Abstract
The ageing in SAP-type (Al–Mg–Si)–Al2O3 composites is investigated using resistivity measurements, hardness measurements, mechanical tests, and electron microscopy observations. An oxide content of about 3% completely inhibits GP zone formation during natural ageing because of the absence of quenched-in vacancies. In contrast, GP zone formation occurs during artificial ageing, the process being maintained by thermal vacancies and by diffusion along grain boundaries. To take advantage of both dispersion and precipitation hardening, it appears advisable to keep the oxide content below 1·5%. A considerable improvement in mechanical properties of the present composites over the conventional alloy is observed.
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