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Ultrafine-grained (UFG) materials typically exhibit high strength but disappointingly low ductility due to the limited dislocation accumulation. To evade the strength-ductility tradeoff of UFG pure aluminum (Al), four kinds of UFG/coarse-grained (CG)/UFG sandwich-structured pure Al laminates were designed via extrusion and cold rolling. Microstructure characterization displayed that the grain size of the CG in the four designed laminates always remained ∼5 times that of UFG, indicating significant grain size heterogeneity between the constituent layers. The finite element analysis indicated that as the CG/UFG layer thickness ratio increased, the distribution of residual stress between the constituent layers was inhomogeneous and became more pronounced near the interface, which reduced the tensile ductility of the laminates. Tensile tests indicated that when the CG:UFG layer thickness ratio was 1:3, the laminate obtained a desirable strength and ductility compared to other laminates and UFG sample, with a yield strength of 150 MPa and a tensile ductility of 13.5 %. The optical microscope (OM)-digital image correlation (DIC) results revealed that the significant strain gradient near the layer interface caused by the heterogeneous deformation contributed to hetero-deformation induced (HDI) strengthening and HDI strain hardening, thereby strengthening and toughening the laminate.
Chen et al. (Wed,) studied this question.