Effect of hydrogen on the tensile ductility of Ti6Al4V .2. Fracture of pre-cracked tensile specimens

Pre-cracked compact tensile specimens of Ti6Al4V charged with hydrogen were slowly strained in tension at room temperature and 2.3 x 10(-5) mm s(-1) and a crack-growth monitor used to detect the early stages of slow crack growth and so confirm the load for its initiation. The micro-fractography and crack propagation path were examined by scanning electron microscopy (SEM). The results confirmed that slow cracking preceded fast cracking in all specimens and at hydrogen contents below 90 p.p.m. the stress intensity factor for slow cracking, K-s, increased with increasing hydrogen, whereas it was reduced at higher levels. The average slow crack growth rate increased on increasing the hydrogen content from 10 p.p.m. to 90 p.p.m., but decreased sharply as the hydrogen content was further increased to 125 p.p.m., and then again increased above 125 p.p.m. hydrogen, but only very slowly. With increasing hydrogen content, the slow crack initiation changed from within the alpha phase to the interface between the alpha and beta phases, the growth path from transgranular to interfacial separation and the fracture mode from the mixed ductile and cleavage to fracture along the alpha-beta interface (greater than or equal to 500 p.p.m.). It is suggested that the mechanism of slow crack growth is different for the different ranges of hydrogen content: at the low hydrogen levels (< 90 p.p.m.) the dominant mechanism is creep-induced slow crack growth, whereas the slow cracking becomes controlled by hydrogen diffusion in both alpha and beta phases when the hydrogen content is above 90 p.p.m. Fast fracture was invariably preceded by slow crack growth at all hydrogen levels up to 500 p.p.m.