A transmission electron microscopy investigation of the early stages of precipitation in an Al-Zn-Mg alloy

The growth of spherical precipitates in an Al-5 wt pct Zn-2 wt pct Mg alloy was monitored by hardness measurements and transmission electron microscopy. The growth of precipitates from 15 to 80Å in diameter was followed for various aging times at temperatures of 30, 70, 100, and 150°C. For short times at low aging temperatures precipitate growth followed at _1/9 law, whereas growth at longer times, or higher temperatures, followed at _1’3 law. Various contrast experiments lead to the conclusion that the precipitates produced at all but the shortest aging times at the lowest aging temperatures have a hexagonal structure and are spherical precursors of the η′ phase. The advantages and disadvantages associated with the various imaging techniques for small precipitates utilized in this study are discussed.