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The Fe-W binary system exhibits a zero heat of mixing in the amorphous state. Thus, there is no chemical driving force for the crystal-to-amorphous transformation. However, the crystal-to-amorphous transition was observed in a mechanically alloyed mixture of pure Fe and W powders. A structural study by x-ray diffraction showed that supersaturated solid solutions of W in Fe [Fe(W) SSS] and Fe in W [W(Fe) SSS] were produced during the early stage of the mechanical alloying. Iron atoms were already totally involved in the solid-state amorphization reaction after 24 h of milling. The end products were W(Fe) SSS and amorphous Fe-W alloy. The amorphization by mechanical alloying of the Fe-W system is attributed to a solid-state amorphization reaction in which lattice distortion induced by supersaturation of W in Fe and a refinement of grain size may raise the free energy of Fe(W) SSS above that of the amorphous phase and make the Fe(W) SSS destabilize.