Effect of thermal longitudinal spin fluctuations on the elastic properties of Ni3Nb

Using an efficient first-principles computational approach, we investigate the influence of the thermal longitudinal spin fluctuations (LSFs) within the nickel sublattices on the thermoelastic properties of Ni3Nb with D022 structure. Leveraging the mean magnetic moment values to represent the spin-fluctuation state at respective temperatures and incorporating the lattice vibration contributions via the Debye model, the equilibrium bulk parameters and elastic constants are derived. We find that, although the volume expansion remains the primary mechanism behind the softening of the elastic constants with increasing temperature, the LSFs effect contributes significantly to the temperature dependence of c11 and c33. Our analysis reveals that including the LSFs makes the theoretical thermal-expansion coefficient highly anisotropic, in line with the experiments. Quantitative agreement between theory and experiment is obtained for the thermal expansion along the c axis at temperatures up to 500 K, whereas the measured thermal expansion along the a axis is underestimated by theory. The present results provide theoretical thermoelastic data for free-standing Ni3Nb precipitates with D022 structure, which can be used to estimate the lattice strains around such precipitates in Ni-based superalloys.