High temperature x-ray and calorimetric studies of phase transformations in quasicrystalline Ti-Zr-Ni alloys

We present the first high temperature x-ray diffraction (HTXRD) studies of in situ quasicrystal-crystal and crystal-crystal transformations in Ti-Zr-Ni alloys. Together with differential scanning calorimetry studies, these x-ray measurements indicate three separate paths for the Ti-Zr-Ni quasicrystal-crystal transformation: single exothermic, single endothermic, or multiple endothermic. The mode of transformation depends on the alloy composition and the level of environmental oxygen. The crystalline products include the Ti₂Ni, MgZn₂ Laves, α-(Ti,Zr), and β-(Ti,Zr) phases. In the absence of oxygen, the endothermic transformation of the quasicrystal demonstrates that it is the lowest free energy (stable) phase at the Ti₅₃Zr₂₇Ni₂₀ composition. Oxygen stabilizes the Ti₂Ni phase, eliminating both the quasicrystal and the MgZn₂ Laves phase, at partial pressures as low as a few hundred ppm.