Crystallization and mechanical behaviour of bulk Zr-Ti-Ni-Cu-Be metallic glasses

摘要

Bulk amorphous samples of Zr-Ti-Ni-Cu-Be alloy have been prepared with the aim of studying the amorphous-ro-crystalline transformation in different conditions. The crystallization process has been studied by means of differential scanning calorimetry. The transformation is accomplished in several stages, the extent of which depends on either the heating rate or the annealing temperature. A phase separation in the amorphous matrix or the formation of crystals of extremely small size probably occurs in the first stage; with further annealing a hexagonal nanocrystalline phase is produced; finally the remaining amorphous part of the alloy transforms to a tetragonal phase. The rate of crystallization is strongly influenced by the atmosphere employed during thermal treatments, and a high resistance to crystallization was observed in vacuum. Rutherford backscattering spectroscopy analysis showed an increase in O content after annealing. The hardness of amorphous and partially crystallized samples has also been measured. An increase in hardness has been found in annealed samples where the formation of nanocrystals was in its early stage. Amorphous and heat-treated samples have been broken in compression; the fracture surface morphology has been investigated to check microstructure-induced changes in the appearance of failure.