HYDROGEN EMBRITTLEMENT OF ALPHA-TITANIUM - INSITU TEM STUDIES

The effect of hydrogen on fracture in Ti-4 wt% Al and the role of titanium hydride in the fracture process have been studied by deforming samples in situ in a high-voltage electron microscope equipped with an environmental cell. Two fracture mechanisms have been observed in a gaseous hydrogen environment at room temperature; one is fracture by localized plastic deformation reduced by the presence of hydrogen, the other is a brittle fracture of the stress-induced titanium hydride which precipitates at elastic singularities. The local stress intensity determines which mechanism predominates. At high stress intensities the crack propagates by hydrogen enhanced localized plasticity; at low stress intensities titanium hydrides form in the vicinity of crack tips and crack propagation proceeds though the hydrides.