Ordering transformations in the Nb3Si phase in Nb-10Si alloy

Abstract

Although the Nb-Si equilibrium phase diagram has been investigated repeatedly, the eutectic and eutectoid temperatures, the equilibrium solubilities of Si in Nb at different temperatures and the stable and metastable phases on the Nb-rich side of the phase diagram are still uncertain. According to the most recent phase diagram, the equilibrium solidification path for hypo-eutectic alloys containing approximately 3.5 to 18.7% Si is liquid Nb-Si undergoing a eutectic transformation to Nb(Si) + Nb{sub 3}Si at 1920{degrees} C with the Nb{sub 3}Si phase further transforming by a eutectoid reaction to Nb + Nb{sub 5}Si{sub 3} at 1770{degrees} C. The calculated metastable extension of the phase boundaries on the Nb-rich side of the phase diagram by Lipschutz et al. indicates that upon suppression of the formation of the Nb{sub 5}Si{sub 3} phase, the intermediate phase, Nb{sub 3}Si becomes a congruently melting compound. In such a case the Nb{sub 3}Si phase does not undergo the above mentioned eutectoid decomposition, and is retained at room temperature. Recent investigations have shown that the Nb{sub 3}Si phase present in the Nb-10Si alloy (atomic %) undergoes ordering transformations. This paper reports for the first time ordering transformations in the Nb{sub 3}Si phase in the Nb-10Si alloy.