Work softening in stoichiometric spinel (MgAl₂O₄)

Abstract

Czochralski-grown single crystals of stoiehiometric spinel (MgAl₂O₄) have been deformed at 1780–1980°C along several different directions. Crystals deformed parallel to [111], [112], and [123] slip on the {111} 01 system, while {101} 01 slip is observed for the specimens deformed parallel to 001. There is thus little plastic anisotropy between these two systems. Work softening occurs at large strains (γ = 0·015) for both types of specimens; the resolved steady-state flow stress for the specimens with {111} slip planes is independent of the number of slip systems being activated and is 7sim;50% higher than for {101} slip planes.

Dislocation structures on the primary and cross-slip planes have been studied as a function of strain for specimens deformed at 1800°C. At small strains (σ ∼0·005), a high density of long straight edge dislocations belonging to the expected primary slip system is observed. At higher strains (γ ∼0·01), a random three-dimensional dislocation network has evolved, corresponding to the maximum stress in the τ-γ curve. Further deformation results in work softening, which is accompanied by a coarsening of the dislocation network until a steady-state flow stress is achieved.

A mechanism to explain the work-softening behaviour is proposed in which the Peierls barrier and the climb dissociation are responsible for an unusually small slip distance of edge dislocations. The small slip distance causes the dislocation density to increase rapidly and overshoot the steady-state dislocation density.