S. Gravier

Interactions between high-temperature deformation and crystallization in zirconium-based bulk metallic glasses

High-temperature deformation of a ZrTiCuNiBe bulk metallic glass (BMG) is investigated by compression tests in the supercooled liquid region. When the temperature is decreased or strain rate increased, the amorphous alloy exhibits the usual Newtonian/non-Newtonian transition behaviour. Using specific heat treatments, partially crystallized alloys are produced, the associated microstructures characterized and the volume fractions of the crystal measured. The interaction between high-temperature deformation and crystallization is investigated by appropriate mechanical testing. According to these measurements, partial crystallization is responsible for a significant increase in flow stress and the promotion of non-Newtonian behaviour. Deformation does not significantly change the volume fraction, composition or size of the crystal. The flow-stress increase with crystallization is analyzed under different hypotheses. We conclude that the flow-stress increase cannot be interpreted through a compositional change in the residual amorphous matrix, either by reinforcement due to hard crystallites or by connections between crystals. It appears that the effect is due to the nanometric size of the crystals alone.

Mechanical Response of Bulk Metallic Glasses: Investigation by Mechanical Spectroscopy and Compression Tests - Elastic, Viscoelastic and Viscoplastic Components

The present paper addresses the mechanical behaviour of several bulk metallic glasses (BMG). Both small and large deformations are investigated, using mechanical spectroscopy and compression tests, respectively. In the case of a given BMG, the influence of temperature and strain rate (or frequency) on the mechanical response exhibits an attractive similarity when either small or large deformations are applied. Equivalence between temperature and time is clearly evidenced. The same behaviour is observed in many BMG, whatever their chemical composition, and therefore whatever their glass transition temperature. This behaviour is also very similar to that reported in other amorphous materials: polymers or oxide glasses. The same physical model enables a good description of this behaviour. It is based on atomic mobility and localized deformation in “soft” zones. nanocrystallization hinders strongly the atomic mobility and induces a drastic hardening at high temperature.