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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.

The kinematic and isotropic strain hardening was investigated in columnar-grained copper with preferentially oriented nanoscale twins deformed at two different strain rates of 5 × 10−5 and 5 × 10−3 s−1. A significant back stress is caused majorly by threading dislocation pile-up and accumulation at the grain boundaries and is strain rate independent. The isotropic hardening associated with an increment in the local effective stress stems from dislocation storage at twin boundaries, and a high strain rate is beneficial for enhancing isotropic strain hardening and tensile ductility. Impact statement Long-range back stress was detected to develop during plastic deformation of nanotwinned structures, majorly caused by pile-up and accumulation of threading dislocations at grain boundary regions.

Gradient-nanostructured material is an emerging category of material with spatial gradients in microstructural features. The incompatibility between gradient nanostructures (GNS) in the surface layer and coarse-grained (CG) core and their roles in extra strengthening and strain hardening have been well elucidated. Nevertheless, whether similar mechanisms exist within the GNS is not clear yet. Here, interactions between nanostructured layers constituting the GNS in a Ni alloy processed by surface mechanical rolling treatment were investigated by performing unique microtension tests on the whole GNS and three subdivided nanostructured layers at specific depths, respectively. The isolated nanograined layer at the topmost surface shows the highest strength but a brittle nature. With increasing depths, isolated layers exhibit lower strength but enhanced tensile plasticity. The GNS sample’s behavior complied more with the soft isolated layer at the inner side of GNS. Furthermore, an extra strain hardening was found in the GNS sample, leading to a greater uniform elongation (>3%) as compared to all of three constituent nanostructured layers. This extra strain hardening could be ascribed to the effects of the strain gradients arising from the incompatibility associated with the depth-dependent mechanical performance of various nanostructured layers.

The effect of specimen thickness on fracture toughness and fracture mechanism was investigated in bulk columnar-grained Cu with preferentially oriented nanoscale growth twins. Below a critical specimen thickness of ~1.0 mm, plane stress state prevailed ahead of the crack tip and the fracture initiation toughness JC decreased with decreasing thickness. Above the critical thickness, JC decreased with increasing thickness until approaching an intrinsic thickness-independent value when the crack front was mainly under plane strain condition. Under plane strain condition, threading dislocations were majorly activated to glide along the nanotwin channels and to produce severe stress concentrations when they piled-up against grain boundaries (GBs). As a result, intergranular cracking mediated the failure of the nanotwinned Cu. On the contrary, under plane stress condition, dislocations slipping-transfer across twin boundaries (TBs) or partial dislocations gliding at TBs were activated to accommodate the plastic deformation. Consequently, stress intensification at GBs was plastically relaxed through enhanced detwinning and shear banding, which suppressed the intergranular fracture and promoted transgranular shear fracture. [ABSTRACT FROM AUTHOR]