TEM investigation of the plastic deformation behaviour of calcium fluoride

Abstract

Cold–stage transmission electron microscopy is shown to eliminate any observable ionization radiation damage in calcium fluoride, thereby providing conditions suitable for performing TEM of the deformation microstructures. Stage II α consists of dislocation walls composed of primary and secondary dislocations. Work hardening during Stage IIα results from the formation of blocking obstacles, forest interactions and jogs. Stage IIβ is characterized by the formation of a dislocation cell structure and a secondary dislocation density much higher than Stage II α. The shear stress during Stage IIβ is correlated to the secondary dislocation density in a manner consistent with forest dislocation models of work hardening based on repulsive trees. The microstructure of Stage III is composed of a' block' cell structure with a cell size less than that of Stage IIβ. Single–crystalline calcium fluoride has been compressed under an initial strain rate of 5–64 × 10–3 per minute at 200, 300 and 400°C to establish the work hardening behaviour. At 200°C two linear regions of work hardening. Stage I la and Stage IIβ, respectively, are observed, while at 300 and 400°C the two linear regions and a third region of parabolic or decraasing work hardening. Stage III, are present. Stage I behaviour was not observed. The effect of the temperature of deformation and the crystallographic orientation ot the compression axis on the deformation behaviour have been established.