On the transition to intergranular fracture during high temperature deformation

Abstract

The nature of the transition from transgranular fracture to intergranular fracture by wedge cracking at grain boundary triple junctions or by cavitation at grain boundary precipitates during high temperature deformation of polycrystalline metals is analyzed. To do this, a model material is constructed that is composed of grain matrix and grain boundary “materials”, each having its own steady state deformation law. The physical and adjustable parameters used in these flow laws are fixed in reference to the high temperature deformation behavior of type 316 stainless steel. The flow incompatibility between the grain matrix and grain boundary reflected in these equations can produce stress concentrations sufficient to cause intergranular separation under certain loading conditions. Characteristic times associated with the build-up and relaxation of stress concentrations at precipitates and triple junctions are calculated using these flow laws and the applicable diffusion equation. These characteristic times are compared with the cavity nucleation incubation time, and the conditions for fracture mode transitions are thereby determined. Simple rate ratio criteria obtained from these results indicate that, at a given temperature, wedge cracking occurs below the applied stress level where the ratio of the boundary strain rate to the matrix strain rate equals the ratio of the grain size to the boundary width: dot ε b / dot ε m = L/δ . “r cavitation” occurs below the even lower applied stress level where dot ε b / dot ε m = 10 7 for type 316 stainless steel. Model predictions based on these criteria compare favorably with published data on material deformed monotonically and in fatigue, including the effects of hold periods.