AB-INITIO MOLECULAR DYNAMICS SIMULATIONS OF MOLTEM Ni-BASED SUPERALLOYS (PREPRINT)

Convective instabilities responsible for misoriented grains (freckle defects) in directionally solidified turbine airfoils are produced by variations in liquid–metal density with composition and temperature across the solidification zone. Here, fundamental properties of molten Ni-based alloys, required for modeling these instabilities, are calculated using ab initio molecular dynamics simulations. Previous calculations on elemental, binary and ternary alloys produced liquid-phase molar volumes (V(c,T)) within 0.6-1.8% of available experimental data and demonstrated that the simulations cells of 500 atoms were sufficient to converge properties of interest. In this work density inversion, an increase in density with increasing temperature is assessed in model Ni-Al-W and RENE-N4 alloys. Calculations are performed using a recently implemented constant pressure methodology (NPT) which enables efficient simulation of highly complex alloys. Density inversion (~2%) is found for the Ni-Al-W alloys for target compositions and temperatures consistent with 0 and 0.5 solid volume fractions in the melt. Recently published parameterizations of V(c,T) developed using binary experimental data from a narrow range of compositions do not predict the expected density inversion. The liquid metal density for a Ni based superalloy, RENE-N4, is calculated at the liquidus and solidus temperatures expected in the solidification (mushy) zone. Multiple 500 atom instantiations of the superalloy melt produce densities 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.