Effect of impurities upon the nucleation of dislocation loops in quenched aluminium

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Abstract
The effect of pre-quench annealing in air upon vacancy precipitation in quenched aluminium of nominal 99·9999 wt. % purity was studied. Aluminium ribbons were resistance-heated for varying lengths of time in air at 600°C and subsequently quenched from 580°C into a controlled bath at 40°C in which the vacancy precipitation procedeed to completion. The electrical resistivity at 4·2°K was then measured for each specimen and compared with its value measured prior to the quench. The specimens were investigated by transmission electron microscopy in order to directly observe the resultant vacancy precipitate ensemble. It was found that the faulted dislocation loop density decreased monotonically with increasing duration of the pre-quench anneal, t a. However, the pre-quench bulk resistivity ratio, ρ(20°C)/ρ(4·2°K), of the specimens was found to be independent of t a, except in a preliminary period of annealing, with a value of 2309±62. It is concluded that the dislocation loops were nucleated heterogeneously at impurity sites over the range of specimen purity investigated. The results are interpreted on the basis that the concentration of impurities active in the dislocation loop nucleation process was reduced during the pre-quench anneal, but that this variation in impurity concentration was small enough to be undetectable in the resistivity ratio measurements. In addition, a value for the stacking-fault resistivity at 4·2°K in aluminium, was determined. The results of the present investigation are discussed in relation to previous work.