Observations of precipitation in thin foils of aluminium +4% copper alloy

A high temperature specimen stage has been used to study precipitation in thin foils of Al +4% Cu alloy directly in the electron microscope. Precipitates a few hundred Angstroms in size am observed to form in a few minutes at temperatures ∼250° to 300°c. This initial precipitation has been identified by electron diffraction as the θ′ phase, and is found to occur in e well-defined single orientation with respect to the matrix. The orientation observed is consistent with one of those of θ′ formed in bulk material. The growth at higher temperatures of larger θ precipitates at the expense of the θ′ precipitates has also been studied. The results show that precipitation in thin foils Mars from that in bulk material due to the fact thee the surfaces act as preferential nucleation sites. Most of the precipitates in the thin foils appear to be at the surfaces. In aged bulk material preferential precipitation occurs on helical dislocations formed from screw dislocations by vacancy climb. The precipitates are observed to go into solution at ∼500°c. Measurement of the rate of dissolution by ciné techniques has enabled an estimate of the diffusion coefficient of copper in aluminium to be made. Other observations include the formation of regions denuded of small precipitates near grain boundaries and large precipitates, and the growth and dissolution of precipitates under a fluctuating temperature.