UQ eSpace

Abstract

Precipitation and age hardening processes in A1 alloy 6061 and Comral-85™ metal matrix composite have been investigated and compared. A number of sophisticated techniques were used to characterise fine-scale precipitation in these materials. These included atom-probe field ion microscopy (APFIM), high resolution transmission electron microscopy (HRTEM), conventional transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). A dark-field high resolution technique was developed that enabled imaging of the crystal structures of fine-scale precipitates. The precipitation sequence in alloy 6061 was found to be: independent clusters of Mg and Si atoms >> co-clusters that contained both Mg and Si atoms >> small precipitates of unknown structure precipitates >> mostly B' and some s' precipitates It was found that the Mg:Si ratio in the small precipitates, precipitates and B' precipitates was close to 1. A new structure was proposed for the precipitate: base-centred monoclinic, a = 15.34Â, b = 4.05A, c = 6.83A, s = 106°. The development of hardness during ageing of alloy 6061 was related to different types of precipitates that formed in the precipitation sequence. The clusters of atoms and also the small precipitates had a negligible effect on hardness. Themajority of age hardening was provided by needle-shaped precipitates. The formation of B' precipitates resulted in substantial overageing of the alloy.