Grain growth mechanisms in ultrafine-grained steel: an electron backscatter diffraction and in situ TEM study

Abstract

In this work, the thermal stability of the strongly elongated and textured ultrafine-grained microstructure of a high-strength low-alloy steel processed by linear flow splitting is investigated. The annealing behavior is studied for α- and γ-fiber orientations, which are dominant in the rolling type texture of the material and are known for their differences in stored energy in conventionally cold rolled steels. Electron backscatter diffraction is used to assess contributions from curvature-driven boundary migration while the contribution of strain-induced migration is investigated by comparing the annealing behavior of prerecovered to non-prerecovered samples. The exact nature of the coarsening process is studied using in situ TEM heat treatments and complementary ACOM analysis. The results show that the observed preferred growth of α-fiber grains can be fully described by curvature-driven grain boundary migration and that there is no indication for the relevance of gradients in dislocation density.