Abstract
The effect of pre-ageing at room temperature on the artificial ageing of an Al-1.2 wt% Mg2Si alloy has been investigated by means of differential calorimetry, electrical resistivity measurement and electron microscopy.
The principal results are summarized as follows.
(1) The power difference vs temperature curves for the specimens measured without pre-ageing and after pre-ageing at room temperature after quenching show a good correspondence to the electrical resistivity vs temperature curves for the respective specimens.
(2) The heat evolution due to the formation of the ordered G. P. zones was found in the power difference vs temperature curve for the specimen aged at 160°C for 20 hr after pre-ageing at room temperature for 33 days after quenching, while it was not recognized in the power difference vs temperature curve for the specimen aged without pre-ageing.
(3) It was found that the resistivity curves of the specimens aged at 180, 200 and 220°C without pre-ageing consist of the two stages, while the curves of the specimens aged after pre-ageing at room temperature consist of a single stage except the ageing at 220°C. The first of the two stages was attributed to the formation of the ordered G. P. zones, and the second was ascribed to the formation of the intermediate β′ phase. The apparent activation energy for the first process was determined to be about 20 kcal/mol.
(4) Coarse acicular precipitates were dispersed in the specimen aged at 200°C for 20 hr after pre-ageing at room temperature for 30 days after quenching, whereas the size distribution of acicular precipitates was regular in the specimen aged without pre-ageing.
(5) It is concluded that the two-step ageing behaviours in an Al–Mg–Si alloy can be explained by the retardation of the formation of the ordered G. P. zones at the second artificial ageing temperature. The retardation occurs as a result of the lowering of the effective concentration of vacancies and solute atoms for the formation of the ordered G. P. zones by the formation of the G. P. zones during pre-ageing. The result was also compared with that of an Al–Mg–Zn alloy, and the contrary two-step ageing behaviours were attributed to the difference of the precipitation sequence in both alloy systems.
