Characterization of silicide precipitates in Nb-Si and Nb-Ti-Si alloys

The present paper describes the morphology, chemistry and crystallography o f silicide precipitates that were observed in Nb-Si and Nb-Ti-Si solid solu tions. Although the stable structure of Nb3Si is tetragonal (tP32), within the bcc Nb solid solution, fine-scale Nb3Si precipitates were observed with a metastable orthorhombic crystal structure. These precipitates were obser ved with an acicular morphology, and with sizes ranging from 2 nm to 1 mum in the as-solidified condition. The orientation relationship between these precipitates and the Nb matrix is (100)//(100) and [010]//[010]. It is sugg ested that the metastable orthorhombic structure forms owing to ease of nuc leation of this structure in the bcc Nb matrix. The crystallography of thes e precipitates is described, and these findings are compared with previous data on the Nb-Si system.