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Solid solution strengthening of magnesium Akhtar, Ainul

Abstract

Solid solution strengthening in magnesium polycrystals containing Zn, Al, Cd, In and Pb as solute has been investigated over the temperatures between 78° and 513°K with particular emphasis on the dilute alloys. The variation of yield stress with concentration occurs in either two or three stages. In stage I, the yield stress increases rapidly and linearly with concentration; in stage II, the rate of increase of yield stress is very much less than in stage I; in stage III, the yield stress decreases with solute additions. The solution hardening rates and the transition concentrations from stage I to stage II (C[subscript T]) depend on the size-difference between Mg and the solutes. The results are discussed in terms of the variation with concentration of the CRSS for both basal and prismatic slip. It is proposed that at concentrations less than C[subscript T], the increase in CRSS for prismatic slip is the dominant factor; beyond C[subscript T], yield is governed by a balance between basal hardening and prismatic softening. The effect of solute on the ductility of magnesium at elevated temperatures is discussed in terms of a stress induced polygonization process and the ductility maxima observed in the Mg-Al alloys are explained. Single crystals of Mg-Zn alloys oriented for basal slip have been deformed in tension over the temperature range from 78°K to 423°K. The variation of the basal dislocation density caused by the addition of solute has been studied using transmission electron microscopy of thin foils. The increase in dislocation density which was found to be proportional to the square root of the solute concentration, cannot account for the observed increase in the athermal stress. A dislocation etch pit technique has been developed and used to measure the variation in the forest dislocation density with solute concentration. The forest density increases linearly and rapidly up to a certain minimum solute concentration, beyond which it remains almost constant. The results are in good agreement with the observed thermally activated flow stress for low solute concentrations. The observed variation in the athermal component of CRSS has been discussed in the light of an increased friction stress arising due to a random distribution of solute. Using rate theory, it has been shown that the forest intersection remains the rate controlling mechanism up to a certain low concentration of solute beyond which the single solute atom pinning of dislocations becomes the rate determining process. The solute dependence of the work hardening parameters are also reported and examined in the light of the existing theories of work hardening. Single crystals of Mg - Zn and Mg- Al alloys have also been deformed so as to suppress basal slip and {1012} twinning and to induce prismatic slip. The results have been explained in terms of an increasing athermal stress and a decreasing Peierls stress with the addition of solute. Peierls stress has been shown to be the rate controlling mechanism below room temperature. The observed variation of CRSS for prism slip with solute concentration accounts adequately for the concentration dependence of yield stress in the polycrystalline aggregate. The results also suggest that the decrease in Peierls stress with solute addition is not necessarily associated with a decreasing c/a ratio and the monovalent nature of the solute.

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