Localization of plastic deformation along grain boundaries in a hardening material

The deformation characteristics of ductile polycrystalline materials at elevated temperatures were studied numerically by considering a square segment of material subjected to different stress modes. The initial polycrystalline microstructure was generated numerically. The micromechanical modeling of the deformation was performed using the material point method. The constitutive behavior was taken to be isotropic, elastic–plastic with linear hardening. To effectively simulate the deformation at high homologous temperatures, the grain boundary region was assumed to be a layer of finite thickness, bearing a lower yield strength than the interior of the grain. Complex deformation patterns were observed. The deformation is dominated by the formation of zones of concentrated instantaneous deformation along grain boundaries. These zones form paths that vary with time, and depend on the microstructure and macroscopic loading mode. The macroscopic stress, as well as the microscopic stresses at various locations within the grain boundary and grain interior, were seen to correlate with the instantaneous deformation pattern throughout the deformation history. For the sample analyzed, with a relatively small macroscopic strain of 0.01, the grain boundary region experienced plastic strains as large as 0.20, a result which provided a strong indication that failure will initiate in the grain boundaries. Implications of this modeling study to actual creep deformation and failure are discussed.