ASPECTS OF FRACTURE MORPHOLOGY EXHIBITED IN THE STRESS-CORROSION BEHAVIOR OF TI-155 IN METHANOL HYDROCHLORIC-ACID SOLUTION

Stress corrosion cracking (SCC) of Ti-155 (UNS R50400) was investigate d in methanol-hydrochloric acid (MeOH-HCl) solution in preexposure and anodic polarization experiments using specimens with tensile axis in the parallel or transverse orientation to the rolling direction. Inter granular penetration data showed a dependence of the intergranular cor rosion process on preexposure time. Susceptibility to SCC, as indicate d by the elongation to failure index (epsilon(m)/epsilon(a)) increased with increasing preexposure time and applied anodic current density. Fractographic analysis by scanning electron microscope (SEM) of fractu red specimen surfaces in preexposure and anodic polarization experimen ts showed a fracture morphology comprised of intergranular at the expo sed surface edge, an intermediate transgranular cleavage zone, and a t erminal ductile region. The transgranular cleavage and associated flut ings were more pronounced in Specimens tested with the tensile axis in the transverse orientation than those tested with their tensile axis parallel to the rolling direction. These observations supported the vi ew that intergranular fracture and transgranular cleavage occuring dur ing SCC of titanium in MeOH-HCl solution is caused by mechanisms relat ed to anodic dissolution and hydrogen embrittlement (HE), respectively . A mechanism to describe the simultaneous Occurrence of dissolution a nd NE at the grain boundary was suggested. In anodic polarization expe riments, outer grains of the intergranular zone exhibited ''sliced pat terns.'' me extent of slicing, which was prominent in specimens tested in the parallel orientation increased with increasing anodic current density. In many cases, the sliced patterns were aligned in what appea red to be crystallographic planes. The occurrence of slicing, which ha s been related to formation of a hydride phase, was considered to be m echanical in origin At applied current densities greater than or equal to 40 mA/mm(2), the slicing effect was extensive, with some slices di sintegrating. me disintegration product of the sliced layer was analyz ed by x-ray diffraction and confirmed to be titanium hydride (TiH1.9).