Thermal and Elastic Properties of an A2/B2 Refractory High Entropy Superalloy and Its Constituent Phases

The microstructure and temperature dependence of linear thermal expansion and elastic properties of a two-phase, A2 + B2, Al10Nb20Ta15Ti30V5Zr20 refractory superalloy (RSA) and its constituent phases are reported. After slow cooling from 1400 °C, this alloy has a nanometer-sized, two-phase microstructure consisting of Nb-rich BCC (A2 crystal structure) cuboidal precipitates at the volume fraction of 0.63 and continuous channels of a Zr-rich ordered B2 matrix phase. This two-phase microstructure forms by spinodal decomposition of a high-temperature BCC phase. Differential scanning calorimetry shows that the order–disorder transformation in the Zr-rich phase occurs in the temperature range of 550 °C to 850 °C. The coefficient of thermal expansion (CTE) of the Nb-rich BCC phase increases slightly from 8.9 × 10−6 to 9.1 × 10−6 K−1 with increasing temperature from 20 °C to 1200 °C. The Zr-rich phase has CTE of 9.1 × 10−6 K−1 in the ordered (B2) state, at 20 °C to 550 °C, and 12.2 × 10−6 K−1 in the disordered (A2) state, at 900 °C to 1200 °C. The Nb-rich phase has higher Young’s and shear moduli, but lower bulk modulus and Poisson’s ratio, as compared to the Zr-rich phase, at 20 °C to 600 °C. The CTE and elastic properties of the two-phase RSA approximately follow the rule of mixtures of the constituent phases in the studied temperature ranges. Correlations between the morphology of the spinodally decomposed microstructure in the two-phase RSA and the thermal and elastic properties of the constituent A2 and B2 phases are established.