HIGH-TEMPERATURE X-RAY AND CALORIMETRIC STUDIES OF PHASE-TRANSFORMATIONS IN QUASI-CRYSTALLINE TI-ZR-NI ALLOYS

We present the first high temperature x-ray diffraction (HTXRD) studie s of in situ quasicrystal-crystal and crystal-crystal transformations in Ti-Zr-Ni alloys. Together with differential scanning calorimetry st udies, these x-ray measurements indicate three separate paths for the Ti-Zr-Ni quasicrystal-crystal. transformation: single exothermic, sing le endothermic, or multiple endothermic. The mode of transformation de pends on the alloy composition and the level of environmental oxygen. The crystalline products include the Ti2Ni, MgZn2 Laves, alpha-(Ti,Zr) , and beta-(Ti,Zr) phases. In the absence of oxygen, the endothermic t ransformation of the quasicrystal demonstrates that it is the lowest f ree energy (stable) phase at the Ti53Zr27Ni20 composition. Oxygen stab ilizes the Ti2Ni phase, eliminating both the quasicrystal and the MgZn 2 Laves phase, at partial pressures as low as a few hundred ppm.