Microstructures of Ti₅₀Al₄₅Mo₅ alloy powders produced by plasma rotating electrode process

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.