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It can be easily deformed at an ultra-low temperature of 258

Time:2020/06/15 丨 source:未知 丨 visit count:

It can be easily deformed at an ultra-low temperature of 258 degrees!
Physicists "see" the multi-stage deformation mechanism of high-entropy alloys for the first time

 
High-entropy alloys (high-entropy alloys) are a new type of structural materials, with a variety of excellent mechanical properties, such as excellent strength and ductility, high fracture toughness and corrosion resistance. It is composed of a variety of main elements, which also leads to complex deformation behavior and distinctive characteristics: in ultra-low temperature environments, when the atoms of most materials are frozen "immobile", high-entropy alloys not only It does not become brittle like other materials, but it has high ductility and can undergo large deformation.
 
This special material property has made researchers very interested in high-entropy alloys, hoping to find out the deformation mechanism behind it.
 
Multi-stage deformation mechanism of high-entropy alloy
 
Recently, an international research team discovered for the first time that the deformation mechanism of high-entropy alloys at ultra-low temperature was originally divided into multiple stages, thus exhibiting extraordinary mechanical properties. This discovery may bring inspiration for the development of new structural materials applied at low temperatures.
 
The research was led by Professor Wang Xunli, Chairperson and Chair Professor of the Department of Physics, City University of Hong Kong (Hong Kong City University), who was just elected to the Fellow of the Neutron Scattering Society of America. He worked with scientists in Japan and the Mainland to conduct an in-situ study on the deformation behavior of high-entropy alloys at ultra-low temperatures. Related research results have been published in the scientific journal "Science Advances" (Science Advances), entitled "Cooperative deformation in high-entropy alloys at ultralow temperatures".
 
Fascinating high-entropy alloy
 
Generally speaking, the material will lose its ability to flow at low temperature because the atoms are "frozen", so it becomes brittle. However, the high-entropy alloy has high ductility at low temperatures and can be stretched to large deformations. "This phenomenon was first discovered in 2014, but the mechanism behind it is still unknown and very interesting." Professor Wang Xunli, who has been studying the deformation mechanism since then, said. Professor Wang Xunli is the corresponding author of this paper.
 
Powerful neutron scattering technology
 
To understand this mystery, the research team led by Professor Wang Xunli used In-situ Neutron Diffraction (a kind of neutron scattering) technique to study the deformation process of high-entropy alloys. Professor Wang Xunli, who is also the director of the neutron science and technology center of CityU of Hong Kong, explained: "Neutron diffraction measurement is one of the few methods to observe the actual deformation process of the material. We can observe each step: what appears first Mechanisms, how (mechanisms) interact with mechanisms, which cannot be observed with traditional experimental methods, such as transmission electron microscopy (TEM)."
 
Professor Wang Xunli continued: "More importantly, in-situ neutron diffraction can be measured at ultra-low temperatures close to absolute zero, and the overall condition of the sample is measured, not just its surface or local area, but also Provide the microscopic mechanism of the deformation process, such as the interaction between different grains of the material."
 

 
Muhammad Naeem, a member of the research team, prepared for the experiment on the TAKUMI, an engineering material spectrometer at the Japan Proton Accelerator Research Center (J-PARC). Using this device, the researchers conducted in-situ neutron diffraction measurements on a variety of high-entropy alloys, and the samples all showed a multi-stage deformation process.
Source: Professor Wang Xunli, City University of Hong Kong
 
For the first time reveal the order of appearance of various deformation mechanisms
 
Using the in-situ neutron diffraction technique, the research team revealed for the first time the order of various deformation mechanisms for high-entropy alloys at ultra-low temperatures. They found that when the temperature drops to 15K (K is the absolute temperature unit, 15K = -258.15℃), the deformation of the high-entropy alloy is divided into four stages.
 
The first stage of the deformation process is "dislocation slip" (dislocation slip), that is, the lattice planes slide against each other. This is a common deformation mechanism of face-centered cubic crystal structure materials. When the dislocation slippage continues, “stacking faults” (stacking faults) gradually become active and begin to dominate the deformation process. At this time, the stacking order of the lattice plane will change due to the deformation. Next, there will be "twinning", the orientation of the lattice plane is wrong, resulting in the mirror image of the mother crystal. Finally, it will transition to "serration rheological behavior" (serration), and high-entropy alloys will exhibit large amplitude deformation stress.
 

 
Illustration of the deformation path of CrMnFeCoNi (one of the high-entropy alloys) samples at 15K. The vertical green dotted line in the figure marks the different stages of the deformation behavior: (1) the start of dislocation slip; (2) the start of stacking faults; (3) the beginning of sawtooth rheological behavior; (4) a large number of sawtooth flows The change is synchronized with the saturation of dislocation slip.
Image source: DOI: 10.1126/sciadv.aax4002
 
The researchers found in experiments that as the temperature decreases, the high-entropy alloy shows higher and more stable strain hardening (strain hardening means that the material becomes stronger and harder after deformation), and exhibits a large extension Sex. The researchers quantitatively analyzed the in-situ experimental data and concluded that the three observed deformation mechanisms of stacking faults, twinning and sawtooth rheological behavior, and the interaction between these mechanisms, are precisely high-entropy alloys. Reasons for superior mechanical properties.
 
New unknown areas:
Deformation of high-entropy alloy at ultra-low temperature

 
It took the research team nearly three years to complete the entire study. However, there are still many areas in this field that deserve further exploration. Professor Wang Xunli said: "The complex deformation mechanism of high-entropy alloys at ultra-low temperatures is a brand new field. Not many people have been involved in exploration, and the results of our research only show the tip of the iceberg in the new field."
 
The research team will next study when stacking faults occur in other alloys and how they deform at different temperatures. Naeem said: "Understanding the deformation mechanism helps to develop new alloys. By using the synergistic effect of different mechanisms, we can adjust these mechanisms to give the material better mechanical properties when used in low temperature environments."
 
In the research team, members from the CityU of Hong Kong include Professor Liu Jinchuan, a distinguished professor in the University, Dr. Wang Feng, associate professor in the Department of Materials Science and Engineering, and Dr. Wang Bing, Dr. Lan Si, Dr. Wu Zhenduo and He Haiyan from the Department of Physics.
 
Other co-authors include Professor Lu Zhaoping and Professor Wu Yuan of the University of Science and Technology Beijing, Dr. Stefanus Harjo and Dr. Kawasaki of the Japan Proton Accelerator Research Complex (J-PARC), and Professor Zhang Zhongwu of Harbin Engineering University.
 
This research was supported by the Qiu Cha Foundation, the Hong Kong Research Grants Council, the National Natural Science Foundation of China, the Shenzhen Municipal Science and Technology Innovation Commission, and the Ministry of Science and Technology of China.
 
DOI: 10.1126/sciadv.aax4002

The article is from Hong Kong City University Research and Innovation
 
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