Ordered structures with functional units offer the potential for enhanced performance in metallic materials. Among these structures, gradient nanotwinned (GNT) microstructures demonstrate excellent controllability. This paper provides a comprehensive review of the current state-of-the-art studies on GNT structures, encompassing various aspects such as design strategies, mechanical properties characterization, spatially gradient strain evolution analysis, and the significant role of geometrically necessary dislocations (GNDs). The primary objective is to systematically unravel the fundamental strengthening mechanisms by gaining an in-depth understanding of the deformation behavior of nanotwinned units. Through this work, we aim to contribute to the broader field of materials science by consolidating knowledge and providing insights for the development of novel metallic materials with enhanced properties and tailored performance characteristics.

We produced a 316L stainless steel with heterogeneous nanometer-thick lamellar structures by severe cold-rolling at room temperature, and conducted micro-scale tensile tests in different orientations to evaluate both the in-plane (parallel to the nano-lamellae) and out-of-plane (normal and 45° inclined to the nano-lamellae) mechanical anisotropy. The parallel orientation demonstrates the greatest tensile strength while the inclined orientation exhibits the least strength. The tensile tests in normal and inclined directions also indicate significant transient elastic-plastic response due to the strain path change. Fractographic examination demonstrates that the specimen fails in the normal direction by premature micro-void nucleation and growth, which restricts its tensile strength; however, we identified zig-zag cracking associated with lamellar shear cracking in the inclined direction.