Abstract
The cluster formation and two–step aging behavior of Al–1.4%Mg2Si and Al–1.4%Mg2Si–0.3%Si alloys were investigated using low-temperature adiabatic calorimetry, hardness measurement, electrical resistivity measurement and transmission electron microscopy. The specific heat changes in the alloys aged at low temperatures clearly demonstrated the existence of two exothermic peaks at temperatures between 273 and 343 K and higher than 343 K. These exothermic peaks were attributed to the formation of two types of pre-precipitates; i.e. Si-rich clusters and GP zones. The formation of these pre-precipitates was also detected by the electrical resistivity and hardness changes. The complicated two-step aging behavior of the present Al–Mg–Si alloys were well understood by taking into account the stability at higher temperatures and the effectiveness on the nucleation sites for the β″ phase of Si-rich clusters and GP zones during the final aging. The Si-rich clusters give a negative effect on the two-step aging phenomena is proposed. As the both pre-precipitates are formed competitively, such heat treatments as to give priority to the GP zone formation are effective to control the negative effect of the two-step aging and to increase age-hardening of Al–Mg–Si alloys.
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