Formation of interface phases in the titanium alloy IMI 834

Abstract

A variety of different preparation techniques have been used in order to identify the experimental conditions under which interface phases are formed in the near-α alloy, IMI 834. In particular, a Ni plated surface examined in cross-section in the TEM produced a gradation in hydrogen content from high at the surface to low levels in the bulk. Two transformation products have been identified, one was a monolithic f c c phase with a lattice parameter of a=0.434 ±0.004 nm, which forms along α-platelet boundaries, and the second was an acicular feature which formed between α-platelet boundaries which was tentatively identified as an h c p phase twin related to the α-titanium. The monolithic interface phase was found to form at both low and high hydrogen levels while the α twins were only seen at high hydrogen contents. In all cases, the transformations were shown to be artefactual and induced solely by hydrogen absorption. No evidence of mechanical strain induced interface phase formation was found. In conflict with the literature, the monolithic f c c phase was found to occur during ion-beam milling and was attributed to water contamination of the milling gas. Formation of the interface phase was suppressed by electropolishing at temperatures ≤ −50 °C.