On the origin of superplastic flow at very low stresses

An important characteristic of the deformation behavior of micrograin superplastic alloys is the experimental observation that the relationship between stress and steady-state creep rate is often sigmoidal. Under creep conditions, such a sigmoidal behavior is characterized by the presence of three regions: region I (the low-stress region), region II (the intermediate-stress region or the superplastic region), and region III (the high-stress region). Experimental results and theoretical consideration have suggested that regions II and I are controlled by the same deformation process, in which the sliding of groups of grains is accommodated by dislocation motion in the blocking grains, and that region I arises from the presence of a threshold stress. The origin of this threshold stress is discussed in terms of impurity segregation either at boundaries or at dispersion particles in the interiors of the blocking grains.