24210009.7, filed on Oct. 31, 2024, which is incorporated herein by reference.

[0002]The present invention concerns articles made of an alloy consisting of two or three of Ti, Zr and Hf, the alloy having a martensitic crystal structure comprising thin plates. Said structure preferably has fine martensite plates of an average length of 20 μm or less and a thickness of 1.5 μm or less, estimated by SEM or by EBSD of a metallographic section. The article optionally comprises a dark oxide layer. The invention further provides a process for obtaining said article comprising a grain refinement treatment step as essential step. The article of the invention is preferably a watch exterior component or a watch movement component.

PRIOR ART

Hf Alloys

[0003]JPS59208080A (Toshiba—1983) mentions a heat treatment of Hf at 370-500° C. under water vapor (steam) pressure to develop a decorative layer that is resistant to corrosion and wear (hard), and of blue, gold, or black color.

Zr Alloys

[0004]U.S. Pat. No. 5,037,438A (Richards Medical Company—1989) and EP0410711A1 (Smith & Nephew Inc.—1989) relate to a thermal oxidation treatment of pure zirconium or zirconium alloys (>about 80% wt Zr with Nb, Ta, or Ti, Y, and additionally Hf). U.S. Pat. No. 5,037,438A discloses commercially available Zirconium alloys such as Zircadyne 702 and 705 and Zircalloy which are suitable for the claimed oxidation treatment. Zircadyne alloys contain 4.5 wt. % of Hf as a maximum corresponding to 2.4 at. %. Zircalloy designates zirconium alloys consisting of more than 90 wt % Zr corresponding to more than 95 at. %, and may contain other metals such as tin (about 1.5 wt %) and Fe, Cr, Ni or Nb (about 2.5 wt %). The oxidized alloy is marketed by Smith & Nephew under the name Oxinium for orthopaedic applications (notably knee and hip prostheses). The treatment consists of an oxidation in air, steam, water, or a salt bath, for example, by a thermal treatment in air at 370-595° C. for 6 hours. This treatment produces a bluish-black zirconium oxide layer with good adhesion to the substrate.

Ti—Hf Binary Alloys

[0005]Ti—Hf binary alloys are known. However, such binary alloys are not known having a hard and black oxide layer and/or a a martensitic crystal structure comprising thin plates.

[0006]Ti—Hf binary alloys are mainly considered in non-patent bio-medical literature focused on their prosthesis application. The full range of composition has been analysed for biocompatibility with respect to: a) corrosion resistance to body liquids; b) mechanical properties targeted to be close to those of bone tissue.

[0007]In WO2013137857A2 principles of identifying binary alloys having stable nanocrystalline structure are disclosed based on thermodynamic parameters. Among many such alloys listed, a Ti—Hf system is especially claimed. However, no experimental preparation of Ti—Hf nanocrystalline alloy was provided. No composition details are disclosed, and no oxidation treatment is mentioned.

[0008]In the article “Development of hafnium metal and titanium-hafnium alloys having apatite-forming ability by chemical surface modification”, J. Biomed. Mat. Res.: Part B—Appl. Biomat. 106, p. 2519-2523, 2018, T. Myazaki et al. investigated the bone-bonding ability of Ti-xHf, x=20%, 40%, 60%, 80% and 100% at, alloys. The pure metals or the alloys were treated with NaOH and heat. It was found that anatase, sodium titanate, hafnium titanate, apatite or hafnium oxide, respectively, are formed on the surface of the alloys depending on the Hf content.

[0009]In the article “Multiple-species ion beams from titanium-hafnium alloy cathodes in vacuum arc plasmas”, J. Appl. Phys. 73, 7184-7187, 1993, J. Sasaki et al. have studied in detail experimentally the system Ti—Hf as a cathode sputtering target, covering the entire compositional range. In particular, a plasma produced by the metal vapor vacuum arc, for the case when the cathode material is a solid solution TiHf alloy of variable composition ratio, was studied.

[0010]In the article “Sputtered Hf—Ti nanostructures: A segregation and high-temperature stability study”, Acta Materialia 108, p. 8-16, 2016, M. Polyakov et al. investigated preparation of Hf—Ti nanostructured alloy by sputtering. They found that upon annealing at 800° C. for 96 h, the Hf-23Ti (% at) alloy shows segregation of Hf and Ti at the nanoscale even though the bulk Hf—Ti phase diagram predicts a homogeneous solid solution.

Zirconium-Hafnium Binary Alloys

[0011]In general, Zr—Hf binary alloys are also known, but not with a hard and black oxide layer and/or a a martensitic crystal structure comprising thin plates.

[0012]U.S. Pat. No. 3,373,017A discloses coins made of Zr-xHf, x=8.5-20% wt, alloy as a replacement of silver and as a counterfeiter measure.

[0013]In JPH04318137A corrosion-resistant binary alloys for nuclear materials processing are disclosed which comprise 1-50% wt of Hf with the balance comprising one or more selected from the group consisting of Ti, Zr, Nb and Ta.

[0014]EP0570308A1 discloses a process for preparation of Nb—Ti and Hf—Zr ingots with a particular crystalline structure by co-electrodeposition of the metals in a molten salt bath. The example of production of Zr-66.2Hf (% wt) is provided.

[0015]JP2013054037A discloses a component of nuclear reactor made of zirconium-hafnium alloy. The use of binary Zr—Hf alloy is claimed but its composition is not described in detail. Instead, alloys with the addition of Nb, Fe, Cr, Sn and Ni are mentioned.

[0016]CN 112195369A discloses a corrosion and neutron resistant Zr-xHf, x=49-51% wt, binary alloy and the method of manufacturing a component from it.

[0017]In the paper “Interdiffusion behaviors and mechanical properties of Zr—X (X=Nb, Ta, Hf) binary systems”, J. Alloys Compds. p. 164910, 2022, J. Wang et al. studied, among a few zirconium binary alloys, the diffusion behavior of Zr—Hf alloy for nuclear, aerospace and prosthesis applications. They also measured the hardness of these alloys by nano-indentation and found that up to approximately 20% at of Hf it is around 4 GPa and then gradually increases to around 6 GPa at 28% at.

[0018]In the paper “The Hf—Zr (Hafnium-Zirconium) System”, Bull. Alloy Phase Diagrams p. 29, 1982, Abriata et al. present a phase diagram for Hf—Zr system based on a literature review. They stated that two elements are completely miscible.

Binary Ti—Zr Alloys

[0019]GB1305879 concerns a Ti-(25-75%) Zr alloy for surgical and dental applications. The % in this reference are wt. %. The alloy may contain at most 2 wt. % of other elements, which apparently are unavoidable impurities. Fe, O₂ and N₂ are specifically disclosed as impurities or rigidity-increasing additions which are contained in commercial grade of pure titanium. The machining of the Ti—Zr alloy remains comparable to titanium. Surface treatments may be carried out, whereby oxides, nitrides or carbides are produced by heat treatment (gas, salt etc.) or by anodic oxidation.

[0020]WO99/04055A1 discloses a surface hardening treatment of pure titanium, pure zirconium or a titanium-zirconium alloy. The hardening is achieved by a two-step heat treatment, i.e. oxidation in air or oxidizing atmosphere (O₂ and N₂) at 700-1000° C. for a short time of 0.1 to 1 h, followed by a heat treatment in vacuum or inert/neutral atmosphere at 700-1000° C. for 10 to 50 h to allow oxygen diffusion. The titanium alloys disclosed are Ti6Al4V, Timet551 (i.e. Ti-4A1-4Mo-4Sn-0.5Si) and Timet 10-2-3 (i.e. Ti-10V-2Fe-3A).

[0021]US2012/0216921A1 (corresponding to U.S. Pat. No. 9,382,606 B2), US 2009/0199932A1 (corresponding to U.S. Pat. No. 8,262,814B2), US2012/0219736A1 (corresponding to U.S. Pat. No. 9,303,306B2) and US2012/0291929A1 (corresponding to U.S. Pat. No. 9,382,607B2), all of Gad Zak (in the following designated as “Zak patents”) disclose the oxidation treatment by heat treatment in air (or oxygen-enriched atmosphere) of a binary Ti—Zr alloy consisting of about 18.4% to about 65.6% zirconium by atomic weight and titanium, or between about 30.9% and about 65.6% zirconium by atomic weight and titanium. A ratio of 34.4% zirconium to 65.6% titanium by atomic weight corresponds to 50% by weight titanium and the balance zirconium, or art-recognized levels of impurities, based on the atomic weights for Ti of 47.867 and for Zr of 91.224. The oxidation treatment can be performed in one step or in two steps (a quenching step, e.g. by water quenching, can additionally be conducted between two heat treatments). The four patents concern binary Ti—Zr alloys, having 18.4 to 65.6 at % Zr, with a first heat treatment at a temperature of 250 to 880° C. and a duration of 10 to 110 minutes. The second heat treatment can be conducted between 480 to 880° C. for about 100 minutes. The different scopes of the patents concern specific characteristics resulting from the process (color obtained such as black or grey or surface properties such as polished, satin or matte finish, or specific process sub-steps, . . . ). The obtained alloys have a dark (i.e. black or grey) surface and can be used for numerous purposes, inter alia watches and watch bracelets.

Ternary Ti—Zr—X Alloys

[0022]WO96/23908A1 discloses a process for surface hardening of a Ti—Zr—X alloy by creating an oxide layer by heating to 200° C. to 1200° C., most preferably to 500° C. The time for heating depends on the temperature used. At 500° C., the time is 6 hrs. Heating is performed in air or oxygen-enriched atmosphere. The document concerns more particularly the ternary alloys Ti—Zr—Nb, of preferential composition Ti, 10-20 wt. % Nb and 0.5 to 20 wt. % Zr, or Ti, 35 to 50 wt. % Nb, and 0.5 to 20 wt. % Zr. The most preferred Ti-13Nb-13Zr alloy is the subject of a patent by the same applicant (U.S. Pat. No. 5,169,597).

[0023]U.S. Pat. No. 5,372,660/WO96/23908A1 discloses that the most commonly used Ti alloy, Ti-6A1-4V is not affected by the process of forming a hard oxide layer, which is ascribed to the absence of Zr. U.S. Pat. No. 5,820,707 discloses a two-step high temperature (T>1000° C.) air oxidation treatment of ternary alloys to obtain a hard surface described as blue/black. The claims cover a material formed of complete or near complete mixed oxides with a Young's modulus of less than 35 GPa. The claimed alloys are ternary alloys of the Ti(Zr,Hf)(Nb,Ta,V) type. There is therefore necessarily a third element such as Nb (possibly combined with Ta and/or V). The concentration ranges mentioned are, for a Ti—X—Y alloy with X and Y in atomic %, X=Zr, Hf or a mixture with 10<X<30 and Y=Nb, Ta, V or a mixture with 5<Y<10. The document discloses that many of the alloys of titanium with zirconium or hafnium are characterized by their extraordinarily rapid oxidation in air at only modestly elevated temperatures, which has severely limited the usefulness of such alloys for many applications.

[0024]U.S. Ser. No. 10/975,462B2 discloses a Ti—Zr—O alloy (in mass %: 83≤Ti≤95.15, 4.5≤Zr≤15 and 0.35≤O≤2) whose oxygen addition, which is considered as a full alloying element, allows it to match or even surpass the mechanical properties of grade 5 ELI (extra low interstitials) titanium while being biocompatible and having excellent ductility. No surface treatment of the alloy is mentioned. The oxygen is included in the alloy by use of TiO₂ and/or ZrO₂ powder in controlled amounts during the melting of the alloy.

[0025]Y. Yamabe-Mitarai et al., Journal of Alloys and Compounds, vol. 911, 6 Apr. 2022, XP087053515, ISSN 0925-8388, DOI: 10.1016/J.ALLCOM.2022.164849 reports on phase stability of Ti-containing high-entropy alloys with a bcc or hcp structure which are for example used in aircraft jet engines. The document discloses a ternary Ti—Zr—Hf alloy with the nominal composition Ti₃₄Zr₃₃Hf₃₃ and comprising 35.7 at. % Ti, 29.2 at. % Zr and 35.1 at. % Hf as analyzed by EDS.

[0026]J. Blacktop et al., Journal of the Less-Common Metals, vol. 109, no. 2, 15 Jul. 1985, pages 375-380, XP024072756, ISSN: 0022-5088, DOI: 10.1016/0022-5088(85)90070-0 describes measurements of the temperature and enthalpy changes of the hcp to bcc transformation in several Ti—Zr—Hf alloys. The document discloses a ternary Ti—Zr—Hf alloy comprising 37.5 at. % Ti, 37.5 at. % Zr and 25 at. % Hf.

Grain Refinement Combined with Thermochemical Surface Heat Treatment of Zr and Hf Alloys

[0027]EP2240616A1 discloses a surface heat treatment of a component of zirconium or hafnium alloy for a nuclear reactor comprising a nano-structuring of the surface layer within 5 μm depth to obtain a grain size less or equal to 100 nm. The treatment is carried out at a temperature lower than that of the last heat treatment during the fabrication of the component and by using conventional shot peening (SP), USSP (Ultrasonic Shot Peening) and/or SMAT (Surface Mechanical Attrition Treatment). Other proposed surface structuring methods comprise discharge peening, laser shot peening, microcavitation (water jet), surface rolling or ultra-fast machining. A surface thermochemical oxidation, nitriding or carburizing or diffusion of such chemical elements is also disclosed. This thermochemical treatment can be performed during or after nano-structuring.

[0028]In CN112853255A the oxidation process of nuclear or chemical reactor components comprising zirconium alloys is disclosed. The important point of the process is to avoid the surface contamination by iron that triggers corrosion, and special measures are proposed for cleaning. Besides the removing of iron contamination, the process comprises the following steps: 1) a nano-structuration of zirconium alloy by conventional shot peening, USSP (Ultrasonic Shot Peening) and/or SMAT (Surface Mechanical Attrition Treatment) in order to achieve a grain size of the order of 10 nm within a surface layer of 10-30 μm thickness; 2) heat treatment under air at 500°-700° C. during 0.5-3 hours to form a compact, hard and black surface oxide layer, namely zirconium oxide layer of 1.5-5 μm thickness.

[0029]The two patent applications described above are focused on components of large size, with estimated typical dimensions more than 5 cm and an estimated minimum thickness from 5 to 10 millimeters, and on non-thermal surface nano-structuring methods based on severe plastic deformation (SPD), which are distinct from the grain refinement in bulk. The SPD processes are acceptable in view of relatively large dimensions of components in prior art, so these components withstand easily high local mechanical stresses. This contrasts with relatively small watch components, with typical dimensions below 5 cm and a thickness from 0.1 to 5 mm, that require nano-structuring methods without the risk of plastic deformation.

[0030]None of the documents cited above discloses a grain refinement treatment of articles other than surface regions of the articles made from alloys, and/or a fine martensitic crystal structure of the alloys disclosed.

Description of the Technical Problem

[0031]The inventors have previously developed a ternary Ti—Zr—Hf alloy, an article made therefrom comprising a dark oxide layer, and a process for obtaining said article, and disclosed this teaching in the European patent application EP23171553.3 in the name of the present applicant, and have filed an International patent application PCT/EP2024/061358 claiming its priority. This previous ternary alloy comprises 18.4 at. % to 80 at. % Zr and 2 at. % to 40 at. % Hf, the balance being titanium. The article has a dark oxide layer and is particularly suitable for watch exterior components and watch movement components due to the excellent mechanical properties.

[0032]The inventors have further developed binary Ti—Hf and Zr—Hf alloys which yield articles having a dark adherent oxide layer and have claimed these articles in a parallel International patent application PCT/EP2024/061360 of the present applicant, claiming the priority of EP23171553.3 mentioned above.

[0033]The inventors aimed at developing an even more adherent oxide layer on various Ti—Zr, Ti—Hf, Zr—Hf binary and Ti—Zr—Hf ternary alloys, which can withstand handling during fabrication and various finishing techniques such as sandblasting without the risk of substantial damage. The problem has been solved by the article made of an alloy of claim 1 having a specific fine crystal structure, i.e. a martensitic crystal structure of fine plates. Preferred embodiments, such as a process for obtaining said article, the process comprising a grain refinement treatment as essential step, are also claimed. The article of the invention preferably is a watch part and/or watch movement part.

[0034]In particular, the following embodiments are provided by the invention:

[0035]An article made of an alloy consisting of two or three of Ti, Zr and Hf, along with unavoidable impurities in an amount of up to 0.3 at. %, the alloy having a fine martensitic crystal structure, as determined by Scanning Electron Microscopy, SEM, or by electron backscatter diffraction, EBSD, of a metallographic section.

[0041]The alloy used for the article of the invention may contain, besides Ti, Hf and/or Zr, accidental impurities as described below.

[0042]The article of the invention has a fine martensitic crystal structure. This is a structure consisting of fine crystal grains in form of plates as observed in the EBSD with average length of plates within a range of 20 μm or less and an average thickness of 1.5 μm or less. It should be noted that the average length of the plates is, at first, limited by the sizes of β grains from which they start to growth. After, during the cooling, their average size is limited by the martensitic plates that already started to growth. The plates' length is defined as the diameter of circumscribed circle and correlates well with the equivalent circle diameter (ECD) known in EBSD, meaning that larger, longer and/or thicker plates correspond to larger values of equivalent circle diameter. The fine martensitic crystal structure is present throughout a component of a comparatively small size such as a watch component due to the bulk heat treatment. But the fine grain structure throughout the article is not obligatory for increased adhesion of the oxide layer. Only the surface region of an article needs to have the fine martensitic grain structure. The necessary depth of this surface fine structure layer is estimated to be comparable to the thickness of the oxide layer, i.e. 3-25 μm.

[0043]Preferably, in the ternary Ti—Zr—Hf alloy of the invention, the amount of Zr is 78 at. % or less, more preferably 75 at % or less, more preferably 60 at. % or less, even more preferably 50 at. % or less, and most preferably 30 at. % or less. The amount of Zr is preferably 20 at. % or more, more preferably 23 at. % or more, most preferably 25 at % or more. Even more preferably, in the ternary Ti—Zr—Hf alloy of the invention the amount of Zr is 18.4-78 at. %, more preferably 23-75 at %, even more preferably 23-50 at. %, most preferably 25-30 at. % Zr.

[0044]In the ternary Ti—Zr—Hf alloy of the invention, the amount of Hf preferably is 35 at. % or less, more preferably 30 at. % or less, even more preferably 25 at. % or less, more preferably 20 at. % or less, even more preferably 10 at. % or less, most preferably 7 at. % or less. Preferably, the amount of Hf is 2 at. % or more, more preferably 3 at. % or more. Preferably, the amount of Hf is 2-20 at % Hf, more preferably 2 to 10 at. %, more preferably 3-7 at. % Hf.

[0045]In the binary Ti—Hf alloy of the invention, the amount of Hf preferably is 99 at. % or less, 95 at. % or less, 80 at. % or less, more preferably 75 at % or less, more preferably 60 at. % or less, even more preferably 50 at. % or less, or 30 at. % or less, and/or 25 at. % or more, preferably 27 at. % or more, more preferably 30 at. % or more, more preferably 50 at. % or more, 60 at. %. The amount of Hf in the binary Ti—Hf alloy of the invention is 20-80 at. %, more preferably 23-75 at %, even more preferably 23-50 at. %, most preferably 25-30 at. %.

[0046]In the binary Zr—Hf alloy of the invention, the amount of Hf preferably is 99 at. % or less, 95 at. % or less, 80 at. % or less, 70 at. % or less, or 50 at. % or less, and/or 10 at. % or more, 20 at. % or more, 30 at. % or more or 50 at. % or more. Even more preferably, the amount of Hf is 10 to 60 at % Hf, more preferably 10 to 50 at. % Hf.

[0047]The amount of Zr in the binary Ti—Zr alloy of the invention preferably is 99 at. % or less, preferably 95 at. % or less, 80 at. % or less, more preferably 75 at % or less, more preferably 60 at. % or less, even more preferably 50 at. % or less, or 30 at. % or less and/or 18.4 at. % or more, more preferably 20 at. % or more, more preferably 23 at. % or more, even more preferably 25 at. % or more. Even more preferably, the amount of Zr in the binary Ti—Zr alloy of the invention is 18.4 to 99 at. %, more preferably 18.4 to 80 at. %, more preferably 18.4 to 60 at. %, or 20 to 80 at. %, preferably 25 to 60 at. %.

[0048]In the ternary Ti—Zr—Hf alloy of the invention, the density of the alloy is between 5 and 12 g/cm³ which can be adjusted as desired by appropriately selecting the amounts of Zr and Hf in the ternary alloy.

[0049]In the binary Ti—Hf or Zr—Hf alloy of the invention, the density of the alloy is between 7 and 12 g/cm³ which can be adjusted as desired by appropriately selecting the amounts of Hf in the respective binary alloy.

[0050]In the binary Ti—Zr alloy of the invention, the density is in the range of 4.9 g/cm³ and 6.4 g/cm³. The density of the article of the invention thus is very variable in an easy manner by suitably selecting the alloying metals and their respective amounts.

[0051]The binary and the ternary alloys used for the article of the invention are paramagnetic. The invention provides an article made of the binary Ti—Zr, Ti—Hf, Zr—Hf or ternary Ti—Zr—Hf alloys described above. Preferably, the article has a dark oxide layer on one or more surfaces.

[0052]For example, the article of the invention is an ingot, a rod, a plate, a wire, a button or any article formed by usual methods such as casting, rolling or powder metallurgy, or additive manufacturing, for example 3D printing. This type of article is also designated as pre-form in the present application. The article is preferably a watch exterior component or watch movement component.

[0053]The thickness of the dark oxide layer, if present, is 5 to 25 μm, preferably 7 to 20 μm, more preferably about 15 μm.

[0054]The hardness of the dark oxide layer, measured by nano-indentation according to ISO 14577-1, 1^st ed. 2002, Metallic materials—Instrumented indentation test for hardness and materials parameters—Part 1: Test method, is at least 10 GPa H_IT, more preferably even higher than 14 GPa H_IT.

[0055]Optionally, the surface of the dark oxide layer is polished partially or completely. Polishing can be achieved by usual techniques. Alternatively, the surface can be partially or completely finished by other commonly used finishing techniques such as sandblasting, brushing, satin finishing or matte finishing.

[0073]The essential step of the inventive process is step 3), whereby the grain refinement of the alloy is achieved.

[0074]A small mechanical stress means the applied stress is below the yield strength Rp of the alloy when it starts to deform plastically.

[0075]The inventors also found that during the grain refinement process by thermal treatment, using the heating under air or oxygen-containing atmosphere and quenching in water, the concomitant formation of a dark and hard oxide layer, with a thickness of 1-5 μm, was observed, water and/or air being the source of oxygen. Thus, in an embodiment of the inventive process, step 3) is carried out in oxygen-containing atmosphere under heating.

[0076]This concomitant oxidation produces a hard, dark and thick oxide layer with good enough adhesion to resist to normal wear and tear of a watch component, especially a watch exterior component. The adherence of such oxide layer was not good enough to withstand quite harsh finishing techniques like sandblasting. However, other “softer” finishing techniques are possible, for example laser surface structuring. Consequently, the concomitant grain refinement/oxidation treatment in step 3) under oxygen-containing atmosphere and/or quenching in oxygen containing solutions (such as water) is preferably used for creating a hard dark oxide layer on finished watch components, preferably watch exterior components, and/or watch movement components.

[0077]The grain refinement treatment in step 3) is preferably carried out by a treatment using no or a small mechanical stress. The grain refinement treatment without using mechanical stress or using a small mechanical stress is preferably carried out by heating in an oven under oxygen-containing atmosphere, e.g. air, or under inert atmosphere, for example argon gas, to a temperature above the β-transus temperature of the alloy for an appropriate time and then quenching (i.e. rapid cooling) with a cooling rate of 100° C./s or more, preferably 200° C./s or more, most preferably 300° C./s or more. For practical reasons, the heating to a slightly higher temperature, around 50° C. higher, than that of a β-transus is preferred. The temperature of the heat treatment in step 3) depends on the composition of an alloy. For example, the heat treatment in step 3) without using mechanical stress is carried out at a temperature of 900° C. or more for a binary Ti—Zr alloy within the complete range of composition; 1000° C. or more for a binary Ti—Hf alloy comprising from 10 to 50 at. % of Ti; 1400° C. or more for a binary Zr—Hf alloy comprising from 10 to 50 at. % of Zr; and 850° C. or more for a ternary Ti—Zr—Hf alloy comprising 70 at. % of Ti and 30 at. % of cumulated Zr+Hf. The duration of the heat treatment at the temperatures indicated previously is, for example, from 20 minutes to 2 hours, preferably 25 minutes to 1. 5 hours, preferably for 30 minutes.

[0078]The oxygen-containing atmosphere in step 3) is air, pure oxygen gas or an oxygen containing environment such as a gas mixture of oxygen and an inert gas such as argon. Preferably it is air.

[0079]Quenching can be achieved by immersing in oxygen-containing solutions such as water or in oil or oil-water mixtures. For small articles, with dimensions below 1-2 mm, quenching by a cold gas is also possible. Quenching is carried out until the temperature of the article falls well below the R-transus temperature of the alloy, such as 100° C. below the R-transus temperature, preferably until room temperature is reached.

[0080]Surprisingly, the grain refinement of these ternary and binary alloys of the invention allows for better adhesion of a thick, hard, and dark surface oxide layer formed subsequently by the optional thermal oxidation step 5). For watch exterior applications, thanks to increased adhesion, various finishing methods can be applied to the thus-obtained dark oxide layer without the risk of substantial damage.

[0081]Preferably, the lightness L* of the oxide layer formed on the surface of grain refined alloys according to the invention in the CIELab color space L*a*b* determined according to EN ISO 11664-4 “Colorimetry—Part 4: CIE 1976L* a* b* Colour space”, ed. 2019, and before the matte finishing, is 50 or less; and after matte finishing, is 40 or less.

[0082]The oxide layer of the invention is adherent, thick, hard, and dark, preferably black.

[0083]The improved wear resistance in tribological pairs for watch components, like the pivot of an axis turning in a ruby bearing of the movement, or a bracelet pin turning in an element of a bracelet or a case, is another advantage of the novel surface oxide.

[0084]By the invention, durable black watch casings become feasible.

[0085]The temperature range of the optional thermal oxidizing treatment in step 5) is chosen regarding the nature of the hardening process by oxide layer conversion, in order to optimize the process parameters, for example the duration of the process. The lower temperature limit is determined by the oxidation reaction that would slow down to an unacceptable level due to the lack of reactivity. The higher temperature limit is determined by the slower oxygen diffusion in the β phase and by the risk of oxide layer delamination at β→α phase transition upon cooling. The β-transus temperature of an alloy can be determined before carrying out the thermal treatment by, for example, DSC (Differential Scanning Calorimetry). It is, further, preferable to find an optimum for using the highest possible temperature to speed up the conversion hardening and the lowest possible temperature to reduce the cost of heating equipment, its operation and the thermal deformation of parts to be treated.

[0086]In one embodiment, the oxidizing heat treatment in step 5) is carried out at 400 to 750° C.

[0087]Preferably, for the ternary alloy, the oxidizing heat treatment of the article in step 5) is carried out for 1 to 420 min, preferably 60 to 400 min, more preferably 100 to 400 min, most preferably 180 to 360 min. In another embodiment, the oxidizing heat treatment of the article is carried out for less than 60 min, preferably less than 30 min.

[0088]Preferably, for the binary Ti—Hf, the oxidizing heat treatment of the article in step 5) is carried at 450 to 600° C. out for 1 to 420 min, preferably 30 to 300 min, more preferably 40 to 240 min, most preferably about 60 min.

[0089]Preferably, for the binary Zr—Hf alloys, the oxidizing heat treatment of the article in step 5) is carried out at 400 to 750° C. for 1 to 420 min, preferably 30 to 300 min, more preferably 40 to 240 min, most preferably about 60 min.

[0090]For the binary Ti—Zr alloy, the oxidizing heat treatment in step 5) preferably is carried out at 400 to 750° C. for out for 1 to 420 min, preferably 60 to 400 min, more preferably 100 to 400 min, most preferably 180 to 360 min.

[0091]The oxygen-containing atmosphere in optional step 5) is air, pure oxygen gas or an oxygen containing environment such as a gas mixture of oxygen and an inert gas such as argon. Most preferably, the oxidizing heat treatment is carried out by thermal heating in an oven.

[0092]The invention provides the use of the alloy described above as a material for watch exterior components and/or watch movement components.

[0093]A watch exterior component or watch movement component made of the alloy of the invention preferably has a dark oxide surface layer as described above or is obtainable by the grain refinement and optional oxidizing heat treatment process of the invention disclosed above. The watch exterior component or watch movement component of the invention is obtainable by the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0094]The Ti—Zr—Hf alloy family used for the article of the invention has been developed by the inventors in order to have a material capable of forming a hard, thick, adherent and dark layer on the surface of a watch component. Such alloys are interesting for applications within the movement (for example, plates, bridges, axes) as well as for the case, bracelet, and also for bracelet pins. This family of alloys also allows the density to be adjusted, notably in a range between 5 and 12 g/cm³, potentially giving more freedom in the conception and design of watch components.

[0095]Ternary Ti—Zr—Hf alloys used for the article of the invention are particularly interesting in this respect due to the ease of alloying, the possibility of manufacturing watch parts, their finishing and the formation of an oxide layer. The oxide layer formed by conversion during oxidizing heat treatment is adherent, thick, hard and dark. It appears surprisingly that Ti, Zr and Hf form a perfect solid solution for both high temperature β phase and low temperature α phase, which favorizes alloying, and allowed the inventors to produce watch components for watch exterior with a satisfactory finish.

[0096]Preferable amounts of the alloying components Ti, Zr and Hf are defined in the appended claims.

[0097]The binary alloy used for the article of the invention comprises at least 25 at. % to less than 100 at. % of Hf, the balance being titanium, or at least 10 at. % to less than 100 at. % of Hf, the balance being zirconium, or at least 18.4 at. % Zr, the balance being titanium, along with unavoidable impurities in an amount of up to 0.3 at. % of the final composition. The upper limit of the amounts of Hf in the Ti—Hf alloy and the Zr—Hf and of Zr in the Ti—Zr alloy is preferably 99 at %, more preferably 95 at. %, more preferably 80 at. %, respectively. Preferable amounts of the alloying components Ti—Hf, Ti—Zr and Zr—Hf are defined in the appended claims.

[0098]Particular examples of binary alloys used for the article of the invention are as follows:

[0119]Particular examples of the ternary alloys used for the article of the invention are as follows:

[0130]The alloy used for the article of the invention preferably consists of two or three of Ti, Zr and Hf. Unavoidable impurities, however, may amount to up to about 0.3 at. % of the final composition. The impurities mainly result from the manufacture of the starting alloying metals and are, e.g. Fe, N, O, C and/or H.

[0131]In the present invention, amounts of metals in alloys are given as at. %. The total of all alloying elements is 100 at. %. The alloys can be represented by the following abbreviation given as an example for a ternary alloy: Ti-yZr-zHf, which means y at. % Zr, z at. % Hf, the balance being Ti if not indicated otherwise, the total being 100 at. %. The parameters y and z are selected according to the amounts of Zr and Hf defined in the claims. Binary alloys can be represented similarly by, e.g., Ti-yZr or Zr-yHf.

[0132]The pure elements as starting materials for forming the alloy are weighed to ensure the correct relative atomic composition of the resultant alloy. Alternatively, the composition of the alloy can be determined in the alloy by usual metal analysis methods known in the art. X-ray fluorescence (EDXRF—Energy Dispersive X-ray fluorescence, WDXRF—Wavelength Dispersive X-ray fluorescence), and Optical Emission Spectroscopy (spark-OES, ICP-OES/MS—Inductively-Coupled Plasma OES/Mass Spectroscopy, LIBS—Laser Induced Breakdown Spectroscopy, SEM/EDX and SEM/WDX—Scanning Electron Microscopy coupled to Energy Dispersive or Wavelength Dispersive X-ray spectroscopy) are methods routinely used.

[0133]In the present specification, the terminology “Ti based alloy”, “Zr based alloy” or “Hf based alloy” is used interchangeably, as well as the notations “Hf—Zr—Ti”, “Zr—Hf—Ti” or “Ti—Zr—Hf”, “Zr—Hf”, “Hf—Zr”, “Ti—Hf”, “Hf—Ti”, “Ti—Zr” or “Zr—Ti” since the microstructure and properties are comparable.

[0134]Ti, Zr and Hf are transition metals belonging to group IVb of the periodic table. Hf has an atomic weight of 178.5 g/mol, compared to 91.2 g/mol for Zr and 47.9 g/mol for Ti. The density of Hf is 13.3 g/cm³, compared to 6.52 g/cm³ for Zr and 4.5 g/cm³ for Ti. Hf is therefore a heavy and dense element; an alloy can be considered an alloy of Hf and not of Ti beyond ˜10 at. % of Hf. However, as mentioned above, in the present specification the terms Ti-alloy, Zr-alloy and Hf-alloy are used interchangeably.

[0135]The density of the alloys of the invention is determined by using a known buoyancy method. For the ternary alloy, it is preferably 5 to 12 g/cm³. For the binary alloys, the density is preferably 5 to 6.5 g/cm³ for Ti—Zr, 7.5 to 12 g/cm³ for Ti—Hf and 7 to 12 g/cm³ for Zr—Hf. The density can be adjusted by appropriately selecting the amounts of Ti, Zr and Hf. Thus, a broad range of densities from 5 to 12 g/cm³ can be achieved using the alloys of the invention.

[0136]The article of the invention has a fine martensitic crystal structure. This designates a martensitic crystal structure of fine plates of an average length of 20 μm or less and an average thickness of 1.5 μm or less as determined by Scanning Electron Microscopy, SEM, or by electron backscatter diffraction, EBSD, of a metallographic section.

[0137]While some metals like silver, gold and copper always have the same crystal structure up to their melting point, other metals like Ti, Zr, Hf and their alloys have two or more different structures (phases) depending on the temperature. Usually, Ti, Zr, Hf and their alloys have two different phases with characteristic microstructures depending on the temperature: there is the high-temperature phase known as a beta-phase (β phase) and the low-temperature phase known as an alpha-phase (α phase). Both can transform into each other by changing the temperature (two-way effect). Martensite is typically formed when Ti, Zr, Hf and their alloys are rapidly cooled (quenched) from the high-temperature beta (β) phase field, by passing the more equilibrium-based transformation routes. This rapid cooling prevents the diffusion necessary for the formation of alpha (α) and other equilibrium phases, leading to the formation of a martensitic phase often called alpha prime (α′). Martensite alpha phase can also be formed upon deformation (it is then called stress-induced martensitic transformation). The transformation occurs as the crystal structure rearranges to a martensitic phase to minimize the overall energy under applied stress.

[0138]Martensite is therefore a specific phase that is formed through diffusionless transformation which results in the subtle but rapid rearrangement of atomic positions and has been known to occur even at cryogenic temperatures for some metals and alloys.

[0139]The nucleation of martensite during quenching is a thermally activated process requiring an energy barrier to be overcome. The martensite start temperature (M_s) in Ti, Zr, Hf and their alloys is the temperature at which the martensitic transformation begins during cooling from the high-temperature beta (β) phase field. Upon cooling from the β phase to temperatures of the α phase, usually by quenching in air, oil or water or oil-water mixtures, alpha prime (α′) needle-like or lath-like structure start to form at M_s within the grains of the β phase. Thus, in a metallographic section and/or EBSD, the traces of β phase grains could be visible as boundaries between regions of particularly oriented martensitic plates.

[0140]The article of the invention, after subjecting the article to the grain refinement treatment of step 3) of the claimed process, has a fine martensitic crystal structure as observed with scanning electron microscope and/or EBSD in a metallographic section, comprising plates with an average length of 20 μm or less, preferably 10 μm or less, even more preferably 5 μm or less, and an average thickness of 2 μm or less, more preferably 1 μm or less, even more preferably 0.5 μm or less.

[0141]In the present specification, the terms “grains” and “crystals” and “crystal grains” are used synonymously.

[0142]This fine martensitic crystal structure of the invention is preferably obtained by heating the article to a temperature above the β transus temperature, and then cooling rapidly (also designated as quenching) at a high cooling rate of e.g. 300° C./s as disclosed above.

[0143]In contrast thereto, if the article is just heated to a temperature above the β transus temperature, but cooled down slowly at a cooling rate of e.g. 1° C./s, a martensitic crystal structure having much larger crystals is obtained, which is designated as coarse martensitic structure in the present invention (not according to the invention).

[0144]In contrast thereto, the crystal grains of the article obtained from a standard pre-form of forged and/or hot rolled titanium, zirconium or hafnium alloys, without the grain refinement treatment of the invention including rapid cooling, have a martensitic structure, i.e. plates, but are much larger, such as an average length of 20 to 50 μm and an average thickness above 2 μm.

[0145]The martensitic crystals according to the invention are plates. The length is defined as the average diameter of circumscribed circle around the section of the plate as observed in metallographic section with SEM or EBSD, whereas the thickness is the shortest length perpendicular to the length of the plate. The values are calculated by measuring for example 100 particles and calculating the average. The plates' length as defined above correlates well with the equivalent circle diameter (ECD) known in EBSD, meaning that larger, longer and/or thicker plates correspond to larger values of equivalent circle diameter.

[0146]In the metallographic sections sometimes, crystals may appear as needles. However, these needles are in fact cross-sections of plates at a particular angle.

[0147]EBSD is the most direct method for determining the crystal structure. SEM indirectly shows contrast between grains by secondary effects. It should be noted that in a given alloy sample, the martensitic plates are oriented isotropically in space. Due to the nature of SEM and/or EBSD, mainly the plates oriented perpendicularly to the metallographic section are detected as the most contrast features.

[0148]The crystal sizes (average length and thickness) are evaluated by Scanning Electron Microscope (SEM) of metallographic sections. They can also be determined by Electron Back Scattering Diffraction of a metallographic section.

[0149]In the preferred embodiment, the fine martensitic structure is formed throughout the article. In this case, there is no coarse grain structure detectable by SEM or EBSD in metallographic sections of the article of the invention.

[0150]In another embodiment, the fine martensitic structure is formed only within a surface region of the article by laser shot peening, typically within the surface layer of 2 to 100 μm thickness. In this case, both coarse grain structure in the bulk and fine grain structure in the surface layer are detectable by SEM or EBSD in metallographic sections of the article of the invention.

[0151]The binary and ternary alloys are obtained by known melting processes of the metal components of the starting alloy as described below. The process may comprise several steps of melting and cooling, for example at least 5 times or 10 times.

[0152]The article of the invention is made from the above-described binary or ternary alloy by routine processes such as cold or hot forming, cutting, milling, casting, drawing or any other suitable method.

[0153]The article of the invention preferably is a watch component, in particular a watch exterior component or a watch movement component. Examples of watch exterior components are watch cases, watch wristbands and/or parts thereof (such as links, pins, clasps, attachments), crowns, bezels, hands, or any other watch exterior parts. Examples of watch movement components are balance wheels, barrels, bridges, base plates, shafts, pinions or any other watch movement part.

[0154]The article of the invention is paramagnetic and is thus unable to be magnetized by magnetic fields.

[0155]The Vickers hardness HV1 (ISO 6507, HV1) of the Ti—Hf alloys (before grain refinement treatment) increases with the Hf content and the maximum hardness attained of HV1=323 corresponds to Ti-50Hf.

[0156]The Vickers hardness HV1 of the Zr—Hf alloy of the invention (before the grain refinement treatment) increases with the Hf content and the maximum attained of HV1=223 corresponds to Zr-50Hf.

[0157]The maximum hardness HV1 of a ternary alloy Ti—Zr—Hf (before the grain refinement treatment) occurs when the cumulated atomic concentration Zr+Hf, that is the sum of atomic concentrations of Zr and Hf, is equal to the atomic concentration of Ti. This has been determined by the inventors for the first time and disclosed in the earlier European patent application EP23171553.3.

[0158]The article of the invention preferably presents a dark oxide layer on one or more surfaces thereof, preferably on all surfaces. The dark oxide layer can be obtained by the process of the invention disclosed below, which comprises a thermal oxidation treatment, either concomitant with the grain refinement treatment carried out in oxygen-containing atmosphere and/or quenching in water or oil-oxygen mixtures (oxygen containing quenching solutions), or subsequently after grain refinement treatment.

[0159]Usually, after forming the dark oxide layer on the article, all surfaces thereof are covered by the oxide layer. However, if desired, the dark oxide layer can be removed from one or more of the surfaces of the article, e.g. by abrasive treatments, machining, laser treatment or the like. The resulting article thus may have only one or several, but not all of its surfaces, covered by the dark oxide layer.

[0160]The thickness of the dark oxide layer, if present, is preferably 5 to 25 μm, more preferably 7 to 20 μm, most preferably about 15 μm.

[0161]The thickness of the dark oxide layer is determined on a metallographic cross-section of the article by Scanning Electron Microscopy or by optical microscopy. This is illustrated in FIG. 4. The dark grey layer shown in FIG. 4 is the dark oxide layer. The black layer is the background, e.g. an organic matrix used to prepare the metallographic section. The bright layer is the alloy. The high oxide layer thickness of up to 25 μm allows for final surface finishing treatments such as sandblasting, satin-finishing, matte finishing, brushing and/or polishing that would not be possible with a lower layer thickness.

[0166]In contrast, alloys with larger grain sizes provide fewer high-energy sites for oxide surface nucleation during growth. As a result, there is higher stress in the growing oxide layers due to the reduced number of stress-relief sites at the grain boundaries. Likewise, the mechanical interlocking can be less efficient on the smoother, more uniform surfaces of larger grains.

[0167]The observed oxide layer (not shown in FIG. 6 and FIG. 7) on both fine (according to the invention) and coarse (comparative) martensitic grain structures appears black on the Electron Backscatter Diffraction (EBSD) maps, indicating the absence of diffraction. This can be linked to amorphous structure or to nanocrystalline structure with non-perfectly aligned nanocrystals, or to a mixture of both amorphous and nanocrystalline structures.

[0168]Concerning an amorphous or nanocrystalline oxide layer, a reduction in accumulated stress is expected, attributable to its isotropic thermo-mechanical properties. Consequently, assuming all other factors, which are described above, are constant, the amorphous or nanocrystalline oxide layer offers a distinct advantage in terms of enhanced adhesion to the substrate alloy when compared to a polycrystalline oxide layer.

[0169]The color of the article of the invention having the dark oxide layer is black or dark grey. There are no or only very slight bluish tones in said dark color. The color of the article of the invention in CIELab color space L*a*b (determined according to EN ISO 11664-4 “Colorimetry—Part 4: CIE 1976 L* a* b* Colour space”, ed. 2019), is preferably L*<50, |a*|<5, |b*|<5, more preferably L*<40, |a*|<1, |b*|<1. In the CIELab color system, L* denotes the perceptual lightness and a* and b* denote the unique colors of human vision: red, green, blue and yellow. L* defines black as 0 and white as 100. The a* axis relates to the red-green opponent colors, with negative values toward green and positive values toward red. The b* axis represents blue-yellow opponent colors, with negative numbers toward blue and positive numbers toward yellow.

[0170]The hardness of the oxide layer measured by nano-indentation according to ISO 14577-1, 1^st ed. 2002, Metallic materials—Instrumented indentation test for hardness and materials parameters—Part 1: Test method, is at least 10 GPa H_IT, preferably 10 GPa H_IT to 14 GPa H_IT, more preferably higher than 14 GPa H_IT. That is, the surface of the article is very hard and thus resistant to scratches and has improved overall mechanical resistance. No difference in the oxide layer hardness was observed between samples with and without the grain refinement by thermal treatment.

[0171]It should be noted that the standard hardness measurement of the oxide layer by indentation, for example Vickers hardness according to the above-cited ISO 6507, 2^nd ed. 1997, is quite difficult because of the very low optical contrast of the indentation mark on the dark oxide surface. To overcome this issue, nano-indentation according to the above-cited ISO 14577-1, 1^st ed. 2002, can be used in the present invention to measure the hardness of the oxide layer, as this technique does not require optical microscopy to analyse the shape and to measure the dimensions of an indentation mark.

[0172]One should distinguish the hardness of a bulk alloy material from the hardness of the conversion oxide layer on the surface. The conversion oxide layer is much harder than the bulk, unoxidized alloy.

[0173]The notion of conversion comes from the gradual formation of the oxide layer from the surface inwards. There is a gradient of oxygen concentration between the oxide layer and the bulk material, where the oxygen concentration goes to almost zero and the hardness goes from that of the oxide layer to the hardness of the bulk alloy. This gradient is typically measurable by GDOES (Glow Discharge Optical Emission Spectroscopy) and can take place within usually 100-1000 nm and, due to this relatively narrow region, is not always resolved (lack of contrast or resolution power) in a metallographic section with optical or electron microscopies.

[0174]The dark surface oxide layer formed on an alloy having a martensitic crystal structure of fine plates as determined by Scanning Electron Microscopy, SEM, or by electron backscatter diffraction, EBSD, of a metallographic section, according to the invention shows a very strong adhesion to the alloy core, i.e. it does not peel off. According to peel-off test ISO 2409, 4^th ed. 2013, with the cutting tool of 1a type, the test result is evaluated as 1 on the scale of from 0 to 5. It has a dense surface with practically no pinholes or surface defects. This is determined by the oxide formation process, when the oxide layer grows inward between the initial thin natural oxide layer of a few nm thickness and the bulk metal alloy, allowing uniform grow of the oxide at the oxide-metal interface as illustrated on the right side in FIG. 3 (O²⁻ migration mechanism). The invention provides a process for obtaining said article optionally having a dark oxide layer. In the first step, the alloy having the desired composition is formed by usual melting processes. The amounts of starting metals are selected and weighed according to the desired composition of the alloy.

[0175]Optionally, the starting materials, e.g. metal chips or slugs of the respective alloying metals, are cleaned before melting, e.g. by ultrasonic cleaning.

[0176]The elements are then mixed and melted in an inert atmosphere or vacuum using any alloy ingot manufacturing method based on melting and solidification, such as vacuum induction melting (VIM) or vacuum arc melting (VAR). Typically, the method includes several melting and cooling steps to produce the alloy ingot, which ensures homogeneity, before allowing it to solidify and cool-down to room temperature inside the inert chamber.

[0177]The obtained alloy is formed to the desired pre-form or article shape by usual processes known in the art, such as hot and/or cold forming, cutting, milling, casting or the like.

[0178]One or more surfaces of the thus obtained pre-form or article can optionally be subjected to surface treatment such as grinding, fine machining, sandblasting and/or polishing, as desired.

[0179]The grain refinement process (Step 3) in the case of a finished watch component, which is the essential step of the invention, is preferably by heat treatment. However, if a finished component can withstand severe plastic deformation on its surface without deterioration of its functionality, surface grain refinement processes described below, which impart mechanical stress, can be used. In the case of a preform, the grain refinement by thermal treatment is preferred.

[0180]In the case of grain refinement by heat treatment, the temperature and duration are chosen as function of the alloy composition and its temperature of beta-transus (the phase change from β phase to α phase).

[0181]The grain refinement can be performed, provided that a component can withstand it, with a known process using mechanical stress like shot peening (SP), ultrasonic shot peening (USSP), surface mechanical attrition treatment (SMAT), microcavitation (water jet), laser peening, surface rolling or ultra-fast machining. Grain refinement using no a or a small mechanical stress, e.g. by laser shot peening or heat treatment, is preferable.

[0182]In one embodiment, the grain refinement treatment imparting no or a small mechanical stress on a component is carried out before machining of the component in step 4) of the claimed process.

[0183]In one embodiment, the grain refinement treatment imparting no or a small mechanical stress on a component is carried out after machining of the component in step 4) of the claimed process.

[0184]Then, in a next step the surfaces of the article can optionally but preferably be oxidized in order to obtain the desired dark surface layer. The surface oxidation can, in one embodiment, be carried out by heat treatment in an oxygen-containing atmosphere at a temperature of 400 to 750° C., for an appropriate time, preferably for 1 to 420 min, more preferably for 60 to 400 min, even more preferably 100 to 400 min, most preferably 180 to 360 min. In another embodiment, the oxidizing heat treatment of the article can be carried out for less than 60 min, more preferably less than 30 min.

[0185]The temperature is kept lower than the transition temperature from the α phase to the β phase. The temperature for alloy oxidation treatment is selected in the range of 400°-750° C. depending on the composition as indicated in TABLE 1 and TABLE 2 below.

[0186]The duration of the oxidation heat treatment is chosen depending on the alloy composition, surface condition, and desired layer thickness.

[0187]In the case of a polished surface, the treatment duration is at least 10 minutes, preferably greater than 60 minutes, typically 180 minutes, with a favorable range between 100 and 400 minutes.

[0188]In the case of a sandblasted surface, the heat treatment duration is at most 10 minutes, preferably no more than 1 minute.

[0189]Other heat treatment parameters can be adjusted as necessary by the skilled person in the field.

[0190]Surface oxidation can be achieved by processes that provide oxygen and the energy necessary for oxygen atoms and/or ions to form the oxide layer and to penetrate the metallic surface and/or the oxide layer being formed, such as ion implantation or PEO—Plasma Electrolytic Oxidation. During the heat treatment, heating can be achieved by induction or by Joule effect in an atmosphere that includes oxygen. Laser heating is also a possibility. However, heat treatment in air remains preferable.

[0191]It was found experimentally that the oxidation time and adherence of the black surface layer on Ti—Zr, Ti—Hf, Zr—Hf and Ti—Zr—Hf alloys, apart from the grain refinement treatment, is also dependent on their surface finishing. Indeed, the sandblasted samples of these alloys required 1 to 10 minutes of oxidation time at the highest temperature before cooling down to achieve an oxide layer thickness of around 5 μm without delamination. Longer times resulted in delamination of the formed oxide layer.

[0192]In contrast, polished or fine ground samples required 1 hour or longer to reach the same oxide layer thickness. No delamination was observed for polished or fine ground samples.

[0193]The oxidizing heat treatment is preferably carried out in an oven by thermal heating, preferably an electric heated oven.

[0194]Alternatively, it is possible to use plasma-electrolytic oxidation or laser heating in oxygen-containing atmosphere as oxidation treatment.

[0195]The oxygen-containing atmosphere can be air, pure oxygen gas or an oxygen-containing environment such as a mixture of oxygen and an inert gas, e.g. argon.

[0196]As described above, the grain refinement treatment in step 3) can be carried out by heat treatment in oxygen-containing atmosphere and/or subsequent quenching in water or oxygen containing solutions like oil-water mixtures (i.e., not in oil) in order to simultaneously achieve grain refinement and formation of the dark oxide layer.

[0197]If desired, the surfaces of the surface-oxidized article obtained as described above can be subjected to common finishing treatments such as sandblasting, brushing, satin finishing, matte finishing or polishing.

[0198]By an appropriate finishing of the oxide surface an even darker color can be obtained than in unfinished or polished specimen, e.g. by matte finishing, as is apparent from the Example of the invention below.

[0199]The invention provides a use of the alloys of the invention as a material for watch exterior components and/or watch movement components as described above. Preferably and usually, the alloy is subjected to grain refinement treatment, and then shaped to obtain the watch part and subsequently surface-oxidized as disclosed above.

[0200]Finally, the invention provides watch components having a dark surface layer. Applications of the alloys of the invention, beyond watch exterior components, are mobile components of watches such as wristband pins and movement pins such as balance shafts, pivots or pinions.

[0201]The hard oxide layer allows for good wear resistance. Furthermore, the component is paramagnetic, which is important for watches.

DESCRIPTION OF THE FIGURES

[0202]FIG. 1 shows binary phase diagrams of Ti—Zr, Hf—Ti and Zr—Hf. The phases “rt” and “ht” mean “room temperature” (α phase), and “high temperature” (p phase), respectively.

[0203]FIG. 2 shows ternary phase diagrams of Ti—Zr—Hf at different temperatures (700° C., 850° C., 1200° C., 1600° C.). The phases “rt” and “ht” mean “room temperature” (α phase), and “high temperature” (p phase) respectively.

[0204]FIG. 3 schematically depicts the oxidation mechanisms of Ti—Zr alloys for different Zr contents.

[0205]FIG. 4 (Comparative) depicts SEM images of the metallographic section of a Ti-30Zr-2.5Hf sample without grain refinement by heat treatment. Left: surface region with the oxide layer. Right: bulk region. Please note the different scale of the images.

[0206]FIG. 5 (Invention) shows SEM images of the metallographic section of a Ti-30Zr-2.5Hf sample after grain refinement by heat treatment. Left: surface region with the oxide layer. Right: bulk region

[0207]FIG. 6 (Comparative) shows EBSD map of the Ti-30Zr-2.5Hf alloy sample without the grain refinement. The image is a composite image according to ISO 13067:2020 that combines the band contrast, inversed pole figure and grain boundary data in order to enhance the visibility of grain structure. The pixel size (step size) is 1.1 μm. According to this image, the estimated average length of plates is 50 μm with a standard mean deviation of 30 μm and the average thickness is 5 μm with a standard deviation of 3 μm.

[0208]FIG. 7 shows EBSD map of the Ti-30Zr-2.5Hf alloy sample with the grain refinement by thermal treatment. The image is a composite image according to ISO 13067:2020 that combines the band contrast, inversed pole figure and grain boundary data in order to enhance the visibility of grain structure. The pixel size (step size) is 0.56 μm. According to this image, the estimated average length of plates is 20 μm with a standard mean deviation of 10 μm and the average thickness is 1.5 μm with a standard deviation of 1 μm.

CRYSTAL STRUCTURE, PROPERTIES

[0209]For a good adhesion of an oxide layer to a metal or metal alloy surface, it is preferable to avoid oxidation at temperatures when a phase transition with crystal structure modification can take place, leading to unwanted stress on the oxide layer with increased risk of its peeling off. All three alloying elements, Ti, Zr and Hf, have a known phase transition at some point above 800° C. from an α phase with a hexagonal close-packed (hcp) structure to a body-centered cubic (bcc) structure known as β phase.

[0210]The inventors discovered experimentally that Ti, Zr and Hf form binary and ternary alloys with perfect solution in a wide concentration range of alloying elements. For example, the supposed α→β transition was determined at around 691±5° C. by DSC technique for 67.5Ti-30Zr-2.5Hf alloy.

[0211]From the perspective of crystalline structure, the equilibrium structure is single-phase a for all compositions at T<˜600° C. for Ti—Zr, at T<˜790° C. for Ti—Hf, at T<˜863° C. for Zr—Hf according to the phase diagrams of the binary compounds (FIG. 1). The equilibrium structure is single-phase a for most of compositions of Ti—Zr—Hf at T<˜850° C. (FIG. 2). The abbreviations “rt” and “ht” stand for “room temperature” and “high temperature,” respectively; for titanium alloys, the “rt” phase corresponds to an a structure (hexagonal close-packed) and the “ht” phase to a p structure (body-centered cubic).

Oxidation Mechanism

[0212]FIG. 3 schematically depicts the oxidation mechanisms of Ti—Zr alloys for different Zr contents.

[0213]For titanium alloys with little or no zirconium, it is the diffusion of Ti⁴⁺ cations at the atmosphere/oxide interface that allows the growth of a porous layer of TiO₂ during an oxidation treatment in air. This layer will not be very adherent and mechanically not very resistant.

[0214]Conversely, for Ti—Zr alloys containing a given concentration of zirconium (about >10 at. %, >17.5 wt. %) as discovered by the inventors, it is the diffusion of oxygen anions through the oxide layer that allows the surface to be converted into a compact, adherent and hard oxide.

[0215]Similarly, for Ti—Hf alloys containing “sufficient” amounts of hafnium (according to the inventors' experiments about >55.4% by mass, >25 at. %), it is the diffusion of oxygen anions through the oxide layer that converts the surface into a compact, adherent, and hard oxide. The claimed amount of Zr/Hf ensures that there is enough zirconium/hafnium in the alloy to have an oxidation mechanism by oxygen diffusion into the substrate and to have good mechanical properties, with zirconium/hafnium having a hardening effect.

[0216]The oxidation mechanisms of titanium alloys are described in C. Leyens, “Oxidation and Protection of Titanium Alloys and Titanium Aluminides”, p. 187-230, in the book: C. Leyens and M. Peters, “Titanium and titanium alloys: fundamentals and applications”, John Wiley & Sons, 2003. However, the reference does not disclose any criteria defining the mechanism of Ti alloys oxidation depending on Zr or Hf content.

[0217]It is the finding of the inventors disclosed in their earlier patent applications cited above that the minimum content of Zr for binary and ternary alloys should be between 10 at. % and 20 at. % in order to allow formation of a dark and adherent oxide layer. Analogously, the minimum content of Hf in binary Ti—Hf alloy should be between 20 at. % and 25 at. %.

[0221]In summary, for the binary Ti—Zr, Ti—Hf and Zr—Hf alloys, as well as ternary Ti—Zr—Hf alloys, the oxidation heat treatment can be conducted for the claimed compositions and at temperatures below the α to β transition.

[0222]For the binary Ti—Zr, Ti—Hf and Zr—Hf alloys, the oxidation heat treatment can be conducted for the claimed compositions and at temperatures below the α to β transition. TABLE 1 and TABLE 2 provide a summary of tested alloy compositions.

EXAMPLES

[0223]In the following the invention is illustrated by Examples.

[0224]EBSD detector built-in the SEM: Oxford Instruments, model Symmetry S3, equipped with AZtecHKL Standard acquisition platform and AZtecCrystalStandard data processing software. The method for determining the average grain size is according to ISO 13067:2020, 2^nd ed. 2020, section 6.5 “Calculation of average grain size”, using equivalent circle diameter calculated from the area of each identified grain and then averaging over the number of all grains.

[0225]One metallographic section of each grain-refined and non-refined sample was used and imaged with SEM and EBSD at three different sites of these metallographic sections.

[0226]In the examples, the total number of identified grains for the samples without the grain refinement was 7500 and for the grain refined samples was 5200. Parameters of grain sizing which were set in the AZtecCrystalStandard software was the following: threshold angle 10°; border grains included; entire dataset; all phases except for zero solutions.

Example 1

[0227]Two samples of Ti-30Zr-2.5Hf ternary alloy were prepared in an arc melter by melting pure elements in a cooled copper crucible. First, the weighted elements in form of small pieces were cleaned in ultrasonic bath. Then the melted samples were turned over and remelted 10 times to homogenize their composition through the volume. In this way buttons were obtained with a size of around 3×3×1 cm.

[0228]The first sample was heated under air in an oven at 750° C. for 0.5 hour and slowly cooled down to room temperature under air at a rate of 1° C./s, after having removed the sample from the oven and left in open air, to provoke large (coarse) martensitic grain growth.

[0229]The second sample was heated under air in an oven at 750° C. for 0.5 hour under air and quenched in water to a room temperature with an estimated cooling rate of 300° C./s to obtain small (fine) martensitic grain sizes.

[0230]The samples in form of a button were cut in slices of 2 to 4 mm thickness. The slices were manually polished with P320 sandpaper. These prepared samples were oxidized by heat treatment in an oven under air at 550° C. for about 2 hours. The resulting oxide layers have thickness in the range of 5-10 μm and a dark color (FIG. 4 (comparative) and FIG. 5).

[0231]The scanning electron microscope images of metallographic sections are presented in FIG. 4 for the first sample and in FIG. 5 for the second one. As one can see, the first sample shows large grain size in form of martensite plates or needles with an estimated length of 10-40 μm and a thickness of 1-2 μm. In contrast, the second sample in FIG. 5 with grain refinement by heat treatment shows finer grain structure below with the length of martensitic plates or needles within the range of 5-10 μm and a thickness below 1-0.5 μm.

[0232]The color measurements in CIELab color space L*a*b* (determined according to EN ISO 11664-4 “Colorimetry—Part 4: CIE 1976L* a* b* Colour space”, ed. 2019) are presented in TABLE 3.

[0233]It was noticed that the adherence of the oxide layer on the first sample was poor, and the oxide layer was damaged during sandblasting even under the least severe conditions corresponding to an air pressure of 2 bars. In contrast, the surface oxide layer on the second sample was not damaged at all even under most severe conditions of sandblasting corresponding to an air pressure of 6-12 bars.

[0234]The progressive force scratch test was carried out on two samples of Ti-30Zr-2.5Hf with coarse and fine grain structures. The alloy button of the dimension 3×3×1 cm was prepared in the arc melter as described above and cut into slices of 4 mm. Then, the first sample was heated in an electrically heated oven under air for 30 min at 750° C. and then quenched in water to obtain fine grain structure. The second sample was heated for 30 min at 750° and then taken out from the oven and left cooled down under air with estimated cooling speed of 1° C./s. Their surface prior to oxidation was grinded and polished successively with sandpaper going down to P4000 size. The samples were oxidized for 1 minute at 600° C. under air in an electrically heated oven and then left cooling down in the oven to a room temperature. The resulting oxide layer thickness was estimated to be 10 μm with SEM of metallographic sections. The Rockwell C indenter with the radius of 200 μm was used on a standard scratch tester with progressive normal force from 0.05 N to 30 N and constant advance speed of 10 mm/min of the indenter. The average critical normal force after 3 runs was 20.74 N for non-grain refined sample and 26.55 N for grain refined one, showing clearly better adhesion of the oxide layer on the sample with grain refined structure.

[0235]The EBSD (Electron Back Scattering Diffraction) measurements of samples without the grain refinement and with grain refinement described above, illustrated FIG. 6 and FIG. 7, respectively, showed that the alloy structure is a martensitic, plate-like one. It also showed that the oxide layer did not have structured diffractions supposing an amorphous and/or nanocrystalline structure.

[0236]The average sectional grain size or projected grain size was determined from EBSD measurements according to ISO 13067:2020 (2^nd ed. 2020) with minimum disorientation angle of 10°, number of pixels per grain more than 10, number of scan field 3 and the total number of grains more than 1000. The average equivalent circle diameter of grain size for the sample without the grain refinement was 14.9 μm with a standard deviation of 8 μm and for the sample with grain refinement it was 3.5 μm with a standard deviation of 2 μm.

[0237]Other samples having the compositions shown in TABLES 1 and 2 above were prepared and heat-treated in the same way as Example 1. The thickness of oxide layers is shown in TABLES 1 and 2.

EXEMPLARY EMBODIMENTS PRESENTED AS CLAUSES

[0238]Clause 1: An article made of an alloy consisting of two or three of Ti, Zr and Hf, along with unavoidable impurities in an amount of up to 0.3 at. %, the alloy having a martensitic crystal structure of fine plates as determined by Scanning Electron Microscopy, SEM, or by electron backscatter diffraction, EBSD, of a metallographic section.

[0244]Clause 3: The article according to clause 2a), wherein the amount of Zr in the ternary Ti—Zr—Hf alloy is 78 at. % or less, more preferably 75 at % or less, more preferably 60 at. % or less, even more preferably 50 at. % or less, most preferably 30 at. % or less, and/or 20 at. % or more, more preferably 23 at. % or more, even more preferably 25 at. % or more.

[0245]Clause 4: The article according to clause 3, wherein the amount of Zr in the ternary Ti—Zr—Hf alloy is 18.4-78 at. %, more preferably 23-75 at %, even more preferably 23-50 at. %, most preferably 25-30 at. % Zr.

[0246]Clause 5: The article according to any of clauses 2 a) to 4, wherein the amount of Hf in the ternary Ti—Zr—Hf alloy is 35 at. % or less, more preferably 30 at. % or less, more preferably 25 at. % or less, more preferably 20 at. % or less, more preferably 10 at. % or less, most preferably 7 at. % or less, and/or 3 at. % or more.

[0247]Clause 6: The article according to any of clauses 2 a) to 5, wherein the amount of Hf in the ternary Ti—Zr—Hf alloy is 2-20 at. %, preferably 2-10 at % Hf, more preferably 3-7 at. % Hf.

[0248]Clause 7: The article according to clause 2 b), wherein the amount of Hf in the binary Ti—Hf alloy is 99 at. % or less, 95 at. % or less, 80 at. % or less, more preferably 75 at % or less, more preferably 60 at. % or less, even more preferably 50 at. % or less, or 30 at. % or less, and/or 25 at. % or more, preferably 27 at. % or more, more preferably 30 at. % or more, more preferably 50 at. % or more, 60 at. % or more, 75 at. %, or 80 at. % or more.

[0249]Clause 8: The article according to any of clauses 2 b) or 7, wherein the amount of Hf in the binary Ti—Hf alloy is 20-80 at. %, more preferably 23-75 at %, even more preferably 23-50 at. %, most preferably 25-30 at. %.

[0250]Clause 9: The article according to clause 2 c) wherein the amount of Hf in the binary Zr—Hf alloy is 99 at. % or less, 95 at. % or less, 80 at. % or less, 70 at. % or less, or 50 at. % or less, and/or 10 at. % or more, 30 at. % or more or 50 at. % or more.

[0251]Clause 10: The article according to clause 2 c) or 9, wherein the amount of Hf in the binary Zr—Hf alloy is 10 to 60 at % Hf, more preferably 10 to 50 at. % Hf.

[0252]Clause 11: The article according to clause 2 d), wherein the amount of Zr in the binary Ti—Zr alloy is 99 at. % or less, preferably 95 at. % or less, 80 at. % or less, more preferably 75 at % or less, more preferably 60 at. % or less, even more preferably 50 at. % or less, or 30 at. % or less and/or 18.4 at. % or more, more preferably 23 at. % or more, even more preferably 25 at. % or more.

[0253]Clause 12: The article according to any of clauses 2 d) or 11, wherein the amount of Zr in the binary Ti—Zr alloy is 18.4 to 99 at. %, more preferably 18.4 to 80 at. %, preferably 18.4 to 60 at. %, or 20 to 80 at. %, preferably 25 to 60 at. %.

[0274]Clause 14: The process according to clause 13, wherein the grain refinement treatment in step 3) is carried out by a treatment using no mechanical stress.

[0275]Clause 15: The process according to any of clauses 13 or 14, wherein the grain refinement treatment in step 3) is carried out by heating in an oven to a temperature above the β transus temperature of the alloy for an appropriate time and then quenching with a cooling rate of 100° C./s or more, preferably 200° C. or more, most preferably 300° C./s or more.

[0276]Clause 16: The process according to any of clauses 13 to 15 wherein the grain refinement treatment in step 3) is carried out an oxygen-containing atmosphere, oxygen containing solution, and/or by quenching in water.

[0277]Clause 17: The process according to any of clauses 13 to 16, wherein the oxidizing heat treatment of the article in step 5) is carried out for 1 to 420 min, preferably 60 to 400 min, more preferably 100 to 400 min, most preferably 180 to 360 min.

[0278]Clause 18: The process according to any of clauses 13 to 17, wherein the oxygen-containing atmosphere in step 3) and/or 5) is air, pure oxygen gas or oxygen containing environment.

[0279]Clause 19: The process according to any of clauses 13 to 18, wherein the oxidizing treatment in step 5) is carried out by thermal heating in an oven or by plasma-electrolytic oxidation.

[0280]Clause 20: A use of the alloy specified in any of clauses 1 to 12 as a material for watch exterior components and/or watch movement components.

[0281]Clause 21: The article made of the alloy consisting of two or more of Ti, Zr and Hf according to any of clauses 1 to 12, additionally having a dark oxide layer on one or more surfaces, wherein the color is determined in the CIELab color space L*a*b* determined according to EN ISO 11664-4 “Colorimetry—Part 4: CIE 1976L* a* b* Colour space”, ed. 2019.

[0282]Clause 22: The article according to clause 21, wherein the lightness L* of the oxide layer in the CIELab color space L*a*b* is 33 or less.

[0283]Clause 23: The article of any of clauses 1 to 12, 21 or 22, which is a watch exterior component or watch movement component.

[0284]Clause 24: The article of any of clauses 21 or 22, wherein the thickness of the oxide layer, as determined on a metallographic cross-section of the article by Scanning Electron Microscopy or by optical microscopy, is 5 to 25 μm, preferably 7 to 20, more preferably about 15 μm.

[0285]Clause 25: The article of any of clauses 1 to 12, 22 or 24, wherein the hardness of the oxide layer measured by nano-indentation according to ISO 14577, 1^st ed. 2002, 1^st ed. 2002, Metallic materials—Instrumented indentation test for hardness and materials parameters—Part 1: Test method, is at least 10 GPa H_IT, preferably 10 GPa H_IT to 14 GPa H_IT, more preferably higher than 14 GPa H_IT.

[0286]Clause 26: A watch exterior component or watch movement component, obtainable by the process of any of clauses 13 to 19.