Observation of beta phase particles in an isothermally aged A1-10 mass% Mg alloy with and without 0.5 mass% Ag

Abstract

Characterisation of the β′ and β precipitate phases formed in the aged A1-10 mass%Ag at 200°C and 240°C has been carried out using TEM. The plate-like precipitate particles were formed in binary alloys aged from 2 h at 200°C. The plate-like particles had habit planes of (100) α, and were identified as the β′ phase. Whereas the very fine scale, uniformly distributed precipitate particles with a spherical shape were formed as the dominant precipitate particle in ternary alloy aged at 200°C for periods of 4 h and 6h. These small spherical particles were identified to be icosahedral phase while no icosahedral phase was observed in the Ag-free alloys when samples were aged at 200°C. Formation of a precipitate cluster consisting of two distinguishable parts has been observed in binary alloys aged 5 h at 240°C, and these precipitate particles were identified as the equilibrium β and metastable β′ phases. Whereas a higher volume fraction of dispersed precipitate phases which comprised a mixture of a coarse-scale, rod-like particles and blocky particles, was observed in ternary alloy aged 5 h at 240°C. With further increase of ageing time up to 24 h at 240°C, the microstructure contained a high volume fraction of coarse-scale, globular precipitate particles, together with a secondary distribution of fine-scale spheroidal precipitates, these coarse globular particles were identified to be the equilibrium β phase. © 2008 The Japan Institute of Light Metals.