Localization of plastic deformation along grain boundaries in a hardening material

The deformation characteristics of ductile polycrystalline materials at ele vated temperatures were studied numerically by considering a square segment of material subjected to different stress modes. The initial polycrystalli ne microstructure was generated numerically. The micromechanical modeling o f the deformation was performed using the material point method. The consti tutive behavior was taken to be isotropic, elastic-plastic with linear hard ening. To effectively simulate the deformation at high homologous temperatu res, the grain boundary region was assumed to be a layer of finite thicknes s, bearing a lower yield strength than the interior of the grain. Complex d eformation patterns were observed. The deformation is dominated by the form ation of zones of concentrated instantaneous deformation along grain bounda ries. These zones form paths that vary with time, and depend on the microst ructure and macroscopic loading mode. The macroscopic stress, as well as th e microscopic stresses at various locations within the grain boundary and g rain interior, were seen to correlate with the instantaneous deformation pa ttern throughout the deformation history. For the sample analyzed, with a r elatively small macroscopic strain of 0.01, the grain boundary region exper ienced plastic strains as large as 0.20, a result which provided a strong i ndication that failure will initiate in the grain boundaries. Implications of this modeling study to actual creep deformation and failure are discusse d. (C) 2000 Elsevier Science Ltd. All rights reserved.