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Journal Pre-proof Ultrastrong and ductile NiFeCrAlV complex- concentrated alloy via dual-morphology brittle intermetallic compound Ultrastrong and ductile NiFeCrAlV complex-concentrated alloy via dual- morphology brittle intermetallic compound
Abstract
Complex-concentrated alloys (CCAs) have received a lot of attention recently due to their exceptional balance between strength and ductility. In this work, a brittle intermetallic compound strengthening Ni46.3Fe21Cr18.5Al10V4.2 (at. %) CCA has fabricated via carefully two-step thermomechanical treatment. The microstructure, phase composition, and mechanical properties of this alloy were investigated systematically. The microstructure characterization that brittle dual-morphology B2 particles are major second phases for the alloy under the two-step heat treatment. The microstructure characterization reveals that the coarse spherical B2 intermetallic compounds pinned grain boundaries and the dispersed dual-morphology B2 intermetallic compounds in the FCC matrix are key factors contributing to excellent mechanical properties. The yield strength and ultimate strength of the CCA enhanced substantially from 920 MPa and 1230 MPa in the annealed state, respectively, to 1400 MPa and 1660 MPa after aging treatment, while the uniform elongation remains above 18.5%. This work reveals the mechanical properties of the CCA can be enhanced by controlling B2 particles morphology and dispersion via suitable thermomechanical processing to meet the requirements of engineering J o u r n a l P r e-p r o o f 2 applications.
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Medium manganese (MMn) steels are a promising class of advanced high-strength steels (AHSS) with potential for application as vehicle panels in the automotive industry. In this study, the deformation behaviour of a representative cold-rolled MMn steel and its dependence on processing parameters are studied using uniaxial tensile testing under low-temperature hot stamping (LTHS) conditions, covering austenitisation soaking times of 60-600 s, deformation temperatures of 500-700 ºC and strain rates of 0.01-5 s⁻¹. A yield point phenomenon is observed for the first time in the early stage of deformation at such temperatures in this MMn steel, which has a submicron grain size. The extent of the yield point phenomenon is reduced, and strain hardening capability and total elongation are enhanced, with longer austenitisation soaking times, which give coarser-grained microstructures. The yield point phenomenon also tends to be weaker at higher deformation temperatures and lower strain rates; under these conditions, both the flow stress and the degree of strain hardening decrease, while the total elongation is insensitive to differences in deformation conditions. The mechanisms for the deformation behaviour and its dependence on test conditions are discussed. In addition, a set of unified constitutive equations is established and calibrated using the experimental data to predict the deformation behaviour of the MMn steel under uniaxial LTHS conditions, and close agreement with experiment, including the yield point phenomenon, is obtained.
Ferritic-martensitic dual-phase (DP) steels are prominent and advanced high-strength steels (AHSS) broadly employed in automotive industries. Hence, extensive study is conducted regarding the relationship between the microstructure and mechanical properties of DP steels due to the high importance of DP steels in these industries. In this respect, this paper was aimed at reviewing the microstructural characteristics and strengthening mechanisms of DP steels. This review article represents that the main microstructural characteristics of DP steels include the ferrite grain size (FGS), martensite volume fraction (MVF), and martensite morphology (MM), which play a key role in the strengthening mechanisms and mechanical properties. In other words, these can act as strengthening factors, which were separately considered in this paper. Thus, the properties of DP steels are intensely governed by focusing on these characteristics (i.e., FGS, MVF, and MM). This review article addressed the improvement techniques of strengthening mechanisms and the effects of hardening factors on mechanical properties. The relevant techniques were also made up of several processing routes, e.g., thermal cycling, cold rolling, hot rolling, etc., that could make a great strength-ductility balance. Lastly, this review paper could provide substantial assistance to researchers and automotive engineers for DP steel manufacturing with excellent properties. Hence, researchers and automotive engineers are also able to design automobiles using DP steels that possess the lowest fuel consumption and prevent accidents that result from premature mechanical failures.
The CrCoNi medium-entropy alloy is not fully utilized at room temperature circumstance due to its low
strength, despite excellent cryogenic mechanical properties. Here we report that, a superior room temperature strength (1.6 GPa and 407.6 Hv) has been obtained in the single phase CoCrNi medium-entropy
alloy, by carefully tuning the formation of multiple lattice defects. Electron microscopy characterizations show that such high-strength CoCrNi alloy contains favorable heterostructures of coarsening grains
(~30 μm) and ultrafine grains (~1 μm), together with the high density of dislocation cells, small stacking
faults, ultrafine nanotwins (~9 nm in thickness) and Lomer–Cottrell locks.
Traditional metallic alloys are mixtures of elements in which the atoms of minority
species tend to be distributed randomly if they are below their solubility limit, or to
form secondary phases if they are above it. The concept of multiple-principal-element
alloys has recently expanded this view, as these materials are single-phase solid
solutions of generally equiatomic mixtures of metallic elements. This group of
materials has received much interest owing to their enhanced mechanical
properties1–5. They are usually called medium-entropy alloys in ternary systems and
high-entropy alloys in quaternary or quinary systems, alluding to their high degree of
configurational entropy. However, the question has remained as to how random these
solid solutions actually are, with the influence of short-range order being suggested in
computational simulations but not seen experimentally6,7. Here we report the
observation, using energy-filtered transmission electron microscopy, of structural
features attributable to short-range order in the CrCoNi medium-entropy alloy.
Increasing amounts of such order give rise to both higher stacking-fault energy and
hardness. These findings suggest that the degree of local ordering at the nanometre
scale can be tailored through thermomechanical processing, providing a new avenue
for tuning the mechanical properties of medium- and high-entropy alloys.
We report deformation bands ahead of the crack tip in a CrCoNi alloy under impact loading, and analyze the effects of deformation twinning on strain localization in the form of shear banding. Due to low stacking fault energy, upon high-strain-rate deformation this medium-entropy alloy forms twins with thickness and spacing of only a few nanometers. These nano-twins are embedded dynamically not only ahead of any developing shear band to retard its advance, but also in its interior to work harden. This dual effect helps to suppress runaway shear banding instability that instigates failure and contributes to the energy absorption as recorded in instrumented Charpy tests.
High-entropy and medium-entropy alloys are presumed to have a configurational entropy as high as that of an ideally mixed solid solution (SS) of multiple elements in near-equal proportions. However, enthalpic interactions inevitably render such chemically disordered SSs rare and metastable, except at very high temperatures. Here we highlight the wide variety of local chemical ordering (LCO) that sets these concentrated SSs apart from traditional solvent-solute ones. Using atomistic simulations, we reveal that the LCO of the multi-principal-element NiCoCr SS changes with alloy processing conditions, producing a wide range of generalized planar fault energies. We show that the LCO heightens the ruggedness of the energy landscape and raises activation barriers governing dislocation activities. This influences the selection of dislocation pathways in slip, faulting, and twinning, and increases the lattice friction to dislocation motion via a nanoscale segment detrapping mechanism. In contrast, severe plastic deformation reduces the LCO towards random SS.
Realizing improved strength–ductility synergy in eutectic alloys acting as in situ composite materials remains a challenge in conventional eutectic systems, which is why eutectic high-entropy alloys (EHEAs), a newly-emerging multi-principal-element eutectic category, may offer wider in situ composite possibilities. Here, we use an AlCoCrFeNi2.1 EHEA to engineer an ultrafine-grained duplex microstructure that deliberately inherits its composite lamellar nature by tailored thermo-mechanical processing to achieve property combinations which are not accessible to previously-reported reinforcement methodologies. The as-prepared samples exhibit hierarchically-structural heterogeneity due to phase decomposition, and the improved mechanical response during deformation is attributed to both a two-hierarchical constraint effect and a self-generated microcrack-arresting mechanism. This work provides a pathway for strengthening eutectic alloys and widens the design toolbox for high-performance materials based upon EHEAs.
Change history: In this Letter, owing to a production error, all the data points (except the two points for O-2 and N-2, respectively) were missing in Fig. 1b. The figure has been corrected online.
Oxygen, one of the most abundant elements on Earth, often forms an undesired interstitial impurity or ceramic phase (such as an oxide particle) in metallic materials. Even when it adds strength, oxygen doping renders metals brittle1-3. Here we show that oxygen can take the form of ordered oxygen complexes, a state in between oxide particles and frequently occurring random interstitials. Unlike traditional interstitial strengthening4,5, such ordered interstitial complexes lead to unprecedented enhancement in both strength and ductility in compositionally complex solid solutions, the so-called high-entropy alloys (HEAs)6-10. The tensile strength is enhanced (by 48.5 ± 1.8 per cent) and ductility is substantially improved (by 95.2 ± 8.1 per cent) when doping a model TiZrHfNb HEA with 2.0 atomic per cent oxygen, thus breaking the long-standing strength-ductility trade-off11. The oxygen complexes are ordered nanoscale regions within the HEA characterized by (O, Zr, Ti)-rich atomic complexes whose formation is promoted by the existence of chemical short-range ordering among some of the substitutional matrix elements in the HEAs. Carbon has been reported to improve strength and ductility simultaneously in face-centred cubic HEAs12, by lowering the stacking fault energy and increasing the lattice friction stress. By contrast, the ordered interstitial complexes described here change the dislocation shear mode from planar slip to wavy slip, and promote double cross-slip and thus dislocation multiplication through the formation of Frank-Read sources (a mechanism explaining the generation of multiple dislocations) during deformation. This ordered interstitial complex-mediated strain-hardening mechanism should be particularly useful in Ti-, Zr- and Hf-containing alloys, in which interstitial elements are highly undesirable owing to their embrittlement effects, and in alloys where tuning the stacking fault energy and exploiting athermal transformations13 do not lead to property enhancement. These results provide insight into the role of interstitial solid solutions and associated ordering strengthening mechanisms in metallic materials.
Significance
Back stress hardening is usually not obvious in single-phase homogeneous grains, but can be made unusually large and sustained to large tensile strains by creating an unusually heterogeneous grain structure in single-phase alloys with low stacking fault energy (SFE), as demonstrated here for the face-centered cubic CrCoNi medium-entropy alloy. The low SFE facilitates the generation of twinned nanograins and stacking faults during tensile straining, dynamically reinforcing the heterogeneity. Large uniform tensile strain can be achieved after yielding even at gigapascal stress, in the absence of heterogeneities from any second phase.
A novel high nitrogen medium-entropy alloy CrCoNiN, which had higher strength and slightly lower ductility than CrCoNi alloy, was successfully manufactured by pressurized metallurgy. The microstructure and corrosion behaviour were investigated by microscopic, electrochemical and spectroscopic methods. The results indicated that nitrogen existed in the form of Cr2N precipitates and uniformly distributed N atoms, and nitrogen alloying significantly refined the grain size. Besides, nitrogen enriched on the outmost surface of passive film and metal/film interface as ammonia (NH3 and NH+ 4) and CrN, respectively. The significant improvement of corrosion resistance of CrCoNiN was attributed to the lower metastable pitting susceptibility together with thicker, less defective and more compact passive film.
Single-phase CrCoNi medium-entropy alloys (MEA) are emerging recently as an interesting class of metallic materials, but the dynamic response of this MEA at high strain rates remains unknown. Here we have produced this MEA with various heterogeneous microstructures, using cold rolling followed by annealing at various temperatures. The high-strain-rate response of the MEAs was characterized using hat-shaped specimens in Hopkinson-bar experiments. A combination of high dynamic shear yield strength and large uniform dynamic shear strain was observed, exceeding all other metals and alloys reported so far. Even better dynamic shear properties was revealed when the experiments were conducted at cryogenic temperature. The strong strain hardening under dynamic shear loading can be attributed to the dynamic grain refinement and deformation twinning that accompany the homogeneous shear deformation. When compared to room temperature, the efficiency of grain refinement was found to be enhanced at cryogenic temperature, with a higher density of multiple twins, stacking faults, Lomer-Cottrell locks, and hcp phase via phase transformation inside the grains, which could be responsible for the better dynamic shear properties under cryogenic environment.
Combining high strength and good ductility is highly-desired yet challenging for conventional structural materials. Newly emerging concentrated multi-component alloys with face-centered-cubic structure provide an ultra-ductile matrix, and the precipitation hardening based on these alloys provides a very effective way to achieve a superior strength-ductility combination. Here, we report a high-strength CoCrNi-based medium-entropy alloy hardened by nanoscale L12-(Ni, Co, Cr)3(Ti, Al)-type particles with a heterogeneous precipitation behavior. Compared to the single-phase CoCrNi medium-entropy alloy, the yield strength and the tensile strength of the precipitation-strengthened CoCrNi medium-entropy alloy were increased by ∼70%–∼750 MPa and ∼44% to ∼1.3 GPa, respectively, whereas a good ductility, ∼45%, was still achieved. The underlying deformation micro-mechanisms were systematically investigated using transmission electron microscope. In the single-phase CoCrNi medium-entropy alloy, the deformation mode was dominated by mechanical twinning. In the precipitation-hardened medium-entropy alloy, however, a high density of stacking faults prevailed. We revealed that the absence of mechanical twinning in this low stacking fault energy precipitation-strengthened medium-entropy alloy could relate to the increasing critical twinning stress affected by the channel width of the matrix. We further calculated that the increment of the yield strength was substantially from precipitation strengthening. Our present findings provide not only a fundamental understanding of the deformation micro-mechanism of the precipitation-strengthened CoCrNi medium-entropy alloy but also a useful guidance for the development of precipitation-hardened concentrated multi-component alloys in the future.
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 199 nm to 111 μm 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.
Combinations of high strength and ductility are hard to attain in metals. Exceptions include materials exhibiting twinning-induced plasticity. To understand how the strength-ductility trade-off can be defeated, we apply in situ, and aberration-corrected scanning, transmission electron microscopy to examine deformation mechanisms in the medium-entropy alloy CrCoNi that exhibits one of the highest combinations of strength, ductility and toughness on record. Ab initio modelling suggests that it has negative stacking-fault energy at 0K and high propensity for twinning. With deformation we find that a three-dimensional (3D) hierarchical twin network forms from the activation of three twinning systems. This serves a dual function: conventional twin-boundary (TB) strengthening from blockage of dislocations impinging on TBs, coupled with the 3D twin network which offers pathways for dislocation glide along, and cross-slip between, intersecting TB-matrix interfaces. The stable twin architecture is not disrupted by interfacial dislocation glide, serving as a continuous source of strength, ductility and toughness.
CoCrFeNi alloy is an exemplary stable base for high entropy alloys (HEAs), but its phase stability is still suspicious. Here, the CoCrFeNi HEA was firstly identified to be thermally metastable at 750 °C. Composition decomposition occurred after annealed at 750 °C for 800 h. The minor addition of Al accelerated the composition decomposition and a second fcc phase with a different lattice constant occurred in the long time annealed CoCrFeNiAl0.1 HEA. The CoCrFeNi HEA cannot be seen as stable alloys anymore and researchers should be very careful when using the current phase selection models to predict stability of HEAs at intermediate temperatures.
Refining a metal’s grain size can result in dramatic increases in strength, and the magnitude of this strengthening increment can be estimated using the Hall–Petch equation. Since the Hall–Petch equation was proposed, there have been many experimental studies supporting its applicability to pure metals, intermetallics and multi-phase alloys. In this article, we gather the grain-size strengthening data from the Hall–Petch studies on pure metals and use this aggregated data to calculate best estimates of these metals’ Hall–Petch parameters. We also use this aggregated data to re-evaluate the various models developed to physically support the Hall–Petch scaling. © 2016 Institute of Materials, Minerals and Mining and ASM International
The identification of orientation relationships (ORs) plays a crucial role in the understanding of solid phase transformations. In steels, the most common models of ORs are the ones by Nishiyama–Wassermann (NW) and Kurdjumov–Sachs (KS). The defining feature of these and other OR models is the matching of directions and planes in the parent face-centred cubic γ phase to ones in the product body-centred cubic/tetragonal α/α′ phase. In this article a novel method that identifies transformation strains with ORs is introduced and used to develop a new strain-based approach to phase-transformation models in steels. Using this approach, it is shown that the transformation strains that leave a close-packed plane in the γ phase and a close-packed direction within that plane unrotated are precisely those giving rise to the NW and KS ORs when a cubic product phase is considered. Further, it is outlined how, by choosing different pairs of unrotated planes and directions, other common ORs such as the ones by Pitsch and Greninger–Troiano can be derived. One of the advantages of our approach is that it leads to a natural generalization of the NW, KS and other ORs for different ratios of tetragonality r of the product body-centred tetragonal α′ phase. These generalized ORs predict a sharpening of the transformation textures with increasing tetragonality and are thus in qualitative agreement with experiments on steels with varying alloy concentration.
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-centered cubic (fcc) 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.
An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to 520 HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of 630 HV at about 450 °C before softening set in at higher temperatures. The isother-mal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 HV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5 min at 450 °C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites.
This textbook fits courses on mechanical behavior of materials in mechanical engineering and materials science and includes numerous examples and problems. It emphasizes quantitative problem solving. This text differs from others because the treatment of plasticity emphasizes the interrelationship of the flow, effective strain, and effective stress and their use in conjunction with yield criteria to solve problems. The treatment of defects is new, as is the analysis of particulate composites. Schmid's law is generalized for complex stress states. Its use with strains allows for prediction of R-values for textures. Of note is the treatment of lattice rotations related to deformation textures. The chapter on fracture mechanics includes coverage of Gurney's approach. Among the highlights in this new edition are the treatment of the effects of texture on properties and microstructure in Chapter 7, a new chapter (12) on discontinuous and inhomogeneous deformation, and the treatment of foams in Chapter 21.
Possibilities of tailoring microstructure and mechanical properties of complex intermetallic containing CoCrFeNi2.1Nbx (x = 0.2, 0.4) high entropy alloys (HEAs) using thermo-mechanical processing were investigated in this work. For this purpose, the homogenized HEAs containing complex Laves phases distributed in the FCC matrix were successfully severely cold-rolled up to 90% reduction in thickness. The cold-rolled HEAs showed a nano lamellar microstructure with fragmented Laves phase particles arranged preferentially along the rolling direction. The microstructure of the Nb0.4 HEA revealed intense deformation zones surrounding the hard Laves phase particles, owing to their higher volume fraction and larger size as compared to those in the Nb0.2 HEA. Upon annealing at 800 °C, the lamellar deformed FCC phase in the two HEAs was transformed into an equiaxed nano-grained structure having fine dispersion of nano precipitates. However, the Nb0.4 showed a decidedly lower recrystallized grain size than the Nb0.2, resulting from the smaller deformed grain size and particle stimulated nucleation. The annealed Nb0.2 showed remarkable strength-ductility combination featured by tensile strength ~1080 MPa and ductility ~21%, originating from the combined effect of a high volume of ductile FCC matrix, remarkable grain refinement, and precipitation hardening. In stark contrast, the annealed Nb0.4 showed complete absence of tensile elongation, likely due to a low volume fraction of the ductile FCC matrix and a high volume fraction of the intermetallic phases. The present study emphasized the critical role of microstructural tailoring for achieving outstanding strength-ductility synergy in complex intermetallic containing CoCrFeNi2.1Nbx HEAs.
The effect of minor Mo doping on the microstructure and mechanical properties of a (CoCrNi)95Mo5 medium-entropy alloy (MEA) was studied by selective laser melting (SLM) to architect the dislocation-formed sub-grains, ultrafine (Mo, Cr)-enriched μ precipitates and various lattice defects. High strength and ductility were achieved in the as-SLMed (CoCrNi)95Mo5 alloy, where the yield strength, ultimate tensile strength (UTS) and fracture strain were 0.79 GPa, 0.97 GPa and 35.9%, respectively. The superior mechanical properties were associated with the typical hierarchical microstructure that were featured by the high-density dislocations-formed equiaxed sub-grains, the columnar sub-grains separated by low-angle grain boundaries (LAGBs), the fine (Mo, Cr)-enriched μ precipitates and the lattice defects including dislocations, stacking faults (SFs) and Lomer-Cottrell locks (LCs) in the matrix. The results have demonstrated the potential of applying precipitation strengthening in CoCrNi-based MEAs through additive manufacturing to achieve superior mechanical properties.
High-/medium-entropy alloys (H/MEAs) with multiple principal elements and promising performance have opened up an unprecedented multi-dimensional compositional space for exploring novel materials. Here, we report a novel B2-strengthening Ni–Co–Cr–Al medium-entropy alloy system with a better combination of strength and ductility. Through simple thermomechanical processing, gigapascal yield strength (∼1 GPa) and ultimate strength (∼1.5 GPa) combined with excellent uniform elongation (∼22%) were achieved. This excellent strength-ductility synergy resulted from the partial recrystallization combined with considerable fine B2 precipitates, and annealing twins in this dual-phase Al13 MEA. This work further broadens material design and optimization space and offers a paradigm for developing heterogeneous microstructures in dual-phase MEAs through alloying and regulating recrystallization and precipitation behaviors.
Based on the results of many studies devoted to exploring the effect of adding different elements to CoCrFeNi alloys in the field of high-entropy alloys, in this study, Ge is included in CoCrFeNi alloys to form CoCrFeNiGex alloys (molar ratio, x = 0, 0.1, 0.2, and 0.3). Ge is used to improve mechanical properties and corrosion resistance of traditional alloys, but its application in high-entropy alloys has been less discussed. CoCrFeNiGex alloys (x ≤ 0.3) exhibit a single face-centered cubic phase. Tensile testing of these alloys revealed that the CoCrFeNiGe0.3 alloy exhibited slightly better mechanical properties—a yield strength of 223 MPa, ultimate tensile strength of 617 MPa, and elongation of 63.2%—due to solid-solution hardening. A potentiodynamic polarization test in deaerated 3.5% NaCl solution showed that Ge addition can effectively inhibit pitting corrosion and can improve the corrosion resistance of CoCrFeNi alloys by reducing the corrosion and passive current density.
In this study, novel CrCoNi–Al2O3 composites with 2.5–7.5 wt% Al2O3 were prepared by mechanical alloying (MA) plus rapid spark plasma sintering (SPS) method, and their microstructure and mechanical properties were systematically investigated. The results demonstrate that the Al2O3 nanoparticles are distributed homogeneously in the CrCoNi medium entropy alloy (MEA) matrix with ultra-fine grain size (≤0.37 μm). Moreover, the composites have clean, regular and well bonded matrix/α-Al2O3 interfaces. Interestingly, a large number of nano twin bundles, large-scale 9R phase and geometrically necessary dislocations (GNDs) were formed near the matrix/α-Al2O3 interfaces. The Vickers hardnesses of the 2.5–7.5 wt% Al2O3 composites reach 521–603 HV0.3, and the compressive yield strength is 1877–2359 MPa with fracture strains of 9.3–31.6%. The yield strength of the composites is by 65.4–107.8% higher than that of the pure CrCoNi matrix, meanwhile they still have considerable ductility, which indicates composite strengthening is a feasible method to improve mechanical properties of high/medium entropy alloys. The composite includes multiply strengthening mechanisms, namely grain refinement strengthening, twin strengthening, Orowan strengthening and dislocation strengthening. The contributions of various strengthening mechanisms were quantitatively analyzed, which were well consistent with the experimental values.
Traditional metallic alloys are mixtures of elements in which the atoms of minority species tend to be distributed randomly if they are below their solubility limit, or to form secondary phases if they are above it. The concept of multiple-principal-element alloys has recently expanded this view, as these materials are single-phase solid solutions of generally equiatomic mixtures of metallic elements. This group of materials has received much interest owing to their enhanced mechanical properties 1-5. They are usually called medium-entropy alloys in ternary systems and high-entropy alloys in quaternary or quinary systems, alluding to their high degree of configurational entropy. However, the question has remained as to how random these solid solutions actually are, with the influence of short-range order being suggested in computational simulations but not seen experimentally 6,7. Here we report the observation, using energy-filtered transmission electron microscopy, of structural features attributable to short-range order in the CrCoNi medium-entropy alloy. Increasing amounts of such order give rise to both higher stacking-fault energy and hardness. These findings suggest that the degree of local ordering at the nanometre scale can be tailored through thermomechanical processing, providing a new avenue for tuning the mechanical properties of medium-and high-entropy alloys. Among the increasing number of medium-to high-entropy alloy systems reported in the literature 8-12 , the CrCoNi-based, face-centred-cubic (fcc) single-phase alloys exhibit an exceptional combination of mechanical properties, including high strength, tensile ductility, fracture toughness and impact resistance 13. Extensive studies have documented the deformation mechanisms in these alloys. Gludovatz et al. reported the outstanding fracture toughness of CrCoNi at cryogenic temperatures 14 , and attributed this to a synergy of deformation mechanisms, including a propensity for mechanical twinning 15. Interestingly, computational work has suggested that the CrCoNi-based fcc single-phase alloys should have near-zero or negative stacking-fault energies (SFEs; γ SF) 15-19. However, these computational predictions do not agree with measured values 20,21 (γ SF_CrCoNi ≈ 22 mJ m −2 and γ SF_CrMnFeCoNi ≈ 30 mJ m −2). Experimentally, the measured SFEs in medium-entropy alloys (MEAs) and high-entropy alloys (HEAs) exhibit a wide distribution 22 , indicating a strong dependence of γ SF on local atomic configuration. Ding et al. 6 showed that the SFE of CrCoNi MEA can be tailored over a wide range by tuning its local chemical order. The work highlights the potentially strong impact of chemical short-range order (SRO) on the mechanical properties of the MEA/HEAs. Later, Li et al. 7 , using molecular dynamics simulations, demonstrated the ruggedness of the local energy landscape and how it raises activation barriers governing dislocation activities. Experimental evidence for the existence of such SRO has so far been limited to X-ray adsorption measurements 23 that are averaged over a relatively large volume of material. Indeed, further efforts are needed to characterize the degree and the spatial extent of the ordering , as well as how both would be affected by thermal history and any associated effects on mechanical behaviour. Here we provide quantitative visualization of the SRO structure, by which we establish a direct effect of this SRO on the mechanical behaviour of MEA/HEA materials. To investigate the presence of chemical SRO, samples of equiatomic CrCoNi alloys were subjected to different thermal treatments after homogenization at 1,200 °C: (1) water-quenched to room temperature to suppress SRO formation; or (2) aged at 1,000 °C for 120 h followed by slow furnace cooling to promote SRO formation. The microstructure and the degree of SRO were characterized with a variety of transmission electron microscope (TEM) imaging techniques. Diffraction contrast from SRO is inherently faint as compared to the fcc matrix lattice dif-fraction signal because the former arises from relatively minor differences in lattice distortion. As a result, measurement of the faint SRO diffraction signal has proven to be challenging. In order to enhance the signal-to-noise ratio of the diffraction contrast from SRO, we minimized the background noise from inelastic scattering by using a Zeiss TEM (LIBRA 200MC) equipped with an in-column Ω energy filter and a camera with 16-bit dynamic range. Energy-filtered diffraction patterns and dark-field images for the two heat treatment conditions are shown in Fig. 1. In the diffraction patterns (Fig. 1a, b), streaks along {111} directions between fcc Bragg spots are clearly observed in the aged sample. Dark-field imaging taken with the objective aperture positioned in the centre of the streaked region shown in Fig. 1b was used to image the https://doi.
A single-phase face-centered cubic (fcc) CoCrNi medium-entropy alloy (MEA) with different boron-doping contents (30–1600 ppm) was fabricated by vacuum arc melting, then followed by cold rolling. It was found that the boron-doping in the CoCrNi MEA results in an obvious partial recrystallization and significant refinement of grain size. The tensile yield strength (YS) and ultimate tensile strength (UTS) are 0.94 GPa and 1.17 GPa with the ductility of 26% in 800 ppm boron doping alloy, which was attributed to the strengthening effect of the large unrecrystallized grains. The tensile YS and UTS is dramatically increased up to 1.35 GPa and 1.48 GPa with the ductility of 11% for in 1600 ppm boron doping alloy. In this study, our results demonstrate that the uneven structure can be accomplished effectively by a proper boron addition and the enhancement of mechanical properties is mainly attributed to the unrecrystallized grains microstructure and grain refinement resulting from microalloying with boron by in situ high-energy X-ray diffraction (HE-XRD) technique. This offers a new perspective in interpreting the heterogeneity of structural materials and its influence on mechanical behaviors.
The influences of nitrogen alloying on the microstructural evolution and tensile properties of CoCrFeMnNi high-entropy alloys (HEAs), which were subjected to cold rolling and subsequent annealing at 773–1173 K, were systematically investigated. The results show cold rolling-induced microbands in the nitrogen-alloyed HEAs instead of the deformation twins and shear bands found in nitrogen-free HEAs. During annealing at 773–873 K, the cold-rolled nitrogen-free HEAs experience a partial annihilation of the deformation twins, while the cold-rolled nitrogen-alloyed HEAs exhibited higher microstructural stability. When the annealing temperature exceeds 973 K, a large number of Cr2N precipitates form in the recrystallized regions of the nitrogen-alloyed HEAs, and the tensile strength of the cold-rolled nitrogen-free HEAs decreases with increasing annealing temperature. However, the tensile strength of the nitrogen-alloyed HEAs annealed at 773 and 873 K experiences an abnormal increase compared to that of the cold-rolled sample. The best combination of strength and ductility are achieved in the nitrogen-alloyed HEAs treated by cold-rolling and annealing at 973 K. When the annealing temperature increases to 1073–1173 K, the nitrogen-alloyed and -free HEAs exhibit completely recrystallized structures with relatively low strength and excellent ductility. The influences of nitrogen alloying on the microstructural evolution are related to the interaction between nitrogen atoms and dislocations, and the multi-mechanism (including nitrogen solid-solution strengthening, precipitation strengthening, and grain refinement strengthening) accounts for the improved tensile properties of the nitrogen-alloyed HEAs treated by cold-rolling and subsequent annealing at 973 K.
The FCC-structured high entropy alloys (HEAs) possess exceptional ductility and fracture toughness, but they generally exhibit insufficient strength for engineering applications. In this work, a precipitation strengthening non-equimolar (FeCoNi)81Cr9Al8Ti1Nb1 HEA was fabricated via arc melting and subsequent two-step aging treatment. The microstructure, phase constitution and mechanical properties of this alloy during aging were investigated systematically. The results indicate that coherent γ′ and incoherent B2 are major precipitates for the alloy under the two-step aging. An excellent balanced tensile property is achieved at room temperature even with extensive B2 grain boundary coverage. Quantitative calculations of the individual strengthening effects demonstrate that particle (γ′) shearing mechanism is the predominant strengthening mechanism. The high work-hardening capability of the FCC matrix could greatly suppress the propagation of microcracks originated at these brittle B2 phases and promote a retention of ductility. In addition, this alloy exhibits outstanding high-temperature tensile properties up to 700 °C. It is attributed to the high thermal stability of the γ′ precipitates as well as the pinning effect of the grain-boundary B2 phases on the grain boundary. Present work will focus on optimizing of the alloy design of HEAs and the precipitation strengthening of HEAs for high-temperature structural applications.
Compositional inhomogeneity of the Mo element in a (CoCrNi)93Mo7 alloy is utilized to obtain a heterogeneous structure with the banded precipitation, which improves the strength and ductility synchronously compared with the conventional structure with σ phase precipitates formed at grain boundaries. The interactions between dislocations and fine precipitates, twinning induced plasticity and inhomogeneous plastic deformation are beneficial to the high capability of work hardening and strength-ductility synergy. This work demonstrates the positive effect of compositional inhomogeneity to develop advanced high entropy alloys with heterogeneous structure via composition design and process optimization.
The present work studied the coherent precipitation and strengthening in a dual-phase AlNi2Co2Fe1.5Cr1.5 high-entropy alloy (HEA) systematically. It was constituted of a primary FCC phase and a small amount of BCC/B2 phase in the inter-dendrites at the as-cast state. After a series of thermo-mechanical processing, including cold-rolling, homogenizing at 1573 K for 2 h followed by water-quenching, and aging at 923 K for 4 h followed by water-quenching, nano-sized ellipsoidal ordered L12 particles with a diameter of 8 nm are formed in the FCC dendrites, and the BCC particle size in B2 inter-dendrites reduces to about 10 nm. Both the tensile strength and microhardness of aged HEA will increase with the aging time first, and then reach a maximum, and decrease finally, which is attributed to the coarsening of these coherent nanoprecipitates. The ultimate tensile strength of the aged HEA can be enhanced up to 1240 MPa, twice as high as that of the homogenized state (637 MPa). The variation of strengths at different states is discussed via the precipitation strengthening mechanisms, which is dominated by the size of L12 and BCC nanoparticles. This work will provide a new approach to enhance the strength and ductility in dual-phase alloys via coherent precipitation, i.e., coherent nanoparticles are precipitated on their respective parent phase matrix simultaneously.
Ultrafine grained dual phase (UFG DP) steels were processed using a thermomechanical treatment designed to obtain UFG ferrite/nano carbide aggregates prior to intercritical annealing. The influences of intercritical annealing temperature on the microstructures and deformation behaviors of DP steels were investigated. The grain average misorientation maps confirmed the prescience of higher amount of strains in martensite phases. In addition, it was shown that the geometrically necessary dislocations which mainly formed at ferrite/martensite interfaces are responsible for continuous yielding of DP structures. The results showed around 3 times improvement in strength-elongation balance (about 33286 MPa% for the UFG DP, compared with the 11501 MPa% for the ferritic-pearlitic structure). The Hollomon, differential Crussard-Jaoul (DC-J) and modified C-J (MC-J) analyses were used to evaluate the deformation behavior of DP steels. The Hollomon and C-J analyses proposed two and three-stage deformation for the present DP steels, respectively, while the modified C-J analysis illustrated four-stage deformation. A relatively high strain hardening rate was observed for all the DP steels in the initial stages of deformation. The initial strain hardening rate was enhanced as a result of decreasing the ferrite grain size and increasing the martensite volume fraction at lower intercritical annealing temperatures and then it began to decrease with further increasing the intercritical annealing temperature.
Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multidimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behavior has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking.
Effect of annealing of a cold-worked CoCrFeMnNi alloy at temperatures of 500–900 °C for 1–50 h on the structure and mechanical properties was studied in the present work. Annealing for an hour resulted in: i) recrystallization of the face-centered cubic (fcc) matrix at 600–900 °C; ii) precipitation of a Cr-rich body-centered cubic (bcc) phase at 500–700 °C or a sigma phase particles at 600–800 °C. Moreover, an increase in the annealing time to 50 h at 600 °C resulted in a continuous growth of both the fcc grans and bcc/sigma particles and in an increase in the fraction of the sigma phase at the expense of the bcc phase particles. The fcc grains growth was found to be controlled by the pinning effect of the second phase particles. Soaking for an hour at 500–600 °C resulted in a substantial increase in strength of the alloy due to the second phases precipitation. Meanwhile annealing at the higher temperatures as well as an increase in the annealing time at 600 °C resulted in softening; however, even after 50 h annealing, the alloy demonstrated reasonably high strength. In the latter case fine fcc grains, preserved due to the pinning effect by the second phases particles, contributed to strength mainly.
Despite having otherwise outstanding mechanical properties, many single-phase medium and high entropy alloys are limited by modest yield strengths. Although grain refinement offers one opportunity for additional strengthening, it requires significant and undesirable compromises to ductility. This work therefore explores an alternative, simple processing route to achieve strength by cold-rolling and annealing an equiatomic CrCoNi alloy to produce heterogeneous, partially recrystallized microstructures. Tensile tests reveal that our approach dramatically increases the yield strength (to ∼1100 MPa) while retaining good ductility (total elongation ∼23%) in the single-phase CrCoNi alloy. Scanning and transmission electron microscopy indicate that the strengthening is due to the non-recrystallized grains retaining their deformation-induced twins and very high dislocation densities. Load-unload-reload tests and grain-scale digital image correlation are also used to study the accumulation of plastic deformation in our highly heterogeneous microstructures.
A novel high nitrogen medium-entropy alloy CrCoNiN, which had higher strength and slightly lower ductility than CrCoNi alloy, was successfully manufactured by pressurized metallurgy. The microstructure and corrosion behaviour were investigated by microscopic, electrochemical and spectroscopic methods. The results indicated that nitrogen existed in the form of Cr2N precipitates and uniformly distributed N atoms, and nitrogen alloying significantly refined the grain size. Besides, nitrogen enriched on the outmost surface of passive film and metal/film interface as ammonia (NH3 and NH+ 4) and CrN, respectively. The significant improvement of corrosion resistance of CrCoNiN was attributed to the lower metastable pitting susceptibility together with thicker, less defective and more compact passive film.
Thermal stability of CoCrFeNi high entropy alloy in as-milled and sintered conditions was investigated using X-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and atom probe tomography. Composite microstructure consists of FCC and carbide with a fine dispersion of oxide was observed in the sintered condition. Unsolicited contamination of carbon and oxygen in the as-milled powder due to the milling medium had led to the formation of composite microstructure. An exceptional thermal stability was observed upon exposure of sintered compact to higher temperatures (0.56 Tm to 0.68 Tm) for the prolonged duration of 600 h. Sintered compact exposed to 700 °C (0.56 Tm) for 600 h showed negligible change in hardness and grain size. Analysis based on the modified Hall-Petch model for two phase alloy indicates the phase boundaries act as a strong obstacle while the major contribution to strengthening comes from grain boundaries.
The excellent cryogenic tensile properties of the CrMnFeCoNi alloy are generally caused
by deformation twinning, which is difficult to achieve at room temperature because of
insufficient stress for twinning. Here, we induced twinning at room temperature to improve
the cryogenic tensile properties of the CrMnFeCoNi alloy. Considering grain size effects on
the critical stress for twinning, twins were readily formed in the coarse microstructure by
cold rolling without grain refinement by hot rolling. These twins were retained by partial
recrystallization and played an important role in improving strength, allowing yield strengths
approaching 1GPa. The persistent elongation up to 46% as well as the tensile strength of
1.3 GPa are attributed to additional twinning in both recrystallized and non-recrystallization
regions. Our results demonstrate that non-recrystallized grains, which are generally avoided
in conventional alloys because of their deleterious effect on ductility, can be useful in
achieving high-strength high-entropy alloys.
We present a systematic microstructure oriented mechanical property investigation for a newly developed class of transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) with varying grain sizes and phase fractions. The DP-HEAs in both, as-homogenized and recrystallized states consist of a face-centered cubic (FCC) matrix containing a high-density of stacking faults and a laminate hexagonal close-packed (HCP) phase. No elemental segregation was observed in grain interiors or at interfaces even down to near-atomic resolution, as confirmed by energy-dispersive X-ray spec-troscopy and atom probe tomography. The strength-ductility combinations of the recrystallized DP-HEAs (Fe 50 Mn 30 Co 10 Cr 10) with varying FCC grain sizes and HCP phase fractions prior to deformation are superior to those of the recrystallized equiatomic single-phase Cantor reference HEA (Fe 20 Mn 20 Ni 20-Co 20 Cr 20). The multiple deformation micro-mechanisms (including strain-induced transformation from FCC to HCP phase) and dynamic strain partitioning behavior among the two phases are revealed in detail. Both, strength and ductility of the DP-HEAs increase with decreasing the average FCC matrix grain size and increasing the HCP phase fraction prior to loading (in the range of 10e35%) due to the resulting enhanced stability of the FCC matrix. These insights are used to project some future directions for designing advanced TRIP-HEAs through the adjustment of the matrix phase's stability by alloy tuning and grain size effects.
Strain hardening behavior and microstructural evolution of non-grain oriented electrical, dual phase, and AISI 304 steels, subjected to uniaxial tensile tests, were investigated in this study. Tensile tests were performed at room temperature and the strain hardening behavior of the steels was characterized by three different parameters: modified Crussard–Jaoul stages, strain hardening rate and instantaneous strain hardening exponent. Optical microscopic analysis, X-ray diffraction measurements, phase quantification by Rietveld refinement and hardness tests were also carried out in order to correlate the microstructural and mechanical responses to plastic deformation. Distinct strain hardening stages were observed in the steels in terms of the instantaneous strain hardening exponent and the strain hardening rate. The dual phase and non-grain oriented steels exhibited a two-stage strain hardening behavior while the AISI 304 steel displayed multiple stages, resulting in a more complex strain hardening behavior. The dual phase steels showed a high work hardening capacity in stage 1, which was gradually reduced in stage 2. On the other hand, the AISI 304 steel showed high strain hardening capacity, which continued to increase up to the tensile strength. This is a consequence of its additional strain hardening mechanism, based on a strain-induced martensitic transformation, as shown by the X-ray diffraction and optical microscopic analyses.
Metals have been mankind’s most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength–ductility trade-off. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys. In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.
Compared to decades-old theories of strengthening in dilute solid solutions, the mechanical behavior of concentrated solid solutions is relatively poorly understood. A special subset of these materials includes alloys in which the constituent elements are present in equal atomic proportions, including the high-entropy alloys of recent interest. A unique characteristic of equiatomic alloys is the absence of “solvent” and “solute” atoms, resulting in a breakdown of the textbook picture of dislocations moving through a solvent lattice and encountering discrete solute obstacles. To clarify the mechanical behavior of this interesting new class of materials, we investigate here a family of equiatomic binary, ternary and quaternary alloys based on the elements Fe, Ni, Co, Cr and Mn that were previously shown to be single-phase face-centered cubic (fcc) solid solutions. The alloys were arc-melted, drop-cast, homogenized, cold-rolled and recrystallized to produce equiaxed microstructures with comparable grain sizes. Tensile tests were performed at an engineering strain rate of 10−3 s−1 at temperatures in the range 77–673 K. Unalloyed fcc Ni was processed similarly and tested for comparison. The flow stresses depend to varying degrees on temperature, with some (e.g. NiCoCr, NiCoCrMn and FeNiCoCr) exhibiting yield and ultimate strengths that increase strongly with decreasing temperature, while others (e.g. NiCo and Ni) exhibit very weak temperature dependencies. To better understand this behavior, the temperature dependencies of the yield strength and strain hardening were analyzed separately. Lattice friction appears to be the predominant component of the temperature-dependent yield stress, possibly because the Peierls barrier height decreases with increasing temperature due to a thermally induced increase of dislocation width. In the early stages of plastic flow (5–13% strain, depending on material), the temperature dependence of strain hardening is due mainly to the temperature dependence of the shear modulus. In all the equiatomic alloys, ductility and strength increase with decreasing temperature down to 77 K.