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Abstract: Isothermal sections of the ternary Cr–Al–Nb phase diagram were experimentally investigated for temperatures of 1150, 1300, and 1450°C by electron-probe microanalysis (EPMA), X-ray diffraction (XRD), differential thermal analysis (DTA) and light-optical (LOM) as well as scanning electron microscopy (SEM). Cr–Al–Nb alloys were prepared by levitation melting and annealed at temperatures between 1150 and 1450°C for up to 1500h. The most striking feature of the ternary Cr–Al–Nb phase diagram is the extended phase field of the cubic C15 and the hexagonal C14 Laves phase Nb(Cr,Al)2. With increasing Al content, the Laves phase polytype changes from C15 to C14. Both phase fields are separated by a small two-phase field, which shifts to lower Al contents with increasing temperature. The lattice parameters of both Laves phase polytypes and of all other phases occurring in this system were studied in dependence on composition and the solubilities of ternary elements in the binary phases were established. [Copyright &y& Elsevier]

Abstract: Partial isothermal sections of the Fe–Al–Nb phase diagram at 1000, 1170, and 1300°C as well as the corresponding part of the liquidus surface were established experimentally for Nb contents in the range from 0 to 33at%. This part of the phase diagram is characterized by an extended two-phase field of the (αFe) solid solution (disordered A2) or FeAl (ordered B2) with the Nb(Fe,Al)2 Laves phase. The solubility of Nb in all Fe–Al phases is small and reaches a maximum in the phase FeAl. A deep eutectic valley runs parallel to the Fe–Al axis across the large two-phase field and continues to the Al corner. The composition dependence of the eutectic temperature and the exact position of the eutectic valley on the liquidus surface including the Al-rich part were determined. The high-temperature phase equilibria are strongly affected by the liquid phase coming from the Al corner. With increasing temperature its phase field grows quickly into the ternary system resulting in a strongly reduced extension of the (αFe)+Nb(Fe,Al)2 two-phase field at 1300°C. [Copyright &y& Elsevier]

Abstract: Isothermal sections of the Co–Al–Nb system at 800, 1000, 1150, 1200, and 1250 °C were experimentally determined by combining results of light-optical as well as scanning electron microscopy, electron-probe microanalysis, and X-ray diffraction measurements with Rietveld analysis of heat-treated alloys and adding some results obtained from solid/liquid diffusion couples. Three polytypes of Laves phases, which are the cubic C15 and the hexagonal C14 and C36 variants, occur as stable phases in the central part of the system. The phase equilibria involving these phases were in the focus of the present study. The composition-dependence of the crystallographic properties of the Laves phases as well as the crystal structure parameters of all other occurring intermetallic phases was investigated. As in case of the binary Co–Nb system, the hexagonal Laves phases C14 and C36 are only stable at off-stoichiometric Nb contents. The preferential site occupations of the Co-, Al-, and Nb-atoms in the hexagonal lattices of the C14 and C36 Laves phase polytypes were determined by Rietveld analyses. For all binary phases except the Nb-rich phase Nb3Al, the solubility for the third element was investigated and the extensions of all phase fields were established. Besides the three Laves phases, the most striking features of the Co–Al–Nb phase diagram are the occurrence of the slowly forming L21-type Heusler phase Co2AlNb and the extended phase field of the μ phase. The existence of a μ′ phase, which was reported in the literature, is not confirmed. [Copyright &y& Elsevier]