The evolution of internal energy released from nanomaterials during grain growth

Abstract

AbstractThe driving force for limited thermal stability of Nano‐Poly‐Crystals is high concentration of internal energy. This makes them to undergo significant changes when exposed to temperatures exceeding one‐third their melting points. Two processes are expected to take place under such temperatures which reduce the internal energy: (a) grain growth i.e. reduction in total grain boundary (GB) surface area or GB volume fraction and (b) reduction of GB energy per surface area. This paper proposes a theoretical approach that couples knowledge from energy method with that from stochastic differential equations to study the energy released from nanometals during grain growth. The basic Physics of the mechanisms leading to grain growth in nanomaterials is applied. The proposed model is tested on polycrystalline nano‐aluminium samples. The impacts of annealing time, mean grain size, grain size dispersion and annealing temperatures are tested. It is anomalously revealed that on comparing the various mechanisms of grain growth, the greatest amount of energy is released from nanomaterial if grain growth is due to Grain Rotation‐Coalescence (GRC) mechanism only, followed by that due to simultaneous GRC and Grain Boundary Migration (GBM) processes and least by that due to GBM only. The energies involved in T1 events, T2 events and in rotating the grains before coalescence during GRC have also been dealt with. On comparing the different physical activities taking place during growth, it is revealed that the largest amount of energy is released from materials in which grains rotate, translate and grow at constant force. This is followed by the amount of energy involved with rotating or translating the grains at constant size and least by the energy released due to changing grain size at constant force. It is also found that the rate of release of energy initially decreases rapidly, and then very slowly approaching zero during grain growth. Thus, one may conclude from the principle of stationary potential energy that the nanomaterial attains energy equilibrium in the long run. Thus, the driving force for grain growth at larger grain sizes is not more the grains' internal energy concentration. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)