In our previous study in steels, it was found that nanocrystalline (NC) structure (grain size smaller than 20 nm) forms by shot peening (SP) and ball milling (BM). The necessary amount of equivalent strain to form NC structure by deformation was proposed to be larger than about 7. In SP and BM, such large deformation occurs by bombarding, and the strain rates of those deformation are roughly estimated as 10~5 - 10~7 s~(-1). Such large deformation with high strain rate generates a large amount of heat, inducing recovery of dislocation after deformation. In the present study, the effect of recovery during discontinuous deformation on strengthening was investigated by HPT (high-pressure torsion) -straining and annealing.
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