High-Entropy Alloy: changing faces under high pressure - Dr. Qiaoshi Zeng

JUNE 2, 2017

A new class of solid solution alloys typically with five or more elements in equal or near-equal proportions—so called the high-entropy alloys (HEAs) are well known for their combination of the desirable properties including high strength, high ductility, high toughness, outstanding wear, fatigue, oxidation and corrosion resistance etc. Rather than the readily formation of multiphase microstructures in traditional multicomponent alloy systems, HEAs usually favor a single simple lattice, which was found to be very stable upon heating or cooling. New study co-led by a HPSTAR staff scientist, Dr. Qiaoshi “Charles” Zeng revealed irreversible polymorphic phase transitions between the fcc and hcp structures in a prototype high-entropy alloy CoCrFeMnNi using in situ high pressure and high temperature x-ray diffraction techniques. Their discovery was just published in Nature Communications (DOI:10.1038/ncomms15687) on June 1st. These results shed new light on the thermodynamics and kinetics of complex HEA systems and also opens a new avenue towards tuning HEAs’ properties via polymorphic structural transitions for applications.

Conventional metallic materials are typically designed based on one or two principal elements, forming various alloys with compositions located at the corners or edges of phase diagrams. Different from this traditional design strategy, HEAs are developed near the center of multicomponent phase diagrams. Due to the high configurational entropy, the complex compositions of HEAs does not induce complex microstructures accompanied by brittleness as expected by traditional metallurgy, but surprisingly stabilize the system in a simple solid solution lattice, and impart attractive properties to these new alloys, forming a new frontier of metallic materials.

“It is generally believed that HEA lattices are severely distorted and atomic diffusion is extremely sluggish due to the chemical complexity and configuration disorder”, said Dr. Qiaoshi Zeng. “This make HEAs have exceptionally high structural stability.

Of all HEAs, the CoCrFeMnNi alloy termed as the Cantor’s alloy, is the first reported HEA and a prototype fcc (face-centered-cubic) HEAs. CoCrFeMnNi can maintain its fcc structure over a large temperature range from cryogenic temperatures up to the melting temperature without any polymorphic phase transition.

“Although CoCrFeMnNi HEA is well known and has been extensively studied by experiments and simulations for more than a decade, a puzzle about its structure stability still remains unresolved. Simulations suggest that the hcp (hexagonal-close-packing) structure should be more stable than its fcc structure at room temperature. However, no hcp structure of the CoCrFeMnNi HEA has ever been observed in experiments. So far, high pressure as a dimension has rarely been explored in HEAs, we are therefore curious whether pressure-induced polymorphism also extensive exist in HEAs like what does in their typical constituent elements, said Qiaoshi.

A team led by Fei Zhang, a visititing Ph.D. student of HPSTAR in Qiaoshi’s group, used a diamond-anvil cell to compress tiny CoCrFeMnNi samples to ~40 gigapascals. To their surprise, the initial fcc CoCrFeMnNi gradually transformed to a hcp structure monitored by in-situ synchrotron radiation x-ray diffractions (XRD).

“Acturally, the structural stability of the fcc CoCrFeMnNi HEA has been extensively studied in a wide temperature range at ambient pressure but no new structure was observed. Using high pressure as a tuning tool, we observed the first polymorphism in the CoCrFeMnNi HEA”, Fei said.

Moreover, the hcp phase could be retained to ambient conditions after pressure release. While further heating experiments on the retained hcp sample at four different pressures indicate that the hcp CoCrFeMnNi HEA will return to the fcc structure at high temperatures, and the transition temperature for hcp-fcc transition increases with pressure

“This means that the well-known fcc phase actually is a stable polymorph at high temperatures, while the hcp structure is more thermodynamically favorable at lower temperatures”, explained Dr. Hongbo Lou, a postdoctoral fellow at HPSTAR in Qiaoshi’s group.

“Since the fcc-hcp polymorphic transition is irreversible and sluggish, it is easy for us to synthesize fcc-hcp dual phase composites with tunable volume fractions. Our results therefore open up a new avenue towards tailoring HEAs properties for novel applications via polymorphic transition-induced HEA composites” Qiaoshi said.

“The polymorphic transition discovered in this work is by no means limited to this specific CoCrFeMnNi HEA, and we expect that this behavior could be general in various HEAs at certain pressure and temperature conditions”, Dr. Zeng added.

Caption: The temperature and pressure metastability boundary (not the equilibrium phase boundary) of the polymorph hcp and fcc CoCrFeMnNi HEA.