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The formation of nanocrystalline structure (NS) on the surface of bulk steel samples by a particle impact and air blast shot peening techniques was studied. Nanocrystalline layers with several microns thick were successfully fabricated by these methods. The nanocrystalline layers produced in the present study have extremely high hardness and separated from adjacent deformed morphology region with sharp boundaries. By annealing, nanocrystalline layers showed slow grain growth without recrystallization. Those characteristics are similar to those observed in the NS produced by ball milling and a ball drop deformation. It was suggested that to produce NS by deformation a large strain is a necessary condition and a high strain rate and low temperature are favorable conditions.

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The effect of strain path in high-pressure torsion (HPT) process on hardening was investigated in commercial purity titanium (Ti-0.03Fe-0.03O, mass%). After monotonic HPT (mHPT) straining up to N ¼ 10 turns at a rotation speed of 0.2 rpm under a pressure of P ¼ 5 GPa, the obtained Vickers microhardness, Hv, was around 3.5 GPa and the microstructure consisted of equiaxed grains of 100 $ 200 nm with high dislocation density. This Hv value was hardly increased even with further strain and strain gradient (further rotation). To investigate the effect of strain path, cyclic (cHPT) and two-steps HPT (2sHPT) processes were carried out. The cHPT-straining performed by repetitive deformation of N ¼ 1=2. In comparison with the mHPT process, the Hv was attained rapidly to the saturated value (Hv 3.5 GPa). However, the maximum Hv value was similar to that obtained by mHPT-straining. In the 2sHPT process, first the disk of 20 mm in diameter was deformed by HPTstraining. Secondly, the disk of 10 mm was cut to contact with the circumference of the deformed disk, and then it was deformed. A higher hardness (Hv 3.8 GPa) was obtained than that by monotonic or cyclic HPT-straining. These results indicate that multi-directional deformations (deformations with different strain paths) contribute to the hardening improvement.

Surface nanocrystallization in various steels by shot peening (both air blast (ABSP) and ultrasonic (USSP)) was investigated. In all the shot-peened specimens, the equiaxed nanocrystals with grain size of several 10 nm were observed at the surface regions. The depth of nanocrystalline (NC) layers was several mm. The NC layers have extremely high hardness and were separated from the deformed structure regions just under the NC layers with sharp boundaries. By annealing, the NC layers show the substantially slow grain growth without recrystallization. These characteristics are similar to those observed in the specimens treated by ball milling, ball drop and particle impact deformation. Comparing ABSP and USSP at the similar peening condition, the produced volume of NC region in ABSP is larger than that in USSP.