Diffusional mass flux accommodating two-dimensional grain boundary sliding in ODS ferritic steel

The interplay between grain boundary sliding (GBS) and atomic diffusion was studied for understanding the fundamental mechanisms of superplasticity and diffusion creep. Two-dimensional GBS was achieved during shear deformation at 900 °C with strain rates of 1.1 × 10⁻⁵–3.3 × 10⁻⁵ s⁻¹ in oxide dispersion strengthened ferritic steel with an anisotropic grain structure, which was designed to minimize the free surface effects including floating grains. Microstructural development during the deformation was observed via electron backscatter diffraction and surface fiducial markers drawn by Ga⁺ focused ion beam. The plastic flow was predominantly mediated by the cooperative process of GBS and grain boundary diffusion, while other mechanisms including intragranular deformation was hardly recognized. The diffusional flux was typically triggered by local principal stress induced at grain boundaries; the matters flew from overlapping (compressive) to splitting (tensile) grain boundaries. In addition, grain boundary morphology changed from wavy to flat patterns via mass flux from convex to concave sides of grain boundaries to minimize the grain boundary energy. Two distinct interplays between GBS and atomic diffusion were confirmed; the most predominant mode was GBS along the shear strain (i.e. Rachinger sliding) and diffusional accommodation via grain boundaries, while a less amount of Coble diffusion creep along macroscopic principal stress was confirmed with GBS accommodation uncorrelated with the shear strain (i.e. Lifshitz sliding).