Effects of solute Mg on grain boundary and dislocation dynamics during nanoindentation of Al-Mg thin films

Using in situ nanoindentation in a transmission electron microscope (TEM) the indentation-induced plasticity in ultrafine-grained Al and Al-Mg thin films has been studied, together with conventional quantitative ex situ nanoindentations. Extensive grain boundary motion has been observed in pure Al, whereas Mg solutes effectively pin high-angle grain boundaries in the Al-Mg alloy films. The proposed mechanism for this pinning is a change in the atomic structure of the boundaries, possibly aided by solute drag on extrinsic grain boundary dislocations. The mobility of low-angle boundaries is not affected by the presence of Mg. Based on the direct observations of incipient plasticity in Al and Al-Mg, it was concluded that solute drag accounts for the absence of discrete strain bursts in indentation of Al-Mg. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.