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 compositionxAl: (1) a nanocrystalline supersaturated solid solution of &agr;‐Zr forxAl≤0.15, (2) an amorphous phase for 0.15<xAl≤0.4, and (3) a metastable face‐centered‐cubic phase forxAl=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.