Microstructure evolution and mechanical property of nanocrystalline Ni–20Fe alloy during cold rolling

We report the microstructure evolution and mechanical property during room-temperature rolling of nanocrystalline Ni–20Fe alloys. The results show that massive dislocations and deformation twins in the deformed samples. Quantitative x-ray analysis reveals deformation induced grain rotation and grain growth. The dislocation density increases firstly and then tend to saturation after an equivalent strain (ɛVM) of ∼0.10 during the deformation. Correspondingly, the hardness increases when the ɛVM increases from 0 to ∼0.10, in spite of the increase in grain size. However, once the ɛVM exceeds 0.10, the hardness starts to decrease. It is suggested that the role of crystal defect as well as grain size should be considered to better understand the mechanical property of nanocrystalline Ni–20Fe alloy. At small strains (ɛVM < 0.10), the increase in hardness is a direct consequence of the high-density crystal defects; while at large strains (ɛVM > 0.10), since the crystal defects are saturated, the hardness decreases with further increase in grain size.

► We found strain hardening and softening in deformed nc Ni–20Fe alloy. ► We quantitatively examine changes in dislocation density and grain size. ► There is a combined effect of defects and grain size on hardness evolution. ► The hardness increases due to defect activities despite of grain coarsening. ► The hardness decreases due to grain coarsening when the defect is saturated.