Abstract: In this study, an AlCu/AlMgSc bimetallic alloy is prepared using a dual-wire arc direct energy deposition method and a triple heterogeneous microstructure (fine/coarse equiaxed grains/columnar grains) is constructed. Additionally, a quantitative comparative analysis of the deformation behavior of triple and dual heterogeneous microstructures during interrupted tensile testing is conducted, with emphasis on the effects of grain morphology and size on the tensile deformation mechanisms in the heterogeneous microstructure. Compared with the AlCu alloy with a double heterogeneous microstructure (equiaxed/columnar grain), the AlCu/AlMgSc bimetallic alloy exhibits a higher ultimate tensile strength of 301.4 ± 7.9 MPa, a yield strength of 181.3 ± 1.4 MPa, and an elongation of 9.7 % ± 1.3 %, which correspond to increases by 19.4 %, 21.2 %, and 24.4 %, respectively. Interrupted tensile testing is performed and a quasi-in-situ approach is employed to investigate the plastic deformation mechanisms of the triple heterogeneous microstructure during tensile deformation. The density of geometrically necessary dislocations (GNDs) in the fine equiaxed grains and the rate of GND accumulation during deformation, surpassed those observed in coarse equiaxed and columnar grains. Furthermore, in micrometer-sized equiaxed grains, the ability to accumulate GNDs decreases as the equiaxed grain size increases, and the equiaxed grains exhibit a higher capacity to accumulate GNDs compared with columnar grains. The triple heterogeneous microstructure provides a more favorable environment for trapping GNDs, thus resulting in enhanced strength and plastic deformation capabilities. This study offers guidance for the formulation and engineering application of heterogeneous microstructure alloys with diverse grain morphologies and multiple length scales. Additionally, novel approaches are introduced to enhance the strength and ductility of Al alloys.

Key words: Dual-wire arc direct energy deposition, AlCu/AlMgSc bimetallic alloy, Triple heterogeneous microstructure, Geometrically necessary dislocations, Mechanical properties