Effect of strain path in high-pressure torsion process on hardening in commercial purity titanium

The effect of strain path in high-pressure torsion (HPT) process on hardening was investigated in commercial purity titanium (Ti-0.03Fe-0.030, mass%). After monotonic HPT (mHPT) straining up to N = 10 turns at a rotation speed of 0.2 rpm under a pressure of P = 5 GPa, the obtained Vickers microhardness, Hv, was around 3.5GPa and the microstructure consisted of equiaxed grains of 100 similar to 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 = 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 0 20 mm in diameter was deformed by HPT-straining. Secondly, the disk of 0 10 mm was cut to contact with the circumference of the deformed disk, and then it was deformed. A higher hardness (Hv 3.8GPa) 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.