Ti-6Al-4V合金の高温低サイクル疲労強度に及ぼす切欠きおよび組織の影響

抄録

The influences of microstructure and notch on the low-cycle fatigue behavior of Ti-6Al-4V alloy at elevated temperature were studied. Three kinds of microstructures in Ti-6Al-4V alloy were prepared under different heat treatment conditions; eqiaxed α structure, lenticular α structure and bimodal (mixed structure of equiaxed α and lenticular α) structure. Low-cycle fatigue tests were performed at 773K in air under a stress controlled triangular wave form for the smooth and circumferentially notched specimens. The following conclusions were obtained.
(1) The low-cycle fatigue strength of the smooth specimen depended upon the microstructure. The highest was the bimodal structure and the lowest the lenticular α structure. However, the difference in low-cycle fatigue life among the specimens with different microstructures was not observed for the notched specimens with stress concentration factors of 2.5 and 6.0.
(2) The crack initiation of the smooth specimen was affected by the microstructure, but for the notched specimen the crack initiation occurred at the stress concentrated region and the fatigue life did not depend on the microstructure.
(3) The crack initiation lifetime of the notched specimen was predicted from the stress distribution around notch root obtained from the finite element method.