A thermodynamic model for the nanocrystalline-to-glass transition in mechanically driven binary alloys

Ultra grain refinement and supersaturation obtained by mechanical alloying performed by ball milling may act as an intermediate state to partial or complete amorphization. Based on an analogy with the wetting of chemical disorder at antiphase boundaries in intermetallic compounds when approaching the order-disorder transition temperature, a thermodynamic model is proposed for the nanocrystalline-to-glass transition. Application to the Zr-based hexagonal solid solution Zr-Ni is given. An isothermal phase diagram of the nanograin size against concentration coordinate system is established, displaying three domains in which nanocrystals, glass and both phases are metastable. The Gibbs energy of the nanograin boundary is found to vary significantly with concentration in the domain where the two phases coexist. It is demonstrated that nanocrystal size is a pertinent variable in describing the nanocrystal-to-glass transition.