Direct Observation of Change from θ′ to θ Phase in Precipitation of an Aluminium-Copper Alloy

Abstract

Transmission electron microscopic studies on precipitation in thin foils of an Al-3.8%wt%Cu alloy have been carried out using a specimen heating attachment and a special 16 mm movie camera. The results obtained are as follows: (1) θ precipitates nucleate at the edge of some θ′ precipitates. θ′ precipitates with θ nucleus gradually grow, but the others having no nucleus gradually disappear. (2) Nucleation of θ phase is never observed in the matrix, except at grain boundaries or the edge of the foil. (3) Newly formed θ precipitates are not perfectly stable; i.e., some of them grow larger but the others shrink to disappear as ageing proceeds. (4) When the foil specimen is very thin, the movement of diffraction contours and their irregularities around precipitates are observed during the change from θ′ to θ phase. They may be due to strain produced by the volume change of precipitates. (5) Rough estimation of diffusion coefficient of copper in aluminium from the measurement of growth rate of θ precipitates gives the value of the same order, but to some degree larger than that obtained from the ordinary diffusion experiment in the bulk materials. (6) Although precipitation in thin foils of supersaturated solid solution at about 300°C appreciably differs from that in the bulk material in some cases, the sequence of precipitation from θ′ to θ phase in thin foils is nearly the same as that occurring in the bulk material.