INFLUENCE OF MODE OF INITIATION ON THE GROWTH OF SMALL SURFACE CRACKSIN TITANIUM ALUMINIDES

Fatigue crack growth behaviour of small surface cracks in two alpha(2) (Ti3Al) based titanium aluminide alloys, Ti-24Al-11Nb (at%) and Ti-25 Al-17Nb-1Mo, was investigated. The primary objective was to monitor th e variations in surface crack aspect ratio (a/c; a = crack depth and c = half surface length) as influenced by the microstructure in which t he cracks were initiated and propagated. The continuous changes in a/c have been estimated using compliance data acquired using a laser inte rferometric system and surface crack length (2c) data collected using a photomicroscopic system. The changes in aic have been used in the ca lculations of stress intensity factor range (Delta K) and crack growth rate. The aspect ratios of small cracks in Ti-24Al-11Nb, primarily in itiated at grain boundaries, varied significantly as the cracks grew, owing to the influence of local microstructure. In the Ti-25Al-17Nb-1M o alloy, cracks initiated from electro-discharge machined (EDM) notche s grew with a nearly semicircular shape. The differences in crack aspe ct ratio variations between the two materials appear to be due to the differences in the microstructural environment through which the crack s grew. The growth rates of large cracks were found to be similar in b oth alloys. The differences in crack growth behaviour of small versus large cracks appear to be due to the reduced closure levels of small c racks relative to large cracks.