Superior strength-ductility synergy and fatigue resistance of heterogeneous structured AZ41 Mg alloy by laser surface processing

In this work, a ∼220 μm-thick heterogeneous remelted layer is achieved in the coarse-grained AZ41 Mg alloy by the laser surface remelting treatment. Microstructural evolution observations reveal that the remelted heterogeneous layer consists of gradient refined equiaxed α-Mg grains (average grain size from ∼4.6 μm in the bottom to ∼ 60 nm at the surface) and dispersed block-shaped nanoscaled β-Mg₁₇Al₁₂ precipitations (∼22.8 nm), which is obviously different from the favored coarse dendritic or cellular α-Mg grains and lamellar β precipitations during conventional laser processing. The formation of this remelted layer is attributed to constitutional supercooling caused by solute elements and the thermal undercooling generated by the high cooling rates based on optimized processing parameters and liquid nitrogen cooling. The heterogeneous nanostructured remelted layer makes the laser-treated sample has a high hardness of 1.51 GPa, a yield strength of 230.4 MPa and an ultimate tensile strength of 313.7 MPa with a good ductility (12.5%). The laser-treated samples also show the higher fatigue lifetimes in low-cycle as well as high-cycle fatigue tests, which are respectively 2.3 and 18.6 times higher than the original samples. In addition, the laser-treated specimen exhibits a 25% higher fatigue limit compared with the original sample. The synergistic strengthening mechanism including gradient grain refinement and dispersed nanoscaled precipitations are quantitatively discussed to improve strength-ductility synergy and strain hardening capacity in the laser-treated samples. This mechanism could effectively facilitate resisting the initiation and propagation of the fatigue microcracks, which was further validated by the in-situ transmission electron microscope observations. Our results in this study could be further applied for the modification of grain morphology and precipitation distribution for additive manufacturing applications, critical to the development of advanced structural Mg alloys with high service performance in practical engineering environments.