High temperature creep of lithium zinc silicate glass-ceramics

Abstract

Two compositionally similar but morphologically different glass-ceramics containing about 20% residual glass phase have been creep-tested in tension and compression between 600 and 800°C. High stresses cause low strain failure in both materials but at low stresses there is considerable plasticity, strains greater than 30% being produced in one material. At the same stress and temperature, creep in tension is much faster than in compression, the ratio of secondary creep rates varying from 10 to 1000. The activation energy for secondary creep, 600 ± 60 kJ mol⁻¹, is effectively independent of stress, and within the scatter of the data is the same in tension and compression for both materials. The stress exponent is independent of temperature, but strongly dependent on stress, varying between 1.0 to 6.0 for the two glass-ceramics over the stress ranges used. This behaviour is similar to that exhibited by basic refractories, and is attributed to voids created within the glassy material between the crystals. The higher the stress, the easier it is to create the voids which are responsible for enhanced creep rates and low failure strain. A simple model is described to illustrate this mechanism.