First-principles study of the third-order elastic constants and related anharmonic properties in refractory high-entropy alloys

Abstract The third-order elastic constants (TOECs) and elastic anharmonic behavior in four body-centered cubic refractory high-entropy alloys (HEAs) based on elements of the fourth, fifth, and sixth groups are investigated using density-functional simulations. We find that the values of the TOECs C 111 are the largest in magnitude among the studied six independent TOECs and strongly increase with increasing average valence electron concentration (VEC). Interestingly, the TOEC C 456 undergos a sign change as a function of the VEC. Using the obtained TOECs, we investigate the mode Gruneisen constants γ i as well as the low temperature limit γ ¯ , derive the long-wavelength acoustic nonlinearity parameters β, and reveal the pressure derivatives of effective elastic constants and polycrystalline moduli as a function of the VEC. Our results show that β displays a different directional order along the pure mode [ 100 ] , [ 110 ] , and [ 111 ] directions for the four considered refractory HEAs. Furthermore, we show that the directional order of β is not correlated to the crystal symmetry. With the help of the obtained pressure derivatives of polycrystalline moduli, we predict the low temperature volume expansion coefficient.