Abstract A high-strength magnesium alloy material developed by Chinese scientists is close to the theoretical strength limit of magnesium-based alloys. In the just published "Nature" magazine, Lu Jian, Vice President of the City University of Hong Kong, and Associate Professor Zhu Linli of Zhejiang University, and other Chinese scientists jointly published a paper "Using...
A high-strength magnesium alloy material developed by Chinese scientists is close to the theoretical strength limit of magnesium-based alloys. 
From the public's point of view, magnesium alloys do not seem to be as famous as aluminum alloys. In fact, as small as a penny of coins, the shell of a mobile phone laptop, as large as the aircraft rocket can not be separated from magnesium alloy materials. Magnesium alloy materials have many advantages such as light weight, good performance and easy processing, and have been a research hotspot in materials science.
In people's common sense, solid metal exists at the normal temperature in the phase of metal crystals. When the ratio of the same elemental metal or alloy is constant, changes in the morphology, proportion, size, etc. of the microstructures (such as crystal grains, twins, etc.) constituting the metal material significantly affect the properties of the metal material, which is one of the materials sciences. It is called an independent branch of metallography. With the development of modern electron microscopy technology, scientists and engineers have been able to observe metal crystals from a microscopic perspective.
In the second half of the last century, scientists discovered that certain properties of materials change as the size of the microstructures that make up the metal material shrinks. These phenomena become especially noticeable when the diameter of a single crystal grain reaches 100 nm or less, for example, the strength and hardness of the material are greatly increased, and the ductility and toughness are lowered. (The reporter's suggestion: the hardness and strength of the material are not the same concept. The diamond with the highest natural hardness is very wear-resistant, but it is vulnerable to a hammer that is much softer than it is, so don't marry at home. Quit test!)
Metal materials composed of such nanoscale microstructures are called nanometal materials, and nanostructured cemented carbides which have been widely used at present are representative of them. For example, tungsten-carbon nano-carbide can be used to make high-strength drills less than one millimeter in diameter.
What is not known to the public is that the metal material can exist in a homogeneous amorphous phase, which is similar to the microstructure of the glass, so the metal in this form is called metallic glass. The metallic glass has good elasticity and resistance to plastic deformation, and the hitting portion of the golf club is made of metallic glass, and can maintain the shape after being subjected to a large impact.
In the past, nano metal materials were difficult to achieve theoretical strength. The main reason is that there are certain defects in the preparation of nano metal crystals, resulting in insufficient strength of the overall material. This is especially true at relatively low stresses. Although the preparation process of nano metal materials has progressed remarkably in recent years, there are limits to improving single phase metal nanomaterials by processes.
Lv Jian et al. tried another idea to coat metal nanocrystal particles with amorphous metal glass. Lu Jian et al. embedded a nanometer magnesium-copper alloy crystal into the amorphous metal shell of magnesium-copper-bismuth alloy to form a new type of magnesium-based two-phase nanoalloyed material, and named it super nanometer. Dual phase glass - crystal structure.
Microstructure of magnesium-based two-phase nanoalloy
The new nanomaterial consists of a single particle with a "shell" of less than 10 nanometers. The core composition of a single particle is a typical crystal composition of magnesium: copper = 2:1 (atomic ratio, the same below). The outer shell is estimated to be Magnesium: Copper: é’‡ = 69:11:20 is composed of a typical amorphous metal. The overall alloy material can be written in the form of magnesium 49 copper 46é’‡9. By testing the currently obtained thin layer material, it can be determined that the strength of the two-phase nano-magnesium-based alloy material reaches 3.3 gigapascals, exceeding all known magnesium-based nanomaterials and approaching the limit of theoretical magnesium-based alloys. This morning, the reporter of Global Science first connected with one of the authors of the cover story of Nature, Associate Professor Zhu Linli of Zhejiang University, and asked him to introduce more information about this research.
Global Science: What are the flaws in traditional nanomaterials? Did the new materials you developed solve these problems?
Zhu Linli: In general, nanostructured metal materials such as nanocrystalline materials have ultra-high strength mechanical properties compared to conventional metal materials. However, as the grain size is further reduced, such as a grain size of less than 10 nm, the strength of the material may soften, ie the material strength no longer increases as the grain size decreases (anti-Hall-Petch relationship) Therefore, the strength of the material cannot reach the desired strength (one tenth or one-twentieth of the elastic modulus E). In the duplex magnesium alloy we developed, the grain size and the thickness of the amorphous region are both less than 10 nm, and the strength of the material is close to the ideal strength E/20 of the magnesium-based amorphous.
Global Science: Why did you choose magnesium-copper alloy materials as research objects?
Zhu Linli: This is because magnesium-based alloys have a large number of potential applications in both industrial and biomedical fields. For example, our choice of magnesium-based alloys is to improve its mechanical properties in medical clinical applications such as reducing the friction coefficient. (Editor's Note: Magnesium is a necessary metal element in the body, and its content in tissues or blood is high. The medical device will not cause toxic side effects after being implanted into the human body).
Global Science: What are the application prospects of the ultra-nano materials described in the article?
Zhu Linli: Since the two-phase geometry of the two-phase super nanomaterials is less than 10 nanometers, we believe that the nanostructured materials of this new structure will exhibit very different mechanical and physical properties. At present, for super-nano metal materials, there will be great potential in industrial applications of ultra-high-strength lightweight structures, such as high-strength, lightweight parts for aerospace and automation.
Global Science: Is the ultra-nano dual-phase material prepared in the experiment suitable for industrial production? What is the cost?
Zhu Linli: We used a magnetron sputtering method to prepare a circular film with a diameter of about 10 cm. The method of magnetron sputtering itself is very mature and can be applied to large-scale material preparation, so the cost is not high. At the same time, we are improving the efficiency of preparing super-dano dual-phase materials by developing other preparation methods.
Paper link:
Dual-phase nanostructuring as a route to high-strength magnesium alloys
DOI: 10.1038/nature21691
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