Simultaneously enhancing strength and plasticity via direct ageing in additive manufactured Al–Ni–Sc–Zr alloys

Eutectic Al alloys processed by laser powder bed fusion (LPBF) frequently display metastable cellular structures. The cells are susceptible to decomposition into nanoparticles during ageing. Furthermore, supersaturated solutes can result in additional precipitation during the ageing process. The complicated microstructure evolution observed in LPBF eutectic Al alloys necessitates a comprehensive investigation into their ageing behaviour, to identify the optimal strength and plasticity. Consequently, this study presents a systematic examination of the impact of direct ageing on microstructure evolution in an LPBF Al‒Ni‒Sc‒Zr alloy, analysing associated changes in strength and plasticity. The optimal ageing parameters for strength and plasticity are determined. The results demonstrate that the reduction in strength resulting from cell decomposition can be offset by the strengthening provided by nanoparticles formed due to cell wall spheroidisation and additional supersaturated solute precipitation, achieving excellent yield strength. Furthermore, the transformation of cells into nanoparticles significantly enhances the plasticity by increasing non-uniform strain, which is not well explained by the conventional work hardening theory. A detailed investigation suggests that direct ageing can alleviate dislocation pile-up and strain localisation around cell walls, and reduce the tendency for crack propagation along melt pool boundaries, resulting in a significant increase in non-uniform strain and ultimately, excellent tensile plasticity. This study demonstrates that direct ageing is an effective strategy for simultaneously enhancing the strength and plasticity of LPBF Al–Ni based alloys. The proposed plasticity mechanism offers a new insight into the plastic deformation behaviour of LPBF eutectic Al alloys.