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

•Deformation behavior of a nc Pd alloy (d = 14 nm) was studied down to 4.2 K.•The variation of deformation mechanisms with temperature was revealed.•A model explaining strong strain hardening at microplasticity stage is proposed.•A stagnation of applied stress at temperatures <77 K was demonstrated.•Serrated flow with extremely high amplitude of applied stress jumps at T < 40 K.

For the first time, the details of plastic deformation in a nanocrystalline Pd–10 at.% Au alloy with an average grain size of 14 nm were investigated in compression tests over the temperature range between 4.2 and 300 K and the corresponding microstructural changes were analyzed. It was established that decreasing the grain size from 10 μm to 14 nm resulted in a 4.7–6.4 increase in the applied stress as the temperature was decreased from 300 to 77 K. 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 40 K. As the samples were deformed in the macroplastic regime, they demonstrated weak strain hardening at room temperature and at 210 K, but strain softening was instead observed at cryogenic temperatures down to 40 K. Subsequent microstructural investigations revealed that the strain hardening behavior was accompanied by significant grain growth indicating a reverse Hall–Petch relationship. Deformation curves at 10 K 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.