Phase relation and microstructure of the Nb–Cr–W alloy system

The phase diagram of the Nb–Cr–W alloy system at 1623 K is constructed by means of X-ray diffraction, optical microscopy and transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy. The NbCr2 Laves phase equilibrates directly with b.c.c. phases with wider range of compositions without forming any intermediate phases. The element W is soluble by about 8at.% in the NbCr2 Laves phase at 1623 K. The Laves phase field extends in a direction such that W substitutes for the Nb site in NbCr2. The metastable C36 and C14 Laves phases in addition to the C15 Laves phase are identified in the as-cast material of the ternary Laves alloy by high resolution transmission electron microscopic observation. It is suggested that the addition of W to NbCr2 reduces the stability of the C15 Laves phase. The alloying behavior of element W in the C15 Laves phase or the retained C36 Laves phase, whether it substitutes for Nb site or Cr site, depends upon the prepared alloy compositions.