SUMMARY

Ultrafine grained (UFG) metals are widely used as thin film materials in various applications. These UFG thin films exhibit mechanical properties very different from their bulk microcrystalline counterparts due to microstructural and geometrical size effects on plastic deformation. These size effects lead to new deformation mechanisms in these thin films which are not observed in microcrystalline metals at room temperature. The operative mechanisms operating in nc metals have been identified using atomistic simulations and independent TEM observations but however obtaining a direct link between the measured mechanical properties and the governing mechanisms is still a challenging task. In this proposal we demonstrate new improvements in an already existing MEMS-based nanomechanical testing setup which expand the envelope of the technique to include calculation of signature parameters of plastic deformation mechanisms like true activation volume, using transient repeated stress relaxation tests. Ex-situ tests conducted using this MEMS device on 100-nm-thick gold and 200-nm-thick aluminium microspecimens result in different true activation volumes for the two. The goal of this proposal is to identify the governing mechanism(s) in these thin film microspecimens corresponding to these activation volumes by repeating these calculations in-situ while simultaneously observing the microstructural changes directly in the TEM, which is possible thanks to the small footprint of the set-up.