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

• Published in: Journal of materials science Vol. 46; no. 8; pp. 2662 - 2671
• Main Authors: Sitarama Raju, K., Ghanashyam Krishna, M., Padmanabhan, K. A., Subramanya Sarma, V., Gurao, N. P., Wilde, G.
• Format: Journal Article
• Language: English
• Published: Boston Springer US 01.04.2011; Springer; Springer Nature B.V
• Subjects: Annealing; Characterization and Evaluation of Materials; Chemistry and Materials Science; Classical Mechanics; Comparative analysis; Crystallography and Scattering Methods; Dependence; Dislocation density; Equal channel angular pressing; Grain size; Hardness; Internal energy; Materials Science; Mechanical properties; Metamorphism (Geology); Microstructure; Nickel; Polymer Sciences; Recrystallization; Solid Mechanics; Thermal stability; Equal Channel Angular Pressing; Accumulative Roll Bonding; Atomic Force Microscopy; Lower Dislocation Density; Equal Channel Angular Pressing Processing
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-010-5122-z

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 × 10 14 m −2 compared with 9 × 10 14 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.