Understanding the effects of recrystallization and strain induced boundary migration on Σ3 twin boundary formation in Ni-base superalloys during iterative sub-solvus annealing

The formation of Sigma 3 twin boundaries in an experimental low stacking fault Ni-based superalloy containing 24 wt % Co was investigated. Normalized microstructures were hot deformed to strain limits of 0.10 and 0.50 at a strain rate of 0.1/s and a temperature of 1020 degrees C. The hot-deformed microstructures were characterized using electron backscatter diffraction, and were subsequently subjected to a series of iterative sub-solvus anneal heat treatments at 1100 degrees C for one to two minutes. By characterizing the changes in the microstructure and grain boundary character distribution as a function of annealing time, this investigation sought to quantify and detail how twin boundary generation and growth occurs during annealing. This behavior was investigated in the same alloy, but with two distinct as-deformed starting microstructures associated with the different strain limits. The starting microstructures possessed varying levels of intragranular misorientation networks, which were found to have greatly affected the resultant formation of Sigma 3 annealing twin boundaries. Strain induced boundary migration (SIBM) was observed to have been restricted in the iteratively annealed sample deformed to a strain of 0.10. Due to the sub-solvus heat treat temperature, grain boundary precipitates were present in the microstructure which limited the mobility of the grain boundaries and suppressed grain growth. As a result, the expansion in the length fraction of Sigma 3 twin boundaries was limited as a function of annealing time. The sample deformed to 0.50 strain, however, was shown to have recrystallized both upon deformation and subsequent annealing. Although recrystallization was found to largely remove much of the pre-existing twin boundaries, the mobility of the newly formed boundaries during growth of the recrystallized grains resulted in a dramatic increase in both the density and length fraction of twin boundaries.