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Dual-phase nanostructuring as a route to high-strength magnesium alloys

Nature volume 545, pages 80–83 (2017)Cite this article

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Abstract

It is not easy to fabricate materials that exhibit their theoretical ‘ideal’ strength. Most methods of producing stronger materials are based on controlling defects to impede the motion of dislocations, but such methods have their limitations. For example, industrial single-phase nanocrystalline alloys1,2 and single-phase metallic glasses3 can be very strong, but they typically soften at relatively low strains (less than two per cent) because of, respectively, the reverse Hall–Petch effect4 and shear-band formation. Here we describe an approach that combines the strengthening benefits of nanocrystallinity with those of amorphization to produce a dual-phase material that exhibits near-ideal strength at room temperature and without sample size effects. Our magnesium-alloy system consists of nanocrystalline cores embedded in amorphous glassy shells, and the strength of the resulting dual-phase material is a near-ideal 3.3 gigapascals—making this the strongest magnesium-alloy thin film yet achieved. We propose a mechanism, supported by constitutive modelling, in which the crystalline phase (consisting of almost-dislocation-free grains of around six nanometres in diameter) blocks the propagation of localized shear bands when under strain; moreover, within any shear bands that do appear, embedded crystalline grains divide and rotate, contributing to hardening and countering the softening effect of the shear band.

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Figure 1: Structure of the magnesium-based SNDP-GC.
Figure 2: Mechanical behaviour of the magnesium-based SNDP-GC at room temperature.
Figure 3: Deformation mechanism for our magnesium-based SNDP-GC.

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Acknowledgements

This work is supported in part by the Major Program of the National Natural Science Foundation of China (NSFC) grant 51590892 and the Hong Kong Collaborative Research Fund (CRF) Scheme (C4028-14G and CityU9/CRF/13G). We thank Q. Wang for technical discussions and Z. F. Zhou for assistance with magnetron sputtering at the City University of Hong Kong.

Author information

Authors and Affiliations

  1. Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong, China

    Ge Wu, Ka-Cheung Chan, Linli Zhu, Ligang Sun & Jian Lu

  2. Department of Engineering Mechanics and Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Hangzhou, 310027, China

    Linli Zhu

  3. Centre for Advanced Structural Materials, City University of Hong Kong, Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen Hi-Tech Industrial Park, Nanshan District, Shenzhen, China

    Jian Lu

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  1. Ge Wu
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Contributions

J.L. designed the project. G.W. and J.L. designed the material and experiments. G.W. conducted the nanoindentation and SEM in situ microcompression experiments and the TEM characterization. K.-C.C. conducted the focused-ion-beam experiments. L.Z. developed the theoretical model. L.S. performed the molecular-dynamics simulation. G.W. and J.L. analysed the data and wrote the paper. All authors contributed to discussion of the results.

Corresponding author

Correspondence to Jian Lu.

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The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks X. Li, J.-F. Nie and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Figure 1 Structure of the Mg-based SNDP-GC.

Left, three-dimensional, reconstructed TEM image of the material. The x–y plane indicates the surface of the sample, and the z axis indicates the depth from the surface. Top right, an SAED pattern. Bottom right, grain-size distribution. The statistical analysis shows the uniform distribution of nanocrystals with an average diameter of 6 nm.

Extended Data Figure 2 Compositional analysis of the Mg-based SNDP-GC.

a, An HAADF STEM image of the material. b, Composition of the nanocrystal within the amorphous shells, as indicated by the yellow arrow (positions on the x axis correspond to positions on the yellow arrow, starting from the end without the arrowhead). Error bars indicate standard deviations for four sets of data points.

Extended Data Figure 3 Molecular-dynamics simulation.

The figure shows the stress–strain results from our molecular-dynamics simulation of a single crystal of MgCu2, taken in the [100] direction. The inset shows a unit cell of Laves phase MgCu2. E is the Young’s (elastic) modulus.

Extended Data Figure 4 Microcompressive stress–strain curves for Mg-based SNDP-GC pillars of different diameters.

The result shows the strength of the material without any size effect.

Extended Data Figure 5 Simulated stress–strain relations.

We used our constitutive model (see Methods) to simulate stress–strain relations for Mg-based BMG, Mg-based alloy and Mg-based SNDP-GC. The simulated results correspond well with our experimental results.

Extended Data Figure 6 Constitutive relations and strain-dependent free-volume concentration for the Mg-based BMG and the metallic-glass phase in Mg-based SNDP-GC.

a, True stress plotted against true strain, showing that failure occurs at a strain of about 2% for the BMG, and about 5% for the metallic-glass phase in SNDP-GC. b, This difference occurs because of a difference in the evolution of free-volume concentration with increasing shear strain.

Extended Data Figure 7 The deformation energy, and the critical energy that is dissipated within a shear band, vary with strain and with the diameter of the sample.

a–d, The critical energy dissipated within a global shear band of our Mg-based SNDP-GC is plotted against true strain for samples of different diameters (D), showing how the deformation energy of the material varies with strain. When the curve of the deformation energy equals the critical dissipation energy, one more global shear band is created. With increasing numbers of shear bands, the critical strain required for one more global shear band to be created is increased.

Extended Data Figure 8 Comparison of predictions from our proposed criterion versus experimental measurements.

a, The number of global shear bands increases as the critical strain increases. b, The number of global shear bands also increases as the diameter of the samples increases. In both cases, our predictions are compared with our experimental results. Our predictions are consistent with our experimental results for samples with diameters of 760 nm, 1,300 nm, 1,800 nm, 3,200 nm and 4,100 nm.

Supplementary information

In-situ micro-compression movie (4× speed) of the 1300-nm-sized pillar

In-situ micro-compression movie (4× speed) of the 1300-nm-sized pillar. (WMV 15121 kb)

In-situ micro-compression movie (4× speed) of the 300-nm-sized pillar

In-situ micro-compression movie (4× speed) of the 300-nm-sized pillar. (WMV 12695 kb)

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Wu, G., Chan, KC., Zhu, L. et al. Dual-phase nanostructuring as a route to high-strength magnesium alloys. Nature 545, 80–83 (2017). https://doi.org/10.1038/nature21691

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