Dual-phase Structure Research Articles

Achieving excellent strength-ductility combination in Co60.8-xNi39.2Alx eutectic medium-entropy alloys

In this study, we design and prepare three eutectic medium-entropy alloys (EMEAs) with the composition of Co60.8-xNi39.2Alx (x = 19, 21.6 and 24). All three alloys exhibit a FCC-B2 dual-phase structure, which evolves from hypoeutectic to eutectic and then to hypereutectic with increasing Al content. The Co39.2Ni39.2Al21.6 EMEA demonstrates a complete lamellar eutectic structure with fine and disperse ordered L12 precipitations in the FCC phase, while the nano-scale strip L10 martensite is present in the B2 phase. The Co39.2Ni39.2Al21.6 EMEA exhibits an exceptional combination of strength and ductility, with high yield strength and elongation that are superior to other FCC-B2 dual-phase EMEA reported in the past five years. The superior ductility was related to the stable K–S interface orientation during the deformation process and the transformation-induced plasticity effect in B2 phases. The high strength is derived from the interface strengthening effect of the FCC-B2 interfaces. Moreover, the hindering of dislocation slipping by the ordered L12 precipitates and the Lomer-Cottrell locks in the FCC lamellar can also enhance the strength of the alloy. These findings have important implications for the structural design of eutectic high/medium-entropy alloys.

Microstructure and oxidation of NiCr alloys studied by analytical in situ environmental TEM

The microstructure and oxidation properties from bulk material to atomic scale of NiCr alloys with different Cr contents from 10 to 50 at% were systematically studied. The oxidation mechanism in NiCr alloys with different Cr contents was analysed by in-situ environmental transmission electron microscopy. The increased porosity of the NiO/NiCr2O4 oxide layer in the 10 at% Cr-containing alloy seriously decreases its oxidation resistance, and Cr contents between 20 and 40 at% offer outstanding oxidation resistance due to the inhibited growth of NiO/NiCr2O4 oxides. Ni-50 at%Cr alloy exhibit poor corrosion resistance due to short-circuit oxidation paths by its FCC/BCC dual-phase structure.

Thermophysical and mechanical properties of dual-phase medium- and high-entropy rare-earth zirconate ceramics

The medium-entropy ceramic (La1/3Nd1/3Gd1/3)2(Zr3/4Ce1/4)2O7 and high-entropy ceramic (La1/5Nd1/5Sm1/5Eu1/5Gd1/5)2(Zr3/4Ce1/4)2O7 were successfully synthesized. The two ceramics had a pyrochlore-fluorite dual-phase structure, and they exhibited excellent phase stability at elevated temperature. Grain refinement or solution strengthening improved the Vickers hardness, fracture toughness and elastic modulus of the dual-phase ceramics. Compared with the single-element ceramic La2Zr2O7, multicomponent doping resulted in a significant increase in the thermal expansion coefficient. Simultaneously, the thermal conductivity of the dual-phase ceramics was lower than that of single-phase ones. In addition, on account of the large size disorder parameter of the medium-entropy ceramic, the thermal conductivity could outperform its high-entropy counterpart. These results indicate that pyrochlore-fluorite dual-phase rare-earth zirconates have enormous potential in applications concerning next generation thermal barrier coatings.

Microstructure, mechanical and corrosion performance of magnetron sputtered (Al0.5CoCrFeNi)Nx high-entropy alloy nitride films

High-entropy alloy nitride (HEAN) films of (Al0.5CoCrFeNi)Nx were deposited on 304 steel and silicon plates using reactive DC magnetron sputtering. The effect of nitrogen flow fraction (RN =0%, 6%, 13%, 23%, 40%) on the microstructure and performance of HEAN films was studied. The deposition rate showed a negative relationship with the nitrogen flow ratio. The N concentration raised with the increasing RN ratio, reaching a maximum value of 33 at% (RN =40%). The as-deposited Al0.5CoCrFeNi film presented an FCC + BCC dual phase structure. With the addition of nitrogen, the HEAN film transformed FCC + BCC structure to a composite structure composed of FCC nanocrystals and amorphous. All the HEAN films grew the smooth surface with nano-clusters. With increasing RN ratio, the average size of clusters decreased from 203 nm to 83 nm, resulting in a denser film. The HEAN films exhibited enhanced hardness, retaining relatively good plasticity. The hardness and elastic modulus reached a maximum value of 14.42 GPa and 212 GPa at RN = 23%, respectively. The HEAN films showed good adhesion with substrate and had no obvious cracks during scratching. Under 3.5 wt% NaCl solution, the increasing RN ratio contributed to the higher Ecorr and lower icorr for HEAN films, with excellent corrosion resistance (Ecorr=0.054 V, icorr=0.72 μA/cm2) at RN = 23%. The incorporation of N showed promising potential in improving the performance of AlCoCrFeNi alloy.

Characterization on microstructure evolution of a bulk nanograined TiZrHfNb refractory high-entropy alloy

A bulk TiZrHfNb nanograined refractory high-entropy alloy (NG-RHEA) with an average grain size of 31.6 nm was prepared by inert gas condensation (IGC) method. In the initial as-IGC sample, it is composed of a body-centered cubic structure and an amorphous phase accounting for about 25%. Then the microstructure was regulated by annealing at 200 °C, 400 °C and 600 °C for 0.5 h. The results show that a dual-phase structure composed of body-centered cubic phase nanograins and amorphous phase was maintained below 400 °C. Significant grain growth was observed when annealed at 600 °C, with obvious phase decomposition detected. The hardness of as-IGC TiZrHfNb NG-RHEA increases from 348 HV to 432 HV after annealed at 400 °C. The relationship between microstructure and hardness was discussed. This work provides insights into the preparation and structure modulation of bulk NG-RHEAs.

Selective laser melting of AlCoCrFeMnNi high entropy alloy: Effect of heat treatment

The dilemma concerning the trade-off between the strength and ductility of alloys has persistently been a significant problem in engineering applications. Conventional techniques such as grain refinement, severe plastic deformation, and precipitation strengthening usually increase strength at the expense of ductility. High entropy alloys with dual-phase structures, on the other hand, generally display excellent strength-ductility synergy. In this study, we attempted to mix pre-alloyed powders of two distinct phases (body-centered cubic - bcc and face-centered cubic - fcc) to fabricate AlCoCrFeMnNi HEAs with a dual-phase structure by selective laser melting. Furthermore, we investigate the impact of varying heat treatment temperatures (900 °C, 1000 °C, and 1100 °C) on its phase composition, microstructure, and mechanical properties. The results show that with an increase in the heat treatment temperature, the volume of the fcc phase increases gradually and the volume of the bcc phase decreases. The columnar structure in the fcc phase transforms into a cellular structure, and a spinodal decomposition is observed with the bcc phase. As the heat treatment temperature increases, the yield strength decreases while the elongation substantially increases, with the samples heat-treated at 1100 °C showing the best comprehensive properties (yield strength ∼700 MPa, tensile strength ∼950 MPa, and ∼15% ductility).

Chemistry-structure-property relations in Al10Cr15(Fe3Mn)75−x(Ni)x medium-entropy alloys

The microstructure, phase behavior, mechanical properties, and corrosion properties of a series of Al10Cr15(Fe3Mn)75−x(Ni)x medium-entropy alloys (MEAs) spanning 0–20 at% Ni were studied to elucidate the chemistry-structure-property relationship of this system as a function of Ni content. This work demonstrates that from an initial BCC phase Al10Cr15(Fe3Mn)75 MEA, Ni additions of 5 and 10 at% result in the formation of ordered B2-phase precipitates due to interaction of Ni with Al, resulting in high hardness (∼475 HV). Further Ni addition to 15 at% leads to a dual-phase FCC+BCC structure, with B2 phase precipitates distributed in the BCC matrix relatively rich in Al and Ni but depleted in Cr. This dual-phase structure has a high yield strength (YS) of 600 MPa with a total elongation of 15%. Additionally, the B2 precipitates in BCC phase serve as preferential sites for corrosion in 0.6 M NaCl. Increasing Ni content to 20 at% results in lower YS of 300 MPa, but a significant improvement in ductility and corrosion resistance due to the increased FCC phase fraction.

A cryogenic ultra-strong and ductile steel induced by stable nanoscale dual-phase structure

A cryogenic ultra-strong and ductile steel induced by stable nanoscale dual-phase structure

Macro-micro analysis of mechanical properties of Al0.6CoCrFeNi high-entropy alloy particle-reinforced Al-based composites

In this study, Al0.6CoCrFeNi high-entropy alloy particle-reinforced 5052Al-based composites with good surface morphology and different diffusion layer thicknesses were successfully prepared using hot-press sintering and annealing, which were subsequently characterized by XRD, SEM, EBSD, micropillar compression, and nanoindentation. The EBSD tests indicate that the Al0.6CoCrFeNi high-entropy alloy has a dual-phase crystal structure with the presence of both FCC and BCC phases inside, and the internal grain size is significantly higher than that of the matrix. Subsequently, the mechanical property mapping images of the particle-diffusion layer-matrix region on the composite surface were obtained using the NanoBlitz 3D method in the nanoindentation experiments. And based on the particle and matrix mechanical property parameters obtained by nanoindentation experiments, they were fitted into different mathematical models to obtain the hardness and elastic modulus of the composite. As a result, the correlation was found to be linear, and the slope of the fitted straight line increased gradually with the prolongation of the annealing time. In conclusion, it is evident from the comparison of the results of mathematical model fitting of mechanical property parameters with the results of macroscopic mechanical property tests that the P. G. Klemens model exhibits an excellent fitting effect on the macro-microscopic mechanical properties of particle-reinforced aluminum matrix composites compared with other models such as the rule of mixtures.

Ultrahigh Permeability at High Frequencies via A Magnetic-Heterogeneous Nanocrystallization Mechanism in An Iron-Based Amorphous Alloy.

The prevalence of wide-bandgap (WBG) semiconductors allows modern electronic devices to operate at much higher frequencies. However, development of soft magnetic materials with high-frequency properties matching the WBG-based devices remains challenging. Here we report a promising nanocrystalline-amorphous composite alloy with a normal composition Fe75.5 Co0.5 Mo0.5 Cu1 Nb1.5 Si13 B8 in atomic percent, producible under industrial conditions, which shows an exceptionally high permeability at high frequencies up to 36000 at 100kHz, an increase of 44% compared with commercial FeSiBCuNb nanocrystalline alloy (25000±2000 at 100kHz), outperforming all existing nanocrystalline alloy systems and commercial soft magnetic materials. The alloy is obtained by a unique magnetic-heterogeneous nanocrystallization mechanism in an iron-based amorphous alloy, which is different from traditional strategy of nanocrystallization by doping non-magnetic elements (e.g., Cu and Nb). The induced magnetic inhomogeneity by adding Co atoms promotes locally the formation of highly ordered structures acting as the nuclei of nanocrystals, and Mo atoms agglomerate around the interfaces of nanocrystals, inhibiting nanocrystal growth, resulting in an ultrafine nanocrystalline-amorphous dual-phase structure in the alloy. The exceptional soft magnetic properties are shown to be closely related to the low magnetic anisotropy and the unique spin rotation mechanism under alternating magnetic fields. This article is protected by copyright. All rights reserved.

Hierarchical crystalline–amorphous nanocomposites with high strength and large deformability enabled by elemental diffusion

Amorphous/nanocrystalline dual-phase structures have recently emerged as an effective way for overcoming the strength–ductility trade-off and breaking the limitation of the reverse Hall–Petch effect. Here, we proposed a new strategy to develop a hierarchical and interconnected amorphous–crystalline nanocomposite arising from the nanoscale elemental interdiffusion and oxygen adsorption behavior during thermal treatment processes. The nanocomposite consisted of a three-dimensional (3D) hierarchical network structure where the crystalline phase (Cr–Co–Ni–Al) was embedded into the Al–O-based amorphous phase network with critical feature sizes encompassing three orders of magnitude (from micrometer to nanometer scale). It can achieve ultrahigh compression yield strength of ∼3.6 GPa with large homogeneous deformation of over 50% strain. The massive interstitial atoms induced lattice distortion and hierarchical amorphous phase boundary contributed to the strength improvement. in situ Uniaxial compression inside a transmission electron microscope (TEM) revealed that the exceptional deformability of the nanocomposites resulted from the homogenous plastic flow of nano-sized amorphous phase and the plastic co-deformation behavior restricted by the nano-architected dual-phase interface. The proposed dual-phase synthesis approach can outperform conventional nanolaminates design strategies in terms of the mechanical properties achievable while providing a pathway to easily tune the microstructure of these nanolaminates.

Eutectic growth kinetics as an indicator for glass-forming ability

A metallic glass is known to be often formed more easily by cooling of an alloy melt whose composition is near a deep eutectic. However, the eutectic solidification kinetics, which is closely related to the glass-forming ability (GFA), is rarely reported. In this paper, directional solidification of such a glass-forming eutectic alloy (Zr48Cu36Al8Ag8) has been conducted to investigate the eutectic growth kinetics in terms of the evolution of eutectic structure as a function of the growth rate (V). It was found that, at lower growth rates, the alloy solidified in a dual-phase lamellar structure characterized by the lamellar spacing (λ), with Vλ2 being a constant. As V increased up to a critical value of 400 μm/s, λ decreased and eventually diminished (λ→0), where the eutectic transitioned into a glass. This critical growth rate (i.e., Vce) can be interpreted as a result of the competition between the glass formation and the eutectic growth, in which the glass formation sets a kinetic upper limit for the eutectic growth. Moreover, Vλ2 is found to be kinetically equivalent to Vce, which can be used as a GFA indicator and is readily measurable at low growth rates.

Molecular dynamic simulations of the martensitic transformation for the dual-phase structure and dislocation activities in Ti80 alloys

Titanium alloy (Ti alloy) has been widely used in various industry areas and attracted extensive attentions due to their excellent properties. In this work, the martensitic transformation (MT) and complex dislocation activities are thoroughly simulated and analyzed for the commercial Ti80 alloy in molecular dynamic (MD) simulations. One specific interatomic potential is developed for Ti80 alloy within the framework of embedded atom method (EAM), which is capable of describing the related physical properties and the plastic deformation mechanism for this alloy. The stabilities of hexagonal close-packed (hcp) and body-centered cubic (bcc) phase boundary and the martensitic transformation mechanism is fully investigated in MD simulations. The orientation relationship between hcp and bcc phases obeys the Burgers relationship. Complex dislocation activities are reproduced in the simulation of the compression of the polycrystalline configuration, including the formation of dislocation wall, (112‾2) compression twin boundaries and dislocation cross slip mechanisms. The MD simulation results are well agreement with our experimental observations and provides an in-depth understanding of the deformation mechanisms of the Ti80 alloy and related Ti alloys. The revealed deformation mechanism and the newly-developed interatomic potential should be a useful guide to the development of the new series of high-performance Ti alloys in the future.

Microstructural Control and Alloy Design for Improving the Resistance to Delayed Fracture of Ultrahigh-Strength Automotive Steel Sheets

The demand for higher-strength automotive steel sheets has increased significantly for lightweight and safe body concepts. However, the increment of the steel strength is often limited by the potential occurrence of delayed fracture. This paper discusses proper microstructure control and alloy design to improve the resistance against the delayed fracture of ultrahigh-strength automotive steel sheets in order to increase the usable upper limit of their strength and provides basic data serving as a practical guide for solving the problem of delayed fracture in ultrahigh-strength automotive steel sheets. It is confirmed that grain refinement, the appropriate dual-phase structure of martensite with ferrite or retained austenite, and surface decarburization, increase the resistance to delayed fracture. In terms of alloy design, the effects of Nb, Mo, and B on the delayed fracture resistance of hot-stamped steels have been investigated. The results suggest that there are other reasons for Nb to improve delayed fracture resistance in addition to grain refinement and the ability to trap hydrogen by its precipitates, as has been conventionally believed. Regarding Mo, it was clearly demonstrated that the segregation of this element at the grain boundary plays a main role in improving the delayed fracture resistance.

Deformation mechanisms in an additively manufactured dual-phase eutectic high-entropy alloy

Nanostructured metals and alloys often exhibit high strengths but at the expense of reduced ductility. Through harnessing the far-from-equilibrium processing conditions of laser powder-bed fusion (L-PBF) additive manufacturing, we develop a dual-phase nanolamellar structure comprised of FCC/L12 and BCC/B2 phases in a Ni40Co20Fe10Cr10Al18W2 eutectic high-entropy alloy (EHEA), which exhibits a combination of ultrahigh yield strength (>1.4 GPa) and large tensile ductility (∼17%). The deformation mechanisms of the additively manufactured EHEA are studied via in-situ synchrotron X-ray diffraction and high-resolution transmission electron microscopy. The high yield strength mainly results from effective blockage of dislocation motion by the high density of lamellar interfaces. The refined nanolamellar structures and low stacking fault energy (SFE) promote stacking fault (SF)-mediated deformation in FCC/L12 nanolamellae. The accumulation of abundant dislocations and SFs at lamellar interfaces can effectively elevate local stresses to promote dislocation multiplication and martensitic transformation in BCC/B2 nanolamellae. The cooperative deformation of the dual phases, assisted by the semi-coherent lamellar interfaces, gives rise to the large ductility of the as-printed EHEA. In addition, we also demonstrate that post-printing heat treatment allows us to tune the non-equilibrium microstructures and deformation mechanisms. After annealing, the significantly reduced SFE and thicknesses of the FCC nanolamellae facilitate the formation of massive SFs. The dissolution of nano-precipitates in the BCC/B2 nanolamellae reduces spatial confinement and further promotes martensitic transformation to enhance work hardening. Our work provides fundamental insights into the rich variety of deformation mechanisms underlying the exceptional mechanical properties of the additively manufactured dual-phase nanolamellar EHEAs.

Designing gradient nanograined dual-phase structure in duplex stainless steel for superior strength-ductility synergy

Similar to other metallic materials, duplex stainless steel dramatically loses its advantage of high ductility as they are strengthened. Here, we produce a gradient nanograined dual-phase structure in the 2101 duplex stainless steel, thus facilitating a superior strength-ductility synergy: a yield strength of 1009.5 MPa being two times higher than that of the as-received sample, a total elongation of 23.4% and a uniform elongation of 5.9%. This novel structure is produced through a processing route of ultrasonic severe surface rolling and annealing, which realizes a superposition of gradient nanostructure and lamellar dual-phase structure with austenite and ferrite. During the tension deformation of gradient nanograined dual-phase structured duplex stainless steel, a significant accumulation of geometrically necessary dislocations occurs. These dislocations are formed to accommodate the deformation incompatibility caused by the layer-by-layer difference in strength and hardness of individual phase domains, as well as the inherent difference in properties between the austenite and ferrite domains. This results in a stronger hetero-deformation induced strengthening and hardening significantly contributing to superior mechanical properties. Our study provides a new avenue to develop advanced steels with high strength and ductility.

Lightweight Ti-Zr-Nb-Al-V refractory high-entropy alloys with superior strength-ductility synergy and corrosion resistance

Refractory high-entropy alloys (RHEAs) are regarded as promising candidates of the next generation superalloys, mainly benefitting from the high strength at high temperatures. However, they are not competitive in lightweight, room-temperature ductility, and corrosion resistance compared with other superalloys. In this work, novel Ti1.6ZrNbAl0.4-xVx (x = 0, 0.2, 0.4) lightweight RHEAs (LRHEAs) were fabricated and systematically investigated. The substitution of V for Al promoted strength-ductility synergy and ameliorated the corrosion resistance of the current LRHEAs. More specifically, an increase in V content caused a phase transformation from a BCC + B2 dual phase structure to a single BCC phase structure and a reduction of grain sizes. Mechanical tests demonstrated that the Ti1.6ZrNbAl0.2V0.2 LRHEA possesses a high yield strength of ∼806 MPa and a moderate tensile ductility of ∼13.2%, which is attributed to synergetic strengthening from grain refinement and solid solution strengthening induced by V addition. In addition to the homogenous phase structure and fine grain sizes, excellent anti-corrosion properties of Ti1.6ZrNbAl0.2V0.2 LRHEA were also attributed to the doping effect of V, leading to the formation of a high content of corrosion-protective products. Our findings provide a new paradigm for achieving such outstanding combinations in LRHEAs.

Rational construction of mushroom-like Ni@N-doped carbon tubes composites with enhanced electromagnetic wave absorption

It is confirmed that electromagnetic wave absorption (EMA) materials with a metal-carbon dual-phase structure has exposed great potential for achieving tunable, strong, and wide EMA. Herein, the mushroom-like Ni@N-doped carbon tubes (NNCTs) are prepared by nickel-catalyzed growth of carbon tubes, realizing carbon encapsulated metal particles. One end of the carbon tube grows and connects to form a bundle of mushroom-like structures, and the other end of the carbon tube is filled with nickel as the mushroom head. The structure control of NNCTs can be achieved by adjusting the amount of catalyst Ni. Appropriate amount of nickel can control the formation of mushroom-like structure. The results show that the structure has a great influence on the EMA performance. Particularly, NNCT-0.5 provides a minimum reflection loss of -60.47 dB with a thickness of 4.0 mm, and the widest effective absorption bandwidth for RL< -10 dB of NNCT-1.5 at a thickness of 2.00 mm was 7.53 GHz (10.47–18.00 GHz). The immobilization of carbon in melamine was achieved by catalytic generation of carbon tubes with metal ions Ni. Metal-carbon biphasic composites introduce dielectric and magnetic losses. The presence of multiple interfaces and voids in the mushroom structure allows tuning the dielectric parameters and optimizing the impedance matching. This work provides an effective strategy for the fabrication of dielectric and magnetic composites with special structures, which has great potential in attenuating electromagnetic wave energy.

Effect of silicon addition on the corrosion resistance of Al0.2CoCrFe1.5Ni high-entropy alloy in saline solution

In the paper, the effect of Si content on the microstructure evolution and corrosion properties of Al0.2CoCrFe1.5NiSix (x = 0, 0.1, 0.2, 0.3) high entropy alloys (HEAs) was systematically investigated. With the addition of Si element, the microstructure changed from single face-centered cubic (FCC) phase in the Al0.2CoCrFe1.5Ni and Al0.2CoCrFe1.5NiSi0.1 HEAs to a dual-phase structure composed of FCC and body-centered cubic (BCC) phases in the Al0.2CoCrFe1.5NiSi0.2 and Al0.2CoCrFe1.5NiSi0.3 HEAs. In addition, the corrosion mechanism has been analyzed by elemental analysis and corrosion morphology. The electrochemical measurement results indicated that the Al0.2CoCrFe1.5NiSi0.1 HEA possessed the best corrosion resistance (Ecorr = − 215 mVAg/AgCl, Icorr = 256 nA/cm2) because the small amount of Si element improves the corrosion resistance of the single-phase FCC HEA. When the Si content increased to 4.08 at% (x = 0.2) and 6.00 at% (x = 0.3), the galvanic corrosion was caused by BCC, resulting in the selective dissolution of the BCC phase and the decrease of corrosion resistance. This research results will help to understand the potential applications of doped trace Si element as a matrix element in single-phase FCC HEAs with good corrosion resistance.

Microstructure and mechanical properties of AlCoCrFeMnNi HEAs fabricated by selective laser melting

To avoid the formation of cracks and to improve the forming capability and performance of the AlCoCrFeNi high entropy alloy (HEA), equiatomic CoCrFeNiMn HEA was added into equiatomic AlCoCrFeNi HEA to form a novel AlCoCrFeMnNi HEA. Three different compositions were prepared by the selective laser melting (SLM) technique, and their formability, microstructure, and mechanical properties were analyzed. Results revealed that AlCoCrFeMnNi shows a composite-like dual-phase structure composed of a soft FCC phase and hard BCC (A2/B2) phase. With the decrease of Al, the SLM forming capability is improved. Besides, the volume fraction of the phase and the nano-hardness decreases with decreasing Al content.