Yield phenomena during the tensile testing of pure titanium sheets of 0.2 mm and 0.4 mm in thickness were investigated in detail focusing on the effects of grain size (d), thickness (t), t/d ratio and tensile direction. With decreasing grain size, the yielding behavior changed from continuous yielding to one accompanied with yield point drop. Upper and lower yield stresses and 0.2％-proof stress followed the Hall-Petch relationship; however, coarse-grained specimens (d ≧ 20 µm) showed larger scatter in 0.2％-proof stress than the others. Consequently, the Hall-Petch coefficient (k) and friction stress (σ₀) derived from 0.2％-proof stress are not accurate enough. The values of k and σ₀ derived from various yield stresses and tensile directions were in the range of 250-600 MPa·µm^0.5 and 30-180 MPa, respectively. Therefore, the validity of stress for yield stress was a concern, and the combination of the lower yield stress in the fine-grain range (d ≦ 20 µm) and 0.2％-proof stress excluding work-hardening in the coarse-grain range (d > 20 µm) was suggested to obtain reliable values of k and σ₀, resulting in values of 370-460 MPa·µm^0.5 and σ₀ 65-140 MPa, respectively. Moreover, it was revealed that k decreased and σ₀ increased with the increasing angle of tensile direction to the rolling direction, regardless of thickness. The anisotropy of k is presumed to be affected by the grain boundary character rather than the Schmid factor, and neither rigidity nor the length of the Burgers vector are responsible. Meanwhile, the anisotropy of σ₀ is verified to be affected by Schmid factors. Furthermore, it was clarified that the t/d ratio hardly affects upper and lower yield stresses (t/d > 14) nor 0.2％-proof stress (1.5 ≦ t/d ≦ 14).

This study aimed to observe the thickening of the S-phase and Sα-phase of various stainless steels subjected to low-temperature direct current plasma nitriding using screen (S-DCPN). Austenitic stainless steel SUS304, ferritic stainless steel SUS430, and duplex stainless steel SUS329J4L were treated using two different screens for comparison, namely, a Ni screen and a steel plate cold commercial (SPCC) screen. Plasma nitriding was performed at 673 K for 300 min under a 75％ N₂ + 25％ H₂ atmosphere at 100 Pa pressure of the mixed gas. After nitriding treatment, the samples were examined using X-ray diffraction (XRD) and glow discharge optical emission spectrometry (GD-OES), and their cross-sectional microstructure and surface microstructure were examined using an electron probe micro analyzer (EPMA). Nitrided samples were also subjected to Vickers hardness and pitting corrosion tests. Examination of the SUS304 samples revealed thickening of its S-phase and higher surface hardness and pitting corrosion resistance when nitriding was done with the Ni screen. This was due to excess nitrogen diffusion into the sample due to presence of the Ni screen than with the SPCC screen. In the SUS430 samples, thickening of the Sα-phase was not be observed. When the Ni screen was used during nitriding, higher surface hardness and less pitting corrosion resistance of sample were observed, along with enhanced nitrogen diffusion than the SPCC screen was used. In the SUS329J4L samples, upon nitriding with Ni screen, thickening of the S-phase was observed and surface hardness and pitting corrosion resistance of the sample were higher, which was attributed to the enhanced nitrogen diffusion into the sample than when nitriding with the SPCC screen.