METASTABLE PHASE FORMATION IN THE ZR-AL BINARY-SYSTEM INDUCED BY MECHANICAL ALLOYING

We have studied the sequence of phase transformations induced in the Zr-Al binary system by mechanical alloying of mixed Zr and Al powders. The structure of these materials has been studied by transmission electron microscopy and by x-ray diffraction measurements. Three different metastable phases have been found experimentally with variation of the initial composition xAl: (1) a nanocrystalline supersaturated solid solution of α-Zr for x Al≤0.15, (2) an amorphous phase for 0.15<xAl≤0.4, and (3) a metastable face-centered-cubic phase for xAl=0.5 with a grain size of 4 nm. The crystallization reaction of the amorphous phase was monitored by differential scanning calorimetry, and the kinetics of the reaction have been examined as well. A possible explanation based on thermodynamic arguments is given for the defect-driven vitrification of the crystalline Zr phase.