A Novel Approach to Determine Variable Solute Partition Coefficients

Proper characterization of solute segregation during the solidification of alloys using partition coefficients (k) is essential to understand and model solidification paths and associated microstructural evolution. Traditional methods to determine k assume a constant value over the course of solidification, which leads to improper descriptions of segregation for elements with variable partitioning behavior. In this study, a novel method to determine partition coefficients as a function of fraction solid using electron probe microanalysis point grid measurements is described. The proposed method, termed the mass balance analysis (MBA), was applied to a cast 90 wt pct Co-10 wt pct Cu alloy with well-documented segregation behavior. The resultant partition coefficients indicated that k_Cu decreased as a function of fraction solid, while k_Co significantly increased in the terminal stage of solidification. The measured k values showed excellent agreement with those taken from the solidus and liquidus lines of Co-Cu binary phase diagram generated using Thermo-Calc. The MBA was also applied to a multicomponent austenitic commercial alloy. It showed a decrease in the partition coefficients for both Cr and Nb as a function of fraction solid and demonstrated the applicability of the MBA method to highly alloyed multicomponent systems.