Effect of microstructure and heat treatment on fracture behaviour of smooth and precracked tensile specimens of Ti6Al4V

Abstract

Following the recognition of a significant difference between the susceptibilities to hydrogen cracking of two mill annealed bars of Ti6Al4V, which were of almost identical composition but had very different microstructures, a systematic investigation of?the effect of heat treatment on the mechanical behaviour of smooth and precracked tensile specimens was carried out. Theoverall hydrogen embrittlement and the initiation and slow crack growth of hydrogen cracking are significantly affected by microstructure. Refinement of the structure of smooth tensile specimens by heat treatment below the β transus improves resistance to embrittlement by hydrogen levels above 2000 wt-ppm unless solution treatment is followed by quenching. Precracking was found to promote susceptibility to hydrogen cracking at much lower hydrogen levels but, as discovered previously in similar materials, there is a narrow range of hydrogen concentrations around 100 wt-ppm where no slow cracking precedes final fast fracture in compact tension specimens. This appears to separate two distinct regions where the mechanism of slow cracking differs. The observed effects of heat treatment on the critical stress intensities for slow cracking at such hydrogen levels are complicated by the occurrence of microcracks at the propagating crack tip.