Experience in developing conventional alloys suggests that the formation of numerous brittle intermetallic compounds with complex structures deteriorates alloy properties when more metallic elements are used to make up the alloy. However, in recent years, researchers have found that by mixing five or more metallic elements together in equimolar or near-equimolar ratios without distinguishing the major elements, the melting of the resulting alloys has structural characteristics such as simplified microstructure, no tendency to intermetallic compounds, nano-precipitates with an amorphous structure, and other performance characteristics such as high strength, high hardness, tempering softening resistance, and wear resistance. These alloys were first defined as multi-principal alloys or high-entropy alloys by Junwei Ye and others from Tsinghua University in Taiwan. Therefore, high entropy alloys have a very broad application prospect and can be used to make high strength, high temperature, and corrosion resistant tools, molds, and machine parts, which is a good opportunity to enter the field of high functional and high value-added special alloy materials.
High-entropy alloys (HEAs) are alloys formed by five or more metals in equal or approximately equal amounts. High-entropy alloys usually contain more than five major elements, with the atomic fraction of each major element ranging from 5% to 35%. Their organization and properties differ from those of conventional alloys in many respects. High entropy alloys have excellent mechanical properties, frictional wear properties, corrosion resistance, and high-temperature resistance, making them one of the most promising new materials for future development. Since high-entropy alloys may have many desirable properties, they have received considerable attention in materials science and engineering.
Entropy” in high-entropy alloys is an important concept. Entropy represents the degree of chaos in a system; the more chaotic the entropy is, the higher the entropy, the more orderly the entropy is, the lower the entropy. According to the second law of thermodynamics, in nature, all isolated systems tend to increase in entropy.
In other words, a whole composed of many objects, no matter how orderly they were before, will always become more and more chaotic over time when no external influences are taken into account.
What are the characteristics of high-entropy alloys?
High entropy alloys have a single crystal structure. Numerous tests have confirmed that high-entropy alloys can form a single body-centered cubic or face-centered cubic structural phase or a simple mixed phase structure of body-centered cubic and face-centered cubic.
This indicates that in the absence of primary elements, the primary elements with equal or approximately equal atomic ratios solidify into simple structures with each other without forming complex intermetallic compounds.
Properties of high-entropy alloys
High entropy alloys have high thermal stability as well as resistance to high-temperature oxidation. High entropy alloys are named for the large confusion of atoms.
At high temperatures, there will be a greater atomic disorder, so high entropy alloys become more stable in both the crystalline and amorphous states, and there is still a solid solution strengthening effect, allowing for extremely high-temperature strength.
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