Formation of a metastable eutectic during the solidification of the alloy Ti60Cu14Ni12Sn4Ta10

Cu-mould cast samples of the alloy Ti60Cu14Ni12Sn4Ta10 have a composite microstructure of micron scale (beta)-Ti dendrites in a nanoscale binary eutectic matrix. Characterisation of the individual phases in the nanoscale eutectic was facilitated by nanobeam electron diffraction, energy dispersive X-ray analysis and Rietveld refinement of X-ray diffraction patterns. The results clearly showed that the eutectic is composed of a (beta)-Ti solid solution, rich in Ta and Sn, and the intermetallic NiTi with ~ 25 at.% Cu substituted for Ni. Analysis of the solidification was achieved by comparison with an analogous ternary Ti-Cu-Ni alloy. In Cu-mould cast Ti60Cu14Ni12Sn4Ta10, after formation of the dendritic phase, the equilibrium phase CuTi2 was kinetically excluded due to sluggish diffusion conditions and the high cooling rate. A metastable equilibrium between (beta)-Ti and NiTi was therefore created, resulting in the observed eutectic. Heat treatment of the Cu-mould cast samples resulted in recovery of the equilibrium CuTi2 phase.