Strong, ductile, and hierarchical hetero-lamellar alloys via microstructural inheritance refinement

What is it about?

The strength−ductility trade-off exists ubiquitously, especially in brittle intermetallic-containing multiple principal element alloys (MPEAs), where the intermetallic phases often induce premature failure leading to severe ductility reduction. Hierarchical heterogeneities represent a promising microstructural solution to achieve simultaneous strength−ductility enhancement. However, it remains fundamentally challenging to tailor hierarchical heterostructures using conventional methods, which often rely on costly and time-consuming processing. Here, we report a multiscale microstructural inheritance and refinement strategy to process ‘structural hierarchy precursors’ in as-cast heterogeneous Al0.7CoCrFeNi MPEAs, which lead directly to a hierarchical hetero-lamellar structure (HLS) after simple rolling and annealing. Interestingly, it takes only 10 minutes of annealing time, two orders of magnitude less than that required to render the state-of-the-art properties during conventional processing of Al0.7CoCrFeNi, for us to achieve record-high strength−ductility combinations via the hierarchical HLS design that sequentially stimulates multiple previously inaccessible deformation and reinforcement mechanisms. In particular, the HLS-enabled high hetero-deformation-induced (HDI) internal stress triggers profuse <111>-type dislocations on over five independent slip systems in the supposedly brittle intermetallic phase and activates extensive stacking faults (SFs) and nanotwinning in the adjoining soft phase with a rather high SF energy. These unexpected, dynamically reinforcing hetero-deformation mechanisms across multiple length scales facilitate high sustained HDI strain hardening, along with a salient microcrack-mediated extrinsic ductilization effect, suggesting that the proposed microstructural inheritance and refinement strategy provides an efficient, fast and low-cost approach to overcome the strength−ductility trade-off in a broad range of structural materials.

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Why is it important?

Brittle intermetallic phases that exist ubiquitously in diverse technologically important structural materials, especially in multiple principal element alloys (MPEAs), hamper practical applications due to premature failure. Here, we report a hierarchical hetero-lamellar structure (HLS) fabricated using a microstructural inheritance and refinement strategy that involves simple rolling followed by rapid annealing in a brittle B2-intermetallic-containing Al0.7CoCrFeNi MPEA. By harnessing high hetero-deformation-induced (HDI) internal stresses, the designed HLS sequentially stimulates multiple previously-inaccessible deformation and reinforcement mechanisms, which not only ductilize the brittle B2 phase, preempting premature failure, but also substantially strain-harden the adjoining soft phase to sustain stable tensile flow, leading to superior strength−ductility enhancement. This fast and low-cost microstructural design strategy is expected to impact a broad range of structural materials.