Intermittent ‘self-locking’ of Grain Growth in Fine-grained Materials

Grain boundary motion may be accompanied with vacancy generation [1–4]. A typical case where a moving grain boundary produces vacancies is in primary recrystallisation, when the growth of a recrystallised grain at the cost of a grain with a large dislocation density involves release of excess free volume. Vacancies are also generated during grain growth, when the total volume of the grain boundary ‘phase’, which is less dense than the bulk, decreases, again leading to release of excess free volume. Interaction of moving grain boundaries with the vacancy atmospheres generated by themselves was shown to affect the grain boundary kinetics [2–4]. The effects are akin to impurity drag of grain boundaries. A kinetic approach taken in Refs. 2–4 made it possible to consider individual grain boundary motion. In the present communication, we look at the vacancy drag of grain boundaries during grain growth in a holistic way. Using a simple thermodynamic argument we shall determine conditions under which grain growth can be ‘suffocated’ by the concomitant vacancy generation. A simple criterion for ‘self-locking’ of grain growth will be presented. On this basis, a picture of intermittent grain growth is proposed in which stages of grain boundary motion are separated by periods of arrest. It is suggested that grain growth induced vacancy generation may be considered as a stabilising effect inhibiting grain growth in fine grained materials. The effect discussed is of particular significance for ultrafine grained polycrystals, notably nanocrystalline materials.