High Entropy Alloy, Method Of Preparation And Use Of The Same

Abstract

A high entropy alloy includes at least five elements selected from Cobalt, Nickel, Titanium, Zirconium, and Hafnium, wherein two of the five elements have a total atomic percentage of 100-x, and the remainder elements have a total atomic percentage of x, where 0<x<100. A method of producing the high entropy alloy. A component for use in a mechanical timepiece. The component is made of the high entropy alloy.

Description

TECHNICAL FIELD

[0001] The present invention relates to a high entropy alloy in particular a high entropy alloy comprising at least five elements. The present invention also pertains to a method of preparing and use of said alloy.

BACKGROUND OF THE INVENTION

[0002] It is appreciated that metallic materials is particularly important in various industrial applications, including but not limiting to actuators, medical devices, and high precision instruments. Several metallic materials may be generally used for the aforementioned applications, yet each of them possesses different kinds of defects.

[0003] For example, bulk crystalline metals plastically deform through dislocations, twinning and/or grain boundary sliding, which limit the elastic strain of bulk crystalline metals to .about.0.2% at room temperature. The use of bulk amorphous alloys in industrial applications may be hindered by the high cooling rate requirement which severely limits the size of the alloys to be produced. On the other hand, whilst shape memory alloys and gum metals may be capable of achieving a large elastic strain limit (.about.2%), such a high strain limit is generally associated with significant mechanical hysteresis and energy dissipation.

[0004] Accordingly, there remains a strong need for developing metallic materials such as alloy materials that are capable of exhibiting linear elastic response to very high strains without hysteresis, and elastic properties which are temperature-insensitive (to high temperature).

SUMMARY OF THE INVENTION

[0005] The first aspect of the present invention relates to a high entropy alloy comprising at least five elements selected from Cobalt, Nickel, Titanium, Zirconium, and Hafnium, wherein two of the five elements have a total atomic percentage of 100-x, and the remainder elements have a total atomic percentage of x, where 0<x<100.

[0006] Advantageously, the inventors have first devised that by tuning the chemical composition of the high entropy alloy, it may engineer the disorder of the high entropy alloy, thereby providing a new route to create temperature-independent, ultra-elastic behavior in a wide range of materials.

[0007] In particular, the high entropy alloy is represented by a chemical formula of (CoNi).sub.100-x(TiZrHf).sub.x, where x is an atomic percentage and 0<x<100.

[0008] Preferably, the high entropy alloy is represented by a chemical formula of (CoNi).sub.100-x(TiZrHf).sub.x, where 45.ltoreq.x.ltoreq.55.

[0009] In an embodiment, the high entropy alloy has an elastic module that is substantially constant with respect to a temperature change from 300K to 900K.

[0010] In another embodiment, the high entropy alloy comprises a body centred cubic (BCC) structure. In particular, atoms of the high entropy alloy are accommodated within the BCC structure by atomic-scale chemical ordering.

[0011] In another embodiment, the high entropy alloy comprises a distorted lattice structure.

[0012] In another embodiment, the high entropy alloy has an atomic size difference of about 11%.

[0013] In another embodiment, the high entropy alloy has an elastic limit of about 2%.

[0014] The present invention in another aspect provides a method of producing a high entropy alloy as described above. The method comprises the steps of: [0015] preparing an alloy precursor by arc melting a predetermined amount of raw materials of each elements constituting the high entropy alloy in an inert atmosphere; and [0016] casting the melted alloy precursor into a cooled mold to obtain the high entropy alloy.

[0017] In an embodiment, the raw materials comprises Cobalt, Nickel, Titanium, Zirconium, and Hafnium. In particular, the raw materials are in atomic percentages of: 0-50% Cobalt, 0-50% Nickel, 0-33.3% Titanium, 0-33.3% Zirconium, and 0-33.3% Hafnium.

[0018] In another embodiment, the raw materials have a purity of >99.9%.

[0019] In another embodiment, the method further comprises the step of flipping and remelting the alloy precursor in a repetitive manner.

[0020] In another embodiment, the arc melting is conducted under a Ti-gettered argon atmosphere with a pressure below 8.times.10.sup.-4 Pa.

[0021] In another embodiment, the mold is a copper mold. In particular, the copper mold is a cylinder or a plate.

[0022] In another embodiment, the cylindrical copper mold has a diameter of 5 mm and a length of 100 mm.

[0023] In another embodiment, the plate copper mold has a dimension of 5.times.10.times.60 mm.sup.3.

[0024] Further provided with the present invention is a component made of the high entropy alloy as described above for use in a mechanical timepiece. In particular, the component is a mainspring and/or a hairspring.

[0025] In an embodiment, the mechanical timepiece is selected from the list comprising mechanical watches, mechanical chronometers, and marine chronometers.

[0026] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variations and modifications. The invention also includes all steps and features referred to or indicated in the specification, individually or collectively, and any and all combinations of the steps or features.

[0027] Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a plot showing the X-ray Diffraction (XRD) patterns of different alloys having a chemical formula of (CoNi).sub.100-x(TiZrHf).sub.x.

[0029] FIG. 2 is a bar chart showing the microhardness of different alloys having a chemical formula of (CoNi).sub.100-x(TiZrHf).sub.x.

[0030] FIG. 3 is a bar chart showing the Young's modulus of different alloys having a chemical formula of (CoNi).sub.100-x(TiZrHf).sub.x.

[0031] FIG. 4 is a plot showing the storage modulus against different temperature of different alloys having a chemical formula of (CoNi).sub.100-x(TiZrHf).sub.x.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by one skilled in the art to which the invention belongs.

[0033] As used herein, "comprising" means including the following elements but not excluding others. "Essentially consisting of" means that the material consists of the respective element along with usually and unavoidable impurities such as side products and components usually resulting from the respective preparation or method for obtaining the material such as traces of further components or solvents. The expression that a material is certain element is to be understood for meaning "essentially consists of" said element. As used herein, the forms "a," "an," and "the," are intended to include the singular and plural forms unless the context clearly indicates otherwise.

[0034] It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

[0035] In general, it is appreciated that the formation of concentrated multi-component alloys (i.e. high entropy alloys, HEAs) requires minimization of overall atomic size misfit in these alloys so as to form single-phase solid solutions, or otherwise it would favors the formation of amorphous phase structure.

[0036] Without wishing to be bound by theories, the inventors have, through their own research, trials, and experiments, devised a high entropy alloy in which the atomic size misfit may be considered to be too large in view of the conventional solid solution alloy design rules. The high entropy alloy of the present invention may also have an atomic-scale distortions within a single crystalline phase, which renders the high entropy alloy an ultra-large elastic strain limit and negligible internal friction at room temperature, and substantially constant elastic properties up to 900K.

[0037] The high entropy alloy provided by the present invention comprises at least five elements selected from Cobalt, Nickel, Titanium, Zirconium, and Hafnium. In particular, two of the five elements have a total atomic percentage of 100-x, and the remainder elements have a total atomic percentage of x, where 0<x<100. In other words, the high entropy alloy may be represented by a chemical formula of (AB).sub.100-x(CDE).sub.x, where A, B, C, D, and E are not identical and each of which is selected from Cobalt, Nickel, Titanium, Zirconium, and Hafnium.

[0038] In one example, the high entropy alloy may be represented by a chemical formula of (CoNi).sub.100-x(TiZrHf).sub.x, where x is an atomic percentage and 0<x<100. Preferably, the high entropy alloy may be represented by a chemical formula of (CoNi).sub.100-x(TiZrHf).sub.x, where 45.ltoreq.x.ltoreq.55.

[0039] In one example embodiment, the inventors have applied the following equation (Eq. 1) and devised that the high entropy alloy of the present invention may have an atomic size difference that is considered to be too large for forming a stable, single phase structure:

Delta= {square root over (.SIGMA..sub.i=1.sup.Nc.sub.i(1-r.sub.i {square root over (r)}).sup.2)} (Eq. 1),

[0040] where N is the number of constituent elements in the alloy, c.sub.i the atomic fraction of the i-th component, r.sub.i the atomic radius of the i-th component and r the average atomic radius; the atomic radii of the constituent elements are 1.251 .ANG. for Co, 1.246 .ANG. for Ni, 1.578 .ANG. for Hf, 1.462 .ANG. for Ti and 1.603 .ANG. for Zr.

[0041] For example, the value of x may be equal to 50, i.e. the high entropy alloy may have a chemical formula of (CoNi).sub.50(TiZrHf).sub.50, the atomic size difference may be calculated to be about 11%. It is appreciated that in general the Delta value may be of 5 or even less in conventional solid solution alloy design rules/theories. The atomic size difference of the high entropy alloy in the present disclosure would therefore have been considered significantly large and such a large atomic size difference would have destabilized the crystalline structure of the as-formed HEAs, leading to either phase separation or the formation of a single, amorphous structure.

[0042] Unexpectedly, the inventors found that the high entropy alloy of the present invention generally comprises a stable, single phase structure. With reference to FIG. 1, there is provided with a plot showing the X-ray diffraction (XRD) patterns of various alloys. As shown, all the high entropy alloys of (CoNi).sub.45(TiZrHf).sub.55, (CoNi).sub.50(TiZrHf).sub.50, and (CoNi).sub.55(TiZrHf).sub.45 possess a single phase structure without any substantial phase separation as compared with the conventional alloys of (CoNi).sub.100 and (TiZrHf).sub.100, which have a single phase face centred cubic (FCC) and a single phase hexagonal closely packed (HCP) structure, respectively. In particular, the high entropy alloys have a single phase body centred cubic (BCC) structure, particularly an ordered BCC-like structure, i.e. a B2 structure.

[0043] Within the BCC structure, atoms of the high entropy alloy may be accommodated by atomic-scale chemical ordering. In one example, the atoms may be accommodated within the BCC structure in an ordered manner. For example, sites in sublattice A of the B2 structure (corner sites of BCC unit cell) may be occupied at random with {Co, Ni} and sites in sublattice B (center sites of the BCC unit cell) may be occupied at random with {Hf, Ti, Zr}. In another example, the atoms may be accommodated in a partially ordered manner such as with 25% of Zr atoms from sublattice A exchanging with Co and Ni atoms on sublattice B.

[0044] In addition to the atomic-scale chemical ordering, the high entropy alloy of the present invention may have a distorted lattice structure. Namely, the structure of the high entropy alloy may have a combination of atomic-scale chemical ordering and lattice distortion.

[0045] The combination of atomic-scale chemical ordering and lattice distortion within the single phase structure of the high entropy alloy may be advantageous for said high entropy alloy to offer outstanding mechanical properties as compared with conventional alloys and/or HEAs with minimized atomic size difference. For example, with reference to FIGS. 2-4, there are provided with a plurality of plots illustrating the microhardness and Young's modulus of the different alloys. As shown in FIG. 2, all the high entropy alloys possess a microhardness of at least 480 HV, which is apparently higher than those of the conventional alloys (CoNi).sub.100 and (TiZrHf).sub.100. In particular, the microhardness of (CoNi).sub.45(TiZrHf).sub.55 is about 800 HV, which is the highest among the three high entropy alloys.

[0046] Turning now to FIG. 3, there is provided with a plot illustrating the Young's modulus of the different alloys at room temperature. The term "Young's Modulus" generally refers as "elastic modulus" which defines a material's resistance to non-permanent, or elastic, deformation. "Elastic deformation" generally refers to the change of physical state such as shape of the material when an external force is applied thereto, and the material will return to its original state after the stress is removed. It is appreciated that once a material experiences an external force that could overcome such resistance, the material will deform permanently, i.e. the material no longer be able to return to its original state even the external force is removed. Namely, a material with a higher modulus would require a larger amount of external force to overcome such resistance, or in other words a material with a small modulus would deform permanently under such the same amount of external force. As shown in FIG. 3, the high entropy alloys of (CoNi).sub.45(TiZrHf).sub.55, (CoNi).sub.50(TiZrHf).sub.50, and (CoNi).sub.55(TiZrHf).sub.45 possess a Young's modulus at room temperature ranging from about 100 to 130 GPa, which is comparable to the conventional alloys (CoNi).sub.100 and (TiZrHf).sub.100. That is, the high entropy alloys have a comparable resistance to non-permanent, or elastic, deformation as if the conventional alloys at room temperature.

[0047] In addition to the comparable Young's modulus at room temperature, it is advantageous that the high entropy alloys may have an elastic module that is substantially constant with respect to a temperature change up to a temperature of at least 600 K, particularly at least 700 K, preferably at least 800 K, most preferably at least 900 K. For example, with reference to FIG. 4, there is provided a plot illustrating a change of Young's modulus of different alloys against different temperatures. As shown, the Young's modulus of the conventional alloys decreases gradually toward 0 as the temperature increases from 300 K to 900 K, in which (CoNi).sub.100 decreases sharply toward 0 at about 550 K whereas (TiZrHf).sub.100 decreases toward 0 at a constant rate with the temperature increase. In sharp contrast, all the high entropy alloys showed a substantially constant Young's modulus when the temperature increases from 300 K to 900 K. In other words, it may be considered that the elasticity of the high entropy alloys of the present invention remains substantially unchanged with respect to a wide range of temperature, or the high entropy alloys have an Elinvar effect over a temperature of 300 K to 900 K.

[0048] The present invention in another aspect provides a method of preparing the high entropy alloy as described above. The method comprises the steps of: preparing an alloy precursor by arc melting a predetermined amount of raw materials of each elements constituting the high entropy alloy in an inert atmosphere; and casting the melted alloy precursor into a cooled mold to obtain the high entropy alloy.

[0049] In one example, the alloy precursor may be prepared by providing the raw materials in atomic percentages of: 0-50% Cobalt, 0-50% Nickel, 0-33.3% Titanium, 0-33.3% Zirconium, and 0-33.3% Hafnium in an arc furnace. The raw materials may be of a high purity such as >90%, particularly >95%, preferably >99%, most preferably >99.90%.

[0050] The aforementioned raw materials may be melted in an arc furnace under an inert atmosphere. Preferably, the arc furnace is pump-filled with Ti-gettered argon gas, for example, 5 times such that the pressure inside the furnace is less than 8.times.10.sup.-4 Pa.

[0051] During the arc melting process, the raw materials may be flipped and remelted in a repetitive manner so as to ensure chemical homogeneity. In other words, to ensure each of the raw material components are uniformly distributed. Preferably, the raw materials are flipped and re-melted for at least five times.

[0052] Once the raw materials are completely arc melted, the resultant material, that is the melted alloy precursor, may be casted into a cooled mold to form the high entropy alloy. In particular, the melted alloy precursor may be casted into a copper mold of different shapes and dimensions so as to obtain a high entropy alloy of desired shape and dimension. In one example, the melted alloy precursor may be casted into a cylindrical copper mold having a diameter of 5 mm and a length of 100 mm. In another example, the melted alloy precursor may be casted into a plate copper mold having a dimension of 5.times.10.times.60 mm.sup.3.

[0053] As mentioned above, the high entropy alloy of the present invention is advantageous as its mechanical properties, particularly the elastic properties remain substantially unchanged over a wide range of temperature. This property may render the high entropy alloy of the present invention particularly suitable for use in components that require to be operated in harsh environments. Thus, further provided with the present invention is a component for use in a mechanical timepiece, where the component is made of the high entropy alloy as described above.

[0054] In one example, the component may be a mainspring and/or a hairspring of a mechanical timepiece. In particular, the mechanical timepiece may be selected from the list comprising mechanical watches, mechanical chronometers, and marine chronometers. It is appreciated that mechanical timepieces are generally driven by a mainspring of which the force is transmitted through a series of gears to power the balance wheel (including a balance spring (i.e. hairspring)), a weighted wheel which oscillates back and forth at a constant rate.

[0055] As temperature increases, it would significantly affect the timekeeping of the balance wheel and the balance spring as a result of decrease in Young's modulus of the balance spring. Whilst it may be overcome by using a temperature-compensated balance wheel, said temperature-compensated balance wheel is generally inoperable at extremes of temperature. Meanwhile, although some "auxiliary compensation" mechanisms may be used to avoid this situation, all of them suffer from being complex and hard to adjust.

[0056] With the high entropy alloy of the present invention to be used in the mechanical timepiece components, the temperature-insensitive elastic property of the high entropy alloy may allow the component to be operated over a wide range of temperature without the need of "temperature compensation" even at an extreme temperature such as at 900 K. Thus, advantageously, it may simplify the mechanical mechanism and therefore the manufacturing process of the mechanical timepieces.

EXAMPLES

Materials and Reagents Used

[0057] The polycrystalline samples of the high entropy alloy Co.sub.25Ni.sub.25(HfTiZr).sub.50 (atomic %) were prepared through arc-melting in a high purity argon atmosphere. The purities of the raw materials for each element were at least 99.9 wt. %. The ingots were remelted at least four times to ensure the chemical homogeneity, and then suction cast into a copper mold. Two different types of copper mold (rod and plate) were used. The dimension of the cylindrical mold was 5 mm in diameter and 100 mm in length while that of the plate mold was 5.times.10.times.60 mm.sup.3. The single crystal Co.sub.25Ni.sub.25(HfTiZr).sub.50 alloys were prepared by a high rate directional solidification method following the standard procedure.

Instrumentation and Methods Applied

[0058] The X-ray diffraction (XRD) instrument (Rigaku Smartlab) was used to identify the crystalline structure.

[0059] Dynamical mechanical analyses were performed in the commercial DMA equipment (Mettler Toledo, DMA1 STAR System). The experiments were carried out by applying a sinusoidal stress at the fixed frequency of 1 Hz during continuous heating at the constant heating rate of 5 K/min. The testing samples had a dimension of 30.times.3.times.1.5 mm.sup.3. The testing was performed in three-point bending mode.

Example 1

Synthesis and Characterization of Co.sub.25Ni.sub.25(HfTiZr).sub.50

[0060] The high entropy alloy Co.sub.25Ni.sub.25(HfTiZr).sub.50 (atomic %) (i.e. (CoNi).sub.50(HfTiZr).sub.50) was prepared via arc melting method. Unlike conventional single phase HEAs, the atomic size difference of Co.sub.25Ni.sub.25(HfTiZr).sub.50 alloy is extremely large, which is calculated to be about 11% based on Eq.1, relative to standard approaches used for single phase alloy design. It is appreciated that the established alloy phase rules/theories suggest that such a large atomic size misfit will destabilize the crystalline structure, leading to either phase separation or the formation of a single phase, amorphous structure. X-ray diffraction (XRD) result demonstrated that the Co.sub.25Ni.sub.25(HfTiZr).sub.50 high entropy alloy is a single-phase crystal with a nominally body-centered cubic (BCC) like structure.

Example 2

Mechanical Properties of Co.sub.25Ni.sub.25(HfTiZr).sub.50

[0061] The mechanical properties were characterized by microhardness device. The hardness and young's modulus values of these alloys are presented in FIG. 2 and FIG. 3. The elastic strain limit of the Co.sub.25Ni.sub.25(HfTiZr).sub.50 high entropy alloy is about 2%.

Example 3

Elinvar Effect of Co.sub.25Ni.sub.25(HfTiZr).sub.50

[0062] With reference to the FIG. 4, the elastic modulus of the Co.sub.25Ni.sub.25(HfTiZr).sub.50 alloy remained constant as the temperature increases, suggestive of an Elinvar effect (temperature-independent elastic constants). Similarly, the elastic modulus of (CoNi).sub.100-x(HfTiZr).sub.x, where x=45 and 55 are nearly constant with increasing temperature.

Claims

1. A high entropy alloy comprising at least five elements selected from Cobalt, Nickel, Titanium, Zirconium, and Hafnium, wherein two of the five elements have a total atomic percentage of 100-x, and the remainder elements have a total atomic percentage of x, where 0<x<100.

2. The high entropy alloy of claim 1, wherein the high entropy alloy is represented by a chemical formula of (CoNi).sub.100-x(HfTiZr).sub.x, where x is an atomic percentage and 0<x<100.

3. The high entropy alloy of claim 1, wherein the high entropy alloy is represented by a chemical formula of (CoNi).sub.100-x(HfTiZr).sub.x, where 45.ltoreq.x.ltoreq.55.

4. The high entropy alloy of claim 1, wherein the high entropy alloy has an elastic module that is substantially constant with respect to a temperature change from 300K to 900K.

5. The high entropy alloy of claim 1, wherein the high entropy alloy comprises a body centred cubic (BCC) structure.

6. The high entropy alloy of claim 5, wherein atoms of the high entropy alloy are accommodated within the BCC structure by atomic-scale chemical ordering.

7. The high entropy alloy of claim 1, wherein the high entropy alloy comprises a distorted lattice structure.

8. The high entropy alloy of claim 1, wherein the high entropy alloy has an atomic size difference of about 11%.

9. The high entropy alloy of claim 1, wherein the high entropy alloy has an elastic limit of about 2%.

10. A method of preparing the high entropy alloy of claim 1, comprising the steps of: preparing an alloy precursor by arc melting a predetermined amount of raw materials of each elements constituting the high entropy alloy in an inert atmosphere; and casting the melted alloy precursor into a cooled mold to obtain the high entropy alloy.

11. The method of claim 10, wherein the raw materials comprises Cobalt, Nickel, Titanium, Zirconium, and Hafnium.

12. The method of claim 11, wherein the raw materials are in atomic percentages of: 0-50% Cobalt, 0-50% Nickel, 0-33.3% Titanium, 0-33.3% Zirconium, and 0-33.3% Hafnium.

13. The method of claim 10, wherein the raw materials have a purity of >99.9%.

14. The method of claim 10, further comprising the step of flipping and remelting the alloy precursor in a repetitive manner.

15. The method of claim 10, wherein the arc melting is conducted under a Ti-gettered argon atmosphere with a pressure below 8.times.10.sup.-4 Pa.

16. The method of claim 10, wherein the mold is a copper mold.

17. The method of claim 16, wherein the copper mold is a cylinder or a plate.

18. The method of claim 17, wherein the cylindrical copper mold has a diameter of 5 mm and a length of 100 mm.

19. The method of claim 17, wherein the plate copper mold has a dimension of 5.times.10.times.60 mm.sup.3.

20. A component for use in a mechanical timepiece, wherein the component is made of the high entropy alloy of claim 1.

21. The component of claim 20, wherein the component is a mainspring and/or a hairspring.

22. The component of claim 20, wherein the mechanical timepiece is selected from the list comprising mechanical watches, mechanical chronometers, and marine chronometers.