Plastic deformation and dislocation structure of single crystal mercuric iodide

Abstract

A systematic investigation of plasticity in single crystal mercuric iodide was carried out. Over eighty compression tests were performed on single crystal specimens oriented variously with respect to the axis of loading. Three characteristic loading responses were observed and classified according to crystal orientation, as follows: (a) the direction of load was perpendicular to the [001] axis, (b) the loading direction was parallel to [001] and (c) the load was neither parallel nor perpendicular to [001]. In cases a and b, under sufficient load, the samples always exhibited brittle failure (without prior plastic deformation) whereas, in case c, the samples were always easily plastically deformed by slip of (001) planes. The case c plastic deformation also exhibited work hardening. A dislocation model for plasticity in mercuric iodide is induced from the experimental results. The model consists of “easy glide” dislocations, the cores of which are parallel to (001) planes, and “hard glide” dislocations that intersect the (001) planes. Movement of the easy glide dislocations causes the (001) slip in case c, whereas interactions between the easy and hard glide dislocations account for the work hardening. The results of other experiments (including creep, annealing, microscopic observation of dislocation etch pits, and bending) are consistent with the model. Deformation during large angle bending tests is a particularly remarkable phenomenon.