A team at Shanghai Jiao Tong University (SJTU), with colleagues at Tsinghua University and Brown University, have developed a means to regain the strain-hardening ability of high-strength metals by incorporating of extrinsic nanofillers at grain boundaries. A paper on their work is published in the ACS journal NANO Letters.

Grain refinement to the nano/ultrafine-grained regime can make metals several times stronger; however, this process is usually accompanied by a significant loss of ductility. Such strength-ductility trade-off originates from a lack of strain-hardening capacity in tiny grains.

The SJTU team demonstrated that the dislocation storage ability in Cu grains can be considerably improved through a novel grain-boundary engineering approach, leading to a remarkably enhanced strain-hardening capacity and tensile ductility (uniform elongation).

In this work, we present a new approach to overcome strength-ductility tradeoff in nanostructured metals by introducing a nano-scaled extrinsic reinforcing phase, referred to as “nanofillers,” with at least one dimension less than 100 nm, into the grain boundaries to form a metal matrix composite. In these composites, the large lattice mismatch between the reinforcement and the matrix drives the nanofillers to migrate from the grain interiors and segregate along the grain boundaries. The nature of the abundant reinforcement/metal interfaces can be elaborately tuned by tailoring the type, configuration, defect state and concentration of the reinforcement, affording additional freedom in designing or tuning boundaries with desired properties.

Here, using nanostructured Cu reinforced with reduced graphene oxide nanosheets (RGO) as a model material, we show that the dislocation storage ability of Cu grains can be greatly enhanced by RGO incorporation at the grain boundaries, resulting in a remarkably elevated strain-hardening capacity and a profound increase in tensile ductility (uniform elongation).

—Li et al.

Experiments and large-scale atomistic simulations revealed that a key benefit of incorporated nanofillers is a reduction in the grain-boundary energy, enabling concurrent dislocation storage near the boundaries and in the Cu grain interior during straining.

The strategy of grain-boundary engineering through nanofillers is easily controllable, generally applicable, and may open new avenues for producing nanostructured metals with extraordinary mechanical properties.

The research was supported by National Key R&D Plan of Ministry of Science and Technology of China, National Natural Science Foundation of China and Science & Technology Commission of Shanghai Municipality.

Resources

• Zan Li, Haotian Wang, Qiang Guo, Zhiqiang Li, Ding-Bang Xiong, Yishi Su, Huajian Gao, Xiaoyan Li, and Di Zhang (2018) “Regain Strain-Hardening in High-Strength Metals by Nanofiller Incorporation at Grain Boundaries” Nano Letters doi: 10.1021/acs.nanolett.8b02375