A triple-scale dislocation-crystal plasticity simulation on yield point drop of annealed FCC ultrafine-grained metal

Annealed ultrafine-grained metals contain some grains with extremely low dislocation density, so that the critical resolved shear stress increases at the first stage of deformation due to the exhaustion of dislocation sources in a grain. In this paper, in order to express the increase of critical resolved shear stress, the conventional Bailey-Hirsh's relationship is extended on the basis of physical consideration for grain boundary that plays a role of dislocation source. A triple-scale dislocation-crystal plasticity FE simulation based on the above model, geometrically necessary crystal defects and the homogenization method is carried out for annealed FCC polycrystals with different initial grain size and initial dislocation density. Yield point drop and propagation of Lüders bands observed in macroscopic specimen with annealed FCC fine-grains are numerically reproduced. Moreover, macroscopic yielding of specimen and microscopic grain yielding are investigated in detail so as to clarify the initial yield behavior of annealed ultrafine-grained metals. It is also shown that plastic deformation is easy to be localized and the tensile ductility decreases as the grain size reduces.