Hydrogen embrittlement of α titanium: In situ tem studies

The effect of hydrogen on fracture in the h.c.p. α Ti-4 wt % Al alloy 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 enhanced 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 the process of hydrogen enhanced localized plasticity, while at low stress intensities titanium hydrides form in the vicinity of crack tips and crack propagation proceeds though the hydrides.