International Journal of Materials Research vol:102 issue:1 pages:109-116
The effect of O on the phase equilibria in Nb-Fe-Cr alloys, more specifically on the formation of a Ti2Ni type phase, has been studied. Nb-xFe-yCr alloys with x = 37.4 - 41.4 at.%, y = 7.6-8.6 at.% were subjected to long term annealing experiments at 950 degrees C after oxygenation. The phases were identified and characterized using scanning electron microscopy, X-ray diffraction and wavelength dispersive spectroscopy. In the pure Nb-37Fe-8Cr and Nb-41Fe-9Cr alloys respectively a two phase equilibrium between (Nb) and mu FeNb and a three phase equilibrium between (Nb), mu FeNb and Fe2Nb are established. In the oxygenated alloys a Ti2Ni type phase with composition (Fe, Cr)(6)Nb6Ox appears mainly at the expense of the mu FeNb phase. With respect to composition and crystallographic parameters the (Fe, Cr)(6)Nb6Ox phase has large similarities with the Fe2.4Nb3.6O and Fe6Nb6O phase. From wavelength dispersive spectroscopy measurements, it is concluded that x equals 0.75 when (Fe, Cr)(6)Nb6Ox is in equilibrium with (Nb), mu FeNb and Fe2Nb. x is higher when this phase is in equilibrium with NbO. The structure of (Fe, Cr)(6)Nb6Ox is also analyzed with Rietveld refinement. This analysis reveals that Cr atoms partially replace Fe atoms only on the 16d site and not on the 32e site.