A fully recrystallized heterogeneous grain structure with dispersed second phases for enhancing strength-ductility synergy in an Al-Cu-Mn alloy

Introducing heterogeneous grain structures has been demonstrated as an effective strategy to overcome the strength-ductility trade-off in metallic materials. Here, we designed a fully recrystallized heterogeneous microstructure via controlling local recrystallization around the grain boundaries coupled with the second phase. The representative feature of this microstructure is the topology of the nearest grain neighbors, where a fully recovered large grain is surrounded by multiple recrystallized small grains. Meanwhile, the second phase distribution around the grain boundary could be remarkably optimized during the processing. Within a commercial Al-Cu-Mn alloy, a remarkable ultimate tensile strength of 365 MPa with uniform elongation of 30 % was achieved through this heterogeneous structure, corresponding to an 18 % increase in ultimate tensile strength and a 29 % improvement in ductility compared with the uniform-grained counterpart. Characterized by quasi-in-situ EBSD, it was found that the multiple small grain neighbors coordinated local strain incompatibility near the hetero-interface by dislocation slips and local reorientations, which can better sustain the strain hardening. In situ tensile tests unveiled the micro-crack blunting by ductile large grains, endowing superior crack-arrest capability. Moreover, tailored dispersed Al₂Cu particles inhibited inter-cluster void coalescence and interface-debonding by eliminating continuous crack growth paths, promoting stable damage evolution. These results give a new insight into the controllable design of heterogeneous-grained alloys with second phases for achieving superior combinations of mechanical properties.