Low Temperature Fabrication Processes for Well-Mixed High-Entropy Alloys

Refractory high-entropy alloys (RHEA) have excellent mechanical properties, which creates the potential for them to replace nickel-based superalloys in turbine engines and hypersonic vehicles. Because of their high temperature stability, however, synthesizing these alloys via melt processing can be difficult and expensive. Furthermore, because the individual metals comprising RHEAs typically have disparate melting points, elemental segregation (which can lead to the formation of multiple phases) is often observed. An improved processing technique is needed that will not only avoid the need for high temperatures and the risk of segregation upon cooldown, but also enable good mixing of the precursors to ensure homogeneity. In this project, Ultramet will perform initial development of simple, low temperature processes for the synthesis of RHEAs and Cantor HEAs. On the theoretical front, proven integrated computational materials engineering (ICME) techniques will be used to identify areas in the processing space (composition, temperature, pressure) where RHEAs with a specified composition will not undergo phase segregation. On the empirical front, rather than using the pure metal powders as the starting material, compounds containing the desired metals will be used instead. For example, forming MoNbTaW or HfNbTaTiZr can start with a mixture of oxides. Unlike metal powders, which can be pyrophoric and which tend to form high temperature intermetallics with poor mechanical properties, oxides are chemically stable (i.e., not pyrophoric), they tend to be miscible with each other, and their mixtures tend to have lower melting points. Once a homogeneous mixture of the desired precursor compounds is formed, the resulting mixture can be reduced to yield a homogeneous metal alloy.