Nanocrystal evolution in bulk amorphous Zr57Cu20Al10Ni8Ti5 alloy and its mechanical properties

Bulk alloys of composite microstructures of nanocrystals and amorphous phases were produced by annealing the bulk Zr57Cu20Al10Ni8Ti5 amorphous alloy prepared by copper mold casting. The mechanism for the evolution of the nanocrystalline microstructures was discussed with respect to the particular crystallization behaviors of the amorphous alloy. The changes of the microstructures due to the annealing conditions were studied. The mechanical properties of the partially nanocrystallized alloys were studied by compression test at room temperature. The fracture stress and the Young's modulus increase with the crystalline fraction to approximate to 45% (i.e. corresponding to a heat release of 45% of the total heat of crystallization) and then decrease. The yield stress also increases with the crystalline fraction to approximate to 45%, but then the deformation shows only an elastic part and no yield point appears with a further increase of the crystalline fraction. The hardness increases linearly with the crystalline fraction. Fracture morphology examination indicates that the fracture mechanism transforms from ductile to brittle nature at a crystalline fraction of about 40-45%. (C) 1998 Elsevier Science S.A.