高熵合金简介

BCC
FCC
The number of phases is far lower than the maximum
F = C – P + 1 …. The Gibbs phase rule
P is number of phases
P = C of freedom, C is number of components, and F is degree + 1
High entropy alloys can be viewed as an atomic-scale composite. Therefore, they exhibit a composite effect from : Basic features of the elements
Interactions among all the elements
FCC 5 elements HE alloys
BCC 5 elements HE alloys
If the number of elements increase, and the atomic size differences is large, what will happen?
The distorted lattice will collapse into an amorphous structure since lattice distortion energy would be very high for retaining a crystalline configuration.
Indirect effects of the various elements on the microstructure
Examples for cocktail effect
If more light elements are used, the overall density will be reduced. If more oxidation-resistant elements are used, such as Al, Cr and Si, the oxidation resistance at high temperatures can be improved.
The simple introduction of new century
High Entropy Alloys (HE alloys)
With Multiple Principal Elements
[1] Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, Tsau CH, Chang SY, nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts And Outcomes, 2004.
[2] Jien-Wei YEH, Recent progress in high-entropy alloys, 2006.
[3] Jien-Wei YEH, 從高熵合金探討未來金屬材料發展趨勢, 2006. [4] Y.J. Zhou , Y. Zhang , T.N. Kim , G.L. Chen, Microstructure characterizations and strengthening mechanism of multi-principal component AlCoCrFeNiTi0.5 solid solution alloy with excellent mechanical properties, 2008. [5] 維基百科,
If an element such as Al is added, which has strong binding force and large different atomic size with the other elements present, such as Co, Cr, Cu, Fe and Ni, and promotes the formation of a BCC phase, the strength will be increased.
Why HE alloys have sluggish diffusion effect ? From the viewpoint of kinetics, the effective diffusion rate for phase separation will be limited by the lattice distortion which hinders atomic movement.
The high entropy of HE alloy significantly lowers the free energy thus lowering its tendency to order and segregate(solubility ↑) , making solid solution more easily form and more stable than intermetallics or other ordered phases during solidification in this alloys, especially at high temperature.
‧ 熱力學上 - High entropy effect (高熵效應) ‧ 動力學上 - Sluggish diffusion effect (遲緩擴散效應) ‧ 晶體結構上 - Lattice distortion effect (晶格扭曲效應) ‧ 性質上 - Cocktail effect (雞尾酒效應)
The result
They lower the rate of nucleation and growth, leading to the formation of nanocrystallines.
TEM bright-field image of AlxCoCrCuFeNi HE alloy at X=1.
The lower limit of 5 elements is imposed.
△Smix is high enough to counterbalance the △Hmix
△Gmix = △Hmix -T△Smix T△Smix
play an important role than mix When entropy increase, Gibbs free energy decrease.
△H
Some intermetallic phases can form in some HE alloys.
Strong bonding between some metallic elements will cause very large heats of formation, so enthalpy can play a more important role.
The high entropy effect plays an important role in simplifying the microstructures.
Form solid solution phases
Such as BCC and FCC rather than ordered phases or intermetallic compounds.
A multi-element lattice is highly distorted because all atoms are solute atoms and their atomic sizes are all different from one another.
FCC original element crystal
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Four major effects Strengthening Mechanism Other features Applications Conclusions
In 1995, the multi-principal-element alloys termed as high-entropy alloys are brought up, which emphasizes their inherent high mixing entropies.
Sluggish diffusion
Difficulty in substitutional diffusion of elements and interactions among interdiffusing species during partitioning. The phase separation during cooling is often inhibited at higher temperatures and therefore delayed until lower temperatures.
The influence of lattice distortion on XRD.
Perfect lattice
distorted lattice
The x-ray diffraction analysis of an equimolar alloy series of Cu-Ni-Al-Co-Cr-Fe-Si from 2 to 7 elements in the as-cast condition.
Such as strong ceramic compounds:
Oxides (氧化物), Carbides (碳化物), Nitrides(氮化物), and Silicides (矽化物).
Why HE alloys have lattice distortion effect ?
The multiple elements is the reason.
There is high entropy in the alloys. 5 to 13 principle elements (5%≦X≦35%) in the alloys. The number of alloy systems (7099) are greater than conventional alloys(30). Having four major effects.
Why high entropy effect can simplify the microstructures?
High mixing entropy enhances the mutual solubility among elements, and prevents phase separation into ordered phases or intermetallic compounds.
The HE alloys can yield the precipitations of nanocrystallines or amorphous structures in as-cast states.
Phase transformations
During cooling, HE alloys might undergo phase transformation.
The influence of lattice distortion effect.
Thermal conductivity Electrical conductivity XRD scattering XRD intensity
Solid solution hardening
Temperature effect