Strain-strain curve and displacement configuration/evolution in MPEA. (a) is a diagram of a DDD simulation for MPEA. Photo credit: Li, Jia et al.
A research team led by materials scientists from the City University of Hong Kong (CityU) recently discovered a new mechanism for increasing the strength and ductility of a high-entropy alloy, two properties that typically interact with each other. are vice versa different. The results provide important insights for the future design of strong yet ductile high-entropy alloys and high-entropy ceramics.
The trade-off between strength and ductility is a chronic problem in traditional alloys, which are usually based on one or two major elements, meaning that increasing strength usually comes at the expense of ductility. In the last decade, a new strategy for alloy design has been proposed: mixing multiple elements to form alloys, referred to as “multi-major element alloys” (MPEAs) or “high entropy alloys” (HEAs). MPEAs exhibit excellent mechanical properties, such as both great ductility and excellent strength.
It is believed that these excellent mechanical properties are due to a large deformation of the atomic lattice caused by the random mixing of several key elements with different atomic sizes, bonding variations and crystal structure differences, which in turn results in a “heterogeneous lattice”. leads to stress”. However, anomalous lattice strain fields (a strain field refers to the stress distribution through a body part) are difficult to measure and characterize, so alloys are strengthened by three-dimensional (3-D) dynamic displacement. Its effect was ignored. until recently.
Video showing the effect of the lattice strain field on the dynamic growth of dislocations under axial tensile loading by DDD simulation; Colored lines represent dislocations at different slip planes. Photo credits: Li, Jia et al.
But the latest experiments and a series of simulations conducted by the research team, led jointly by Professor Yang Yong of CityU’s Department of Mechanical Engineering and Professor Fang Qihong of Hunan University, suggest that anomalous stress fields can lead to advanced mechanical Contribute to new properties of MPEA through heterogeneity can be stress-induced strengthening mechanisms leading to synergies between strength and ductility in alloys. Their results were published in the Proceedings of the National Academy of Sciences (PNAS) under the title “Strengthening anomalous lattice stress in heavy deformed crystal solids”.
“Textbooks on materials science and engineering traditionally list four ductility strengthening mechanisms: displacement strengthening, tortuous strengthening, grain boundary strengthening, and precipitation strengthening,” Professor Yang explained. “This textbook has been taught at scientific universities for students of materials science, mechanical engineering and applied physics for hundreds of years.”
Characterization of dislocation motion in alloys using discrete dislocation dynamics (DDD) simulations. Photo credit: Li, Jia et al.
“We have now discovered, through experimentation and numerical simulations, a new ductility strengthening mechanism which we call ‘anomalous lattice stress strengthening’.”
Unlike traditional strengthening mechanisms, which typically result in a compromise between strength and ductility, this newly discovered strengthening mechanism promotes synergies between strength and ductility, allowing researchers to simultaneously achieve high-entropy alloys. Can increase the strength and ductility of the metal. “The new results help explain several recent findings whose mechanisms are currently under discussion, and guide the development of new strong, yet ductile metals and ceramics,” Professor Yang said.
In the experiments, the research team first characterized the lattice strains in the high-entropy alloy FeCoCrNiMn using techniques such as geometric phase analysis (GPA) based on high-resolution transmission electron microscopy (TEM). This was followed by microcolumn compression tests to study how dislocations slide and cross-slip in the alloy. Next, the team performed extensive Discrete Dislocation Dynamics (DDD) simulations using experimentally measured lattice strains.
Experiments showed that lattice strain not only restricted the rate of displacement and thus improved yield strength, but also promoted displacement lateral slip to increase ductility. The results showed a significant influence of the heterogeneous stress field on the mechanical properties of the alloy. They offer a new approach to study the origin of high-strength, high-entropy alloys and open new avenues for the development of advanced crystalline materials.
The combined efforts of experiments and computer simulations revealed the physical mechanisms underlying the force-flexibility synergies observed in the experiments. “The results of this study provide a fundamental mechanism for overcoming the strength-ductility trade-off faced by conventional alloys,” said Professor Yang.
Small precipitates make a big difference in minimizing the trade-off between strength and ductility
Jia Li et al., Amplification of anomalous lattice strain in highly deformed crystalline solids, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2200607119 Provided by City University of Hong Kong
Citation: Scientists Discover New Mechanism for Increasing Strength and Ductility of High Entropy Alloys (2022 August 11), received August 11, 2022
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