The Ageing of a Quenched Aluminium-22.5% Zinc Alloy

Abstract

The effect of drastic water-quenching on the kinetics and mode of precipitation in an aluminium-22.5% zinc alloy has been studied during ageing at elevated temperatures. As with direct-quenching, decomposition occurs in four stages: G.P. zones → platelets of rhombohedral transition phase → cubic α′ → zinc. The loops and helical dislocations introduced by quenching from 475°C do not appreciably affect the initial stages of precipitation but accelerate by a factor of ∼ 5 the conversion of the rhombohedral precipitate into cubic α′ and ultimately zinc. The effect is believed to be due to the more rapid loss of coherency and coarsening of precipitates nucleated on the dislocations. Other factors studied or discussed are the geometry of the precipitation of rhombohedral α′ platelets on dislocations, the direct precipitation of zinc at temperatures above the metastable α−α′ solvus, and the conversion of the α′ platelets into zinc during storage at room temperature after the completion of heat-treatment. In the last instance, globules of zinc form within each plate, probably as the result of a discontinuous precipitation process.