Editorial Feature

High-Entropy Alloy with Hexagonal Close-Packed (HCP) Structure

Through high pressure processing techniques performed in a symmetric diamond anvil cell, a group of researchers from the Department of Geological Sciences at Stanford University have successfully developed a hexagonal close-packed phase high-entropy alloy.

In their pioneering endeavor, the research team led by Rodney Eqing and Wendy Mao developed a highly customizable alloy mixture that has the potential to be applied to a diverse range of material science applications.

Conventional metal alloys describe a metal mixture that is comprised of one primary metal and one or more metals, non-metals or relating elements in order to combine the beneficial properties of the individual metals to create a more useful final product for certain applications.

As the development and enhancement of alloys continues to thrive, recent work on a new class of alloys known as high-entropy alloys (HEA) has emerged. Comprised of at least five or more principal metals of the same atomic weight, HEA compositions lack the impurities that are often found in conventional alloys.

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To date, HEAs have been reported to either exhibit a face-centered cubic (fcc) or body-centered cubic (bcc) structure. While the bcc lattice contains one metal particle in the center of the cube, the fcc structure contains one atom in the center of each face of the cube, which is a more common alloy lattice arrangement.

The hexagonal close-packed (hcp) structure is a highly rare phase of high-entropy metals, however, this highly desirable structure is often attributed to increasing the hardness and ductility of the metal composition in which it is found. Of the known hcp HEAs include scandium (Sc), yttrium (Y), and lanthanides solid solutions, however, these rare earth metals have been difficult to reproduce in the laboratory setting.

Within a diamond anvil cell, the Stanford researchers placed a CrMnFeCoNi powder. By applying pressures to the diamond anvil cell at gradually increased rates that led to a final pressure of 54.1 GPa, angel-dispersive X-ray powder diffraction (XRD) measurements were taken to monitor the structural changes of the alloy.

This rapid analytical technique is often used to identify the composition of crystalline materials, as well as provide important information regarding the individual cell dimensions of a particular sample of interest.

While the initial appearance of the CrMnFeCoNi alloy showed an fcc structure, the gradual pressure beginning with 14 GPa added to the metal mixture transformed this HEA to exhibit an HCP structure. Despite the typical recoil that will allow for structures to return to their original state once the applied pressure is eliminated, this study showed that the newly formed HCP structures remained.

Under such dramatic applied pressures, the magnetic pressure between the atoms will typically oppose the formation of the HCP structure, however, in this case, that magnetic force exist instead pushed the atoms past these forces in order to maintain its final desired structure. Once the CrMnFeCoNi mixture returned back to ambient pressures, most of the hcp phase remained, with only approximately 40% of the structure reverting back to its initial fcc phase.

The ability of a transition metal element to retain such pressure-induced structural changes from fcc to hcp has not been experimentally achieved until this time. The Stanford researchers are hopeful that the development of multiphase materials, such as the fcc-to-hcp CrMnFeCoNi HEA developed in this study, will allow for unlikely metal combinations to be created in order to manipulate the individual ductility, strength and structural toughness of the metals within the mixture to create a highly customizable final product for specific industrial applications.

With current HEA applications found in thermoelectric, soft magnetic and radiation tolerant materials, the ability to enhance the hardness, corrosion resistance and thermal and pressure stability of these alloys is promising.

References:

“High pressure synthesis of a hexagonal close-packed phawe of the high-entropy alloy CrMnFeCoNi” C. Tracy, S. Park, et al. Nature Communications. (2017). DOI: 10.1038/ncomms15634.

Image: Shutterstock.com/Glebnisk

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