Effect of ageing on the evolution of precipitates in AlSiCuMg alloys

Hardness changes and structure of precipitates in aluminium alloy containing 9.0 wt.% Si and 3.5 wt.% Cu (1) were studied in relation with the magnesium addition of 0.5 wt.% (2) after ageing at 160 °C. The hardness maximum of 140 HV was obtained in alloy 1 after 1 day of ageing, while that of alloy 2 was about 30 HV higher. In both alloys elongated needle like precipitates parallel to the 〈100〉 matrix directions were observed after 24 h of ageing at 160 °C. Their length approached 200 nm in the alloy 1 and was altered down to about 80 nm by the magnesium addition. The electron diffraction pattern at [001] zone axis allowed to identify Θ′ precipitates and weak S′ (Al2CuMg) spots within streaks. It is in agreement with X-ray studies detecting one type of precipitates in AlSiCu and two types in AlSiCuMg alloys.