The potential of new metal alloys

Traditionally, alloy development has focused on refining the synthesis process or adjusting the component combination of conventional metal alloys.

In 2003 Brian Cantor, now Professor emeritus at Oxford University, and other colleagues published research into developing new alloys. A separate paper published a few months later by Jien-Wei Yeh, a materials scientist at National Tsing Hua University, named them high-entropy alloys (HEAs).

These HEAs (also named compositionally complex alloys, multicomponent alloys, or multi-principal component alloys) altered the traditional way alloys were made which was by combining one base element with a small percentage of another 1 or 2 elements.

In HEAs, no single element dominates – rather, they are combinations of 4 or more metallic elements, often (though not exclusively) mixed in equal proportions. The opportunity to use 60 to 70 elements instead, creates the potential for many new metal alloy materials to be discovered – besides the additional possibilities offered by ceramics and oxides.

Because combining various elements alters the thermodynamic factors driving the formation of different phases (homogenous states of matter) in the materials, it affects a material’s macroscopic properties such as strength, hardness and ductility.

Cantor and colleagues found areas within the alloys they made in which a single phase, consisting of cobalt, nickel, chromium, manganese and iron, predominated. Together, they combined ductility and high strength and instead of becoming increasingly brittle at ultracold temperatures, they became tougher.

Clearly HEAs, as a source of new structural materials which don’t melt at hotter temperatures, are lighter, tougher, and more resistant to hydrogen embrittlement or corrosion, have potential to transform many functional applications such as batteries, thermo-electrics, photovoltaics and catalysts.

With the search for specific properties such as density or melting temperature in mind, the development of appropriate new alloys could be enhanced by computational screening applying tests which eliminate incompatible contenders.

A computational tool called calculation of phase diagrams (CALPHAD) is uses data to predict the phase structures of alloys at different temperatures, pressures, and compositions, highlighting the potential of certain compounds for specific applications. Besides eliminating unsuitable alloy combinations, the subsequent testing data can then be re-used to refine the machine learning for better subsequent predictions.

Though still a challenging area, combinational and high-throughput (CHT) screening can test new alloys to gather information about their properties. New materials will open the door to new technologies and the potential for new alloys may provide a rich seam for exploration.