Eutectoid Phase Transformations in Nb-Silicide In-Situ Composites
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Eutectoid Phase Transformations in Nb-Silicide In-Situ Composites B.P. Bewlay1, S.D. Sitzman2, L.N. Brewer1 and M.R. Jackson1 General Electric Global Research1, Schenectady, NY 12301. HKL Technology Inc.2, Pasedena, CA 91104.
ABSTRACT Nb-silicide based composites have excellent potential for future high-temperature structural applications. Nb-silicide composites possess Nb together with high-strength silicides, such as Nb5Si3 and Nb3Si. Alloying elements such as Ti and Hf, are added to obtain a balance of properties such as creep performance and oxidation resistance. In Nb-silicide composites generated from Nb-rich binary Nb-Si alloys, Nb3Si is unstable and experiences eutectoid decomposition to Nb and Nb5Si3. The present paper describes a low temperature eutectoid phase transformation during which (Nb)3Si decomposes into (Nb) and (Nb)5Si3, where the (Nb)5Si3 possesses the hP16 structure, as opposed to the tI32 structure observed in binary Nb5Si3.
INTRODUCTION Nb-silicide based composites contain phases such as Nb5Si3 and Nb3Si, and are being developed for high-temperature structural applications in gas turbine engines [1-5]. These composites contain Hf and Ti alloying additions to improve the high-temperature strength and oxidation performance. In systems with high Hf and high Ti concentrations, Nb5Si3 has been observed with the hP16 structure [6, 7], as opposed to the tI32 structure observed in binary Nb5Si3. The equilibrium phases at the potential use temperatures for the Nb-silicide based composites are bcc Nb and tI32 Nb5Si3. However, the composites generally solidify with the Nb3Si phase during casting. The binary Nb-Si phase diagram contains a eutectic, L → Nb3Si + Nb at 1880°C, and a peritectic L + Nb5Si3 → Nb3Si at 1980°C [4]. The eutectic occurs at Nb-18.2%Si (all compositions are given in atom percent). The binary phase diagram indicates that tI32 Nb5Si3 is generated by a eutectoid phase transformation of the form: Nb3Si → Nb5Si3 + Nb [4]. However, the decomposition kinetics of binary Nb3Si were found to be very slow [5]. When Nb-Si alloys are alloyed with Ti, the Nb3Si is stabilized and the eutectoid transformation can be suppressed [7]. The Nb-Ti-Si phase diagram has been described by Subramanian et al. [1], and Bewlay et al [7]. It was shown that Ti: 1) stabilizes Nb3Si to lower temperatures (Ti3Si is isomorphous with Nb3Si and forms by a peritectoid reaction between Ti and Ti5Si3 at a temperature of 1170oC); and, 2) promotes the formation of a hexagonal hP16 Ti5Si3 phase. Additions of Ti to Nb-silicide based composites improve oxidation resistance and fracture toughness. There has also been extensive work on the effect of Hf additions to Nb-Si alloys [7], but there have been no previous reports on phase equilibria in the Nb-Hf-Ti-Si system. The binary Nb5Si3 and Nb3Si have tI32 and tP32 crystal structures, respectively. In binary Nb5Si3 two crystal forms have been reported [4]; low- and high-temperatures forms of the tI32
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structure, as shown in Table I. In the present study, only
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