Equiatomic Medium-entropy Alloy Research Articles

Extensive phase transformation in an equiatomic CrCoNi medium entropy alloy under extreme uniaxial tension

The dynamic tension behavior of an equiatomic CrCoNi medium entropy alloy (MEA) has been studied at both room temperature (RT, 298 K) and liquid nitrogen temperature (LNT, 77 K). Microstructural observations show that extensive phase transformations were activated in this alloy under dynamic tension at LNT instead of the activation of the deformation twins under high strain rate only or quasi-static loading at LNT. The appearance of massive nanoscale phase transformation bands along multiple slip directions even created a network structure of HCP lamellae in the FCC grains. This leads to defect accumulation at the intersection regions and results in the formation of new HCP variants, inverse phase transformations to FCC structure and even amorphous domains. Such transformation induced plasticity (TRIP) effects including the mutual interactions of crossover phase transformation bands on the plasticity and tension fracture of the MEA under extreme loading conditions were further discussed. These findings provide a deeper understanding of the deformation mechanisms in the equiatomic MEA under extreme loads.

Multi-objective optimization and machine learning assisted design and synthesis of magnesium based novel non-equiatomic medium entropy alloy

Non equiatomic compositionally complex alloys have gained significant attention due to their unique combinations of properties including high strength, good ductility, excellent thermal stability and corrosion resistance. The non equiatomic compositions provide flexibility in tailoring the mechanical, thermal, magnetic, biomedical and other functional properties to meet the specific application requirements. In this study, a novel design approach was proposed to identify quaternary non equiatomic medium entropy alloy for biomedical application. The design methodology employs multi-objective optimization of empirical phase formation parameters and ML algorithm to design solid solution containing non equiatomic medium entropy alloy. The suggested approach scans over a large composition space to find possible non-equiatomic compositionally complex alloy compositions (CCA) with desired phases. The proposed methodology is an alternative to trial-and-error alloy fabrication and CCA design approach reported in the literature. This step significantly reduced the time and resources utilized in CCA design by decreasing the number of experimental runs. Based on the design strategy, Mg60Ti24.24Zn9.66Nb6.1 alloy was designed and selected for fabrication through powder metallurgy route by employing microwave sintering. The Mg rich alloy exhibited the presence of two solid solution phases viz HCP and BCC in both prealloyed powder and final sintered specimen. The developed alloy showed a high microhardness value of 153.9 ± 0.85HV with a measured density of 2.82 ± 0.03 g/cm3. The mechanical properties such as yield strength, ultimate compressive strength, elastic modulus and fracture strain of the developed alloy were found to be superior to that of pure Mg and AZ31 fabricated through the same manufacturing route. Mechanical properties of local phases present within the alloy were assessed through nanoindentation technique. Elastic modulus (E) and hardness (H) values for the Ti rich BCC phase were 118.9±7.2 GPa and 4.65±1.1 GPa, respectively, whereas the Mg rich HCP phase showed values of 65.2±7 GPa and 1.6±0.43 GPa, respectively. Furthermore, with minor changes to the constraint equations and correct ML dataset selection, the suggested design technique can be applied to any multi-principal element alloy system to design non-equiatomic compositions.

Mechanical and Structural Characterization of Laser-Cladded Medium-Entropy FeNiCr-B4C Coatings.

Equiatomic medium-entropy alloy (MEA) FeNiCr-B4C (0, 1, and 3 wt.% B4C) coatings were deposited onto an AISI 1040 steel substrate using pulsed laser cladding. Based on an SEM microstructural analysis, it was found that the cross-sections of all the obtained specimens were characterized by an average coating thickness of 400 ± 20 μm, a sufficiently narrow (100 ± 20 μm) "coating-substrate" transition zone, and the presence of a small number of defects, including cracks and pores. An XRD analysis showed that the formed coatings consisted of a single face-centered cubic (FCC) γ-phase and the space group Fm-3m, regardless of the B4C content. However, additional TEM analysis of the FeNiCr coating with 3 wt.% B4C revealed a two-phase FCC structure consisting of grains (FCC-1 phase, Fm-3m) up to 1 µm in size and banded interlayers (FCC-2 phase, Fm-3m) between the grains. The grains were clean with a low density of dislocations. Raman spectroscopy confirmed the presence of B4C carbides inside the FeNiCr (1 and 3 wt.% B4C) coatings, as evidenced by detected peaks corresponding to amorphous carbon and peaks indicating the stretching of C-B-C chains. The mechanical characterization of the FeNiCr-B4C coatings specified that additions of 1 and 3 wt.% B4C resulted in a notable increase in microhardness of 16% and 38%, respectively, with a slight decrease in ductility of 4% and 10%, respectively, compared to the B4C-free FeNiCr coating. Thus, the B4C addition can be considered a promising method for strengthening laser-cladded MEA FeNiCr-B4C coatings.

Effect of local chemical order on the irradiation-induced defect evolution in CrCoNi medium-entropy alloy

High- (and medium-) entropy alloys have emerged as potentially suitable structural materials for nuclear applications, particularly as they appear to show promising irradiation resistance. Recent studies have provided evidence of the presence of local chemical order (LCO) as a salient feature of these complex concentrated solid-solution alloys. However, the influence of such LCO on their irradiation response has remained uncertain thus far. In this work, we combine ion irradiation experiments with large-scale atomistic simulations to reveal that the presence of chemical short-range order, developed as an early stage of LCO, slows down the formation and evolution of point defects in the equiatomic medium-entropy alloy CrCoNi during irradiation. In particular, the irradiation-induced vacancies and interstitials exhibit a smaller difference in their mobility, arising from a stronger effect of LCO in localizing interstitial diffusion. This effect promotes their recombination as the LCO serves to tune the migration energy barriers of these point defects, thereby delaying the initiation of damage. These findings imply that local chemical ordering may provide a variable in the design space to enhance the resistance of multi-principal element alloys to irradiation damage.

Friction stir welding of CoCrNi medium-entropy alloy: Recrystallization behaviour and strengthening mechanism

Dynamic recrystallization (DRX), deformation-induced mechanical twins, and strengthening mechanism of CoCrNi equi-atomic medium-entropy alloy (MEA), welded by friction stir welding (FSW) at different welding speeds, were systematically investigated in this study. The results indicated that FSW led to grain refinement in the stir zones (SZs) with diameters fluctuating in the range of 2.1–9.6 μm. The hardness was improved from ∼154HV (base material, BM) to 238–263HV (SZ) by FSW. The yield strength and ultimate tensile strength of the optimal SZs were 601 MPa and 844 MPa, respectively, which were 260% and 134% higher than those of the as-received material, respectively. Analyses of DRX behaviour indicated that discontinuous DRX (DDRX), continuous DRX (CDRX), and geometric DRX (GDRX) were successively activated with increasing strain during FSW to facilitate grain refinement. The larger the grain size, the higher the fraction of mechanical twins. However, the thickness of the twins had a negative correlation with the grain size. Although a few HCP structures were formed on the twin boundaries, grain refinement and dislocation hardening mechanisms remained dominant in enhancing the mechanical properties of the SZs. The strength-ductility synergy by FSW, under low heat input conditions, was caused by the formation of thin mechanical twins in fine equiaxed grains. Thinner twins were more effective in transferring and homogenising plastic deformation, thereby contributing to the postponing of plastic instability and further promoting an excellent combination of strength and ductility. These findings confirm that FSW is a promising approach for strengthening the MEA for superior performance.

Liquid helium temperature deformation and local atomic structure of CoNiV medium entropy alloy

Equiatomic CoNiV is now known to be the strongest fcc-structured equiatomic medium entropy alloy (MEA) with a high lattice friction stress. The local structure of CoNiV is measured using the extended x-ray absorption fine structure (EXAFS) and fitted to a fully disordered CoNiV lattice structure. EXAFS analysis suggests the bond length between V and its neighbors is significantly larger compared to the bond length of Co and Ni to its neighbors. Density functional theory (DFT) calculations reveal that the high friction stress is due to fluctuating atomic bond lengths and charge transfer between V and Co/Ni atoms. Cryogenic tensile tests also show that the tensile strength and ductility both increase with decreasing temperature up to liquid helium temperature (4.2 K). EBSD and TEM investigations suggest dislocation-mediated plasticity persists from ambient to liquid helium temperature due to the high stacking fault energy of the CoNiV MEA. This makes CoNiV the first of its kind in displaying enhancements in strength and ductility without any additionally activated deformation mechanisms.

Elucidating the Origination of Annealing-Induced Hardening in an Equiatomic Medium-Entropy Alloy

Elucidating the Origination of Annealing-Induced Hardening in an Equiatomic Medium-Entropy Alloy

Microstructure and texture of CoCrNi medium entropy alloy (MEA) processed by severe cryo-rolling: A study vis-a-vis cold-rolling

The microstructure and texture of severely cryo-rolled and annealed CoCrNi equiatomic medium entropy alloy (MEA) were investigated in the present work. To highlight the impact of cryo-rolling, these aspects were critically compared with similarly cold-rolled and annealed MEA. The alloy was cryo- and cold-rolled to 90% reduction and annealed at temperatures ranging from 700 °C to 1200 °C. Formation of deformation-induced nanostructure was observed in both materials. However, the slightly higher hardness of the cryo-rolled material was indicative of finer nanostructure and higher defect density as compared to those in the cold-rolled material. The cold-rolled material showed a perceptibly stronger brass texture than the cryo-rolled alloy. Extreme grain refinement and high density of intersecting shear bands appeared to weaken the brass texture in the cryo-rolled material. The lower grain size in the cryo-rolled material after different annealing treatments was consistent with a greater density of potential nucleation sites. The annealing texture of both cryo- and cold-rolled materials showed the presence of weak α-fiber (ND//<110>) components. Annealing resulted in a pronounced decrease in hardness, although sufficiently high hardness could still be obtained at a lower annealing temperature regime. Moreover, the hardness results were represented in a Hall-Petch plot which revealed a very high lattice friction stress for the MEA.

Ultrastrong medium entropy alloy with simultaneous strength-ductility improvement via heterogeneous nanocrystalline structures

Medium Entropy Alloy (MEA) has been of great research interests in materials science. However, the strength and ductility, like most metals, cannot be obtained in MEAs simultaneously so far, without changing their chemical compositions. Here, we present a new way to fabricate gradient heterogenous nanograined structure for overcoming strength-ductility trade-off of the equiatomic Medium Entropy Alloy (MEA) at multiple length scales. Ultrasonic assisted cyclic thermal dynamic solid-state physical process was developed to fabricate ultrastrong gradient heterogenous nanograined FeCoNi MEA structures with heterogenous nanocrystalline (NC). The microstructure and mechanical behaviors of the gradient heterogenous nanograined MEA are investigated. We studied the mechanical responses of the FeCoNi MEAs with designed crystalline structures. The yield strength of the heterogenous nanograined FeCoNi MEA (1.45 GPa) was increased by 8 times compared to that of coarse-grained FeCoNi MEA, while ductility was increased from 33% to 45% after the processing. The strengthening mechanism in the NC FeCoNi MEA is found to be dominated by grain refinement and heterogenous grain structure. We also found an identical strain rate sensitivity exponent for the FeCoNi medium entropy alloy at multiple length scales, implying that dislocation mediated deformation mechanism dominates in the heterogenous FeCoNi MEA. This study provides a new way to fabricate novel gradient heterogenous nanograined structures to achieve both high strength and ductility in MEAs and provide fundamental understanding of the mechanical strengthening and deformation mechanisms of the nanograined MEAs.

Study of Texture Analysis on Asymmetric Cryorolled and Annealed CoCrNi Medium Entropy Alloy

CoCrNi equiatomic medium entropy alloy sheets were prepared by asymmetric rolling, cryorolling, and asymmetric cryorolling. The asymmetric cryorolled samples exhibited a noteworthy ultra-fine-grain heterogeneous lamella structure. The microstructure and corresponding hardness obtained by different rolling processes and subsequent annealing are compared. It can be seen from the results that the cryogenic deformation temperature had a stronger effect on the mechanical properties of the medium entropy alloys (MEA), compared with the shear strain caused by the asymmetric cryorolling. The effect of annealing temperature on texture components and volume fractions of the specially rolled samples was also analyzed. The result revealed that the recrystallized MEA exhibited similar texture components and the corresponding volume fraction, which indicated that the rolling process had limited influence on the formation of annealing texture. The recrystallized texture after annealing retained the deformation texture and twin related orientations appeared. Asymmetric rolled MEA showed strong random composition than symmetric rolled MEA regardless of rolling temperature. The recrystallized textures of the species obtained by the three rolling processes did not exhibit a significant dependence on the annealing temperature.

Experimental and simulation studies of solidification behaviour in undercooled CuCoNi equiatomic medium entropy alloy

Undercooling studies were carried out in equiatomic CuCoNi system with an aim to understand the growth kinetics and microstructural variations as a function of undercooling. The morphological change in microstructure was observed and correlated with the undercooling obtained. Non-linear variation of growth velocity with respect to the increase in undercooling was obtained from high-speed video measurements. The variation in growth velocity was compared with the dendritic growth model with a modified kinetic undercooling term. The segregation profile was predicted using multi-phase field method and compared with the experimental data. Micro-hardness variation was correlated with the undercooling obtained.

Mechanism of grain refinement in an equiatomic medium-entropy alloy CrCoNi during hot deformation

With promising mechanical properties, the newly developed medium-entropy alloy (MEA) has aroused widespread interest recently. Plastic deformation processes including hot working are usually adopted to refine the grain of many alloys and thereby enhance the strength, hardness, and wear resistance at low temperatures. So far, few discoveries have reported the mechanism of grain refinement during hot deformation on MEA, especially for CrCoNi. In this article, plastic deformation at different conditions was intentionally designed to uncover the underlying mechanism of grain refinement in CrCoNi MEA alloy. Unlike many other alloys with low stacking fault energy, the occurrence of continuous dynamic recrystallization during hot deformation of CrCoNi alloy has been unexpectedly discovered, which is proved as the dominating mechanism for grain refinement of the current alloy.

Influence of synthesis method on microstructure and mechanical behavior of Co-free AlCrFeNi medium-entropy alloy

The influence of synthesis method on a Co-free equiatomic medium-entropy alloy (MEA), AlCrFeNi, has been systematically investigated in the present study. Three synthesis methods, melting and casting, spark plasma sintering (SPS) of the mechanically alloyed powders and direct SPS of the mixture of elemental powders, were respectively used to prepare bulk samples. All bulk samples primarily consisted of a disordered body-centered cubic (BCC, A2) phase and an ordered BCC (B2) phase, suggesting that synthesis method had a negligible influence on the phase compositions. However, the synthesis method showed a significant influence on the microstructure of the AlCrFeNi MEA, leading to altering the mechanical properties. A rare chrysanthemum-like eutectic microstructure was obtained in the as-cast sample, while a spinodal decomposed microstructure was observed in the SPS sintered samples. All bulk AlCrFeNi samples exhibited a relatively high yield strength (>1040 MPa) with a fairly high strain-to-failure (>28%). The CALPHAD (Calculation of Phase Diagrams) approach was used to explain the phase formation and stability in the bulk AlCrFeNi samples. In addition, the microstructural evolution and microstructure-property relationship were also investigated deeply.

Ultrastrong Medium-Entropy Single-Phase Alloys Designed via Severe Lattice Distortion.

Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering. Yet, the yield strength values of medium- and high-entropy alloys investigated so far do not substantially exceed those of conventional alloys owing to the insufficient utilization of lattice distortion. Here it is shown that a simple VCoNi equiatomic medium-entropy alloy exhibits a near 1 GPa yield strength and good ductility, outperforming conventional solid-solution alloys. It is demonstrated that a wide fluctuation of the atomic bond distances in such alloys, i.e., severe lattice distortion, improves both yield stress and its sensitivity to grain size. In addition, the dislocation-mediated plasticity effectively enhances the strength-ductility relationship by generating nanosized dislocation substructures due to massive pinning. The results demonstrate that severe lattice distortion is a key property for identifying extra-strong materials for structural engineering applications.

Ultra-high tensile strength nanocrystalline CoCrNi equi-atomic medium entropy alloy processed by high-pressure torsion

A nanocrystalline CoCrNi alloy of ~50 nm grain size with the ultra-high ultimate tensile strength of ~2.2 GPa and fracture strain of ~9% was fabricated using high-pressure torsion. The presence of high density of nano-twins, stacking faults, dislocations, and nano-grains is attributed to the superior mechanical properties.

Medium entropy alloy CoCrNi coatings: Enhancing hardness and damage-tolerance through a nanotwinned structuring

Medium entropy alloys (MEA) are defined as alloys consisting of three equiatomic elements, such as CoCrNi. MEAs are reported to have superior mechanical properties and high thermodynamic stability, as well as excellent fracture toughness at cryogenic temperatures. Here, we investigate a series of equiatomic medium entropy alloy coatings, containing three elements, Co, Cr, Ni. These coatings were deposited onto M2 steel substrates with a range of coating thicknesses using a DC magnetron sputtering system using a CoCrNi alloy target (1:1:1at.%). The microstructure and mechanical properties were examined by a number of characterization techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation. XRD analysis showed that the coatings were dominated by a fcc CoCrNi phase, with a smaller amount of hcp Co. TEM analysis demonstrated that the elongated grains contained a high density of {111} nanotwins. In addition, the residual stresses in the coatings were analysed using X-ray diffraction by adopting the conventional sin2ψ method. A high hardness value, ~10GPa, was determined by nanoindentation of these coatings. Exceptional damage-tolerance was also found in these coatings under contact loading. It is believed that the nanotwinned structure is responsible for the high hardness and damage tolerance observed in the new coatings.

Friction stress and Hall-Petch relationship in CoCrNi equi-atomic medium entropy alloy processed by severe plastic deformation and subsequent annealing

The present work focuses on the friction stress and the Hall-Petch relationship in CoCrNi equi-atomic medium entropy alloy (MEA). The CoCrNi equi-atomic MEA and a Ni-40Co alloy were processed by high-pressure torsion and subsequent annealing. The specimens with fully-recrystallized microstructure with different grain sizes ranging from 199nm to 111μm were obtained. The Hall-Petch plot of the yield strength of the present specimens indicated that the friction stress of the CoCrNi MEA was much higher than Ni-40Co and pure metals, suggesting that the local lattice distortion in the equi-atomic alloy played an important role for the dislocation activity.

Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures.

High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centred cubic solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature, the alloy shows tensile strengths of almost 1 GPa, failure strains of ∼70% and KJIc fracture-toughness values above 200 MPa m1/2; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa m1/2. Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning.