The origin of ultrahigh-strength in GWZ1021K alloy fabricated by wire-arc directed energy deposition

Research on the preparation of over-sized lightweight magnesium rare-earth (Mg-RE) components using the wire-arc directed energy deposition (WA-DED) is progressively gaining attention in the advanced manufacturing fields. Herein, to satisfy the demand of ultrahigh load-bearing, the Zn-modified Mg-10Gd-2Y-1Zn-0.5Zr (GWZ1021K) alloy was designed for exploring the influence pattern of Zn element on the microstructure and properties of Mg-10Gd-2Y-0.5Zr (GW102K) with high RE content. Specifically, the Zn element enables finer and more homogeneous grains in the as-built GWZ1021K (18.2 µm) than that in the as-built GW102K (23.9 µm), owing to more nucleation sites and precipitation of nano-γ" and nano-γ' to impede grain growth during the intrinsic heat treatment. After solution treatment, the eutectic phases and RE-rich regions completely disappear in both GW102K and GWZ1021K, yielding elongations of up to 14.6% and 13.2%, respectively. Notably, the high-temperature solution process allows the growth of nano-γ" and nano-γ', as well as the segregation of RE/Zn clusters and subsequent atomic rearrangements to form the 14H long period stacking ordered (LPSO) structures. Following peak-aging treatment, although dense nano-β' is precipitated in both GW102K and GWZ1021K, the Zn element facilitates the precipitation of nano-β₁ to relieve the stress concentration induced by the two adjacent nano-β'. Generally, Zn elemental addition enhances strength with a sacrifice of ductility, which can be ascribed to the prismatic nano-β′ and basal 14H-LPSO structures work together to hinder the movement of both basal and non-basal dislocations. As a result, the GWZ1021K alloy achieves an ultra-high strength with an ultimate tensile strength of 403 MPa and a yield strength of 278 MPa, which far exceeds the reported average level of the WA-DED Mg-RE alloys. This study thus sheds new light on the fabrication of ultrahigh-strength Mg-RE alloy components by WA-DED process through appropriate composition modification.