Additive manufactured Mg-Gd-Y-Zr alloys: Effects of Gd content on microstructure evolution and mechanical properties

Laser directed energy deposition (LDED), as one of the most promising additive manufacturing processes, is an excellent technique for preparing large-scale engineering structural components. Herein, a series of Mg- x Gd-3Y-0.2Zr (wt%, x = 9, 12, 15, and 18) alloys was manufactured by LDED, respectively. The microstructure evolution, hardness, tensile properties and fracture behavior were characterized and the effects of Gd addition were systematically explored. With the increment of Gd content, epitaxial growth at the bottom of melting pool transforms to equiaxed growth. The size of equiaxed dendritic grains shows negligible changes, whereas the intermetallic phase increases to a large extent from 1.51% to 15.92% (volume fraction). The Vickers hardness presents a linear growth against Gd content, and peaks at x = 18 wt% (118 Hv). Furthermore, the variation trend of the yield strength (YS) was rationalized by using the superposition law, according to which solute atoms in supersaturated solid solution (i.e . α-Mg matrix) and the intermetallic phase provided the main contribution. The highest YS (230 MPa) appears in Mg-15Gd-3Y-0.2Zr alloy, in contrast, the ultimate tensile strength (UTS) peaks at x = 12 and 15 wt%, at around 260 MPa. Noticeably, the hardness, YS, UTS, and elongation of Mg- x Gd-3Y-0.2Zr alloys manufactured by LDED are generally superior to those of their as-cast counterparts.