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

Abstract

The present paper describes the morphology, chemistry and crystallography of silicide precipitates that were observed in Nb–Si and Nb–Ti–Si solid solutions. 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 observed with an acicular morphology, and with sizes ranging from 2 nm to 1 μm in the assolidified condition. The orientation relationship between these precipitates and the Nb matrix is (100)//(100) and [010]//[010]. It is suggested that the metastable orthorhombic structure forms owing to ease of nucleation of this structure in the bcc Nb matrix. The crystallography of these precipitates is described, and these findings are compared with previous data on the Nb–Si system.