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High-Entropy Alloys

June 20, 2016

One of the first bits of chemical knowledge that I learned as a child was that oil and water don't mix. This was also a common saying at that time that described a variety of political and social situations. Oil and water don't mix since water molecules are polar, and oil molecules are non-polar. This results in oil being hydrophobic. As a young scientist, however, I was determined to find a process to mix oil and water.

I had among my tools a vibratory wood saw. This was a child's version of a jigsaw built as a powerful electric buzzer coupled to a jig saw blade. It was relatively safe, since it would only cut thin pieces of soft wood, such as balsa. I found that one of my mother's tall spice bottles could be inserted in place of the blade to form a vibratory mixer for liquids.

I placed equal volumes of vegetable oil and water into the bottle and mixed them. At first, I thought that I had succeeded, but the mixture separated after a time back to a layer of oil atop the water. This was my first lesson in phase separation, a thermodynamic phenomenon so critical to many chemical processes and the formation of high strength alloys.

Any quantity of water can be mixed with itself, an idea that seems trivial in its telling, but an idea that has deeper consequences. How far different can a substance be to still mix with water? Heavy water (D2O) is somewhat different from H2O, having a density of 1.107 g/cc, about 10% greater than that of H2O. Heavy water is completely miscible in normal water.

Extending ourselves further, copper and nickel are both transition metals with the face-centered cubic crystal structure. Nearly all metals are miscible in their liquid state, but copper and nickel are so similar that they are completely miscible in the solid state, as can be seen in their phase diagram.

Copper-nickel phase diagram

The copper-nickel phase diagram.

(Created from available data using Inkscape.)


Atoms of two-component solid solutions such as Cu-Ni are in a high entropy state, since all the crystal lattice sites of the alloy are identical, and any one of the two components can reside at any lattice location. Consider the unusual and unlikely case in which nearly equal quantities of many component atoms form a solid solution. The configurational entropy of such a multi-component alloy would be large; and, surprisingly, such high entropy alloys exist.

Boltzmann's entropy equation is

Configurational entropy equation

where KB is Boltzmann's constant of 1.38065 x 1023 joule/kelvin (J/K), and Ω is the number of states accessible to a system; that is, the ways in which the atoms can be differently arranged. In the case of combining atoms of i different elements of mole fractions Xi, this equation becomes

Configurational entropy sum for multicomponent system

where R is the gas constant of 8.314 J/K/mol. For the special case of equal mixtures of components, we get the configurational entropy values as shown in the graph below.

Ideal configurational entropy for equi-atomic multicomponent alloys

Ideal configurational entropy for equi-atomic multicomponent alloys as a function of the number of components.

(Created with Gnumeric.)


As any metallurgist knows, liquid mixtures of several different types of metal atoms will give you a solid consisting of multiple crystal types when cooled. This thermodynamic phenomenon has been used to advantage in the creation of high strength alloys, such as those used in turbine engine blades. For example, MAR-M-247, a Martin Marietta nickel-based superalloy has the following composition (mass-%):[1]

 ElementPercentElementPercent
 Nickel59Tantalum3.0
 Tungsten10Titanium1.0
 Cobalt10Molybdenum0.7
 Chromium8.25Iron0.5
 Aluminum5.5Boron0.015

Additionally, a little hafnium or zirconium is also added. This alloy has a crystal matrix of the face-centered cubic structure of nickel containing gamma-prime and carbide phases that provide its high temperature strength.

There was much head-scratching among metallurgists (especially the older, balding ones) in 2004 when B. Cantor, I.T.H. Chang , P. Knight, and A.J.B. Vincent of the Oxford University Department of Materials produced a single phase alloy of equal portions of five components.[2] This alloy was Fe20Cr20Mn20Ni20Co20, which forms a face-centered cubic solid solution. This discovery marked the beginning of serious study on high-entropy alloys.

atomic structure model of FeCrMnNiCo

Atomic structure model of the Oxford University FeCrMnNiCo alloy.

In this image, magenta = Fe, green = Co, blue = Cr, cyan = Ni, and yellow = Mn. Aside from slight differences in atomic size, swapping colors would serve just as well, since the composition is equi-atomic.

(Image by Shaoqing Wang, via Wikimedia Commons.)


So, are there properties of high-entropy alloys that are especially useful? One important application is their potential for use in nuclear reactors. Energetic particles such as neutrons that impinge on an alloy will cause displacement of atoms from their positions in the crystal lattice. This generates lattice defects and dislocations that change the alloy's mechanical properties. Equi-atomic multicomponent alloys have shown good resistance to such radiation damage.

In an open access paper in Physical Review Letters, scientists from the University of Helsinki (Helsinki, Finland), Oak Ridge National Laboratory (Oak Ridge, Tennessee), and theUniversity of Michigan (Ann Arbor, Michigan), report on experiments and modeling that show a substantial reduction of radiation damage in single-phase NiFe and NiCoCr alloys, as compared to elemental Ni.[3-4]

They ascribe this effect to a reduction in dislocation mobility, with a result that dislocation structures grow more slowly. The radiation damage did not show any phase separation or amorphization of the alloys.[3] In conventional alloys, a matrix element such as iron exists in greater concentration, and its radiation exposure is proportionately greater. In a high-entropy alloy, the dislodging probability is distributed among all the elements.

Scientists from the Max-Planck-Institut für Eisenforschung (Düsseldorf, Germany) and the Massachusetts Institute of Technology (Cambridge, Massachusetts) have reported in a recent issue of Nature a way to transform a high-entropy alloy into a material that exhibits simultaneous ductility and strength.[5-6]

In conventional alloys, you can have high ductility and low strength, or high strength and low ductility, while high-entropy alloys have high strength, but they're brittle and not ductile.[6] Their alloy, composed of 50% iron, 30% manganese, 10% cobalt and 10% chromium, has two coexisting crystal structures, and these transform from one into the other, giving the desired mechanical properties. (see figure).[6]

Phases of the Fe50Mn30Co10Cr10 alloy

Phases of the Fe50Mn30Co10Cr10 alloy, as imaged using electron backscatter diffraction.

(MPI f. Eisenforschung image, © 2016 Nature.)


References:

  1. W. Danesi, J. Hockin, and C. Lund, "Tungsten containing alloy," US Patent No. 3,759,707, September 18, 1973 (via Google Patents).
  2. B. Cantor, I.T.H. Chang, , P. Knight, and A.J.B. Vincent, "Microstructural development in equiatomic multicomponent alloys," Materials Science and Engineering, vols. 375-377 (July 2004), pp. 213-218, doi:10.1016/j.msea.2003.10.257.
  3. F. Granberg, K. Nordlund, M. W. Ullah, K. Jin, C. Lu, H. Bei, L. M. Wang, F. Djurabekova, W. J. Weber, and Y. Zhang, "Mechanism of radiation damage reduction in equiatomic multicomponent single phase alloys, Physical Review Letters, vol. 116, no. 13 (April 1, 2016), Document No. 135504. This is an open access paper with a PDF file available here.
  4. Akshat Rathi, "A new kind of metal could make nuclear reactors stronger and last longer," Quartz, March 21, 2016.
  5. Zhiming Li, Konda Gokuldoss Pradeep, Yun Deng, Dierk Raabe, and Cemal Cem Tasan, "Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off," Nature (May 18, 2016), doi:10.1038/nature17981.
  6. Strength and ductility for alloys, Max-Planck-Gesellschaft Press Release, May 24, 2016.

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