Translation-Rotation Plastic Flow in Polycrystals under Creep

Abstract

Grain boundary sliding is the primary process of plastic flow in polycrystals under creep which is accommodated by multiscale translation-rotation modes of intragranular plastic deformation. The steady stage of creep is characterized by constant creep rates which is related to slow rotation of sliding dislocations and material mesofragmentation. The rate of the tertiary creep is being increased which is related to the similarity of grain boundary sliding and plastic flow within near-boundary zones, where high lattice curvature arises. Here we develop a nonlinear theory of micro, meso- and macroscale plastic flow and propose a constitutive equation for its velocity, rotation modes, local irreversible stress, temperature, and heat and mass flux densities with regard to structural transformation. The theory agrees well with experimental data on multiscale translationrotation in Al polycrystals under creep, suggesting that fracture in tertiary creep is caused by a multiscale increase in lattice curvature in a deformable polycrystal. Fracture is initiated at the interface of grain boundary sliding and near-boundary regions where lattice curvature results in cracks.