Figure 9

Apparent stresses for nano-Ni and micron-Ni plot as functions of pressure and temperature, which include both microstrain and instrument-baseline effects. The “Yielding” points are derived by the intersections of elastic loading and plastic work-hardening/softening stages. The onset-pressures for the yielding are apparently different for the two samples. The corresponding high-P yield-strengths are the stress differences Δσ between the yielding and the initial states. The labels of high-T stress relaxation and grain growth are simply to distinguish the dominant mechanisms at different temperature stages. The residual stresses of the samples should be read from the instrument-baseline to the initial stress states at the ambient conditions. The purple open-square symbol is to mark the recovered samples, which come back completely to the initial micron-Ni in terms of stress/strain and grain sizes. Pressure has a noticeable decrease of 0.1–0.2 GPa at high temperatures due to the cell assembly adjustments, however, the derived stress relaxations for the grain-to-grain contacts are much more significant, about 10-folds bigger for the micron-Ni and over 50-folds for the nano-Ni. The plot shows a good comparison of constitutive properties of nano-Ni and micron-Ni under high P–T conditions