High Entropy Alloy Synthesis by Arc Melting
The all metallic element pieces in equi-atomic proportion are placed in the Copper crucible under a vacuum or inert gas environment and then melted by electric arc using a Tungsten electrode. The melted material is solidified by cold water. This process is repeated multiple times to ensure the homogeneous mixing of all metallic elements. A casted ingot is characterized for its single-phase formation using X-ray diffraction method.
Four Core Effects - High entropy alloys exceptional properties explained using four know effects. some more expected in near future
Cocktail Effect
High entropy alloy cocktail
It is one of the effect in four core effects of high entropy alloys, which introduced the functional properties in high entropy materials. It is similar to making a cocktail of many juices mixed with flavouring ingredients.
“Cocktail is a stimulating liquor, composed of spirits of any kind, sugar, water, bitters, and shaken with ice”
or
Specific drink formula rather than a category of mixed drinks.
Similarly, in High entropy alloys:
A specific metal formula to be mixed (metallic solution) and activate specific functional property.
See in the Figure in catalyst materials design, the metals have been selected from two categories materials. The metal with high absorption strength (Co, Pd, Pt, Ni) of CO and H2 and metal with low absorption strength (Zn, Ag, Au, Ga) of CO and H2 were taken and designed a specific formula AgAuCuPdPt. This metal combination works ideally compared to other noble metals or other catalysts.
How this work: Suppose Pt catalyst has high absorption strength with Carbon mono-oxide gas (CO) and these gas molecules poison the catalyst surface by blocking the active sites. However, diluting the Pt with low absorption strength metals (HEA alloy formation) enhance the desorption of CO, and the catalyst work with high efficiency.
2. Lattice distortion
If, All five elements of high entropy alloy are arranged in ascending or descending order of their atomic radius. There is a sufficient difference between the first and last elements radius to impart lattice distortion in a single unit cell.
Nanocrystalline High Entropy alloys as Multifunctional Materials
HEAs way toward multifunctional materials and machine learning
The high entropy alloys have become the most intensely researched materials in recent times. They offer the flexibility to choose a large array of metallic elements in the periodic table, a combination of which produces distinctive desirable properties that are not possible to be obtained by the pristine metals. Over the past decade, a myriad of publications has inundated the aspects of materials synthesis concerning HEA. Hitherto, the practice of HEA development has largely relied on a trial and- error basis, and the hassles associate with this effort can be reduced by adopting a machine learning approach. This way, the “right first time” approach can be adopted to deterministically predict the right combination and composition of metallic elements to obtain the desired functional properties.
Kumar et al. , Emergent Materials, (2021)
Mechanical milling/Pulverization/Comminution/Grinding
Free standing nanocrystalline High entropy alloy Nanoparticles
The preparation of free-standing high entropy alloy nanoparticles is not as simple as other materials. It has many challenges due to five or more than five components or elements. In the case of wet chemical synthesis, there is a huge challenge as each metallic element salt/ precursor has a different rate of reaction, leading to the formation of intermetallics in the materials. Suppose the first two precursors having a high rate of reaction will form the product first compared to the other three. Cumulative reactions with each other are leading to form multiphase materials and loss the unique HEA materials properties.
Therefore, I have developed a method to retain high entropy single-phase materials and obtain the HEA nanoparticles without support from other substrates or free nanoparticles. It is Casting-cum-comminution (CCC) method. In this method, the casted high entropy alloy ingot parted into pieces and then milled at 123 K temperature to form free-standing HEA nanoparticles.
The milling below 123 K is known as cryomilling, and it has many benefits to synthesize free-standing nanoparticles.
1) The reduced oxidation rate of metallic components or nanoparticles
2) Suppression in the cold welding
3) Early-stage ultra-refinement
4) A negligible amount of foreign contamination from milling tools
5) The synthesis process is a purely Green synthesis
Casting-Cum-Comminution method-An approach to synthesise Nanocrystalline high entropy alloys
1) Oxidation of metallic surface at low or cryo-temperature
In ball milling, the native surfaces formed due to multi fracture events, which surface prone to oxidize rapidly.
The natural oxide skin of several nm thickness on the surface of the particles is a severe problem of metallic powders. This oxidation problem can be severely reduced at cryotemperature, as shown in Figure. The oxide growth on the Copper surface at different temperatures with time. At 78 K there is no oxide film growth due to low temperature.
2) Suppression in the cold welding
Thermal Spray Coating of High Entropy Alloy at LSBU
vide_therm.mp4