Ordering Transformations in the Nb\u3csub\u3e3\u3c/sub\u3eSi Phase in Nb-10Si Alloy

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 [1]. 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) + Nb3Si at 1920°C with the Nb3Si phase further transforming by a eutectoid reaction to Nb + Nb5Si3 at 1770°C [2]. The calculated metastable extension of the phase boundaries on the Nb-rich side of the phase diagram by Lipschutz et al. [1] indicates that upon suppression of the formation of the Nb5Si3 phase, the intermediate phase, Nb3Si becomes a congruently melting compound. In such a case the Nb3Si phase does not undergo the above mentioned eutectoid decomposition, and is retained at room temperature. Various investigations have shown that following solidification at cooling rates normally observed in conventional castings, the cast microstructure consists of Nb and Nb3Si phases [3, 4]. The presence of Nb3Si has been attributed to difficulties in initiation of the eutectoid reaction Nb3Si → Nb5Si3 + Nb [4]. The phase diagram also predicts that the Nb3Si phase should have an ordered tP-32 (Ti3P type, space group # 86) structure over the entire temperature range of its stability (1770 ° to 1990°C). Recent investigations have shown that the Nb3Si phase present in the Nb-10Si alloy (atomic %) undergoes ordering transformations [5]. This paper reports for the first time ordering transformations in the Nb3Si phase in the Nb-10Si alloy