Thermodynamics and kinetics of grain boundary triple junctions in metals: Recent developments

摘要

This paper assesses the contribution of grain boundary triple junctions to the driving force for grain growth and the "energetic" effect of boundary junctions on grain growth in nanocrystalline materials. The first measurement of grain boundary line tension allows the quantitative estimation of the fraction of the driving force due to boundary triple junctions. For polycrystals with a grain size in the range approx 50 nm, it is comparable with the driving force from grain boundaries. The traditional concepts of grain growth are based on the assumption of the dominant role of grain bound-aries. A most obvious example of such an approach is the well-known von Neumann-Mullins relation for two-dimensional (2D) grain growth. According to this theory, boundary junctions (triple junctions) do not af-fect grain growth, and their role is reduced to maintain-ing the thermodynamically prescribed equilibrium angles at the points where boundaries meet. A more re-cent approach where triple junctions are considered as grain boundary elements with specific kinetic properties was introduced in Refs. [1-4]. Crystallographically defined grain boundary configurations in tricrystals and novel experimental techniques [1,2] have made it possible to study the steady-state motion of grain boundary systems with triple junction and to measure accurately the mobility of the grain boundaries and triple junctions. Moreover, molecular dynamics simula-tions of triple junction migration were performed for the same geometrical configurations as used in experiment. Overall, the simulations support the exper-imental observations of non-equilibrium triple junction angles and ascertain a substantial triple junction drag [5].