Dynamic recrystallisation via nucleation at distorted twins in a Ni-based superalloy

Abstract

Annealing twins of Σ3 character are common special grain boundaries in face centred cubic metals with low-to-medium stacking fault energy such as Ni-based superalloys. Despite their prevalence, little is known about the detailed evolution of these interfaces during hot deformation, then often referred to as distorted twins, and how their presence contributes to nucleation during dynamic recrystallisation. Here, we developed an automated methodology to systematically analyse large numbers of distorted Σ3 twins in microstructures after hot compression tests using electron backscatter diffraction. We show that annealing twins lose their Σ3 character via a deviation in misorientation angle from the ideal 60°. This is accompanied by a rotation axis shift from 〈111〉 towards 〈110〉. These distorted twins then facilitate discontinuous dynamic recrystallisation. Newly recrystallised grains generally form via twinning phenomena and maintain a special orientation relationship to their neighbouring deformed grains by sharing a close to common 〈111〉 direction. At high deformation temperatures above 1130 °C, most pre-existing twin boundaries are fully transformed into recrystallised grains. This highlights the importance of twin boundaries as nucleation sites for recrystallisation. Furthermore, we demonstrate that automated methodologies are required to enable systematic studies of the interplay between recrystallisation and microstructural features such as Σ3 twins.