Overall effects of initial melt undercooling, solute segregation and grain boundary energy on the grain size of as-solidified Ni-based alloys

An analysis of the effects of nucleation, growth and recalescence on the primary solidification of undercooled Ni-based alloy melts was presented. If the initial melt undercooling, ΔT, is above the critical value, ΔT*, grain refinement occurs as a result of solid-state recrystallization arising from the strain effect of the recalescence. The grain size of the as-solidified alloy is dominated by its response to recrystallization, i.e. by ΔT. According to Gibbs' adsorption equation and McLean's grain boundary (GB) segregation model, a thermodynamic model was then proposed that attributes the grain size to the combined effects of ΔT, solute segregation, and GB energy, σ_b. In the example systems studied, DD3 superalloy and Ni₇₅Cu₂₅ alloy, the modeling showed close agreement with the experimental results. As compared to Ni₇₅Cu₂₅ alloy, it was interpreted as due to larger enthalpy change of solute segregation and higher solute concentration that lead to smaller grain size in the as-solidified superalloy.