Additive manufacturing high-strength and ultra-high-rare-earth magnesium alloys: Excellent long-time aging hardening and strengthening behavior

• An ultra-high rare-earth content Mg-23Gd-2Zn-0.4Zr alloy is successfully fabricated using LDED technology, surpassing the compositional limits of rare-earth magnesium alloys in additive manufacturing. • The LDED-T6 GZ232K alloy demonstrates an outstanding tensile strength of 425 MPa, representing the highest reported strength among additive-manufactured magnesium alloys to date. • High hardness platforms are observed at aging temperatures of 220 °C and 250 °C. • The high density and three-direction cross-interlocked structure of the β' phase contribute to the high yield strength in the over-aging phase. Gadolinium (Gd) is one of the most effective strengthening elements for magnesium alloys. The development of commercially available Mg-Gd alloys with high Gd content and the optimization of their preparation processes have been a major focus in magnesium alloy research. In this study, a Mg-23Gd-2Zn-0.4Zr alloy with ultra-high Gd content is designed, and high-quality fabrication is achieved using laser-directed energy deposition (LDED) technology. Through heat treatment and microstructure control, a balance between tensile strength (425 MPa) and elongation (3.4 %) is achieved. The ultra-high strength of the LDED-T6 VZ232K alloy is primarily attributed to precipitation strengthening caused by the ultra-high density (2.4 × 10 4 µm -2 ) of β′ phase. The high ductility is mainly due to the modification of the fracture mode, facilitated by the introduction of a substantial number of stacking fault structures during solution heat treatment. The extended hardness plateau (exceeding 138 Hv) and high yield strength (exceeding 300 MPa) are associated with the three-directional cross-interlocked structure of the β′ phase in the over-aged state at 220 °C and 250 °C. The analysis of the LDED-VZ232K alloy indicates that reduced heat input during the additive manufacturing (AM) process is critical for the defect-free fabrication of alloys with ultra-high Gd content.