Microstructure and Mechanical Behavior of High-Entropy Alloys

Materials, 2021

Microstructural phase evolution during melting and casting depends on the rate of cooling, the collective mobility of constituent elements, and binary constituent pairs. Parameters used in mechanical alloying and spark plasma sintering, the initial structure of binary alloy pairs, are some of the factors that influence phase evolution in powder-metallurgy-produced HEAs. Factors such as powder flowability, laser power, powder thickness and shape, scan spacing, and volumetric energy density (VED) all play important roles in determining the resulting microstructure in additive manufacturing technology. Large lattice distortion could hinder dislocation motion in HEAs, and this could influence the microstructure, especially at high temperatures, leading to improved mechanical properties in some HEAs. Mechanical properties of some HEAs can be influenced through solid solution hardening, precipitation hardening, grain boundary strengthening, and dislocation hardening. Despite the HEA syste...

The phase-formation rule of high-entropy alloys (HEAs) with different microstructures is discussed, based on the atom-size difference in multicomponent alloys. For the single-phase HEA with the composition of AlCoCrFeNi, the yielding strengths and fracture strengths at cryogenic temperatures increase distinguishingly, compared to the corresponding mechanical properties at ambient temperature. However, the plasticity at 298 and 77 K changes very gently, while the fracture modes are intergranular and transgranular, respectively.

Metals

The phase stability, microstructural evolution, and mechanical properties of Al0.6CoCrFeNi high-entropy alloy (HEA) subjected to different thermo-mechanical treatments were systematically investigated in the present study. The face center cubic (FCC) matrix, B2, and minor Body Center Cubic (BCC) phases were observed in the as-cast state. The morphology of the B2 precipitates evolved from needle-like to droplet-shaped when annealed at 900 °C, 1000 °C, and 1100 °C. The resulting yield stress of this FCC/B2 duplex-phase HEA after annealing heat treatments was successfully analyzed based on the contributions from solid solution strengthening, precipitate strengthening, grain boundary hardening, and dislocation hardening.

Microscopy and Microanalysis, 2023

The high-entropy alloys were discovered in 2004, and they represent a new group of materials with excellent application potential. They are composed of 5 or more elements with content in the interval 5-35 at.% which preferably form a solid solution. This is due to high configurational entropy and other effects like sluggish diffusion, deformation of the crystal lattice, and cocktail effect [1]. Typical representatives are Cantor's equiatomic alloy CoCrFeNiMn [2] and CoCrFeNiAl [3], which is characterized by better mechanical properties. In this research the influence of Al addition to the quinary CoCrFeNiMn alloy on microstructure, phase composition, and mechanical properties was studied. The system of (CoCrFeNiMn) 100-X Al X (X = 5, 10, 16.6; further denoted as Al5, Al10 and Al16.6) was prepared by a combination of mechanical alloying (MA) and spark plasma sintering (SPS). The process of MA was performed in a planetary mill Retsch PM 100 for 8 hours under protective Ar atmosphere. The rotation speed during milling was 400 rpm. Further MA-alloys were compacted using the SPS-FCT Systeme HP D 10. The compaction conditions comprised of heating rate 200°C•min-1 up to the compaction temperature of 1000°C after which the sample was compressed by a pressure of 48 MPa with a dwell time of 9 min. The chemical and phase composition of prepared alloys was determined using XRF (ARL XP 2400) and XRD (PANalytical X'Pert Pro) analysis. Microstructure was observed using SEM (Tescan VEGA 3 LMU) with EDS (Oxford Instruments 80 mm 2) utilizing the combination of+ BSE + SE (80:20) detectors. Further, the Vickers microhardness was measured using the Future-Tech FM-700 device using 1 kg load with a dwell time of 10 s. The mechanical properties of the alloys were determined by the compressive stress-strain testing on the universal testing device LabTest 5.250SP1-VM with strain speed 0,001 s-1. The measured values were accompanied by a confidence interval with a level of significance α = 0.05. Using powder metallurgy methods (MA + SPS), the high-entropy alloys of required chemical composition were prepared. It was found that in terms of phase composition, that the addition of Al leads to a decrease in the content of the FCC phase increasing the content of PC phase which formed in the CoCrFeNiMnAl10 alloy and its content further increased in the CoCrFeNiMnAl16.6. The microstructure of prepared alloys was ultrafine-grained and homogeneous. The SEM + EDS analysis showed that the microstructure was formed by solid solutions that differed only slightly in chemical composition. Chromium carbides were also present in all alloys, which in the case of CoCrFeNiAl16.6 (Fig. 1.) were identified as Cr 23 C 6. Further, a significant influence of the Al addition on the mechanical properties was observed. With increasing Al content, there was an improvement in compressive yield strength (CYS) and ultimate compressive strength (UCS). Alloy CoCrFeNiMnAl16.6 achieved the best mechanical properties-CYS = 2173 ± 264 MPa and UCS = 2367 ± 305 MPa. The results of the Al16.6 alloy documented the increased brittleness of the alloy that, compared to identically prepared alloy partially substituting the Mn content by Al (CoCrFeNiMn5Al15, in at. %) [5], significantly affected the overall performance of the alloy. The results of microhardness measurements (Fig. 2.) shows that the hardness of CoCrFeNiMnAl5 and CoCrFeNiMnAl10 is comparable within the frame of considered deviation, but in the case of CoCrFeNiMnAl16.6 it is significantly higher. Differences in mechanical properties were caused by changes in phase composition. Following the work Wang et al. [4], increasing Al content leads to an increase in the proportion of hard phase with BCC or PC structure. [5]

for their considerable efforts and guidance to my Master of Science Degree study. My sincere thanks also go to Dr. Oleg N. Senkov and Dr. Daniel B. Miracle from Air Force Research Laboratory (AFRL), for contributing their immense knowledge and insightful comments to my study. I would like to thank Dr. Chad M. Parish from Oak Ridge National Laboratory (ORNL) for microstructural characterization. I am grateful to Drs. Chuan Zhang and Fan Zhang from CompuTherm LLC. for their thermodynamic calculations. I am also grateful to Prof. Fuqian Yang and Mr. Guangfeng Zhao from The University of Kentucky for their nanoindentation tests. I would like to thank Mr. Douglas E. Fielden, and his team, Mr.

Crystals, 2022

In this research, we systematically investigated equiatomic CoCrFeNi and CoCrFeMnNi high-entropy alloys (HEAs). Both of these HEA systems are single-phase, face-centered-cubic (FCC) structures. Specifically, we examined the tensile response in as-cast quaternary CoCrFeNi and quinary CoCrFeMnNi HEAs at room temperature. Compared to CoCrFeNi HEA, the elongation of CoCrFeMnNi HEA was 14% lower, but the yield strength and ultimate tensile strength were increased by 17% and 6%, respectively. The direct real-time evolution of structural defects during uniaxial straining was acquired via in situ neutron-diffraction measurements. The dominant microstructures underlying plastic deformation mechanisms at each deformation stage in as-cast CoCrFeNi and CoCrFeMnNi HEAs were revealed using the Convolutional Multiple Whole Profile (CMWP) software for peak-profile fitting. The possible mechanisms are reported.

Intermetallics, 2016

Starting from three typical equiatomic CoCrFeNiMn, CoCrFeNiAl and CoCrFeNiCu high entropy alloys (HEAs), we systematically investigated the compositional dependence of phase formation and mechanical properties of 78 alloys by varying the atomic ratio of the constituent elements. It was found that the simple phase structures, including a single face-centered cubic (FCC) or body-centered cubic (BCC) phase, duplex FCC phases, duplex BCC phases, instead of intermetallics, could form within a broad compositional landscape in 68 out of the 78 alloys not limited to the equiatomic composition where the configurational mixing entropy is maximum. This fact indicates that it may be the nature of the constituent elements that leads to simple phase structure formation. With compositional variation, the microstructure and mechanical properties including hardness and tensile properties show corresponding changes. It was found that the hardness variation of samples within the same structure is smaller for the FCC than that of the BCC. Tensile results indicated that the tensile elongation of (CoCrFeMn) (100Àx) Ni x (x ¼ 0, 10 and 20) alloys increases with Ni addition due to the decreasing volume fraction of sigma phase. For the (CoCrFeAl) (100Àx) Ni x (x ¼ 27.3, 33.3, 38.5, 42.9 and 50) alloys, the yield strength decreases and tensile elongation increases with Ni addition due to decreasing volume fraction of BCC phase which is hard yet brittle. The present results are important to understand the phase formation and relationship between microstructure and mechanical properties in HEAs.

Zeitschrift für Kristallographie - Crystalline Materials, 2015

The term " high-entropy alloys (HEAs) " first appeared about 10 years ago defining alloys composed of n = 5 -13 principal elements with concentrations of approximately 100/ n at.% each. Since then many equiatomic (or near equiatomic) single-and multi-phase multicomponent alloys were developed, which are reported for a combination of tunable properties: high hardness, strength and ductility, oxidation and wear resistance, magnetism, etc. In our paper, we focus on probably single-phase HEAs (solid solutions) out of all HEAs studied so far, discuss ways of their prediction, mechanical properties. In contrast to classical multielement/multiphase alloys, only single-phase multielement alloys (solid solutions) represent the basic concept underlying HEAs as mixing-entropy stabilized homogenous materials. The literature overview is complemented by own studies demonstrating that the alloys CrFeCoNi, CrFeCoNiAl 0.3 and PdFeCoNi homogenized at 1300 and 1100 ° C, respectively, for 1 week are not singlephase HEAs, but a coherent mixture of two solid solutions.

2019

Today’s challenge is focused on the reserch and development of best quality and nobel property materials. So new alloys have been developed at the turn of the new millennium referred to as high entropy alloys. These alloys have been achieved through equiatomic substitution, by replacing individual components with multi-component equiatomic or near-equiatomic mixtures of chemically similar species. The present work includes the processing of Multi-component high entropy alloys (HEA) FeNiCoCrCu. The HEA is synthesized by ball milling, Milling hour varies from 0 hour to 40 hour. For the structural analysis X-Ray Diffraction (XRD) as characterization technique is adopted. XRD analyses shows that the intensity of the peaks corresponding to BCC and HCP crystal structures get reduced with increase in milling time. An affinity to FCC phase formation is observed with increase in milling time. The lattice parameter of individual component was determined viz. Bragg’s method.

High-entropy alloys (HEAs) are alloys with five or more principal elements. Due to the distinct design concept, these alloys often exhibit unusual properties. Thus, there has been significant interest in these materials, leading to an emerging yet exciting new field. This paper briefly reviews some critical aspects of HEAs, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors. Current challenges and important future directions are also pointed out.

JOM, 2012

This article reviews the recent work on the high-entropy alloys (HEAs) in our group and others. HEAs usually contain five or more elements, and thus, the phase diagram of HEAs is often not available to be used to design the alloys. We have proposed that the parameters of d and X can be used to predict the phase formation of HEAs, namely X ‡ 1.1 and d £ 6.6%, which are required to form solid-solution phases. To test this criterion, alloys of TiZrNbMoV x and CoCr FeNiAlNb x were prepared. Their microstructures mainly consist of simple bodycentered cubic solid solutions at low Nb contents. TiZrNbMoV x alloys possess excellent mechanical properties. Bridgman solidification was also used to control the microstructure of the CoCrFeNiAl alloy, and its plasticity was improved to be about 30%. To our surprise, the CoCrFeNiAl HEAs exhibit no apparent ductile-tobrittle transition even when the temperatures are lowered from 298 K to 77 K.

Entropy, 2019

Refractory high entropy alloys (HEA) are promising materials for high temperature applications. This work presents investigations of the room temperature tensile mechanical properties of selected 3 and 4 elements medium entropy alloys (MEA) derived from the HfNbTaTiZr system. Tensile testing was combined with fractographic and microstructure analysis, using scanning electron microscope (SEM), wavelength dispersive spectroscope (WDS) and X-Ray powder diffraction (XRD). The 5 element HEA alloy HfNbTaTiZr exhibits the best combination of strength and elongation while 4 and 3 element MEAs have lower strength. Some of them are ductile, some of them brittle, depending on microstructure. Simultaneous presence of Ta and Zr in the alloy resulted in a significant reduction of ductility caused by reduction of the BCC phase content. Precipitation of Ta rich particles on grain boundaries reduces further the maximum elongation to failure down to zero values.

The majority of studies on high-entropy alloys are focused on their phase, microstructure, and mechanical properties. However, the physical properties of these materials are also encouraging. This paper provides a brief overview of the physical properties of high-entropy alloys. Emphasis is laid on magnetic, electrical, and thermal properties.

Acta Materialia, 2019

Phase decomposition is commonly observed experimentally in single-phase high entropy alloys (HEAs). Hence, it is essential for the consideration of HEAs for structural applications to study and understand the nature of phase decomposition in HEAs, particularly the influence it has on mechanical behavior. This paper describes the phase decomposition in the equiatomic CoCuFeMnNi HEA and how the reported secondary phases influence mechanical behavior. Thermomechanical processing, followed by systematic post deformation annealing treatments, revealed the formation of two distinct secondary phases within the equiatomic face-centered cubic (FCC) matrix phase. Low temperature annealing treatments at 600 °C and below led to the nucleation of Fe-Co rich ordered B2 precipitates that contributed precipitation hardening while sufficiently small in size, on the order of 140 nm in diameter. At temperatures < 800 °C Cu segregation , due to its immiscibility with the other constituents, eventually forms a Cu-rich disordered FCC phase that is determined to increase the yield strength of the alloy while reducing the ductility, likely attributable to the presence of additional interfaces. The thermal stability and chemistry of these phases are compared to those predicted on the basis of calculated phase diagram (CALPHAD) analyses.

Microstructures, 2022

The CoCrFeMnNi alloy is one of the most notable first-generation high-entropy alloys and is also known as a Cantor alloy. This alloy was first proposed in 2004 and shows promising performance at cryogenic temperatures (CTs). Subsequent research has indicated that the equiatomic ternary CoCrNi medium-entropy alloy (MEA), as a subset of the Cantor alloy family, has better mechanical properties than the CoCrFeMnNi alloy. Interestingly, both the strength and ductility of the CoCrNi MEA are higher at CTs than at room temperature. CoCrNi-based alloys have attracted considerable attention in the metallic materials community and it is therefore important to generalize and summarize the latest progress in CoCrNi-based MEA research. The present review initially briefly introduces the discovery of the CoCrNi MEA. Subsequently, its tensile response and deformation mechanisms are summarized. In particular, the effects of parameters, such as critical resolved shear stress, stacking fault energy and short-range ordering, on the deformation behavior are discussed in detail. The methods for strengthening the CoCrNi MEA are then reviewed and divided into two categories, namely, modifying microstructures and adjusting chemical compositions. In addition, the mechanical performance of CoCrNi-based MEAs, including their dynamic shear properties, creep behavior and fracture toughness, is also deliberated. Finally, the development prospects of CoCrNi-based MEAs are proposed.