EP3543368B1 - High-entropy alloys for covering components - Google Patents

Description

TECHNICAL AREA

The present invention relates to a high entropy alloy and to a covering component for a watch or a piece of jewelry made from this alloy.

PRIOR ART

Different alloys are now commonly used for the manufacture of watch trim components which are components generally exposed to the external environment and which may be in contact with the skin. These are, for example, austenitic stainless steels, titanium alloys or even precious metals. These alloys indeed have certain important properties for this type of part, namely high corrosion resistance, good polishability for aesthetic reasons, and no ferromagnetism. In addition to these characteristics, other properties are currently in high demand in watchmaking. These characteristics are high biocompatibility, in particular by reducing or eliminating potentially allergenic elements such as nickel or cobalt, and high hardness and scratch resistance. Alloys that meet all of these criteria are rare. Precious metals have low hardnesses (<200 HV in the annealed state). Austenitic stainless steels generally contain nickel and also have limited hardnesses (<300 HV in the annealed state). Martensitic stainless steels are hard (> 600 HV) but ferromagnetic. Finally, titanium alloys, such as grade 5 titanium (Ti6Al4V), certainly represent the best compromise among the properties presented above but they have a particular color and hardness (around 350 HV for grade 5 titanium) not significantly higher. higher than some austenitic stainless steels for example. For comparison, amorphous metals, also very interesting for cladding components, may have hardnesses greater than 500 HV. However, very specific implementations are necessary to obtain components made of amorphous metal, which further limits their use as a trim element.

In the field of watch dressing, there is therefore always a strong interest in obtaining hard crystalline alloys (> 400 HV in the annealed state), non-ferromagnetic, corrosion resistant and exhibiting good polishability. In this context, high entropy alloys, currently very studied and which constitute a new class of alloys, are particularly promising. According to the initial definition, high entropy alloys are considered to be all alloys containing at least 5 main elements with an atomic fraction between 5 and 35%, the elements having an atomic fraction of less than 5% being considered as minor. To date, it is accepted that alloys containing 4 main elements can also be considered as high entropy alloys. At the thermodynamic level, the high entropy resulting from the mixing of numerous main elements would make it possible to stabilize the phases in solid solution with respect to the formation of potentially embrittling intermetallic phases. As a result, unique and little observed properties in traditional alloys based on one or two main elements are obtained. For watchmaking, obtaining simple phases in solid solution is very advantageous, because this promotes good polishability and good corrosion resistance. In addition, the mixing of many different elements results in solid solution hardening. Among the single-phase high entropy alloys, high hardnesses have thus already been demonstrated, particularly for those which have a centered cubic structure. These single-phase high entropy alloys with a centered cubic structure, such as for example NbTiVZr, AINbTiV, Al0.4Hf0.6NbTaTiZr or even Hf0.5Nb0.5Ta0.5Ti1.5Zr, more specifically target high temperature applications, for aeronautics especially. However, they contain a lot of elements expensive, very reactive or having high melting temperatures, such as Nb, Zr, Hf, Ta. To facilitate the implementation of the trim components, it is important to avoid or limit the amount of these elements, as high temperature resistance is not a desired property.

SUMMARY OF THE INVENTION

The object of the invention is to provide a high entropy alloy with a composition specifically adapted to the needs of the trim components. The present invention thus aims more particularly to develop an alloy having, after processing, a hardness greater than or equal to 400 HV, a non-ferromagnetic behavior and a high corrosion resistance.

For this purpose, the alloy contains 3 main elements which are Cr, Fe and V, each having atomic concentrations between 20 and 40%. It also contains as main element Al and / or Si having the effect of destroying the ferromagnetic behavior of the alloy. These elements each have an atomic concentration greater than or equal to 5% with a total atomic concentration for Al and Si less than or equal to 25%.

The alloy can also optionally comprise one or more main elements chosen from Mn, Mo, Ti and Ni each having an atomic concentration greater than or equal to 5% with a total atomic concentration for these 4 main elements less than or equal to 35 %. According to the invention, the Ni content is specifically maintained at a value less than 20% to avoid, during implementation and in particular heat treatments, the formation of undesirable phases which weaken the material and reduce corrosion resistance. . Some grades are also Ni-free to ensure high biocompatibility.

The remainder can be composed of possible impurities and / or of one or more minor elements each having an atomic concentration of less than 5%.

The material obtained after processing has, depending on the composition and the thermomechanical treatments, a single phase with a centered cubic structure, which promotes good resistance to corrosion and good polishability for a better surface finish or in the case of of multiphase alloys, a matrix (main phase) with a centered cubic structure reinforced with nanoprecipitates. It has the other advantage of having a color close to that of austenitic stainless steels.

Other advantages will emerge from the characteristics expressed in the claims, from the detailed description of the invention illustrated below with the aid of the appended drawings given by way of non-limiting examples.

BRIEF DESCRIPTION OF THE FIGURES

• The figure 1 shows a watch case made with the alloy according to the invention.
• The figure 2 represents the diffractogram of an Al6Cr30Fe30Mo5V29 alloy after casting and heat treatment for 3 h at 1300 ° C followed by cooling in a furnace with an average cooling rate of the order of 100 ° C / min.
• The figure 3 represents for this same alloy the hysteresis curve.

DETAILED DESCRIPTION

The present invention relates to high entropy alloys and to their use for trim components for watches or jewelry, in particular for components intended to be in contact with the skin. The covering component can be a caseband, a back, a bezel, a push-piece, crown, bracelet link, dial, hand, dial index, clasp, etc. By way of illustration, a watch case 1 produced in the alloy according to the invention is shown in figure 1 .

According to the invention, the alloys have between 4 and 9 main elements. The term “main elements” is understood to mean elements having an atomic concentration greater than or equal to 5%. Alloys contain the following 3 main elements: Cr, Fe, V with an atomic concentration of between 20 and 40%. They also contain 1 or 2 main elements chosen from Al and Si with a total atomic concentration for these two elements less than or equal to 25%. They also optionally comprise one or more main elements chosen from Mn, Mo, Ti and Ni with a total atomic concentration for these 4 main elements of less than or equal to 35%.

According to the invention, the total atomic concentration for all of the aforementioned main elements is greater than or equal to 80%. The balance may optionally contain minor elements chosen from the list comprising Si, Mn, Mo, Al, Nb, H, B, C, N, O, Mg, Sc, Ti, Cu, Ni, Zn, Ga, Ge, Sr, Y, Zr, Rh, Pd, Ag, Sn, Sb, Hf, Ta, W, Pt and Au. The term “minor elements” is understood to mean elements having an atomic concentration of less than 5%. The balance may also contain residual impurities from processing.

In order to obtain the alloys according to the invention, all the forming processes can be envisaged. It is in particular possible to obtain these alloys by casting, by powder metallurgy processes, by additive manufacturing techniques or even by layer deposition technologies. This also includes possible thermomechanical treatments (heat treatment, hot deformation, cold deformation) and stages of sintering and hot isostatic compression (HIP).

After shaping and carrying out any thermomechanical treatments, the alloys according to the invention mainly exhibit a centered cubic structure (BCC), which can be disordered (structure A2, space group Im3m ) or ordered (structure B2, space group Pm3m ). In particular, a single-phase microstructure can be obtained at ambient temperature for the alloys according to the invention which contain neither Ni, nor Ti as main elements, nor minor elements, which promotes corrosion resistance and polishability. . Nevertheless, depending on the composition and the heat treatments carried out, the alloys according to the invention can exhibit a microstructure with a second phase in the form of precipitates which in certain cases make it possible to improve the mechanical properties (hardness, ductility, toughness, etc. .). When the precipitates are small with sizes which may be nanometric and when the matrix has an almost unchanged composition, i.e. that it retains a composition which satisfies the definition of the alloys according to the invention (phases in multi-element solid solution), good polishability, high corrosion resistance and the absence of ferromagnetism are retained. In particular, the addition of Ni or Ni and Ti is particularly advantageous, since this makes it possible to obtain very hardening nanoprecipitates.

In summary, the alloys according to the invention have, after processing, the following properties required for the trim components: non-ferromagnetic behavior, hardness greater than or equal to 400HV, high corrosion resistance, with in particular no sign of corrosion after salt spray test according to ISO 9227.

Some examples of alloy compositions which meet all these criteria after processing are given in Table 1 below. The alloys were produced by arc melting ( arc melting ) without further heat treatment. In the table, atomic fractions have been rounded to the nearest whole number and hardnesses have been rounded to the nearest ten. <u> Table 1 </u> Compositions (% at) Hardness (HV10) AI10Fe25Cr40V25 450 Al10Fe40Cr25V25 410 Al10Fe25Cr25V40 500 Al10Fe30Cr30V30 410 Al5Cr30Fe30Mo5V30 480 Al6Cr30Fe30Mo5V29 480 Al5Cr30Fe30Si5V30 460 Al5Cr30Fe30Mn5V30 410 Al13Cr25Fe25Ni12V25 650 Fe25Cr25V25Al10Ni10Ti5 630 Cr31Fe31V31Si7 500

It is observed in particular that the addition of nickel makes it possible to significantly increase the hardness, thanks to the formation of nanoprecipitates of NiAI in the matrix with a centered cubic structure.

After casting and a heat treatment of 3 hours under argon at 1300 ° C to homogenize the casting structure, a single-phase microstructure is obtained, in particular for alloys containing only major elements without Ni or Ti, such as, for example, for the alloy Al6Cr30Fe30Mo5V29.

X-ray diffraction analysis (Bragg-Brentano configuration) was performed on this alloy and confirmed that only one phase is present with three lines corresponding to the centered cubic structure. The diffractogram is shown at figure 2 .

Regarding the magnetic properties of this same alloy, a hysteresis curve was measured at room temperature with a vibrating sample magnetometer (magnetization M as a function of the applied field H). Although exhibiting a relatively high volume susceptibility high (4.8 10 ^-3 ), the alloy exhibits a linear behavior signature of the paramagnetic behavior as shown in figure 3 .

It is still possible to improve the properties, particularly the mechanical properties, by adding certain minor elements while maintaining a main phase which satisfies the definition of the alloys according to the invention. It is, for example, possible to add a small amount of boron as a minor element. By adding 0.1% at. of boron to the Al10Cr30Fe30V30 alloy, the hardness is unchanged compared to the same boron-free alloy (410 HV) but, on the other hand, the addition of boron makes it possible to reduce the granular growth after heat treatment and thereby improve ductility and polishability. Adding interstitial atoms such as C, N and O as minor elements also helps to increase hardness.

Claims (11)

1. High entropy alloy with a composition containing between 4 and 9 major elements chosen from the list comprising Cr, Fe, V, Al, Si, Mn, Mo, Ti and Ni with:

   - 3 major elements which are Cr, Fe and V, each having an atomic concentration comprised between 20 and 40%,

   - 1 or 2 major elements chosen from Al and Si each having an atomic concentration greater than or equal to 5% with a total concentration for these 2 major elements of less than or equal to 25%,

   - 0, 1, 2, 3 or 4 major elements chosen from Mn, Mo, Ti and Ni, each having an atomic concentration greater than or equal to 5% with a total atomic concentration for these 4 major elements of less than or equal to 35%,

   the total atomic concentration for all of the 4 to 9 major elements being greater than or equal to 80% and the remainder being made up of impurities and/or one or more minor elements each having an atomic concentration of less than 5%.
2. Alloy according to claim 1, characterised in that the minor element(s) are chosen from the list comprising Si, Mn, Mo, Al, Nb, H, B, C, N, O, Mg, Sc, Ti, Cu, Ni, Zn, Ga, Ge, Sr, Y, Zr, Rh, Pd, Ag, Sn, Sb, Hf, Ta, W, Pt and Au.
3. Alloy according to claim 1 or 2, characterised in that it contains between 0.005 and 0.1% atomic concentration of B as minor element.
4. Alloy according to any one of the preceding claims, characterised in that it contains between 7 and 15% atomic concentration of Ni as major element.
5. Alloy according to any one of the preceding claims, characterised in that the alloy meets one of the following formulae expressed in atomic fractions: Al10Fe25Cr40V25, Al10Fe40Cr25V25, Al10Fe25Cr25V40, Al10Fe30Cr30V30, Al5Cr30Fe30Mo5V30, Al6Cr30Fe30Mo5V29, Al5Cr30Fe30Si5V30, Al5Cr30Fe30Mn5V30, Al13Cr25Fe25Ni12V25, Cr31 Fe31 V31 Si7 or Fe25Cr25V25Al10Ni10Ti5.
6. Alloy according to any one of the preceding claims, characterised in that it includes a single-phase, body-centred cubic solid solution.
7. Alloy according to any one of claims 1 to 5, characterised in that it has a two-phase structure including a body-centred cubic matrix and nanoprecipitates.
8. Alloy according to any one of the preceding claims, characterised in that it exhibits non-ferromagnetic behaviour and does not exhibit signs of corrosion after being subjected to the salt spray test according to ISO standard 9227.
9. Alloy according to any one of the preceding claims, characterised in that it has a hardness HV10 greater than or equal to 400.
10. External component for horology or jewellery, characterised in that it is made from the alloy according to any one of the preceding claims.
11. Component according to claim 10, characterised in that it is chosen from the list including a case middle, a case back, a bezel, a pusher, a crown, a bracelet link, a clasp, a buckle, a prong, a dial, a hand, and a dial index.

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