In situ transmission electron microscopic observations of deformation and fracture processes in nanocrystalline palladium and Pd90Au10

In situ tensile tests were performed in a transmission electron microscope (TEM) so that the deformation and fracture processes in nanocrystalline Pd and Pd90Au10 could be observed directly. High-resolution electron microscopy was used to characterise the events. Material with average grain sizes of 10 and 65 nm, prepared by inert-gas condensation and repeated cold rolling and folding (RCR), respectively, were used in this investigation. Intergranular fracture was the materials' main response obtained in tensile testing and it was observed for both grain sizes, which indicates an early onset of the limit to plastic flow in the samples. This, in our opinion, is because dislocation-based deformation processes are not able to operate sufficiently within such small grains to relax the stress concentration prior to fracture. Moreover, it was found that deformation twins had formed due to the stress field of the advancing crack tip during the in situ tensile test and subsequently provided favourable intragranular propagation paths for the crack tip. However, it is also emphasised that such deformation twins could be observed only in very few grains (about 1-2% of the total number of grains) next to the crack.