Uncovering the impact of Gd content on the microstructure and mechanical properties of wire-arc directed energy deposited Mg-Gd-Y-Zr alloys

The preparation of large-sized magnesium rare-earth (Mg-RE) alloy parts using wire-arc directed energy deposition (WA-DED) has clear advantages such as high-efficiency and cost-effective. The impact of Gd content, which is one of the most important RE elements, on the microstructure evolution and mechanical response of the as-deposited and heat-treated Mg-Gd-Y-Zr alloys, deserves to be thoroughly unveiled. Herein, multi-scale microstructure characterization and mechanical evaluation of Mg-xGd-2Y-0.5Zr (wt%, x = 4, 7, and 10) alloys were carried out. Specifically, the increased Gd content facilitates the grain refinement, micro-segregation, and precipitation during the deposition process. As a result, the strength of the as-deposited samples with increased Gd content was improved through refined grain and dispersion strengthening of nano-β", but the ductility was severely deteriorated due to the premature failure caused by excessive β-Mg₂₄(Gd, Y)₅ eutectic phases. Besides, the increased Gd content successfully restrains grain coarsening through higher content of eutectic phase and larger RE-rich region during solution treatment. Following peak-aging treatment, while the increased Gd content does not affect the precipitation types, the content of nano-β' was remarkably enhanced, which leads to excellent strength. Ultimately, a superior yield strength of 239 MPa, an ultra-high ultimate tensile strength of 371 MPa and an elongation of 4 % are achieved in the solution plus aging-treated Mg-10Gd-2Y-0.5Zr alloy. This study thus provides guidelines on the composition modification and post-treatment of WA-DED Mg-Gd-Y-Zr alloys suitable for engineering applications.