Dependence of Yield Stress on Diameter and Density of Prismatic Dislocation Loops

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Abstract
An alloy of aluminum with 0.5 at.% magnesium was quenched from 600° to −10°C and aged at various temperatures and times. The supersaturation of vacancies generated by these treatments produced a large number of prismatic dislocation loops, which appreciably affected the mechanical properties of the alloy. In particular, the yield stress was raised to as much as three times the value of the fully annealed alloy. The loop density and loop diameter depended upon the treatment, and both showed influence on the yield stress. By means of electron microscopy, the structure of quenched and aged alloy specimens was observed before and after deformation, and related to the treatments. The mechanism of interaction between prismatic dislocation loops and glide dislocations has been analyzed, and the best fit for the experimental results is given by the expression for the increase in flow stress, Δτ=μb(dN)1/2/ω, where μ, b, d, N are the shear modulus, Burgers vector, loop diameter, and loop density, respectively, and ω is a constant found experimentally to be 2.5. This model accounts for the experimentally measured values of the yield stress for loop diameter up to about 500 Å, whereas for larger loops, the type of interaction presumably changes.