Homogeneous deformation of bulk metallic glasses in the super-cooled liquid state

Different Zr-Ti-Al-Ni-Cu bulk metallic glasses have been processed. They have been characterized by neutron and X-ray diffraction in the glassy state and after DSC heating up to crystallization. The homogeneous visco-plasticity of the glasses has been studied in the under-cooled liquid state by compression tests at constant strain-rate and by differential mechanical tests with strain-rate jumps at different temperatures. The flow stress appears to be strongly thermally activated. Another characteristic feature of the deformation of the fully amorphous state is that the flow stress is independent of the strain-rate path followed to reach a given level of strain. The stress-strain curves and their dependence on temperature and strain-rate are then discussed in terms of Newtonian and non-Newtonian viscosity. It is shown that a reduced viscosity can be scaled on a master curve, independently of the strain-rate and temperature. A second analysis of the results is made, in terms of free volume. The temperature and stress variation of the plastic flow are compared with Spaepen model. A constitutive equation of the deformation is proposed. Finally, the effect of crystallization on mechanical behaviour of metallic glasses is treated trough a model involving backstress due to a composite behaviour.