SULFUR SEGREGATION IN TITANIUM AND SELECTED TITANIUM-ALLOYS

The segregation of S in electrotransport-purified polycrystalline alph a-Ti and in several polycrystalline Ti alloys has been studied by Auge r electron spectroscopy and ion scattering spectroscopy in the tempera ture range extending from 20 to 1000-degrees-C. The chemisorbed oxygen and carbon on Ti were observed to disappear at T > 400-degrees-C afte r which the S signal increased to levels approaching 0.5 monolayer. At lower temperatures the presence of the surface oxygen and carbon appe ared to inhibit the segregation, presumably because there were no avai lable surfaces sites for the S emerging from the bulk. The activation energy for the S segregation in pure polycrystalline Ti was determined to be 16.7 kcal/mol, which, when compared to S segregation from singl e-crystal Ti, is quite small and suggests grain boundary or defect dif fusion segregation kinetics. In the Ti-aluminide alloys, the presence of Al appeared to enhance the retention of surface oxygen which, in tu rn, substantially reduced the S segregation. The gamma alloy, with its high Al content, exhibited the greatest retention of surface oxygen a nd the smallest quantity of S segregation (T almost-equal-to 1000-degr ees-C). On the other hand, the beta-21S alloy exhibited a greater segr egation rate for the S than did alpha-Ti.