チタン合金の窒化層形成および疲労特性に及ぼす窒化微粒子ピーニングの影響

抄録

Conventional nitriding improves the tribological properties of titanium alloys; however, reduces their fatigue strength owing to grain-coarsening. The purpose of this study is to examine the effect of the nitrided-fine particle peening (N-FPP) on the formation of nitrided layer and fatigue properties of Ti-6Al-4V alloy. This approach forms the nitrided layer on Ti-6Al-4V alloy by bombardment with nitrided commercially pure (CP) titanium fine particles. Plasma nitriding and gas nitriding were performed at 873 or 973 K to form a nitrided layer on the surface of CP-titanium fine particles, and then N-FPP was performed for 1, 10, 30 s in air at room temperature. Nitrided layer with high hardness and compressive residual stress could be found on the N-FPP treated surface of Ti-6Al-4V alloy. Nitrogen concentration in nitrided layer tended to increase with the particle nitriding temperature and N-FPP treatment time. Fatigue life and fatigue limit of the N-FPP treated specimen under four-point bending fatigue tests were higher than those of conventional nitrided one, whereas were lower than those of the un-peened specimen. This was because a fatigue crack in the N-FPP treated specimen was initiated from the surface dent formed by N-FPP due to the stress concentration. In contrast, FPP using un-nitrided CP-titanium fine particles increased the fatigue limit of Ti-6Al-4V alloy.