The strengthening of polycrystalline metals based on grain refinement has previously been reported to be no longer effective below a critical grain size of approximately 10-15 nm (Refs. 1, 2). That report imposed a limit on grain size tuning for synthesizing stronger materials. Here, we report our study using a diamond-anvil cell coupled with radial X-ray diffraction to track in situ the yield stress and deformation texturing of pure nickel samples with various average grain sizes. Continuous strengthening isobserved from 200 nm to the minimum grain size of 3 nm. Strengthening as a function of grain size is enhanced in the lower grain size regime below 20 nm. We achieved an ultra-high strength of ~ 4.2 gigapascals in nickel, 10 times larger than the values for commercial nickel material. The maximum flow stress of 10.2 gigapascals is reached in 3 nm nickel in the pressure range of this study. Plasticity simulation and transmission electron microscopy (TEM) examination reveal that the high strength observed in 3 nm nickel is caused by the superposition of strengthening mechanisms: partial and full dislocation hardening plus grain boundary plasticity suppression. These results rejuvenate the search for ultra-strong metals via materials engineering.Understanding the strengthening of nanograined metals has been puzzling, as both mixed results of size softening and hardening have been reported [3][4][5][6] . The main challenges in resolving this debate are the difficulty in synthesizing high quality, ultrafine metal samples for traditional tension or hardness tests and making statistically reproducible measurements. Some researchers have pointed out that reported size softening may be related to materials preparation 7 . Porosity, amorphous regions and impurities may be introduced during sample preparation methods like inert gas condensation and electrodeposition, leading to softening in

An ultrafine grained Al-0.3wt. %Cu has been produced by cold rolling to a thickness reduction of 98% (ε vM =4.5). The deformed structure is a typical lamellar structure with a boundary spacing of 200nm as characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Coarsening of the deformed structure to recrystallization is achieved by heat treatment in the range of 100~300℃.Good thermal stability has been observed up to 175℃ with some segregation of Cu to the boundaries as observed by 3D atom probe characterization. Tensile tests have shown a flow stress (0.2% offset) of 198MPa with continuous flow with no yield drop and Lüders elongation. To quantify the contribution of boundary strengthening to the flow stress, dislocation strengthening and solid solution hardening have been calculated and subtracted from the flow stress. It has been found that boundary strengthening can be expressed by a Hall-Petch relationship and that these constants in this equation are in very good agreement with precious observation of recrystallized pure polycrystalline aluminium with a grain size in the tens of micrometer range. Thereby the Hall-Petch relationship of aluminium can be extended an order of magnitude from the micrometer to the sub-micrometer range, which is of both scientific and technical importance.

Strength and ductility are mutually exclusive if they are manifested as consequence of the coupling between strengthening and toughening mechanisms. One notable example is dislocation strengthening in metals, which invariably leads to reduced ductility. However, this trend is averted in metastable austenitic steels. A one-step thermal mechanical treatment (TMT), i.e. hot rolling, can effectively enhance the yielding strength of the metastable austenitic steel from 322 ± 18 MPa to 675 ± 15 MPa, while retaining both the formability and hardenability. It is noted that no boundaries are introduced in the optimized TMT process and all strengthening effect originates from dislocations with inherited thermal stability. The success of this method relies on the decoupled strengthening and toughening mechanisms in metastable austenitic steels, in which yield strength is controlled by initial dislocation density while ductility is retained by the capability to nucleate new dislocations to carry plastic deformation. Especially, the simplicity in processing enables scaling and industrial applications to meet the challenging requirements of emissions reduction. On the other hand, the complexity in the underlying mechanism of dislocation strengthening in this case may shed light on a different route of material strengthening by stimulating dislocation activities, rather than impeding motion of dislocations.

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To obtain some basic laws for bird-strike resistance of composite materials in aeronautical application, the high-velocity impact behaviors of composite laminates with different materials were studied by numerical methods. The smoothed particle hydrodynamics (SPH) and finite element method (FEM) coupling models were validated from various perspectives, and the numerical results were comparatively investigated. Results show that the different composite materials have relatively little effect on projectile deformations during the bird impact. However, the impact-damage distributions can be significantly different for different composite materials. The strength parameters and fracture energy parameters play different roles in different damage modes. Lastly, modal frequency was tentatively used to explain the damage behavior of the composite laminates, for it can manifest the mass and stiffness characteristics of a dynamic structure. The dynamic properties and strength properties jointly determine the impact-damage resistance of composite laminates under bird strike. Future optimization study can be considered from these two aspects.

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