Homogeneous deformation of Zr–Ti–Al–Cu–Ni bulk metallic glasses

Zr–Ti–Al–Ni–Cu bulk metallic glasses have been prepared by copper mould casting and were characterized using neutron and X-ray diffraction and scanning calorimetry in the as-cast glassy state and during crystallization. Depending on chemical composition, the under-cooled liquid domain separating the glass transition temperature (Tg) from the onset of crystallization (Tx) ranges from 82 to 102 K, for a heating rate of 20 K/min. The visco-plastic behaviour of the material has been studied in the supercooled liquid state, between Tg and Tx, using constant strain-rate compression tests and strain-rate jump tests. The stress–strain curves display an overshoot at large strain-rate and low temperatures, followed by a constant flow stress up to very large plastic strains. The flow stress is strongly thermally activated. Another characteristic feature of the deformation of the fully amorphous state is the independence of the flow stress on the strain-rate path. The stress–strain curves and their dependence on temperature and strain-rate are first discussed in terms of glass viscosity. It is shown that the data obtained at various temperatures and strain-rates can be scaled on a master curve. The master curve is directly deduced from the stationary strain-rate proposed by Spaepen for homogeneous flow based on the free volume concept. Deformation constitutive laws are then established in the out of equilibrium general case, taking into account the strain induced free volume creation and relaxation. They allow an accurate description of the transients regimes commonly observed such as overshoot, stress oscillations and strain-rate changes. Finally, the hardening resulting from partial crystallization is described by the introduction of a back-stress originating in the undeformability of the crystallites.