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.
Other researchers in this team include HPSTAR’s Zhidan Zeng, and Yuan Wu, Xiongjun Liu, Zhaoping Lu from University of Science and Technology, Beijing, Vitali B. Prakapenka, Eran Greenberg from University of Chicago, Yang Ren and John S. Okasinski from APS, Jinyuan Yan from ALS, Yong Liu from Central South University. This work was supported by NSFC under grand U1530402.
含有五个基元的CoCrFeMnNi高熵合金是面心立方（fcc）结构高熵合金的代表体系，它具有极好的低温强度和断裂韧性、延展性和抗氢脆性。人们把这个合金作为高熵合金模型体系开展了大量的研究。然而，到目前为止，人们对其结构的认识还不是很清楚。理论计算认为该合金的密排六方结构（hcp）可能是常温下更稳定的结构，但是以往的实验从没有观察到该hcp结构的存在。从液氮温度到其熔点的大范围加热实验也表明CoCrFeMnNi高熵合金的fcc结构很稳定，不存在任何多形态相变，和其构成元素的丰富多形态现象形成鲜明反差。最近，北京高压科学研究中心的曾桥石研究员所领导的国际团队和北京科技大学吕昭平教授课题组合作，利用高压作为独特的调制参量，采用原位高压同步辐射X射线衍射技术对CoCrFeMnNi这一典型的fcc高熵合金的结构进行了系统研究，发现了该合金在加压过程中存在fcc到hcp的不可逆多形态相变现象。该研究团队又利用高压原位激光加热技术结合同步辐射X射线衍射技术，对fcc和hcp结构稳定性做了进一步研究。结果表明，fcc其实是高温下的稳定相，而hcp才是低（室）温稳定相。该研究成果揭示了看似极度稳定的高熵合金中也能存在多形态相变现象，澄清了CoCrFeMnNi高熵合金中fcc和hcp结构相对稳定性的长期疑问，也为从原子结构层面调控和设计这类材料的性能提供了可能。而高压为解决这种挑战，提供了独特而有效的途径。