Mechanical Behavior Of High-entropy Alloys Research Articles

Microstructural evolution and mechanical properties of novel nanoparticles strengthened CoCrFeNi based high entropy alloy

A novel nanoparticles-strengthened CoCrFeNi(CuTi)x alloys (x = 0.2–1.0 at.%) were designed and synthesized to study the influence of CuTi concentrations on the microstructural evolution, phase composition and mechanical behavior of high-entropy alloys (HEAs) fabricated by Mechanical Alloying (MA) and Spark Plasma Sintering (SPS). The microstructure of the (CuTi)0.2 alloy is composed of CoCrFeNi-rich grey matrix, CrFe rich flap-like structure and Cu rich phases. With increasing CuTi content, the crystal structure changes from FCC to FCC + laves+η phases because of the formation of novel nanoparticles. A large amount of CoTi rich spherical particles and NiTi rich needle-like phases are observed in the (CuTi)0.6 alloy. The alloy with a density of 99.19% after SPS exhibits excellent comprehensive properties. Compared to single-phase FCC CoCrFeNi alloy, the compressive fracture strength increases from 871 MPa to 2083 MPa (∼139%). The solid solution strengthening of the FCC matrix and the formation of the secondary phases (Laves and η phases) are the main factors involved in improving the strengthening of the alloy. Most importantly, the Vickers hardness of the (CuTi)1.0 alloy is probably 1.7 times higher than that of the (CuTi)0.2 alloy.

Tensile deformation behavior of twist grain boundaries in CoCrFeMnNi high entropy alloy bicrystals

High entropy alloys (HEA) are a class of materials that consist of multiple elemental species in similar concentrations. The use of elements in far from dilute concentrations introduces a multi-dimensional composition design space by which the properties of metallic systems can be tailored. While the mechanical behavior of HEAs has been the subject of active research recently, the role of grain boundaries (GBs) in their deformation behavior remains poorly understood. Motivated by recent experiments on HEAs demonstrating that GBs act as nucleation sites for deformation twins, herein, we leverage atomistic simulations to construct a series of equiatomic CoCrFeMnNi HEA bicrystals with langle 110 rangle and langle 111 rangle symmetric twist GBs and examine their tensile behavior and underlying deformation mechanisms at 77 K. Simulation results reveal that plastic deformation proceeds by the nucleation of partial dislocations from GBs, which then grow with further loading by bowing into the bulk crystals leaving behind stacking faults. Variations in the nucleation stress exist as function of GB character, defined in this work by the twist angle. Our results provide future avenues to explore GBs as a microstructure design tool to develop HEAs with tailored properties.

Microstructural characteristics and mechanical behavior of microwave-assisted sintered ferromagnetic FeCoNi1.5CrCu HEAp/Al matrix composites

The preparation of metal particle reinforced aluminum matrix composites by powder metallurgy has been widely studied in recent years. In this work, the effect of microwave sintering temperature on the microstructure and mechanical behavior of high-entropy alloys (HEAs) reinforced aluminum matrix composites prepared by powder metallurgy was investigated. Due to the outstanding microwave absorbing property of ferromagnetic HEAs particles, the composites sintering proceeded efficiently. At 480 °C, the aluminum matrix in the composite material is more compact, and the HEAs particles in the composite material are more evenly distributed, and the interface between HEAs particles and aluminum matrix is smooth. At 500 °C, the intensity of HEAs diffraction peak in the composite material changes significantly. At 520 °C, AlNi solid solution phase with BCC structure appears in the HEAs region, and the HEAs structure changes from FCC to FCC + BCC, and Al5Cr8, Al5Co2 and Al2Cu phases precipitate. At 480 °C, the nanohardness and elastic modulus at the interface reaches 1245.31 MPa and 72.45 GPa, respectively, showing relatively excellent interfacial bonding strength. The elastic modulus of both HEAs reinforcement and aluminum matrix decrease at a sintering temperature above 480 °C, and the composites have hard and brittle characteristics. At 480 °C, the macroscopic compression and yield strengths of composites are ~ 421.2 MPa and ~ 289.5 MPa, and the maximum shrinkage rate is ~65.3%, exhibiting excellent strength and toughness.

Tuning the mechanical behavior of high-entropy alloys via controlling cooling rates

The microstructures determine the macro-mechanical properties in metals and alloys. However, the formation mechanism of microstructures in high-entropy alloys (HEAs) rapidly cooled to room temperature still remains unknown at nanoscale. Here, we present the AlCoCrCuFeNi HEAs prepared by various cooling rates and the deformation behaviour using molecular dynamic simulation. The results show that the radial distribution function of HEAs is in good agreement with the previous experimental result. The amorphous HEA is formed at high cooling rates, while the nanocrystalline HEA is generated at low cooling rates. The hybrid amorphous-crystallization structured HEA occurs at middle cooling rates. For low cooling rates, the considerable amount of face-centered-cubic and hexagonal-close-packed short-range orders clearly show the crystal-nucleation process in the supercooled liquid of HEAs. The high residual hydrostatic stress around grain boundaries is observed in the nanocrystaline HEAs. Moreover, the nanocrystal HEA presents a high strength, compared to the amorphous HEA. The amorphous HEA exhibits the stable flow stress due to no obvious local stress concentration. This result could help guide the preparation of HEAs with a high strength by controlling the cooling rate, which generates the structural hierarchy.

Effects of Constituent Elements and Fabrication Methods on Mechanical Behavior of High-Entropy Alloys: A Review

High-entropy alloys (HEAs) have become a research hotspot in recent years. The nature of the multi-principal elements, high mixing entropy, and mutual interactions between elements render this novel material outstanding mechanical and functional properties, in which most research efforts are focused on mechanical properties. There are many aspects that can influence the mechanical behavior, such as constituent elements and fabrication methods. This paper will mainly summarize and discuss the effects of constituent elements and fabrication techniques on the mechanical properties of HEAs, by reviewing relevant papers, to have a better understanding of the variation ranges resulting from the above two factors and the reasons for the properties changes. Future directions are provided at the end of this article.

Fundamental understanding of mechanical behavior of high-entropy alloys at low temperatures: A review

Abstract

Tribological Properties of AlCrCuFeNi2 High-Entropy Alloy in Different Conditions

In order to understand the environmental effect on the mechanical behavior of high-entropy alloys, the tribological properties of AlCrCuFeNi2 are studied systematically in dry, simulated rainwater, and deionized water conditions against the Si3N4 ceramic ball at a series of different normal loads. The present study shows that both the friction and wear rate in simulated rainwater are the lowest. The simulated rainwater plays a significant role in the tribological behavior with the effect of forming passive film, lubricating, cooling, cleaning, and corrosion. The wear mechanism in simulated rainwater is mainly adhesive wear accompanied by abrasive wear as well as corrosive wear. In contrast, those in dry condition and deionized water are abrasive wear, adhesive wear, and surface plastic deformation. Oxidation contributes to the wear behavior in dry condition but is prevented in liquid condition. In addition, the phase diagram of Al x CrCuFeNi2 is predicted using CALPHAD modeling, which is in good agreement with the literature report and the present study.

Microstructure and Mechanical Behavior of High-Entropy Alloys

High-entropy alloys (HEAs) have generated interest in recent years due to their unique positioning within the alloy world. By incorporating a number of elements in high proportion, usually of equal atomic percent, they have high configurational entropy, and thus, they hold the promise of interesting and useful properties such as enhanced strength and alloy stability. The present study investigates the mechanical behavior, fracture characteristics, and microstructure of two single-phase FCC HEAs CoCrFeNi and CoCrFeNiMn with some detailed attention given to melting, homogenization, and thermo-mechanical processing. Ingots approaching 8 kg in mass were made by vacuum induction melting to avoid the extrinsic factors inherent to small-scale laboratory button samples. A computationally based homogenization heat treatment was given to both alloys in order to eliminate any solidification segregation. The alloys were then fabricated in the usual way (forging, followed by hot rolling) with typical thermo-mechanical processing parameters employed. Transmission electron microscopy was subsequently used to assess the single-phase nature of the alloys prior to mechanical testing. Tensile specimens (ASTM E8) were prepared with tensile mechanical properties obtained from room temperature through 800 °C. Material from the gage section of selected tensile specimens was extracted to document room and elevated temperature deformation within the HEAs. Fracture surfaces were also examined to note fracture failure modes. The tensile behavior and selected tensile properties were compared with results in the literature for similar alloys.

Microstructure and mechanical behavior of a novel Co20Ni20Fe20Al20Ti20 alloy fabricated by mechanical alloying and spark plasma sintering

A novel equiatomic Co20Ni20Fe20Al20Ti20 (at%) alloy was designed and synthesized to study the effect of high atomic concentrations of Al and Ti elements on the microstructure, phase composition and mechanical behavior of high-entropy alloys (HEAs) fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). Following the MA process, the Co20Ni20Fe20Al20Ti20 alloy was composed of a primary body-centered cubic (BCC) supersaturated solid solution and a face-centered cubic (FCC) supersaturated solid solution. However, following SPS, a primary FCC solid-solution phase, a BCC solid-solution phase and a trace amount of Al3Ti intermetallics were observed. Transmission electron microscopy (TEM) results confirmed the presence of the FCC solid-solution phase, the BCC (B2-type) solid-solution phase and Al3Ti intermetallics in the bulk alloy. The FCC and B2-type phases are ultrafine-grained, and Al3Ti intermetallics is nano/ultrafine-grained. Our results suggest that consideration of a single existing empirical design criterion is inadequate to explain phase formation in the Co20Ni20Fe20Al20Ti20 alloy. Solid-solution strengthening, grain-boundary strengthening, twin-boundary strengthening, the presence of the strong B2-type BCC phase, and precipitate strengthening due to the presence of a trace amount of Al3Ti are responsible for the ultra-high compressive strength of ~2988MPa and hardness of ~704Hv. The strain-to-failure of ~5.8% with visible ductility is dominated by the FCC solid-solution phase.

The microstructure and mechanical behavior of modern high temperature alloys

Article history: Received September 6, 2014 Accepted 2 January 2015 Available online 5 January 2015 Over the past decades, high entropy alloys (HEAs) have attracted continuously increasing research efforts because of their technological promise for structural applications and their scientific interest as a multi-component alloy exhibiting an overall random solid solution structure with high mixing entropy at high temperature. In this summary, we briefly review the recent studies focused on the structure and mechanical behavior of HEAs, covering the important issues from phase stability to elastic modulus, mechanical strength, hardness and fatigue resistance. Finally, we highlight a few key findings recently reported for HEAs and discuss the outstanding issues yet to be resolved. © 2015 Growing Science Ltd. All rights reserved.

Aluminum Alloying Effects on Lattice Types, Microstructures, and Mechanical Behavior of High-Entropy Alloys Systems

The crystal lattice type is one of the dominant factors for controlling the mechanical behavior of high-entropy alloys (HEAs). For example, the yield strength at room temperature varies from 300 MPa for the face-centered-cubic (fcc) structured alloys, such as the CoCrCuFeNiTix system, to about 3,000 MPa for the body-centered-cubic (bcc) structured alloys, such as the AlCoCrFeNiTix system. The values of Vickers hardness range from 100 to 900, depending on lattice types and microstructures. As in conventional alloys with one or two principal elements, the addition of minor alloying elements to HEAs can further alter their mechanical properties, such as strength, plasticity, hardness, etc. Excessive alloying may even result in the change of lattice types of HEAs. In this report, we first review alloying effects on lattice types and properties of HEAs in five Al-containing HEA systems: AlxCoCrCuFeNi, AlxCoCrFeNi, AlxCrFe1.5MnNi0.5, AlxCoCrFeNiTi, and AlxCrCuFeNi2. It is found that Al acts as a strong bcc stabilizer, and its addition enhances the strength of the alloy at the cost of reduced ductility. The origins of such effects are then qualitatively discussed from the viewpoints of lattice-strain energies and electronic bonds. Quantification of the interaction between Al and 3d transition metals in fcc, bcc, and intermetallic compounds is illustrated in the thermodynamic modeling using the CALculation of PHAse Diagram method.