A Method to Predict Transitions in Material Behavior

This work addresses two transitions in material behavior: one, the initial peak in stress response associated with dynamic recrystallization and two, the rapid increase in grain growth rate associated with pore separation from grain boundaries. A criterion is derived that predicts the initial peak in stress response associated with dynamic recrystallization, and another criterion is derived that predicts the rapid increase in grain growth rate associated with pore separation from grain boundaries. The criterion for the initial peak in stress associated with dynamic recrystallization shows the interaction between the rate of dislocation accumulation and the rate of recrystallization, modified by the individual contribution of dislocation density and recrystallized volume fraction. It is the first criterion for dynamic recrystallization that shows explicitly the interaction of internal structure with temperature and strain rate. The criterion for the rapid increase in grain growth rate associated with pore separation shows the interaction between the grain growth rate and the densification rate, modified by the grain size and relative porosity of the material. It is the first criterion for pore separation that explicitly shows the effect of variations in temperature, pressure, and material parameters on internal structure; none of the existing criteria account for all of these quantities at once. Both criteria derived in this work show good agreement with experimental data. The criteria's sensitivity to uncertainties in parameter values is shown. The criteria are presented in two equivalent forms: as algebraic expressions and graphically as processing envelopes. In either form the criteria can assist the planning of component fabrication processes, such as hot rolling or sintering, because components made from materials that sustain an unintended transition in material behavior are rendered useless. Finally, work is presented on a general, structured method to derive a convergence rate criterion for complex transitions in material behavior governed by coupled, simultaneous kinetic processes. Thesis Supervisor: Professor Stuart B. Brown Richard P. Simmons Associate Professor of Materials Manufacturing