A castability model based on elemental solid-liquid partitioning in advanced nickel-base single-crystal superalloys

Abstract

A castability model that accounts for the characteristic segregation behavior of constituent elements in Ni-base superalloys has been developed and experimentally verified in production scale casting trials. The model ranks alloy compositions with respect to their susceptibility to freckle formation during directional solidification. Thirty-nine distinct Ni-base single-crystal superalloys encompassing a broad range of compositions were investigated to assess the influence of the constituent elements on their solidification characteristics. Linear regression was applied to the fitted solid-liquid partition coefficients of the major constituent elements to develop formulas capable of describing elemental interactions. The high-density refractory elements Ta, W, and Re were found to segregate most severely during solidification. Increasing the amount of Cr and Mo in high-refractory single-crystal alloys reduced the extent of W and Re microsegregation during solidification. This effect was found to minimize the occurrence of freckle defects due to the corresponding decrease in the density inversion term, which is effectively the driving force for thermosolutal convective instabilities known to cause macroscopic grain defects during single-crystal solidification. Model predictions were validated using production scale casting trials where additions of 1.5 wt pct (1.9 at. pct) Cr and 3.0 wt pct (2.0 at. pct) Mo to a high-refractory superalloy more than halved the number of solidification-related grain defects formed. These findings suggest that elemental interactions between Cr, Mo, W, and Re need to be considered when optimizing alloys for high-temperature creep properties.