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Dual-phase nanostructuring as a route to high-strength magnesium alloys, Wu, Chan, Zhu, Sun, Lu, Science, 2017

Novelty/impact/significance:

Create a magnesium-based supra-nanometer-sized dual-phase glass-crystal (SNDP-GC) material that approaches the near-ideal strength, 3.3 GPa and a Young’s modulus of 65 GPa. The fabrication is easy to scale up (magnetron sputtering) and the material is 10cm x 10cm with 10 µm in thickness, which is promising for various industrial applications.

Scientific question:

How to achieve the ideal strength of metals/alloys (E/10, E is the elastic modulus) making full use of the strengthening advantages while avoiding the softening disadvantages?

Key of how:

Making a dual-phase structure consisting of MgCu2 crystals (~6 nm in diameter) uniformly embedded in magnesium-enriched amorphous shells (~2 nm thick).

The central crystalline phase (almost dislocation-free) blocks the propagation of localized shear bands and the grains rotate and divide within the shear bands, to provide hardening and counter the softening effect of amorphous shell, while the amorphous shells impede the gliding of the central grains and the dislocations motions so that to prevent the reverse Hall-Petch softening effect.

The high strength comes from (1) the ultrafine-grained crystals, (2) the strain hardening of the nanocrystals, (3) the restriction in grain sliding and dislocation motion by the amorphous shell (prevent reverse Hall-Petch effect), (4) the shear-band arrest by the core crystals (prevent amorphous metal softening) and the multiplication of main shear bands;

Major points:

1. Most strengthening mechanisms in crystalline metals are based on controlling defects (e.g., impede the motion of dislocations); but these cannot be increased definitely due to the change from a dislocation-related process to a defect-softening behavior. For example, the reverse Hall-Petch effect when grain sizes below 10 nm. The strength of nanocrystalline/nanotwinned materials is usually around E/85.

2. Amorphization, as another strengthening in metals with the deformation mechanisms of shear bands, also suffers from a softening effect by the localization of shear strain (maximum strain has to be within 2%). So amorphous materials cannot reach ideal strength either.

3. Materials with near-ideal strength are usually nanometer-sized (crystalline nanowires, nano-metallic glasses, etc.) with complicated fabrications, while enlarging the size with similar strength is more challenging.

It is proposed in this work that a ‘supra-nanometer-sized dual-phase glass-crystal’ (SNDP-GC) structure, nanocrystalline cores (each grain size <10 nm) surrounded by an amorphous shell (several nanometer thick), can make full use of both high strengths and meanwhile prevent the reverse Hall-Petch effect (the grain gliding and dislocation motion are impeded by the amorphous shell) and inhibit the shear-band softening (shear bands are arrested/controlled by the central crystals).

4. By magnetron sputtering with appropriate heating, the SNDP-CG consists of ~6 nm MgCu2 crystalline grains (volume fraction of 56%) uniformly embedded in amorphous shells (composition of Mg69Cu11Y20) that are ~2nm thick. High resolution TEM in different orientations, FFT, and SAED confirmed the homogeneous structure in three dimensions (differ from common granular/columnar by sputtering) and the crystallinity and amorphous nature.

The nanocrystalline MgCu2 grains are almost free from dislocations, a low-energy state.

5. This structure shows a near-ideal strength of 3.3 GPa and a strain limit of 4.5% in microcompression, remarkably superior than magnesium-based crystalline alloys (0.46 GPa, 5%) and magnesium-based glasses (~1 GPa, 2%).

The E/20 strength reaches the ideal strength of metallic glasses, is close to the theoretical strength (E/10) of all materials, and is the highest among reported magnesium alloys.

The synthesis is easy for manufacturing and the samples are large (10 cm x 10cm x10µm), with conventional materials of near-ideal strength generally existing in nanometer-sized form.

6. The SNDP-CG has a Young’s modulus of 65 GPa that is comparable with that of nanocrystalline magnesium alloys and magnesium-based glasses.

7. The high strength results from new mechanisms (HRTEM, constitutive modeling), the constrained behavior/interactions of the crystalline grains (elasticity) and amorphous shells (shear-band), which innovatively use both advantages and avoid both softening disadvantages.

The quasi-dislocation-free nanocrystals with further strain hardening are inherently strong, and the amorphous phase too.

The shear-band softening of amorphous shells is prevented in such a way: the main shear band is arrested by the nanoscrystals, and generates multiple embryonic shear bands (demonstrated by the multiple, small ‘pop-in’ events in stress-strain curves and TEM). The similarity in size of shear band and crystals allows the structure to recover immediately, and the nanocrystals are divided and rotated within the shear bands. These contribute to retaining the stress and strain (demonstrated by the no stress-drops after each pop, thus no catastrophic failure as normal metallic glasses upon yielding).

The reverse Hall-Petch softening effect of nanocrystals is prevented by the amorphous shells that impede grain gliding and dislocation motion.

It is an exciting work, and may serve as a link with composites either polymer- or ceramic-based in certain fundamental mechanisms. Here is the link of the fulltext: https://www.nature.com/articles/nature21691