Phase Stability and Mechanical Behavior of BCC Refractory High Entropy Alloys

High entropy alloys (HEAs) have become a popular framework for alloy design, due to the multitude of compositional knobs that can be adjusted; in contrast to the dilute solutions of previous generations. Refractory HEAs (RHEAs) have garnered interest due to their potential usage as new structural materials for high-temperature applications, as their high melting temperatures exceed those of nickel-based superalloys. Here, body-centered cubic RHEAs are explored using model alloy systems based on Ti-Zr-Nb-Hf-Ta and V-Nb-Mo-Ta-W.

The first work looks at the equiatomic TiZrNbHfTa and VNbMoTaW, and all the equiatomic quaternaries, ternaries, and binaries that can be formed from their constituent elements, to assess compositional effects on single-phase stability. Three constituent elements in TiZrNbHfTa (Ti, Zr, Hf) have the HCP crystal structure at room temperature while the remaining two (Nb, Ta) are BCC. In contrast, all the elements in VNbMoTaW have the BCC structure. Phase stability was assessed after isothermal aging at 800°C, 1000°C, and 1200°C for times of 72 to 7200 h. It was found that some compositions have an island of instability at lower temperatures, but most can be single-phase above that. From these results, it appears that design of single-phase alloys should take into account the crystal structures of the elements and the mutual solubilities of their constituent binary pairs.

The second work looks at a pseudobinary (TiZrHf) – (NbTa) system, designated as (HCP_eq) – (BCC_eq), where HCP_eq is the sum of the concentrations of elements with HCP crystal structure at room temperature and BCC_eq that of the BCC elements. Five compositions ranging from 0.0 – 0.4 BCC_eq were investigated. Three compositions (0.2/0.3/0.4 BCC_eq) were able to be cold-rolled to 80% reduction, recrystallized, and tensile-tested. They showed that as the BCC_eqincreased, so did the yield stress and uniform elongation. This increase in yield strength is attributed to increasing shear modulus with increasing BCC_eq.

The final work looks at off-equiatomic TiZrNbHfTa derivatives: a Ti-rich and Zr-rich composition. Mechanical testing was performed at 293 K and 77 K. While the Ti-rich alloy exhibited cryogenic ductility, the Zr-rich elongation was reduced by ~30% and the equiatomic composition was embrittled at 77 K.