SUBSURFACE CRACK INITIATION IN HIGH CYCLE FATIGUE OF TI-5AL-2.5SN EXTRA-LOW INTERSTITIAL ALLOY AT LIQUID-HELIUM TEMPERATURE

Subsurface fatigue crack initiation in a Ti5Al2.5Sn extra-low interstitial alloy at liquid helium temperature was characterized. The subsurface crack initiation occurred at low stress level and neither defects nor foreign material were detected at the subsurface initiation site. The subsurface crack initiation site was composed of "small grains" of diameter about 5-10 μm. The concentration of aluminium at the initiation site was much lower than in the bulk. From the viewpoint of the microstructure this alloy had partly a two-phase region (small α grain region). It was composed of small α grains with low aluminium content and an iron-enriched β phase at their grain boundaries. The size, morphology and composition of the subsurface crack initiation site fitted those of the small α grain region. Hence it is concluded that the subsurface crack was initiated in the small α grain region. © 1990.