Theoretical study of the dislocation structure in HgI2

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Abstract

The structures of dislocations that participate in plastic deformation processes in mercuric iodide (α-HgI2) are studied theoretically. The idealized atomic configurations of the dislocations are determined and planar views are used to illustrate the geometric arrangements of atoms in both edge and screw configurations. Slip of glissile dislocations on (001) planes is responsible for plastic deformation in the model of James and Milstein [J. Mater. Sci. 18 (1983) 3249]. Here it is argued that these are (001)a〈100〉 dislocations (i.e. their Burgers vector has a magnitude equal to the lattice parameter a and is in a〈100〉 direction). It is shown that these dislocations can be subdivided into type a or b, according to whether the Hg atoms in the (001) planes that are adjacent to the (001) I-I slip planes move (from their “unslipped” equilibrium positions) closer to or further from nearest I atoms in the next nearest neighbor I slip plane as slip initiates. Atomic based calculations indicate that the critical shear stresses required for movements of types a and b dislocations can be appreciably different. Other topics discussed include (i) interactions of glissile (001)a〈100〉 dislocations with forest {010}a〈100〉 edge or {010}c[001] screw dislocations and the relevance of these interactions to work hardening, (ii) the generation of Frank-Read mills formed by the successive traversal of forest {010}a〈100〉 edge dislocations by glissile (001)a〈100〉 dislocations, and (iii) the separation of {010}c[001] edge dislocations into Shockley partial dislocations.

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