An In Situ Resistance-Based Method for Tracking the Temporal Evolution of Recovery and Recrystallization in Ni-Base Single-Crystal Superalloy at Super-Solvus Temperatures

Abstract

This study explores the influence of thermo-mechanical behavior and microstructure on recovery and recrystallization in single-crystal superalloys during casting and subsequent solution heat treatment, using miniature testing. Here, the temporal evolution of resistance was measured using in situ electrothermal mechanical testing (ETMT) to track the process of recovery and recrystallization (RX). It was found that recrystallization is dominant only when recovery is incomplete and is dependent on both the history dependence of the strain path as well as the magnitude of the accumulated plastic strain. A precursor to recrystallization is the occurrence of subgrains and deformation twins on the sample surface, where a characteristic butterfly-type morphology of γ′ precipitates is always observed in highly strained regions. The migration of RX grain boundaries is accompanied by the elimination of lattice curvature associated with the density of geometrically necessary dislocations. Homogenized samples provide the most reliable results, while interpreting resistivity changes with recovery and recrystallization is more challenging when inhomogeneity (microsegregation, local variation of mechanical properties) in as-cast material prevails.