The influence of temperature and storage time at RT on nucleation of the β" phase in a 6082 Al-Mg-Si alloy

The age hardening process of a 6082 Al-Mg-Si alloy by isothermal ageing at 100, 125 and 150 degreesC for 0 to 40 days has been investigated by both conventional and high-resolution transmission electron microscopy. Changes in sizes and densities of the pre-beta" and beta" phases during ageing can explain the evolution of hardness, in particular a plateau in the curve of the hardness versus ageing time. Fully coherent pre-beta" particles (GP-1 zones) are formed from atomic clusters that were created upon annealing or at natural ageing. Room temperature (RT) storage reduces the number of pre-beta" particles by a factor of 5. Ageing at 125 and 150 degreesC causes the phase pre-beta" to transform to the phase beta". An ageing temperature of 100 degreesC was found to be too low to initiate the transformation of pre-beta" to beta". (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.