First author: X. Y. Li

Corresponding author: X. Y. Li, K. Lu

Communication unit: Institute of Metals, Chinese Academy of Sciences

Research highlights:

1. The constrained minimum interface structure in the ultrafine grain polycrystalline copper material was found

2. It provides a new idea for the stability of surface nanomaterials

The surface grain nanometer greatly improves the mechanical strength and hardness of metal, but also increases the surface grain boundary density. Due to the high surface energy of nanocrystals, the high density boundary of nanocrystals leads to the thermodynamic instability of nanocrystals, which is prone to coarsening at high temperature or even room temperature, resulting in the loss of nanocrystals effect and the reduction of mechanical properties.

As the miniaturization of microelectronic devices, high integration, the thickness of the metal wire and line width is now in nanometer scale, and the use of electronic devices in inevitably leads to the rise of temperature, nano-sized metal particles or film the melting temperature of general equilibrium melting point is lower than the corresponding block material, and with the particles diameter or declined significantly with the decrease of the thin film thickness.

In view of this, Lu Ke, academician of Institute of Metals, Chinese Academy of Sciences, Li Xiuyan et al. discovered the minimum interface structure of polycrystalline copper materials with 10 nm ultra-fine grain, which can maintain high strength at high temperature near the melting point. This discovery puts forward a new idea for the stabilization of metallic nanomaterials!

FIG. 1. Microstructure of ultrafine grain polycrystalline copper material

In a solid material, the atoms of a single crystal are arranged in an ordered lattice, as opposed to an amorphous solid or glass, where the atoms are arranged only in a short or medium order. Metals usually exist as polycrystalline solids, somewhere between these extremes. Metallic polycrystals are composed of smaller grains separated by various boundaries where the arrangement of atoms is usually disordered, called disordered grain boundaries.

The disordered grain boundary (GB) is responsible for the thermodynamic instability of polycrystalline metals. From the thermodynamic point of view, when the temperature rises, the grain boundary tends to be coarsened and even eliminated, and the polycrystalline material tends to become more stable until it finally becomes a single crystal. Alternatively, when grains are small enough, grain boundaries can be eliminated by switching to a metastable amorphous state.

Grain boundary migration usually occurs at temperatures less than half the melting point, and the coarsening temperature decreases with the decrease of grain size, even to room temperature in some nanocrystalline metals. The transition to metastable amorphous state is another option for a fine polycrystal with high enough grain boundary density. The method of reducing the grain size to below a few nanometers is thermodynamically sound.

A basic question then arises: when polycrystalline grains are steadily refined to extremely small sizes, can other metastable structures be formed?

Figure 2. TEM of a single grain

In 2018, the research team found that when the grains of pure Cu and Ni were refined to tens of nanometers by plastic deformation, the dissociation of grain boundaries would trigger spontaneous relaxation and enter a low energy state. The thermal and mechanical stability of nanocrystals are greatly improved due to the decrease of grain boundary energy, which can prevent the coarsening of nanocrystals at smaller sizes. The results indicate that by approaching the limit of grain size, nanocrystalline structure is likely to evolve into a more stable state.

Recently, based on the above findings, the researchers refined the polycrystalline Cu grains with a purity of 99.97wt % into nanometer size through a two-step plastic deformation process of surface mechanical grinding and high-pressure torsion under liquid nitrogen conditions. Through experiments and molecular dynamics simulations, another metastable state of ultrafine grain polycrystalline pure copper has been discovered. After the grain size is reduced to several nanometers by strain, the grain boundary in polycrystals evolves into a three-dimensional minimum interface structure constrained by twin boundary network.

FIG. 3. Ultrahigh thermal stability and strength

This polycrystalline structure prevents grain coarsening even when near the equilibrium melting point. At the same time, this polycrystalline Cu material can maintain high thermal stability, but also show strength near the theoretical value, which can be said to have it both ways.

In conclusion, this study provides a new way of thinking about the balance between thermal stability and mechanical properties of nanomaterials.

Figure 4. Atomic model and MD simulation

References:

X. Y. Li et al. Constrained minimal-interface structures in polycrystalline copper with extremely fine grains. Science 2020, 370, 831-836.

https://science.sciencemag.org/content/370/6518/831

Source: Nano

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