In-situ nanoscale TEM observation of cooperative deformation mechanisms in nanocrystalline-Au films

Abstract

nGrain boundaries activities are important for the deformation of nanocrystalline metals. Studies revealed that deformation mechanisms of nanocrystalline metals switch from dislocation-mediated regime to grain-boundary-mediated regime as approaching the lstrongestr grain size. Basically, stress-driven grain growth, grain rotation and grain boundary sliding have been widely studied and discussed in the aspect of grain-boundary-mediated deformation mechanisms in nanocrystalline metals. Based on the previous studies, these grain-boundary-mediated deformation mechanisms could perform in either individual forms or cooperative forms. However, limited studies have been focused on the cooperative form mechanisms for the grain-boundary-mediated deformation mechanism, especially for the cooperative grain boundary sliding mechanisms which could lead to more ductile performance than the pure grain boundary sliding. Particularly, only a few molecular dynamic simulation studies and theoretical prediction studies investigated the cooperative grain boundary sliding mechanism in nanocrystalline metals. Therefore, it is necessary to practically examine the true deformation behaviours in straining nanocrystalline metals.In this MPhil project, with utilising the homemade tensile device equipped with a transmission electron microscopy heating holder, in-situ transmission electron microscopy tensile tests are conducted for the magnetic-sputtering-fabricated nanocrystalline Au thin films (thickness ~ 15nm, average grain size ~ 16nm). In the in-situ experiments, the dynamic crack propagation processes of strained nanocrystalline Au films are clearly recorded under nanoscale bright-field TEM mode. It has been found that grain boundary sliding and stress-driven grain boundary migration are prevalent during the deformation processes. As cooperative deformation mechanism, stress-driven grain boundary migration assisted grain boundary sliding is confirmed experimentally. It is also found that cooperative grain boundary sliding mechanismncan be activated both at the crack tips and near the crack tips. On the other hand, the grain coalescence and active partial dislocation activities are occasionally accompanied with the cooperative grain boundary sliding mechanism during the crack tip blunting processes. Additionally, the statistics studies show that the nanotwinned grains located at the blunting crack tips tend to undergo obvious stress-driven grain growth, while the none-nanotwinned grains at the crack tips tend to slightly shrink into small size. However, the tendency of stress-driven grain growth in the nanosized grains that located at the blunting crack tips is independent to the existence of nanotwins. It is found from the statistics results that the tiniest grains (grain size l 10nm) located near the blunting crack tips tend to be totally absorbed by the adjacent larger grains.