Microstructure evolution and hardness variation during annealing of equal channel angular pressed ultra-fine grained nickel subjected to 12 passes

Abstract

The microstructure, thermal stability and hardness of ultra-fine grained (UFG) Ni produced by 12 passes of equal channel angular pressing (ECAP) through the route Bc were studied. Comparing the microstructure and hardness of the as-ECAPed samples with the published data on UFG Ni obtained after 8 passes of ECAP through the route Bc reveals a smaller average grain size (230 nm in the present case compared with 270 nm in 8-pass Ni), significantly lower dislocation density (1.08 × 10¹⁴ m^-2 compared with 9 × 10¹⁴ m^-2 in 8-pass Ni) and lower hardness (2 GPa compared with 2.45 GPa for 8-pass Ni). Study of the thermal stability of the 12-pass UFG Ni revealed that recovery is dominant in the temperature range 150-250°C and recrystallisation occurred at temperatures >250 °C. The UFG microstructure is relatively stable up to about 400 °C. Due to the lower dislocation density and consequently a lower stored energy, the recrystallisation of 12-pass ECAP Ni occurred at a higher temperature (∼250 °C) compared with the 8-pass Ni (∼200 °C). In the 12-pass Nickel, hardness variation shows that its dependence on grain size is inversely linear rather than the common grain size^-0.5 dependence.