Effects of loading strain rate and stacking fault energy on nanoindentation creep behaviors of nanocrystalline Cu, Ni-20 wt.%Fe and Ni

Creep behaviors of nanocrystalline (NC) Cu, Ni-20(wt.%)Fe and Ni with comparable grain sizes were examined by nanoindentation tests. It was showed that the creep behaviors depend on loading strain rate and stacking fault energy (SFE). At high loading strain rate, the initial creep rates of three NC materials are far higher than those predicted by the models of the Coble creep and the grain boundary sliding and are the highest and lowest for NC Cu and NC Ni, respectively, while that of NC Ni-20Fe lies between them due to their different SFE. At low loading strain rate, the creep behaviors do not exhibit significant difference among three NC materials. Our analysis revealed that grain boundary dislocation sources can be activated at high loading strain rate and the emitted dislocations from grain boundaries can be effectively stored in the loading regime, but cannot at low loading strain rate. The very high initial creep rates of three NC materials are attributed to the rapid absorptions of the stored dislocations in the loading regime and the newly nucleated dislocations in the holding regime. The higher creep rate of NC Cu as compared with those of NC Ni-20Fe and NC Ni arises from the higher density of the stored dislocations caused by the enhanced interactions of dislocations with twins and stacking faults due to its lower SFE.