Icosahedral-phase formation in as-cast Ti-Zr-Ni alloys

Previously, the formation of the thermodynamically stable icosahedral Ti-Zr- Ni i-(Ti-Zr-Ni) phase has been observed in rapidly quenched alloys, or by a low-temperature solid-state transformation from crystal phases that form at higher temperatures. Here we report the nucleation and growth of the icosahedral (i) phase directly from the liquid in as-cast Ti33Zr46Ni21 and Ti37Zr42Ni21 alloys. Diffraction patterns obtained by transmission electron microscopy and X-ray diffraction confirm the phase identity. Differential scanning calorimetry measurements show an endothermic transformation from the i- phase to a phase mixture of a C14 Laves phase and a solid-solution phase, demonstrating that this i- phase is also stable. The short time that the liquid remains in the Laves-phase-forming field and the higher nucleation rate for the i- phase, owing to strong similarities in the local structures of the i- phase and liquid, allow i- phase nucleation and growth directly from the liquid.