Molecular-dynamics simulations of Ni-based superalloys

Fundamental properties of molten Ni-based alloys are calculated using ab initio molecular dynamics simulations based on Density Functional Theory. In this work predictions for density inversion, an increase in liquid metal density with increasing temperature, are made for a model Ni-Al-W alloy and the Ni-based superallloys RENE-N4 and CMSX-4. Previous calculations on simple, binary and ternary Ni alloys illustrated proof of concept of this method, predicting liquid-phase molar volumes (V(c,T)) within 0.6-1.8% of available experimental data. Simulation cells of 500 atoms, run for approximately 10 psec, were adequate to converge liquid metal densities and diffusion rates. Predicted liquid metal densities for Ni-Al-W alloys, for target compositions and temperatures consistent with 0 and 0.5 solid volume fractions in the melt, produce approximately a 2% density inversion. The liquid metal density for a Ni based superalloy, RENE-N4, is calculated at the liquidus and solidus temperatures expected in the mushy zone. A series of 500 atom instantiations of the superalloy melt produce self-consistent densities that are well within expected numerical error. This illustrates that simulations of moderate spatial and temporal scales sample enough degrees of freedom to properly represent the configuration entropy found in a liquid metal superalloy. Density inversions are predicted for both RENE-N4 and CMSX-4 for chemistries and temperatures consistent with 0 and 0.4 solid volume fractions in the melt.