A THERMODYNAMIC MODEL FOR THE NANOCRYSTALLINE-TO-GLASS TRANSITION IN MECHANICALLY DRIVEN BINARY-ALLOYS

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