Ductile-brittle transition investigated by micro-indentation: results for quartz and olivine

A microindentation technique has been used for the measurement of fracture surface energy Γ in olivine and single crystals between 20 and 900°C. Γ is found to decrease in olivine with increasing temperature up to about 500°C. Microscopic observations on olivine show that some plasticity develops at the tip of the crack above this temperature. Quartz behaviour is different because of the α?β transition: Γ decreases very rapidly when the transition is approached. Above the transition, Γ increases with temperature. These results suggest that increasing the temperature enhances the fragile behaviour up to a critical temperature which depends on the nature of the material. At higher temperatures the trend is reversed. Microscopic observations in the ductile-fragile regime show that the mechanical results can be interpreted microscopically in terms of crack-dislocations interactions. Phase changes can, however, strongly perturb this behaviour.