Phase relation and microstructure of Nb-Cr-V and Nb-Cr-Mo alloy systems

The phase diagrams of ternary Nb-Cr-V and Nb-Cr-Mo alloy systems at 1623 K are investigated by metallography, X-ray diffraction (XRD) and transmission electron microscopy (TEM) equipped with energy-dispersive X-ray (EDX) analysis. In both alloy systems, the C15 NbCr2 Laves phases are equilibrated directly with b.c.c. phase without forming any intermediate phases. The C15 NbCr2 Laves phase accommodates a relatively large amount of ternary elements V and Mo, and extends along directions so that V and Mo occupy sites of the component elements Cr and Nb, respectively. The b.c.c. phase has a larger solid solution range in the V (or Mo)-rich side than in the Nb- and Cr-rich sides. A variety of duplex microstructures are observed depending on the alloy system and alloy composition. Also, it is shown by TEM that the constituent C15 phase containing V consists of featureless microstructure while that containing Mo involves the metastable C36 phase in addition to microtwins and stacking faults in the C15 phase.