The effect of Si on precipitation in Al-Cu-Mg alloy with a high Cu/Mg ratio

The precipitations in an Al-5.0Cu-0.3Mg (wt%) alloy and an Al-5.0Cu-0.3Mg-0.3Si (wt%) alloy have been systematically investigated by high-angle annular dark-field scanning transmission electron microscopy. The results are compared to clarify the effect of Si addition. The nucleation and growth process of theta' (Al2Cu) phase in Si-containing alloy during isothermal ageing at 180 degrees C is revealed in detail. The formation of Q ''-type precipitates, on which the theta' precursors nucleate heterogeneously, contributes to the considerable increase in the ageing kinetics and higher strength at the early ageing stage. The thickening of the theta' precipitate is largely confined due to the rather small size of fine Q ''-type precipitate. As a result, a large proportion of theta' phase precipitates possess a specific thickness of 2c(theta'), and change slightly during the entire observed duration of ageing. The theta' growth mechanism distinct from the Al-Cu-Mg alloy finally leads to a refined theta' morphology regarding the thickness and aspect ratio (diameter/thickness). As is counterintuitive, the theta' precipitate thickness distribution is demonstrated to have little effect on the mechanical property steadiness at the late ageing stage of the Al-Cu-Mg-(Si) alloys. (C) 2014 Elsevier B.V. All rights reserved.