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• The NS surface layer supported by CG substrate displays significant strain delocalization and achieves improved uniform elongation. • The strain delocalization is realized by forming dispersed stable strain bands (SBs) and promoting the deformation of non-strain banding zone. • The intra-layer microstructure inhomogeneity and the limited strain hardening capability of intra-layer domains lead to SBs nucleation. • The inter-layer microstructure inhomogeneity causes extra inter-layer shear constraints, which stabilize SBs and promotes the deformation of non-strain banding zone. • The NS indeed becomes ductile, due to the extensive nanotwinning and dislocation activity enabled by strain delocalization. How to suppress strain localization in tensioned nanograined layer and make it ductile? This remains a great challenge. Here we explore the effects of microstructure inhomogeneity on the plastic behavior of nanostructured layer. A nanostructured CrMnFeCoNi high entropy alloy layer supported by coarse-grained substrate (in gradient architecture) is taken as an example. In the tensile deformation, strain examinations find that the nanostructure layer experiences significant strain delocalization by activating dispersed stable strain bands (SBs) and promoting the deformation of non-strain banding zone. Finite element analysis reveals that it is the intra-layer microstructure inhomogeneity which enables dispersed SBs nucleation from the mechanical weak sites. The inter-layer microstructure inhomogeneity introduces extra shear constraints from the coarse-grained substrate, which stabilizes SBs and improves the stress of non-strain banding zone, thereby suppressing the early catastrophic strain localization. Importantly, the strong nanostructure layer indeed becomes ductile (enables further work hardening) due to the activation of hierarchical nanotwinning and dislocation activity. [Display omitted] [ABSTRACT FROM AUTHOR]