PNNL

Abstract

The evolution of microtwins during high temperature creep deformation in a strengthened Ni-base superalloy has been investigated through a combination of creep testing, TEM characterization, theoretical modeling and computer simulation. Experimentally, microtwin nucleation sources were identified and their evolution was tracked by characterizing the deformation substructure at different stages of the creep deformation. Initially, deformation is highly localized around stress concentrators such as carbides, borides and serrated grain boundaries, which act as sources of a/2 matrix type dislocations. Due to microstructural effects such as fine channels between particles and low matrix stacking fault energies, the a/2 matrix dislocations dissociate into a/6 Shockley partials, which were commonly observed to be decorrelated from one another, creating extended intrinsic stacking faults in the matrix. As deformation progress further, microtwins form via partial dislocations cooperatively shearing both and phases on adjacent {111} glide planes. The TEM observations lead directly to an analysis of dislocation-precipitate interactions. Through phase field simulations and theoretical analysis based on Orowan looping, the important processes of dislocation dissociation and decorrelation are modeled in detail, providing comprehensive insight into the microstructural features and applied stress conditions that favor the microtwinning deformation mode in strengthened Ni-based superalloys.