The Step Growth Hypothesis for Abnormal Grain Growth

摘要

The implications of the hypothesis that the abnormal grain growth, with or without liquid matrix, occurs by two dimensional nucleation on the boundaries of defect free grains are examined and compared to the observations. The model of coarsening by the growth on steps, produced by two dimensional nucleation, predicts three types of coarsening behaviour: the stagnant growth where the growth of all the grains with singular boundaries is limited by the production of surface steps; the abnormal grain growth (AGG) where some of the grains with singular boundaries grow rapidly by kinetic roughening under relative by large driving forces; and normal growth where the boundaries are rough. The transition from the stagnant growth to AGG and from AGG to nearly normal growth can occur by increasing the heat-treatment temperature and the driving force (by deformation, composition change, or grain size change) and by decreasing the step free energy. The normal growth, however, is possible only if the boundaries are thermodynamically rough with 0 step free energy. Some experimental observations of the dislocation effects are also consistent with the step growth hypothesis for singular boundaries. More quantitative analysis and observations are needed to test this hypothesis.