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Compression behaviour of bulk nanocrystalline Ni-Fe

The mechanical behaviour of fully dense nanocrystalline Ni-19Fe (grain size \mbox{36-37} nm) was investigated by compression and deformation-trace observations with optical microscopy and atomic force microscopy in the temperature range from -162 to 600 °C. The results indicated that yield stress (σ0.2) increased with increasing strain rate with strain-rate sensitivity exponent m = 0.01 at ambient temperature. At the temperatures T<100 °C or T>400 °C yield strength decreased gradually with increasing temperature, while it dropped catastrophically in the range 100 °C<T<400 °C. Low-temperature deformation produced two sets of macroscopic bandlike traces which oriented at around 55° to the compression axis and consisted of ultra-fine straight lines; in contrast, high-temperature deformation led to formation of microcavities accompanying which some nanograins arranged locally in regular form. The results on both mechanical behaviour and deformation morphology suggest that the deformation mechanism at low temperatures was different from that at high temperatures, with the transition temperature locating at Tc~340 °C.