Tailoring Heterogeneous Microstructures Governs Precipitate Coarsening Kinetics and Strength-Ductility Trade-Off in additively manufactured Al-Mn-Mg-Sc-Zr alloys

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Abstract
The thermal stability of additively manufactured aluminum alloys is often dictated by a complex interplay between high-temperature precipitation strengthening and grain- boundary-driven degradation. However, the presence of heterogeneous microstructures, composed of coarse- and fine-grained regions, introduces competing mechanisms that are rarely disentangled. Here, we report on two LPBF-processed TiC-modified Al-Mn-Mg-Sc-Zr alloys with distinct Sc/Zr contents and grain architectures, subjected to thermal exposure at 250-400 ℃ for up to 100 h. Our results establish a clear hierarchy of precipitate stability, wherein Al3(Sc,Zr) resists coarsening most effectively, followed by icosahedral quasicrystals, whereas Al6Mn and Al30Mg23 exhibit marked temperature sensitivity and anisotropic growth, particularly above 300 ℃. Critically, we uncover that the fine-equiaxed-grain-dominated alloy, despite its higher initial strength, accelerates grain-boundary-mediated coarsening of Al6Mn and Al30Mg23, leading to a paradoxical strength-ductility trade-off during prolonged thermal exposure, even though it retains yield strengths above 530 MPa after 100 h at 250-300 ℃. In contrast, the coarse-columnar-grain-dominated counterpart exhibits more sluggish microstructural degradation and superior ductility retention. These findings demonstrate that the long-term performance of these alloys is governed by a competition between stable nanoscale precipitation and grain-boundary-mediated phase evolution. Our work thus underscores that optimizing thermal stability requires not only enhancing the resistance to precipitate coarsening but also tailoring grain boundary architecture, a design criterion that extends beyond conventional alloy development for high-temperature applications.

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