Phase Equilibria in the Nb-Rich Region of Al-Nb-Sn at 900 and 1200 °C

The Al-Nb-Sn phase diagram was studied experimentally in the Nb-rich region to provide important phase equilibria information for alloy design of Nb-silicide based materials for aero engine applications. Three alloys were produced: Nb-17Al-17Sn, Nb-33Al-13Sn and Nb-16Al-20Sn (at.%). As-cast and heat-treated alloys (900 and 1200 °C) were analysed using XRD (X-ray diffraction) and SEM/EDS (scanning electron microscopy/ electron dispersive x-ray spectroscopy). Tin showed a high solubility in Nb(2)Al, reaching up to 21 at.% in the Sn-rich areas, substituting for Al atoms. Tin and Al also substituted for each other in the A15 phases (Nb(3)Al and Nb(3)Sn). Tin showed limited solubility in NbAl(3), not exceeding 3.6 at.% as it substituted Al atoms. The solubility of Al in NbSn(2) varied from 4.8 to 6.8 at.%. A ternary phase, Nb(5)Sn(2)Al with the tI32 W(5)Si(3) crystal structure, was found to be stable. This phase was observed in the 900 °C heat-treated samples, but not in the 1200 °C heated samples.