Variation of the deformation mechanisms in a nanocrystalline Pd–10 at.% Au alloy at room and cryogenic temperatures

Abstract

For the first time, the details of plastic deformation in a nanocrystalline Pd–10at.% Au alloy with an average grain size of 14nm were investigated in compression tests over the temperature range between 4.2 and 300K and the corresponding microstructural changes were analyzed. It was established that decreasing the grain size from 10μm to 14nm resulted in a 4.7–6.4 increase in the applied stress as the temperature was decreased from 300 to 77K. However, a further decrease in the temperature did not lead to an additional increase in the applied stress in these nanocrystalline samples. The nanocrystalline samples revealed an extended microplasticity stage with parabolic strain hardening up to 4.2% strain at room temperature. With decreasing temperature, the strain range over which microplasticity occurred shrank and was down to 2% at 40K. As the samples were deformed in the macroplastic regime, they demonstrated weak strain hardening at room temperature and at 210K, but strain softening was instead observed at cryogenic temperatures down to 40K. Subsequent microstructural investigations revealed that the strain hardening behavior was accompanied by significant grain growth indicating a reverse Hall–Petch relationship. Deformation curves at 10K displayed serrated plastic flow. To explain the observed details of the deformation behavior of nc alloys, a specific deformation mechanism for the nc state is proposed where plasticity was governed by grain boundary sliding (GBS) and was accommodated by the slip of dislocations emitted from grain boundaries. This is based upon the fact that GBS allows for the stress concentration necessary for dislocation emission and that dislocations provide a way to accommodate geometric incompatibilities arising along the GBS path.