Revealing effects of solute Ta on solidification and precipitation of Al-7Si-0.3Mg based alloys

Effect of Ta additions up to 0.12 wt.% on the grain refinement and the stability of β-MgSi series precipitates of Al-7Si-0.3Mg based alloys was investigated. It was found that (i) Ta can replace Ti for an effective grain refinement of Al-7Si-0.3Mg based alloys. Similar to the case of free Ti, a Ta-rich layer on the interface (basal plane) of TiB₂ was observed by using TEM and further supported by DFT calculation, while no significant interaction of Ta with other solutes (i.e. Si) was observed. More interestingly, Ta partitions into TiB₂, forming (Ti,Ta)B₂, which was observed for the first time in this alloy system. With the addition of 0.12 wt.% Ta, the grain size of primary Al can be refined down to 150 µm. More importantly, the eutectic grain size can be also refined down to 1500 µm even after Sr modification. (ii) DFT calculation shows that Ta stabilizes the β-MgSi series precipitates, while no significant effect of Ta on activation energy of β-MgSi precipitates was observed by using DSC due to its low concentrations. But, the activation energy follows an ascending order, i.e., β''-MgSi < β-MgSi. (iii) TEM shows that the dominant strengthening effect is related to the β''-MgSi precipitate. DFT indicates that Ta can improve the stability of the β''-MgSi precipitate. DSC confirms that Ta can enhance the precipitation kinetics of the β''-MgSi precipitate. This investigation can provide a novel methodology to control solidification and precipitation microstructure of Al-7Si-0.3Mg based alloys via the addition of Ta.