Fatigue crack initiation in hot isostatically pressed Ti-6Al-4V castings

Abstract

Precision castings of the Ti-6Al-4V alloy containing pore defects were hot isostatically pressed (HIP) in an attempt to improve the high cycle fatigue strength. Although all defects were healed, the fatigue strength was still low when compared to β-processed wrought material with a similar microstructure. Fatigue-crack initiation analysis, which included precision sectioning, indicated that early fatigue-crack initiation sites were related to relatively large α-platelet colonies and massive grain-boundary α(GBα) phase. Shear across the large colonies or along the GBα interfaces provided large initial cracks which resulted in the fatigue-life degradation. Large colonies and massive GBα developed in the HIP healed zones of the casting pores. In spite of the total HIP pore closure, the fatigue-strength improvement was small when compared to wrought material due to the coarse microstructure that developed in some locations during the HIP cycle. Large planar-shear initiation facets across several colonies were also observed. The multicolony faceted shear is the result of the Burgers relation between the colony orientations and increases the chance of early fatigue-crack inititation.