Improving the corrosion resistance of an ultra-lightweight BCC Mg-Li-Zn alloy via controlling the microstructure by heat treatment

Microstructure and corrosion resistance of an as-cast Mg-14Li-8Zn (in wt. %, LZ148) alloy subjected to solid solution treatment (T4) and aging treatment (T6) were investigated. The results revealed that the large eutectic MgLi₂Zn (θ’) phase formed in as-cast LZ148 alloy were completely dissolved into β-Li matrix after T4 treatment, and meanwhile nano θ’ phase could be rapidly precipitated in T4 sample, and then the θ’ nanoprecipitates transformed into MgLiZn (θ) phase with a moderate size after T6 treatment. The as-cast sample presented the worst corrosion resistance due to the occurrence of strong micro-galvanic effect between the cathodic eutectic θ’ phase and anodic β-Li matrix. After T4 treatment, the corrosion resistance of as-cast LZ148 alloy was dramatically improved, which was primarily ascribed to the trivial micro-galvanic effect due to the absence of noble eutectic θ’ phase. In addition, the dispersed MgLi₂Zn nano-precipitates and/or the homogenized distribution of alloying elements in matrix were also beneficial for the improved corrosion resistance of T4 sample. Furthermore, the lower local strain energy in the matrix of T4 sample could also contribute to the excellent corrosion resistance. However, the precipitated θ phases in T6 sample were detrimental to further improve corrosion performance due to the reactivated micro-galvanic effect. At last, the main compositions of surface film on the three samples were also revealed in detail, attributing to the different corrosion resistance of three samples as well.