Ti-6Al-4V合金の疲労特性に及ぼす応力比の影響

Effect of stress ratio on fatigue properties was investigated for 900 and 1100 MPa-class Ti-6Al-4V alloys. Numbers of heats were 3 for each tensile strength class. Fatigue tests were conducted under stress ratios of R=-1, 0 and 0.3, and under the condition fixing the maximum stress at the yield stress (σmax=σy test). Although heat C for 900 MPa-class and heat F for 1100 MPa-class did not show internal fracture under R=-1, the internal fracture occurred in all heats under stress ratios above R=0. The fracture sites of the internal fracture revealed no inclusion but matrix including several facets under stress ratios above R=0. The size of a facet was almost equal to the α grain size and the areas of the internal fracture origin containing the facets were 100-300 μm. The fatigue limits determined at 108 cycles were below a modified Goodman line under R=0 and 0.3, although they were above under around R=0.7. The drops of fatigue limits under around R=0 suggested that the internal fracture properties were more sensitive to the stress ratios in case of the Ti-6Al-4V alloys. Moreover, in comparing the fatigue limits with ΔKth, the dependency of the fatigue limits on the stress ratios was similar to that of ΔKth.