Solidification path and microstructure evolution of Mg-3Al-14La alloy: implications for the Mg-rich corner of the Mg-Al-La phase diagram

The solidification path and microstructure evolution in Mg-3Al-14La alloy have been investigated by in-situ synchrotron radiography, thermal analysis and characterisation of the final microstructure. It is shown that the solidification sequence begins with the precipitation of primary Al₅La₂, followed by a eutectic with the products α-Mg+Al₅La₂, then a mono-variant binary eutectic with the products α-Mg+Mg₁₂La and lastly a ternary eutectic with the products α-Mg+Mg₁₂La +(Al,Mg)₃La. This solidification sequence is found to be significantly different to the current Mg-Al-La thermodynamic description. The two main discrepancies are: (i) with Al₅La₂ being the primary phase, the Mg-3Al-14La alloy is a hyper-eutectic alloy rather than a hypo-eutectic alloy as currently described and (ii) the (Al,Mg)₃La phase, which was recently identified as a new phase in the Mg-Al-La system, is also present in the Mg-3Al-14La alloy. The results from this study will be useful for the improvement of the current thermodynamic description of the Mg-Al-La system.