Development of a high strength Zr/Sc/Hf-modified Al-Mn-Mg alloy using Laser Powder Bed Fusion: Design of a heterogeneous microstructure incorporating synergistic multiple strengthening mechanisms

Laser Powder Bed Fusion (L-PBF) provides great advantages in creating supersaturated solid solutions due to its intrinsic ultrafast cooling and high solidification rate, which is particularly desired for enhanced solid solution strengthening and precipitation strengthening. In the present work, a novel high strength Zr/Sc/Hf-modified Al-Mn-Mg alloy with a good L-PBF processability is investigated. The as-built alloy exhibits a heterogeneous microstructure featuring a bi-modal grain structure with coarse columnar and fine equiaxed grains and heterogeneously distributed nanometer and submicrometer-sized cuboid-shaped primary Al₃(Sc,Zr,Hf) particles. The Al₃(Sc,Zr,Hf) phase, exhibiting a homogeneous elemental distribution of Zr, Sc, and Hf, adopts a cubic L1₂ structure with a lattice parameter of 0.4044 ± 0.009 nm, showing a good coherency with α-Al. Additionally, the rapid L-PBF solidification enables the formation of submicrometer-sized elongated and globular primary Al₆Mn and a supersaturated Al matrix with a high dislocation density. This results in a good combination of strength (yield strength of 438 ± 3 MPa and ultimate tensile strength of 504 ± 2 MPa) and ductility (elongation at fracture of 10.9 ± 1.4 %), with a moderate work hardening strength of 113 ± 12 MPa. Direct-ageing at 325 °C for 10 h promotes the formation of a large amount of rod-shaped Al₆Mn precipitates and a few spherical Al₃(Sc,Zr,Hf) nanoprecipitates. The heat treatment increases the hardness from 166 ± 2 HV in as-built condition to 173 ± 4 HV and enhances the tensile strength (yield strength of 487 ± 2 MPa and ultimate tensile strength of 542 ± 3 MPa) but slightly reduces the ductility to 7.4 ± 0.7 %. The high strength was achieved by the synergistic effect of grain boundary strengthening, solid solution strengthening, and Orowan strengthening mechanisms.