Mg-Li-Al alloys with a body-centred cubic (BCC) structure can exhibit exceptional specific strengths in combination with excellent ductility and corrosion resistance. In general, the strength of these alloys is very sensitive to the processing temperature due to the occurrence of various phase transformations. Although different phases have been identified in these alloys, their corresponding transformation mechanisms and unique role played in controlling the mechanical properties have never been studied in depth. In this work, we identified the phase transformation sequence by in-situ synchrotron X-ray diffraction. Moreover, we investigated the evolution of precipitation and their morphology using transmission and scanning electron microscopy, together with simulations based on the phase field modelling and first-principles calculations. Phase transformation sequence of Al-rich zone → θ (D0₃₋Mg₃Al) → AlLi was confirmed during anisothermal ageing. A braided structure resulting from spinodal decomposition was found to be the optimized microstructure for achieving the peak strength. Nanocrystalline α-Mg phase at the interface between θ and the matrix was identified as the main reason for softening in the alloy. The core-shell model for θ → AlLi transformation is observed and verified. Our findings deepen the understanding of BCC Mg-Li-Al alloys and pave a pathway to develop new generation of ultralight alloys with stronger strength and better stability.

具有体心立方 (BCC) 结构的 Mg-Li-Al 合金可以表现出卓越的比强度以及出色的延展性和耐腐蚀性。一般来说，由于会发生各种相变，这些合金的强度对加工温度非常敏感。尽管在这些合金中已经确定了不同的相，但从未深入研究过它们相应的转变机制和在控制机械性能方面所起的独特作用。在这项工作中，我们通过原位识别相变序列同步加速器 X 射线衍射。此外，我们使用透射和扫描电子显微镜以及基于相场建模和第一性原理计算的模拟研究了降水的演变及其形态。富铝区相变序列→θ (D0 _{3 −} Mg ₃Al)→AlLi 在等温老化过程中得到确认。发现由旋节线分解产生的编织结构是实现峰值强度的优化微观结构。θ与基体界面处的纳米晶α-Mg相被确定为合金软化的主要原因。观察并验证了 θ → AlLi 转变的核壳模型。我们的研究结果加深了对 BCC Mg-Li-Al 合金的理解，并为开发具有更强强度和更好稳定性的新一代超轻合金铺平了道路。