Microstructure Of High-entropy Alloy Research Articles

The Effects of the Al and Zr Contents on the Microstructure Evolution of Light-Weight AlxNbTiVZry High Entropy Alloy.

To investigate the comprehensive effects of the Al and Zr element contents on the microstructure evolution of the AlNbTiVZr series light-weight refractory high entropy alloys (HEAs), five samples were studied. Samples with different compositions were designated Al1.5NbTiVZr, Al1.5NbTiVZr0.5, AlNbTiVZr, AlNbTiVZr0.5, and Al0.5NbTiVZr0.5. The results demonstrated that the actual density of the studied HEA samples ranged from 5.291 to 5.826 g·cm-3. The microstructure of these HEAs contains a solid solution phase with a BCC structure and a Laves phase. The Laves phase was further identified as the ZrAlV intermetallic compound by TEM observations. The microstructure of the AlNbTiVZr series HEAs was affected by both the Al and Zr element contents, whereas the Zr element showed a more dominant effect due to Zr atoms occupying the core position of the ZrAlV Laves phase (C14 structure). Therefore, the as-cast Al0.5NbTiVZr0.5 sample exhibits the best room temperature compression property with a compression strength (σp) of 1783 MPa and an engineering strain of 28.8% due to having the lowest ZrAlV intermetallic compound area fraction (0.7%), as characterized by the EBSD technique.

Effect of laser energy densities on the phase composition change in laser-cladded AlCoCrNbMo high-entropy alloy coatings

This work designed a new high-entropy alloy (HEA), namely, AlCoCrNbMo, and prepared it by using laser cladding (LC). The effects of different values of laser energy density (Es) on the phase composition, microstructure, mechanical properties and corrosion resistance of the HEA coating were investigated via X-ray diffractometry; scanning electron microscopy; energy dispersive spectrometry and microhardness, friction wear and electrochemical tests. Results indicate that increasing Es (Es = 35, 55, 75 J/mm2) leads to the formation of different phases, including bcc1, bcc1 +bcc2 and bcc1 +bcc2 +Al3Nb, in the coating. The bcc1 phase is rich in Co and Cr elements, whereas the bcc2 phase is rich in Nb and Mo elements. As Es increases, the HEA coating shows increasing hardness and wear resistance but decreasing corrosion resistance. Moreover, the AlCoCrNbMo alloy prepared through vacuum arc melting does not conform to the form of HEAs but is instead composed of various intermetallic compounds, reflecting the unique advantage of LC. This paper discusses the influence of LC process parameters on the microstructure of HEAs and the unique advantages of LC for HEA preparation.

Effect of synergistic cavitation erosion-corrosion on cavitation damage of CoCrFeNiMn high entropy alloy layer by laser cladding

The CoCrFeNiMn high entropy alloy (HEAs) clad layer was prepared on the surface of 304 stainless steel using coaxial powder feeding laser cladding. The effects of laser energy density on the macroscopic residual stress, crystallographic characteristics, microhardness, corrosion resistance and cavitation erosion (CE) resistance of HEAs clad layer were studied. With the decrease of laser energy density from 45 J/mm2 to 36 J/mm2, pore defects and abnormal coarsening of microstructure in HEAs cladding layer can be effectively avoided, the average grain size can be refined to 74.3 ± 1.7 μm, the average Schmidt factor is 0.43, which also shows the highest dynamic passivation tendency and corrosion resistance in artificial seawater. After 10 h-CE, the carrier density ND in the passivation film of HEAs clad layer can be as low as 3.24 ± 0.08 × 1022 cm−3, and the lowest mean depth erosion rates (MDER) is 0.68 ± 0.07 μm/h, which is much lower than that of 304 stainless steel substrate (4.23 ± 0.13 μm/h). The excellent CE resistance of the HEAs clad layer is attributed to the fact that its grains can effectively resist the plastic deformation and the surface passivation film can maintain a relatively high degree of integrity and continuous level after long aging CE, which has a considerable application prospect in marine engineering.

An overview of microstructure, mechanical properties and processing of high entropy alloys and its future perspectives in aeroengine applications

Modern engineering applications continually strive to develop greater performance mechanical components with good microstructural stability, improved mechanical properties, corrosion resistance and decreased cost of repairing and maintenance. This necessitates the broad use of advanced high performance materials like high entropy alloys (HEAs). These alloys are created by combining five or more alloying elements in equal or substantial amount. About 5 to 35 at. % of the alloying element is present in HEAs. It is characterized primarily by greater entropy, slow diffusion, severe lattice distortion, and cocktail effects. Due to its advanced microstructural stability throughout a larger temperature span and for longer length of time, it demonstrates improved mechanical characteristics at ambient temperature, cryogenic temperature, and elevated temperature. The diversity of elemental contents and significantly higher mixing entropy of HEAs make them mechanically superior to classic metals and alloys. It also shows better strength to weight ratio. Hence, it qualifies as a possible structural and functional material for aeroengine applications. In this work, the studies on the HEAs are briefly reviewed. A basic explanation of the four core effects of HEAs is given. Discussion is held on microstructure and mechanical properties of HEAs. The processing routes for manufacturing of HEAs (arc melting, bridgman solidification, mechanical alloying and vapour deposition) are presented briefly. The influence of heat treatment on mechanical behavior and microstructure of HEAs is presented. The simulation approach of CALPHAD modeling for designing of HEAs is discussed briefly. The future scope for research and development of HEAs in aeroengine applications is briefed.

Hot deformation mechanisms of dual phase high entropy alloys

The microstructure of high entropy alloys can finally be designed via thermomechanical treatments to tune the mechanical properties. This work investigates the modification of the microstructure after treatments at 1100 °C for three hypo-eutectic high entropy alloys. Two phases were indexed according to the BCC and FCC crystal structures using electron backscattered diffraction. Their microstructure is investigated for three hot deformation tests: at a constant strain rate of 0.001s−1, at a strain rate jumps from 0.001s−1 to 1s−1 and from 1s−1 to 0.001s−1. The BCC size and fraction strongly influence the deformation of the FCC matrix. Due to its typical semi-interconnected hypo-eutectic structure, the BCC phase carries the load at the beginning of the deformation. Progressively, the FCC phase deforms to accommodate the plastic strain due to the bending and fragmentation of the BCC phase. Fine particles of the BCC phase are formed within the FCC matrix at high temperatures, and they pin the high-angle grain boundaries formed by continuous dynamic recrystallisation. The fragmentation of the BCC phase occurs faster for thinner eutectic BCC particles, and it is a consequence of I) the formation of boundaries during plastic deformation via dynamic recovery followed by continuous dynamic recrystallisation; II) the movement of phase boundaries consuming the formed boundaries within the BCC phase, fragmenting them. A fine substructure with a high density of high-angle grain boundaries is formed at 1100 °C for the alloy with initial fine BCC eutectic particles.

Microstructural Evaluation of Thermal-Sprayed CoCrFeMnNi0.8V High-Entropy Alloy Coatings

The aim of this work is to improve the understanding of the effect of the cooling rate on the microstructure of high-entropy alloys, with a focus on high-entropy alloy coatings, by using a combined computational and experimental validation approach. CoCrFeMnNi0.8V coatings were deposited on a steel substrate with high velocity oxy-air-fuel spray with the employment of three different deposition temperatures. The microstructures of the coatings were studied and compared with the microstructure of the equivalent bulk high-entropy alloy fabricated by suction casting and powder fabricated by gas atomization. According to the results, the powder and the coatings deposited by low and medium temperatures consisted of a BCC microstructure. On the other hand, the microstructure of the coating deposited by high temperature was more complex, consisting of different phases, including BCC, FCC and oxides. The phase constitution of the bulk high-entropy alloy included an FCC phase and sigma. This variation in the microstructural outcome was assessed in terms of solidification rate, and the results were compared with Thermo-Calc modelling. The microstructure can be tuned by the employment of rapid solidification techniques such as gas atomization, as well as subsequent processing such as high velocity oxy-air-fuel spray with the use of different spray parameters, leading to a variety of microstructural outcomes. This approach is of high interest for the field of high-entropy alloy coatings.

Compressive vs. tensile yield and fracture toughness behavior of a body-centered cubic refractory high-entropy superalloy Al0.5Nb1.25Ta1.25TiZr at temperatures from ambient to 1200°C

The microstructure of high-entropy alloys with refractory elements and Al as constituents can be considered to be analogous to superalloys. These so-termed refractory high-entropy superalloys (RHSAs) can show remarkable compressive strength up to temperatures exceeding 1200 °C. Here, we examine the microstructure and properties - compressive, tensile, and fracture toughness - of a precipitation-hardened, body-centered cubic, RHSA, Al0.5Nb1.25Ta1.25TiZr, at ambient temperature (RT) to 1200 °C. Two dual-phase microstructures comprising ordered B2 (brittle) and disordered A2 (ductile) phases were produced in this alloy - one with B2 as the matrix, the other with A2 - for evaluation of the mechanical properties. Under compression, both microstructures display RT compressive strengths above 1.5 GPa and considerable ductility exceeding 40% at elevated temperatures; the alloy with the A2 matrix has ∼15% compressive ductility even at RT. However, properties are very different under tensile loading; at all temperatures, both microstructures fail predominately in an intergranular mode in the elastic regime at a fracture stress less than 200 MPa and ductility below 0.15%. The microstructure with the A2 matrix has a KIc fracture toughness of ∼15 MPa√m at RT, although at all temperatures above 800 °C, measured KIc values for both dual-phase microstructures are less than 5 MPa√m. In this study, we investigate the microstructural origin of these mechanical properties, and emphasize the importance of evaluating these alloys in tension.

Strengthening CoCrFeNi high entropy alloys via additive manufacturing with laser shock modulation of melt pool

A novel synchronous laser shock modulation of melt pool (LSMMP) method was proposed to enhance melt pool convections and inhibit columnar crystal growth mainly found in additive manufacturing high entropy alloys (HEAs) via an additional second pulsed laser. The effect of laser shock on the microstructure of HEAs under the same additive manufacturing parameters was discussed assisted by finite element method (FEM). Results of electron backscattered diffraction (EBSD) show that grain refinement occurs in LSMMPed HEAs under optimal parameters. The strengthened plane texture is observed in the LSMMPed HEAs from three crystallographic orientations of [100], [110], and [111]. The Vickers hardness of HEAs is enhanced by grain refinement and there is no elemental segregation in the LSMMP process. The validation of LSMMP by tensile tests also indicates that the LSMMPed HEAs have better ductility. The present results demonstrate the fast real-time interference capability of LSMMP in metal additive manufacturing.

Effect of Laser re-melting on the microstructure of High Entropy Alloys

• A low density equiatomic High Entropy Alloy Zr-Nb-V-Ti-Al was synthesized. • The as-cast alloy had coarse dendritic structure with compositional inhomogeneities. • Re-melting the as-cast alloy with Nd-YAG Laser resulted in formation of fine grains. • Laser Re-melting significantly reduced inhomogeneities in distribution of Zr & Al. • The microhardness of Laser melted region was found to be similar to the BCC phase. A low density equiatomic High Entropy Alloy of composition Nb-Zr-V-Ti-Al was synthesized for nuclear reactor applications. The alloy in as-cast condition showed a coarse dendritic structure with a major BCC/B2 type dendritic phase while segregation of Zr and Al in the form of intermetallics was noted in the inter-dendrtitic region. Re-melting a part of the alloy surface with an Nd-YAG Laser resulted in formation of very fine grains (∼500 nm) with a significant reduction in compositional segregations with respect to Zr and Al. The microhardness of the Laser melted region was found to be similar to the BCC phase in the as – cast microstrucure. This indicates that Laser melting could be an effective method in reducing compositional and microstructural inhomogeneities and push the multiphase structure towards single phase.

Microstructural evolution, mechanical properties and corrosion resistance of CoCrFeNiW0.5 high entropy alloys with various annealing heat treatment

High entropy alloys (HEAs) with excellent mechanical properties and corrosion resistance show promising potential for use in many fields including marine environment. In order to make a good tradeoff between the mechanical properties and corrosion resistance, the microstructure of HEA is adjusted by proper heat treatment. Herein, the non-equiatomic CoCrFeNiW 0.5 HEA with dual FCC and μ phase was prepared by using an electromagnetic levitation melting method, and the microstructure, mechanical properties and corrosion resistance of these alloys with annealing temperature range from 600 to 1200 °C for 3 h have been investigated. After annealing at 1000 °C, a large number of needle-like μ phase precipitates was firstly observed in the matrix, the volume fraction of the μ phase is 26.64 %, which is about twice as higher as under the as-cast conditions (13.35 %). This alloy exhibited the highest compression yield strength of 1253.9 MPa with a considerable fracture strain of 40.8 % and the highest Vickers hardness value of 369.6 HV. The decomposition of μ phase precipitates was observed when annealing temperature up to 1200 °C, which lead to the mechanical properties of the alloy decreased slightly. All the alloys were found to show excellent corrosion resistance in 3.5 wt% NaCl solution, and corrosion occurs preferentially on the FCC matrix and phase boundaries, the increase of μ phase volume fraction result in more severe galvanic corrosion of alloys. Our results offer insights into analyzing the evolution of both mechanical properties and corrosion resistance of HEAs caused by heat treatment. • Heat treatment between 600 and 1200 °C lead to the change of μ phase precipitates. • CoCrFeNiW 0.5 HEA exhibited the highest compression yield strength of 1253.9 MPa with a fracture strain of 40.8 %. • The potential difference between FCC and μ phase intensifies the corrosion. • Phase volume ratio can influence the mechanical properties and corrosion resistance.

Effect of Sn and Mo on Microstructure and Electrochemical Property of TiZrTaNb High Entropy Alloys

The effects of Sn and Mo alloying elements on the microstructure and electrochemical properties of TiZrTaNb high entropy alloys were studied by optical microscope (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrochemistry. TiZrTaNb, TiZrTaNbMo and TiZrTaNbSn alloys with equal atomic ratio were prepared by the arc melting method. The results showed that the microstructure of the high entropy alloys was dendritic structure with single BCC structure. The addition of Mo and Sn elements promoted the growth of the dendritic structure and accelerated the interdendritic segregation of the TiZrTaNb alloy. The TiZrTaNbMo alloy exhibited excellent corrosion properties compared to TiZrTaNb and TiZrTaNbSn alloys based on corrosion parameters Icorr, φcorr, Ipass. The corrosion mechanism is discussed based on the corrosion morphology. The alloying elements have an important effect on the microstructure and electrochemical properties of a high entropy alloy.

A comparative study and development of a novel deep learning architecture for accelerated identification of microstructure in materials science

The properties and processing of materials depend on the microstructure at various stages. Identifying the microstructure is essential for the accelerated development of advanced materials and Material Informatics initiatives. In the current study Convolutional Neural Network (CNN) models such as ResNet, VGG, DenseNet, Inception based, MobileNet based, SVC, Tree based, MLP based are trained and compared for microstructure classification. The models are developed with fewer parameters by decreasing the depth and width of the network. The model based on DenseNet shows high testing, training, and validation accuracy with an optimum number of parameters. The developed model based on DenseNet has a total number of parameters of 421674, including 10,930 non-trainable parameters and 4 Dense blocks, 3 Transition blocks, and the fully connected network. The current model achieved a training accuracy of 97.45%, validation accuracy of 96.26%, and testing accuracy of 94.89%. The developed model is used for classifying the microstructure of high entropy alloys, and results show a good agreement. These state-of-the-art models will accelerate the material development and Material Informatics initiatives.

On the phase constituents of four CoCrFeNiX (X = Y, Ti, Zr, Hf) high-entropy alloys after prolonged annealing

Despite the large amount of literature on high entropy alloys (HEAs) in recent years, reports on the equilibrium/near-equilibrium phase and microstructure of HEAs are still very limited. Here, we investigate the phase constituents of four CoCrFeNiX (X=Y, Ti, Zr, Hf) HEAs after prolonged annealing. It is found that the alloys typically contain two major phases – a dominating intermetallic (IM) phase and a BCC phase. The major phases in general remain unchanged regardless of annealing temperature. Annealing at lower temperatures, however, typically leads to the formation of new minor phases. In particular, the formation of σ phase during annealing is observed in all the alloys. Existing phase formation theories are applied to the alloys. The theories successfully predict that the present alloys do not form simple solid solution phases only. Phase-specific stability criteria also explain the prevalence of Laves and σ phases quite well.

Materials and manufacturing renaissance: Additive manufacturing of high-entropy alloys

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Hierarchical Eutectoid Nano-lamellar Decomposition in an Al0.3CoFeNi Complex Concentrated Alloy

This paper reports a novel eutectoid nano-lamellar (FCC + L12)/(BCC + B2) microstructure that has been discovered in a relatively simple Al0.3CoFeNi high entropy alloy (HEA) or complex concentrated alloy (CCA). This novel eutectoid nano-lamellar microstructure presumably results from the complex interplay between Al-mediated lattice distortion (due to its larger atomic radius) in a face-centered cubic (FCC) CoFeNi solid solution, and a chemical ordering tendency leading to precipitation of ordered phases such as L12 and B2. This eutectoid microstructure is a result of solid-state decomposition of the FCC matrix and therefore distinct from the commonly reported eutectic microstructure in HEAs which results from solidification. This novel nano-lamellar microstructure exhibits a tensile yield strength of 1074 MPa with a reasonable ductility of 8%. The same alloy can be tuned to form a more damage-tolerant FCC + B2 microstructure, retaining high tensile yield stress (~900 MPa) with appreciable tensile ductility (>20%), via annealing at 700 °C. Such tunability of microstructures with dramatically different mechanical properties can be effectively engineered in the same CCA, by exploiting the complex interplay between ordering tendencies and lattice distortion.

Effective hydrogen diffusion coefficient for CoCrFeMnNi high-entropy alloy and microstructural behaviors after hydrogen permeation

Herein, the first observation of the effective hydrogen diffusion coefficient of CoCrFeMnNi high-entropy alloy (HEA) was performed using electrochemical hydrogen permeation; further, it was compared with those of stainless steels (SS) 304 and 316L. HEA and SS 316L showed similar effective hydrogen diffusion coefficient of 1.75 × 10−11 m2/s and 1.91 × 10−11 m2/s, respectively. SS 304 showed the smallest that of 0.58 × 10−11 m2/s in the study. Hydrogen diffusion through the grain boundary was dominant in face-centered cubic metals. Hydrogen permeation resulted in no change in the microstructure of HEA and SS 316L; however, it caused a martensitic transformation in SS 304.

Effective Hydrogen Diffusion Coefficient and Microstructural Behaviors Measured by Electrochemical Hydrogen Permeation for CoCrFeMnNi High-Entropy Alloy

Herein, the first observation of the effective hydrogen diffusion coefficient of CoCrFeMnNi high-entropy alloy (HEA) was performed using electrochemical hydrogen permeation; further, it was compared with those of Fe-17Cr-10Ni and Fe-18Cr-8Ni stainless steels (SS). HEA and Fe-17Cr-10Ni SS showed similar effective hydrogen diffusion coefficient of 1.75×10-11m2/s and 1.91×10-11 m2/s, respectively. Fe-18Cr-8Ni SS showed the smallest that of 0.58×10-11 m2/s in the study. Hydrogen diffusion through the grain boundary was dominant in face-centered cubic metals. Hydrogen permeation resulted in no change in the microstructure of HEA and Fe-17Cr-10Ni SS; however, it caused a martensitic transformation in Fe-18Cr-8Ni SS.

Effect of Cold Deformation and Heat Treatment on the Microstructure and Mechanical Behavior of High Entropy Alloy CuCrFeNi2Al0.5

CuCrFeNi2Al0.5, a high entropy alloy (HEA) with a single FCC phase, was deformed at room temperature and then heat treated. The small particles precipitated from the deformed FCC solid solution matrix after heating at 700 °C, softening the solid solution matrix. This outcome confirms that the microstructure of HEAs can be controlled by heat treatment in addition to controlling the chemical composition and the rate of cooling from liquid alloys. The hardness of the samples increased due to precipitation hardening, and the specimen heated at 700 °C with 10% predeformation had the highest hardness. As the predeformation increases, the precipitates tend to interweave in the network morphology. Meanwhile, grains in all specimens became more severely coarse after heat treatment at 1100 °C. Predeformation followed by annealing can obviously slow down the creep rate, indicating that annealing at an appropriate temperature can improve the mechanical properties of deformed HEA.

Current and emerging practices of CALPHAD toward the development of high entropy alloys and complex concentrated alloys

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Thermodynamic Analysis for Microstructure of High-Entropy Alloys

Abstract The Gibbs free energy of liquid and crystal, intermetallic compounds, and amorphous phases were investigated by thermodynamic analysis for microstructure of high-entropy alloy (HEA). Results show that the value of Δ S r Δ H h(sol.) plays an important role in estimating the microstructure of HEAs. The alloys with lower value of Δ S r Δ H h(sol.) have stronger tendency of forming single fcc or bcc solid solution, and the alloy with complicated microstructure always has larger value of Δ S r Δ H h(sol.) .