A comparison of the microstructures of As-cast and laser surface melted Ti-8Al-4Y

A Ti-8 at. pct Al-4 at. pct Y alloy has been cast in small quantities and subjected to laser surface melting treatments in an effort to produce a uniform distribution of fine-scale dispersoids by internal oxidation. The microstructures of both as-cast and laser-modified material have been characterized using transmission electron microscopy, energy dispersive X-ray spectroscopy, and convergent beam electron diffraction, and the identity and distribution of second phase dispersoids established. An ability to refine significantly the dendritic structure of the as-cast alloy by laser surface treatment is demonstrated. The matrix phase of the modified alloy is martensitic α′, in contrast to the α phase of the as-cast material. The interdendritic phase observed in both as-cast and laser-modified material and the fine-scale dispersed particles (50 to 70 nm) in the as-cast alloy are shown to be rich in Y, with a structure consistent with that of the oxide Y₂O₃. Dispersoids within the laser modified zone are typically 8 to 20 nm in diameter and show a tendency to form preferentially at interlath boundaries of the martensite. The origins of these structures and their potential effects on the mechanical properties of the alloy are discussed briefly.