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 alpha-Zr for xAl<=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