High entropy alloy structure and a method of preparing the same

A method for preparing a high entropy alloy (HEA) structure includes the steps of: preparing an alloy by arc melting raw materials comprising five or more elements; drop casting the melted alloy into a cooled mold to form a bulk alloy with eutectic microstructure therein; and subjecting the bulk alloy to an acidic condition to form a bulk porous structure with eutectic microstructure therein. A high entropy alloy structure is also provided as prepared by the method.

TECHNICAL FIELD

The present invention relates to a high entropy alloy structure and a method of preparing the high entropy alloy structure, specifically, although not exclusively, to a high entropy alloy with eutectic microstructures and a method of preparing a high entropy alloy with eutectic microstructures.

BACKGROUND

With respect to the human history, human civilization has striven to develop, discover and invent new materials for more than thousands of years. Since the Bronze Age, alloys have traditionally been developed according to a “base element” paradigm. That is, choosing one or rarely two principle elements such as iron in steels or nickel in superalloys for its properties, and a minor alloying approach to obtain the alloys. This kind of alloys may be used as coins, gate valves, tools, weapons, etc.

SUMMARY

In accordance with the first aspect of the present invention, there is provided a method for preparing a high entropy alloy (HEA) structure comprising the steps of: preparing an alloy by arc melting raw materials comprising five or more elements; drop casting the melted alloy into a cooled mold to form a bulk alloy with eutectic microstructure therein; and subjecting the bulk alloy to an acidic condition to form a bulk porous structure with eutectic microstructure therein.

In an embodiment of the first aspect, the method further includes step B1, after step B, of rotatably cooling the bulk alloy.

In an embodiment of the first aspect, the method further includes step B1′, after step B, of heat-treating the bulk alloy to form a bulk structure with coarsened eutectic microstructure therein.

In an embodiment of the first aspect, step B1′ includes step B2′ of annealing the bulk alloy to facilitate growing of eutectic microstructures.

In an embodiment of the first aspect, step B1′ further includes step B3′, after step B2′, of water quenching the annealed alloy.

In an embodiment of the first aspect, step C includes step C1 of immersing the alloy into an acidic solution to form the bulk porous structure.

In an embodiment of the first aspect, the method further includes step C0, prior to step C, of cutting the annealed alloy into smaller piece.

In an embodiment of the first aspect, the raw materials are provided in approximately equal atomic ratios.

In an embodiment of the first aspect, the raw materials are Cobalt, Chromium, Iron, Nickel and Niobium.

In an embodiment of the first aspect, Cobalt, Chromium, Iron, Nickel and Niobium are provided in the atomic ratios of 1:1:1:1:0.48.

In an embodiment of the first aspect, the raw materials have a high purity of >99.90%.

In an embodiment of the first aspect, the mold is made of copper.

In an embodiment of the first aspect, the alloy in step A is arc melted within an argon atmosphere with a pressure less than 8×10−4Pa

In an embodiment of the first aspect, the alloy in step B1 is rotatably cooled within an argon atmosphere with a pressure less than 1×10−3Pa.

In an embodiment of the first aspect, the bulk alloy is annealed at a temperature of at least 800° C. or at least 60% of the alloy melting point for at least 5 hours.

In an embodiment of the first aspect, the alloy in step C is immersed into an acidic solution including dilute Aqua Regia at 50-100° C. for at least 2 hours.

In an embodiment of the first aspect, the alloy is rinsed for at least 3 minutes with ethyl alcohol.

In accordance with the second aspect of the invention, there is provided a high entropy alloy structure prepared by the method in accordance with the first aspect.

In an embodiment of the second aspect, the distance between the ligaments of the alloy structure is positively correlated with the temperature and duration of the heat treatment of the alloy in step B1′.

In an embodiment of the second aspect, the hydrophobic property of the alloy structure is positively correlated with the distance between the ligaments.

In an embodiment of the second aspect, the Hydrogen Evolution Reaction (HER) property of the alloy structure is positively correlated with the specific surface area of the alloy structure.

In an embodiment of the second aspect, the specific surface area of the alloy structure is negatively correlated with the size of the ligaments of the alloy structure.

In an embodiment of the second aspect, the high entropy alloy structure processed by the method includes strong and hard ligaments.

In an embodiment of the second aspect, the hardness of the structure is in the range of 60-260 HV.

In an embodiment of the second aspect, the structure obtained in step B1 or B1′ is a dual phase eutectic structure.

In an embodiment of the second aspect, the dual phase includes face centre cubic (FCC) phase and Laves phase.

In an embodiment of the second aspect, the structure obtained in step C is a single phase eutectic structure.

In an embodiment of the second aspect, the single phase includes a Laves phase.

DETAILED DESCRIPTION

High Entropy Alloys (HEAs) are a new kind of alloy typically composed of five or more elements with near equi-atomic ratio and no principal/dominant element. These alloys, however, usually possess relatively a single phase structure, which may lead to a failure in combining different mechanical properties such as strength and ductility.

Without wishing to be bound by theories, the inventors have, through their own research, trials, and experiments, devised a new alloy material, eutectic high entropy alloys (EHEAs) and a method of preparing the same. The EHEAs may contain multiphases with nanometer length scale. Comparing with conventional eutectic alloys, EHEAs having multiple elements in each phase may result in a synergistic effect of multicomponents such that optimal mechanical and functional properties may be achieved. In some embodiments, the EHEAs may further be processed to possess porous microstructures therein, which may allow the EHEAs to be used in various applications.

With reference toFIG. 1, there is provided a block diagram showing the process flow of a method for preparing a high entropy alloy (HEA) structure. The method comprises the steps of: preparing an alloy by arc melting raw materials comprising five or more elements; drop casting the melted alloy into a cooled mold to form a bulk alloy with eutectic microstructure therein; and subjecting the bulk alloy to an acidic condition to form a bulk porous structure with eutectic microstructure therein.

As shown, in step102, an alloy is prepared by arc melting raw materials comprising five or more elements. For instance, specific composition of elements may be selected for forming alloy with desirable eutectic microstructure that would be suitable for various applications. The raw materials may be independently selected from the elements of groups 4-12 in period 4-7 in the periodic table or the elements of lanthanide series in the periodic table, particularly from the elements of groups 4-12 in period 4-7, preferably from the elements of groups 4-12 in period 4-5. Most preferably, the raw materials are Cobalt, Chromium, Iron, Nickel and Niobium. The total weight of the raw materials may be at least 40 grams or above. The elements may also be provided in approximately equal atomic ratios. In this example, the atomic ratios of the raw materials are 1:1:1:1:0.48. Specifically, the raw materials, Cobalt, Chromium, Iron, Nickel and Niobium are provided with an atomic percentage of 22.32%, 22.32%, 22.32%, 22.32%, and 10.72%. The raw materials may be of a high purity such as >90%, particularly >95%, preferably >99%, further preferably >99.90%, or most preferably >99.95%.

The aforementioned raw materials may be melted in an arc furnace under an inert atmosphere. Preferably, the arc furnace is pump-filled with argon gas for at least 5 times such that the pressure inside the furnace is less than 8×10−4Pa.

Once the raw materials are arc melted, the resultant material, that is the melted alloy, may be drop casted into a cooled mold to form a semi-finished product in step104. Preferably, the melted alloy may be drop casted into a copper mold cooled with water so as to obtain a bulk alloy with eutectic microstructure.

The thus-obtained bulk alloy may then be subjected to a specific heat treatment107so as to tune the optimum size of the microstructure therein. The heat treatment107involves steps108and110. In step108, the bulk alloy is annealed to facilitate growing of the eutectic microstructures i.e. microstructure evolution by ligament coarsening to micro meter scale from nanometer scale. To carry out the annealing process, the bulk alloy may be heated to at least 800° C., particularly at least 900° C., preferably at least 1000° C. or to a temperature at least 60% of the alloy melting point for at least 5 hours, preferably at least 6 hours in the furnace. In one example, the temperature and the duration of the annealing process may influence the growth of microstructures. It is appreciated that a skilled person may adjust the annealing temperature and duration according to their technical needs to provide different properties for serving different purposes.

The annealed alloy is then taken out from the furnace and directly quenched with water so as to obtain a bulk alloy with coarsened eutectic microstructures therein in step110.

Afterwards, the annealed alloy may be further processed by cutting into smaller pieces in step112, followed by immersing these small pieces into an etching solution so as to obtain a bulk porous structure in step114. The process may also refer as a dealloying process. The etching solution may be an acidic solution particularly a dilute Aqua Regia. Preferably, the etching process is carried out by immersing the small pieces of annealed alloy into the dilute Aqua Regia under a water shower at 50-100° C. for at least 2 hours. Finally, the aforementioned alloy may be rinsed for at least 3 minutes with ethyl alcohol to remove any residues or acidic solution left behind in step116. As such, a porous HEA with eutectic microstructures therein is obtained.

In another embodiment, the melted alloy obtained in step102may be drop casted to a mold such as a copper mold cooled by water, liquid nitrogen or the like in step104to obtain a bulk alloy. The thus-obtained bulk alloy i.e. the as-cast alloy may be directly proceeded to steps112to116to form a porous as-cast HEA structure with eutectic microstructures therein.

In yet another embodiment, the melted alloy obtained in step102may be rotatably cooled in step106to form an as-spun alloy. Preferably, the melted alloy is rotatably cooled during a melt spinning process under an inert atmosphere. In this example, the melt spinning process is carried out in an area pump-filled with argon such that the pressure within the area is less than 1×10−3Pa. The as-spun alloy obtained in step106may then be directly etched in step114and rinsed in step116to obtain a porous as-spun HEA structure with eutectic microstructures therein

By going through different preparation steps, the HEAs prepared may have various morphologies and microstructures. For example, the microstructures may have different forms, spaces, distances, etc. which may in turn affect the properties of the HEAs. The morphologies and microstructures of the prepared HEAs may be characterized by methods known in the art such as scanning electron microscopy (SEM).

With reference toFIGS. 2A and 2B, there are provided the SEM images of HEAs prepared by the method as described above. In this example, the HEA is an as-cast alloy202obtained in step104without undergoing the annealing process108. The as-cast alloy202was directly cut into smaller pieces and etched prior to SEM imaging. As shown inFIG. 2A, the HEA structure202showed a uniform lamellar structure. Upon magnifying the surface to 8000× (FIG. 2B), it was found that the surface of HEA structure202was occupied by ligament- or lamellar-like microstructures with around 100 nm length scale. The microstructures were closely packed with limited space between each of the ligaments. This may be advantageous in that the HEA structure202may provide a tremendous specific area for various applications such as catalysis.

With reference toFIGS. 3A to 3D, there are provided the SEM images of annealed HEAs prepared by the method as described above. In this embodiment, the HEAs302and304are as-cast alloys obtained in step104and being annealed at 1000° C. and 1200° C. for 6 hours, respectively. The annealed HEA was cut into smaller pieces and etched prior to SEM imaging. As shown, the surfaces of the annealed HEA structures302(FIG. 3A) and304(FIG. 3B) were much rougher as compared the HEA structure202(FIG. 2A). The surfaces of the annealed HEA structures302and304include a plurality of porous space and isolated ligament-like structures. In particular, the ligaments in HEA304(FIG. 3D) was found to be less continuous as compared with those in HEA302(FIG. 3B). That is, the distance between each of the ligaments in HEA304(FIG. 3D) were generally larger than those in HEA302(FIG. 3B). This may suggest that the distance between each of the ligaments in HEA is positively correlated with the temperature and/or duration of the annealing process. As such, it may be advantageous in that the distance between the ligaments and therefore the properties of the HEA may be tuned readily.

With reference toFIGS. 4A and 4B, there is provided the SEM images of an as-spun HEA as prepared by the method described above. In this example, the as-spun HEA402obtained from step106was directly etched, followed by being characterized with SEM. As shown inFIG. 4A, the as-spun HEA402possesses a rough surface. The magnified images ofFIGS. 4A to 4Bindicated that the surface was occupied by a plurality of globular structures being connected with a ligament network. That is, there are two forms of structure observed in the as-spun HEA402resulting from the rapid cooling of the melt spinning process and the dealloying process.

Without wishing being bound by the theories, the inventors devised that the HEA prepared by the aforementioned method possesses multiphases particularly dual phases. With reference toFIG. 5A, there is provided an X-ray diffraction (XRD) diagram showing the X-ray diffraction pattern of the HEA structures prepared by the aforementioned method without undergoing the etching step114. As shown, the as-cast HEA202, the annealed as-cast HEAs302and304as well as the as-spun HEA402all possess dual phases, namely face centre cubic (FCC) and Laves phases.

The inventor further devised that any of the HEA structures mentioned above may have a single phase structure upon subjecting to the etching step114. As shown inFIG. 5B, there is provided an XRD diagram of an as-cast HEA structure502obtained in step104of the aforementioned method. The HEA structure502was etched in accordance with step114prior to XRD analysis. It is clear fromFIG. 5Bthat the HEA structure502possesses a single phase, namely the Laves phase. This may be a consequence of a selective dealloying of the FCC phase microstructures, leaving the Laves phase microstructures behind and therefore a porous HEA structure. As a result, large specific area of mixed multi transitional metal is exposed, which enable them a great potential of variety of applications.

With reference toFIGS. 6A to 6C, there are provided the SEM secondary electron images of indentation on the HEAs as prepared in the aforementioned embodiments. In each of the figures, there is a solid square with dashed diagonal lines indicating a residual indent, which represents the location where an indenter was applied. Each of the HEAs discussed below were subjected to the etching process in step114prior to analysis. As shown inFIG. 6A, the as-cast HEA202, in the absence of the annealing step108, possesses a plurality of residual cracks602. Such residual cracks are missing from the annealed HEAs302and304as shown inFIGS. 6B to 6C, which suggests the importance of the annealing process108in providing strong and hard ligaments in the HEAs. It is aware by the skilled person in the art that the indentation size may be used to measure the hardness and estimate the density of a material. In this example, the hardness of the HEAs was measured as 60-260 HV whereas the density was estimated to be about 4 g/cm3.

As mentioned above, one of the advantages of the present invention is that the distance and size of the ligaments of HEAs may be tuned by adjusting the annealing temperature and duration. This may in turn adjust the specific surface area as provided by the HEAs for various applications such as catalysing hydrogen evolution reaction (HER) to generate hydrogen.

The size of the ligaments of the HEAs as described above may affect the HER property of the HEAs. Preferably, the HER property of HEAs is positively correlated with the specific surface area of the HEAs, whilst the specific surface area of the HEAs is negatively correlated with the size of the ligaments of the HEAs.

With reference toFIG. 7, there is provided a plot of current density against potential showing the HER property of HEAs prepared in the aforementioned embodiments with respect to a commercial Ni foam. As shown, with reference to a constant current density such as 10 mA cm−2, the over-potential values of the porous as-cast HEA202as well as the porous annealed as-cast HEAs302,304obtained in step114were determined to be 0.17V, 0.26V, and 0.27V respectively whereas the over-potential value of the commercial Ni foam was determined to be 0.2V. It is appreciated that since a smaller over-potential value indicates a higher reactivity of a material, the porous as-cast HEA202with the smallest ligament size among all three HEAs showed a superior HER property over the commercial Ni foam.

The distance between the ligaments of the HEAs as described above may also affect the hydrophobic property of the HEAs. Preferably, the hydrophobic property of the HEAs is positively correlated to the distance between the ligaments.

With reference finally toFIGS. 8A and 8B, there are provided images showing the contact angles of the porous annealed HEAs with silanization modification. The term “contact angle” is the angle between a drop of water and a flat and horizontal surface i.e. the surface of the HEAs upon which the droplet is placed. It is appreciated that a material may be considered as hydrophobic if the contact angle for water of greater than about 90°, preferred greater than about 100° and more preferred of about 110°. In this example, the contact angles for the porous annealed HEAs302and304were determined to be 134.5° and 140° respectively. That is, the hydrophobicity of the HEA304annealed at a higher temperature is higher than that of the HEA302annealed at a lower temperature.

The present invention is advantageous in that the HEA possesses microstructures that can be tuned by adjusting the processing conditions such as the annealing temperature and duration. The length scale of the microstructures may be tuned from several tens of nanometers to several microns. With different microstructures, the HEAs may have superior hardness, total density, hydrophobicity as well as large specific surface area for catalytic applications. In particular, the total density of the presently claimed HEAs may approach the commercial light weight alloy such as TiAlV alloy and can be used to fabricate small light weight devices.

In addition, the method of the present invention involves easy and inexpensive procedures. The method may also be used to produce a HEA structure with a size of, for example 100 mm by 10 mm by 1 mm, which is larger than similar structure fabricated by other techniques.