Fatigue behavior of beta processed titanium alloy IMI 685

The low cycle fatigue behavior of IMI-685 alloy withβ-processed andβ-annealed microstructures was investigated. Material with large colony structure ofα-platelets oriented in the same direction, resulting from insufficientβ-work and slow cooling rate from theβ-phase region, exhibited lower fatigue strength than material with basketweave arrangement of theα-platelets. Most of the fatigue crack initiation and propagation processes were dominated by cracking related to intense shear across a colony. The size of the shear related initial cracks could be limited by reducing the colony size, which resulted in an increased fatigue strength. In the large colony microstructure, it was possible to cause a substantial fatigue life debit by introducing a small planar defect on the surface or by applying 5 min dwell time at peak load. The combination of planar defect and dwell time caused the highest life debit. Residual porosity of negligible size caused, in the large colony microstructure, a fairly large, subsurface, cleavage-like planar defect that participated in the initiation of fatigue cracks. Due to its appearance on the fracture surface, the defect which is characterized in detail in the paper, was named cleavage rosette. J. A. HALL, formerly with the Air Force Materials Laboratory