KSEF RDE: Combinatorial Thin Film Approach to Creating New Single-Phase High Entropy Alloys

The objectives of the proposed research are to use a combinatorial experimental approach to identify promising new high entropy alloy (HEA) materials, and to understand the structure and mechanical behavior of candidate alloys that will serve as models for other HEA systems. HEAs represent a relatively new area of alloy research, with reports of novel materials exhibiting greatly improved mechanical properties. HEAs, also called "multi-principal-elements alloys", generally contain five or more alloying elements. This leads to a wide range of possible alloy compositions and a broad spectrum of microstructures and properties. Systematic evaluations of certain HEA systems have yielded promising HEAs, usually with near-equiatomic stoichiometries. However, identification of "optimal" compositions for a given property or application is hindered by the experimental difficulty of evaluating broad alloying ranges. The first hypothesis for this proposal is that the optimal HEA composition is not necessarily equiatomic - after all, why would such a compound necessarily exhibit the best microstructure or properties? The second hypothesis is that a thin film combinatorial approach is an efficient way to identify promising candidate HEA compositions, for subsequent fabrication in bulk form for comprehensive mechanical testing. It is experimentally impractical to fabricate vast arrays of individual HEA samples with slight composition variations. Instead, combinatorial materials science and thin film fabrication will yield samples with two-dimensional gradients, and interrogate a broad composition range to identify HEAs with relatively simple microstructures (single phase; homogeneous distribution of HEA elements across grains; no intermetallic phases). These candidate compositions and microstructures will guide the fabrication of bulk HEAs via arc melting and then heat treatment, followed by mechanical behavior studies. Funding of this proposal will foster the initiation of new materials research activities in alloy development at the University of Kentucky, and will serve as a basis for additional proposals to NSF.