Phenomenological aspects of fatigue life and fatigue crack initiation in high strength alloys at cryogenic temperature

Abstract

Based on the S N (stress cycling) data of Ti6Al4V alloys and high CrNi steels (JN1 and YUS170) at 4, 77 and 293 K, the fatigue strength and crack initiation behavior in the long-life range were investigated in this study. Ti6Al4V rolled alloys and YUS170 steel exhibited a good fatigue strength in the long-life range at cryogenic temperatures. In Ti6Al4V forged alloys and JN1 steel, however, there was a rather sharp drop in the fatigue strength at 4 and 77 K between 10 6 and 10 7 cycles. Subsurface crack initiation occurred in the long-life range for each material, and the location of its site was determined. The size of the subsurface crack initiation site obviously increased for Ti6Al4V forged alloys and JN1 steel as the number of cycles increased. We then proposed that the fatigue strength in the long-life range for the present alloys is related to the microcrack growth behavior in their microstructure.