Monday, 23 September, 2013
High Entropy Alloys, a New Class of Structural Materials:
What are they, why do they form, how do we predict them and how many of them are likely to exist?
Presented by Dr. G. Malcolm Stocks, Oak Ridge National Laboratory
The development of metallic alloys is arguably one of the oldest of sciences, dating back at least 3,000 years. It is therefore very surprising when a new class of metallic alloys is discovered. High Entropy Alloys (HEA) represent such a class, a class that is receiving a great deal of attention due to their unusual combinations of strength, ductility, thermal stability, corrosion and wear resistance that make them candidates for technological applications. The term "high entropy alloys" typically refers to alloys that are comprised of 5, 6, 7 or even more elements, at or near equi-atomic composition, that form simple solid solution alloys on simple underlying lattices such as FCC and BCC. The appellation "High Entropy Alloys" refers to an early conjecture that these unusual systems were stabilized as solid solutions by the high entropy of mixing associated with the large number of components. In this presentation, I will present a simple model that provides answers to a number of fundamental questions posed by the very existence of these alloys, not the least of which are: what are the driving mechanisms for their unexpected stability, which combinations of elements can give rise to HEAs, and how does the number of possible alloys depend on the number of constituents species?