Evidence of the existence an α/β interface phase in a near-α titanium alloy

Abstract

There have been contradictory reports about the existence of an α/β interface phase in titanium alloys over the last two decades. Recently some publications tend to favour the notion that the interface phase is an artifact formed in the preparation of the thin foil by electropolishing. However our studies on an advanced near-α titanium alloy (Ti-5.5Al-3.98Sn-2.16Zr-0.98Mo-0.85Nd-0.28Si) especially by means of High Resolution Transmission Electron Microscopy (HRTEM) showed that the α/β interface phase in the alloy existed before electropolishing and it could decompose during prolonged aging. In this paper, these evidences will be presented. It was observed from a strained region of a tensile sample along the longitudinal direction that the dislocation slip bands exist across the α platelets. At a higher magnification, it was noted that the dislocations that pass through the α platelet boundary regions and also cut through the α/β interface phase. From the HRTEM image of the ledge formed by the dislocations cutting through a platelet boundary, the glide plane of the dislocations is identified. It clearly showed that the α/β interface phase had existed before the tensile test was conducted. Moreover, it was also found that the decomposition of the α platelet occurred during the prolonged aging up to 3000 h at 550°C and the resultant product, silicide, was formed on the platelet boundary after aging. Around the decomposed β phase, the α/β interface phase disappeared, but still existed around the non-decomposed part of the boundary β phase. The evidences imply that the α/β interface phase also underwent decomposition or transformation while the β phase was decomposed. Such evidence would not have existed if the α/β interface phase is formed during electropolishing. The evidence also demonstrated that the α/β interface phase not only is a real phase in the alloy employed in this case but also unstable and may change dramatically under thermal exposure, which should affect the mechanical properties of this alloy.