Microstructures of Ti50Al45Mo5 alloy powders produced by plasma rotating electrode process

Abstract

Microstructures of Ti50Al45Mo5 (at.%) alloy powders produced by the plasma rotating electrode process (PREP) were investigated. The powders have inhomogeneous structures, which consist of dendrites and rounded grains. The dendrites, which show a “rosettelike” morphology, are formed on the powder surface and around the rounded grains. The rosettelike dendrites are of hexagonal α2 (D019) phase even though the dendrites have an equiaxial morphology, and a small amount of β2 (B2) phase is also contained inside. It is suggested that the solidification to α (hcp-A3) phase occurred by the peritectic reaction between the primary β (bcc-A2) dendrites and the liquid: L+β→L+β+α. The rounded grains, on the other hand, are of β2 phase in which acicular α or α2 laths are precipitated with the Burgers orientation relationship. Antiphase domain boundaries in the β2 matrix are intersected by α(α2) laths. It is interpreted that the α(α2) laths were formed by the solid-state transformations: β2→β2+α→β2+α2. The formation of the two different microstructures in the powder particles is rationalized in terms of the changes in local composition of the liquid phase during the rapid solidification process.