Tensile Properties and Phase Stability of Refractory High-Entropy Alloys

Off-equiatomic and pseudo-binary subsets of the TiZrHfNbTa refractory high-entropy alloy (RHEA) were tensile tested to determine effects of composition on strength and ductility. Compositional effects on strength are largely due to effects on shear modulus. On a modulus-normalized basis, strength is relatively constant over a large range of compositions as long as the alloys retain their single-phase BCC solid solution structure. Ductility generally exhibited an inverse correlation with strength (i.e., followed the usual strength-ductility tradeoff). The equiatomic TiZrHfNbTa alloy was creep tested at temperatures to 1373 K. At the tested temperatures, this RHEA was significantly weaker in creep than a two-phase Ni-base superalloy (CMSX-4). It was also weaker than the single-phase solid solution matrix of CMSX-4 which has the close-packed FCC structure as opposed to the more open BCC structure of the RHEA. Additionally, stress-assisted phase decomposition was observed after the creep tests. To investigate phase stability in the absence of stress, the quinary VNbMoTaW and TiZrHfNbTa RHEAs and their lower-order equiatomic subsets (quarternaries, ternaries and binaries) were investigated at temperatures to 1473 K and times to 300 days. Consistent with Hume-Rothery rules, the former alloy and its subsets were mostly stable as single-phase BCC solid solutions given that their constituent elements are all BCC over the entire temperature range. In contrast, the latter alloy and its subsets, being composed of HCP and BCC elements, tended to phase separate. Effects of temperature (vibrational entropy), number of alloying elements (configurational entropy), and crystal structure were quantified and correlated with phase stability.