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 inhom ogeneous structures, which consist of dendrites and rounded grains. The den drites, which show a "rosettelike" morphology, are formed on the powder sur face and around the rounded grains. The rosettelike dendrites are of hexago nal alpha (2) (D0(19)) phase even though the dendrites have an equiaxial mo rphology, and a small amount of beta (2) (B2) phase is also contained insid e. It is suggested that the solidification to alpha (hcp-A3) phase occurred by the peritectic reaction between the primary beta (bcc-A2) dendrites and the liquid: L + beta --> L + beta + alpha. The rounded grains, on the othe r hand, are of beta (2) phase in which acicular alpha or alpha (2) laths ar e precipitated with the Burgers orientation relationship. Antiphase domain boundaries in the beta (2) matrix are intersected by alpha(alpha (2)) laths . It is interpreted that the alpha(alpha (2)) laths were formed by the soli d-state transformations: beta (2) --> beta (2) + alpha --> beta (2) + alpha (2). The formation of the two different microstructures in the powder part icles is rationalized in terms of the changes in local composition of the l iquid phase during the rapid solidification process.