Affordable FeCrNiMnCu high entropy alloys with excellent comprehensive tensile properties

Fe_0.4Cr_0.4NiMn_xCu (0 ≤ x ≤ 1.4) high entropy alloys (HEAs) were prepared by copper-mold casting. The phase selection, microstructure, tensile properties and fracture morphologies were investigated. The microstructure with dual FCC phases was formed in the as-cast HEAs with x ≤ 1, and BCC phase was crystallized from the central FCC dendrites of HEAs with x = 1.2 and 1.4. In homogenized Fe_0.4Cr_0.4NiMnCu HEA, needle-like shaped BCC phase was formed resulting in a slight enhancement of yield strength. Compositional heterogeneity was detected in both FCC and BCC dendrites. These HEAs exhibit excellent comprehensive tensile properties, e.g. the yield strength, ultimate strength and elongation of the HEA with x = 1 reaches 439 MPa, 884 MPa and 23.4%, respectively. High density of dislocations in FCC matrix was formed after tensile deformation. FCC type of fine polyhedra, which is mainly composed of Cr, Mn and O, is formed in dendrites. In this work, the phase selection and strengthening mechanism were evaluated based on atomic size factor. It was found that two criteria can be employed to predict the phase regions of current alloys. The solid solution strengthening for this HEA system is the most important among the four kinds of strengthening mechanisms.     

FeCoNiCrCu0.5Alx高熵合金的结构和性能（英文）

研究Al含量和热处理对FeCoNiCrCu0.5Alx多主元高熵合金的相结构、硬度和电化学性能的影响规律。随着Al含量的增加,铸态合金的相结构由FCC相向BCC相转变。当x从0.5增加到1.5时,FeCoNiCrCu0.5Alx高熵合金的稳定结构由FCC结构向FCC+BCC双相结构转变。BCC相的硬度高于FCC相的,在氯离子及酸性介质中BCC相的耐腐蚀性均优于FCC相的。FeCoNiCrCu0.5Al1.0铸态合金具有高硬度和良好的抗腐蚀性能。     

The high-entropy alloys with high hardness and soft magnetic property prepared by mechanical alloying and high-pressure sintering

The equiatomic multiprincipal CoCrFeCuNi and CoCrFeMnNi high-entropy alloys (HEAs) were consolidated via high pressure sintering (HPS) from the powders prepared by the mechanical alloying method (MA). The structures of the MA'ed CoCrFeCuNi and CoCrFeMnNi powders consisted of a face-centered-cubic (FCC) phase and a minority body-centered cubic (BCC) phase. After being consolidated by HPS at 5 GPa, the structure of both HEAs transformed to a single FCC phase. The grain sizes of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were about 100 nm. The alloys keep the FCC structure until the pressure reaches 31 GPa. The hardness of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were 494 Hv and 587 Hv, respectively, much higher than their counterparts prepared by casting. Both alloys show typical paramagnetism, however, possessing different saturated magnetization. The mechanisms responsible for the observed influence of Cu and Mn on mechanical behavior and magnetic property of the HEAs are discussed in detail.     

Effects of annealing at 800 and 1000 °C on phase precipitates and hardness of Al7Cr20FexNi73−x alloys

The effects of Fe content on the microstructure, phase constituents and microhardness of the as-cast, 800 °C- or 1000 °C-annealed Al₇Cr₂₀Fe_xNi_73?x (x=13?66) alloys were investigated. Not all these alloys are composed of the single FCC phase. The BCC and B2 phases are found. It is confirmed that the BCC phase in the Al₇Cr₂₀Fe₆₆Ni₇ alloy is transformed from the FCC phase at about 900 °C during cooling. While in the 800 °C-annealed Al₇Cr₂₀Fe₆₀Ni₁₃ alloy, the FCC phase is stable and the hardness decreases. After annealing at 1000 °C, for the precipitation of the B2 particles, the Al content in the FCC phase decreases, which results in decreasing of the alloy hardness. Moreover, after annealing at 800 °C, a small amount of Al-rich B2 particles precipitate at the phase boundary and some nanocrystal BCC phase precipitates in the FCC matrix, which increases the hardness of the Al₇Cr₂₀Fe_xNi_73?x (x=41?49) alloys. These results will help to the composition design and processing design of the Al?Cr?Fe?Ni based high-entropy alloys.     

Effects of Nb additions on the microstructure and mechanical property of CoCrFeNi high-entropy alloys

A series of five-component CoCrFeNiNb_x high entropy alloys (HEAs) were synthesized to investigate alloying effects of the large atom Nb on the structure and tensile properties. Microstructures of these alloys were examined using scanning electron microscopy and the phase evolution was characterized and compared using the ΔH_mix–δ and ΔX criteria. It was found that the microstructure changes from the initial single face-centered cubic (FCC) to duplex FCC plus hexagonal close-packed (HCP) structure with additions of Nb. The current alloy system exhibits a hypoeutectic structure and the volume fraction of the Nb-enriched Laves phase with the HCP structure increases with increasing the Nb content, which is mainly responsible for the increment in the yield and fracture strength. Particularly, the Nb_0.155 alloy containing a 9.3% Nb-enriched Laves phase exhibits the most promising mechanical properties with the yield strength and plastic strain as high as 321 MPa and 21.3%, respectively. The ΔH_mix–δ criteria well describe the phase selection for the thermally treated alloys, while the physical parameter ΔX fails to predict the appearance of the Nb-enriched Laves phase in this alloy system.     

Composition dependence of structure,physical and mechanical properties of FeCoNi(MnAl)x high entropy alloys

This paper has provided a systematic study of the crystal structure, physical and mechanical properties of FeCoNi(MnAl)_x (0 ≤ x ≤ 2) high entropy alloys. As x increases, the crystal structure changes from FCC to FCC + BCC then to BCC solid solution. High saturated magnetization, low coercivity together with high electric resistivity are found in this system, indicating their potential applications as soft magnets. Meanwhile, the strength and hardness increase monotonously as x increases. Optimal balance of physical and mechanical properties are obtained in this system.     

Phase formation in mechanically alloyed AlxCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) high entropy alloys

Alloying behavior and phase transformations in Al_xCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) multi-component high entropy alloys that are synthesized by mechanical alloying were studied. Two FCC phases along with a BCC phase were formed in Al_0.45CoCrCuFeNi and AlCoCrCuFeNi, while a single B2 phase was observed in higher Al containing alloys Al_2.5CoCrCuFeNi and Al₅CoCrCuFeNi. DSC analysis indicates that BCC phase present in the alloys could be Fe–Cr type solid solution. A detailed analysis suggests that two melting peaks observed during DSC in lower Al containing alloys can be attributed to that of Cu–Ni and Fe–Ni FCC solid solutions. The BCC phase disappears in Al_0.45CoCrCuFeNi and AlCoCrCuFeNi at high temperatures during DSC. However, Al₅CoCrCuFeNi retains its B2 structure despite of heating in DSC. Further, phases present in these alloys retain nanocrystallinity even after exposure to high temperatures. A critical analysis is presented to illustrate that solid solution formation criteria proposed for high entropy alloys in the literature are unable to explain the phase formation in the present study of alloys. Besides, these criteria seem to be applicable to high entropy alloys only under very specific conditions.     

AlCrCuFeNbxNiTi高熵合金组织与性能研究

采用电弧熔炼制备了AlCrCuFeNb_xNiTi (x = 0, 0.25, 0.5, 1.0)高熵合金,研究不同Nb含量对AlCrCuFeNb_xNiTi高熵合金显微组织和力学性能的影响。研究表明：AlCrCuFeNb_xNiTi (x = 0, 0.25, 0.5, 1.0)高熵合金物相主要包含有序FCC的L2₁相和Laves相,还有少量的BCC(A2)和FCC相;Nb元素的添加能促进Laves相的生成且对Cu元素的偏析具有一定的抑制效果;通过相判据参数计算找到了适合AlCrCuFeNb_xNiTi高熵合金的相形成判据;添加适量的Nb元素能够改善AlCrCuFeNiTi六元高熵合金的力学性能;AlCrCuFeNb_0.5NiTi 高熵合金具有较好的综合力学性能,抗压强度达到1587.4 MPa,硬度达到568.8 HV;Nb元素含量过高时会形成过多的Laves相使合金表现出过早脆化现象。     

Microstructure and Mechanical Properties of Low-Activation Fe-Cr-V Multicomponent Single-Phase Alloys

A ternary equiatomic and nearly equiatomic alloy composed of the low-activation elements Fe, Cr, and V was designed for potential application as low-activation structural materials for the first wall of fusion reactors. The optimal composition of the multicomponent alloy was determined by minimizing the Gibbs free energy for solid-solution formation. The microstructure, phase stability, and mechanical properties of the equiatomic and optimized Fe-Cr-V alloys were studied. The equiatomic alloy consisted of a body-centered-cubic (BCC) solid-solution phase with a small amount of a face-centered-cubic (FCC) solid-solution phase, whereas the optimized alloy consisted of a single BCC solid-solution phase in the as-cast state. The crystal grains of both alloys were equiaxial. No phase transition was observed in the optimized alloy after homogenization; however, for the equiatomic alloy, the FCC solid-solution phase disappeared and surface relief was observed. The Vickers microhardness and yield strength of the Fe-Cr-V alloys increased after homogenization compared with those of the as-cast alloys.     

Structural and magnetic properties of Ni2CoSi: First-principles calculation and experimental realization

BCC Heusler phase Ni₂CoSi has been predicted to be a promising candidate to realize magnetic field induced martensitic transformation. We tried to prepare Ni₂CoSi single phase using different methods. Single phase Ni₂CoSi cannot be synthesized by arc-melting and annealing. Then we used mechanical alloying method to synthesize Ni₂CoSi. But a FCC phase rather than BCC was obtained after ball-milling. The lattice constant of FCC Ni₂CoSi is 3.52 Å and the Curie temperature is around 900 K. The saturation magnetization at 5 K is 2.44μ_B/f.u. This FCC phase is stable and no transition is observed when heating to 1173 K. The electronic structure and phase stability of the FCC and BCC Heusler phase have been investigated by first-principles calculations. The FCC Ni₂CoSi has lower total energy compared with BCC, agreeing with the experimental observation. But the calculated total moment is much smaller than the M_s at 5 K. This difference is related to the atomic disorder and was discussed by KKR-CPA calculation.     

Alloy Design and Properties Optimization of High-Entropy Alloys

This article reviews the recent work on the high-entropy alloys (HEAs) in our group and others. HEAs usually contain five or more elements, and thus, the phase diagram of HEAs is often not available to be used to design the alloys. We have proposed that the parameters of ?? and ?? can be used to predict the phase formation of HEAs, namely ??????1.1 and ??????6.6%, which are required to form solid-solution phases. To test this criterion, alloys of TiZrNbMoV _x and CoCrFeNiAlNb _x were prepared. Their microstructures mainly consist of simple body-centered cubic solid solutions at low Nb contents. TiZrNbMoV _x alloys possess excellent mechanical properties. Bridgman solidification was also used to control the microstructure of the CoCrFeNiAl alloy, and its plasticity was improved to be about 30%. To our surprise, the CoCrFeNiAl HEAs exhibit no apparent ductile-to-brittle transition even when the temperatures are lowered from 298?K to 77?K.     

Effects of Tungsten Addition on the Microstructure and Mechanical Properties of Near-Eutectic AlCoCrFeNi<Subscript>2</Subscript> High-Entropy Alloy

The effects of tungsten addition on the microstructure and mechanical properties of near-eutectic AlCoCrFeNi₂ high-entropy alloy were investigated in this paper. The AlCoCrFeNi₂W _x alloys comprised the primary BCC phase plus eutectic FCC/BCC phases. It was found that W element can both promote the formation of the primary BCC phase and act as a solid solution strengthening element. The hardness of the AlCoCrFeNi₂W _x alloys increased from HV 293 to HV 356.2 with the increase in W content. The addition of W element improved the strength of alloys but reduced ductility. Thereinto, the AlCoCrFeNi₂W_0.2 alloy showed the most excellent compressive properties with the fracture strength of 2785.9 MPa and the plastic strain of 0.42, respectively, which implied the potential industrial application values.     

Mechanical properties and oxidation characteristics of TiNiAl(Nb) intermetallics

The structural evolution, mechanical properties and the oxidation characteristics of Ti₅₀Ni_50−xAl_x alloys were investigated. The influence of additive Nb on the microstructure, mechanical properties as well as the high temperature oxidation resistance was examined. It is found that the substitution of Al for Ni can remarkably reinforce the TiNi-based intermetallics by precipitation of (Ti,Al)₂Ni phase and by solid-solution strengthening effects. It is also found that the addition of Nb has beneficial influence on the mechanical properties of TiNiAl intermetallics, and that the oxidation resistance of TiNiAl(Nb) alloys can be effectively improved by the Nb addition. The effect of Nb on the high temperature oxidation behavior of TiNiAl(Nb) alloys has been discussed.     

Microstructure and mechanical properties of FeCoCrNiMn high-entropy alloy produced by mechanical alloying and vacuum hot pressing sintering

FeCoCrNiMn high-entropy alloys were produced by mechanical alloying (MA) and vacuum hot pressing sintering (VHPS). Results showed that the nano-crystalline alloy powders were obtained by MA and the corresponding phase structures were composed of FCC matrices and low amounts of BCC and amorphous phases. After VHPS, the BCC phases almost disappeared, simultaneously with the precipitation of σ phases and M₂₃C₆ carbides. An increase of sintering temperature resulted in grain growth of the precipitated phases. As the sintering temperature was increased from 700 to 1000 °C, the strain-to-failure of the alloys rose from 4.4% to 38.2%, whereas the yield strength decreased from 1682 to 774 MPa. The bulk FeCoCrNiMn HEAs, consolidated by VHPS at 800 °C and 900 °C for 1 h, showed relatively good combination of strength and ductility.     

Effect of Al on microstructure and mechanical properties of cast CrCoNi medium-entropy alloy

Cast Cr Co Ni Alx(x=0-1.2) medium-entropy alloys(MEAs) were produced by arc melting and flip cast to investigate the alloying effect of Al addition on the microstructure, phase constituent and mechanical properties. The crystal structure changes from an initial face-centered cubic(FCC) to duplex FCC and body-centered cubic(BCC) and finally a single BCC with increasing Al content. In the duplex region, FCC and BCC phases form under a eutectic reaction in the interdendrite region. In the single BCC region, the dendrites transform to ordered B2 and disordered A2 BCC phases resulting from spinodal decomposition. Corresponding to their phase constituents, yield strength increases accompanied with an elongation reduction with increasing Al addition. A very interesting phenomenon of very weak ordered FCC(001) spots appearing in Al-0.4 alloy was observed, indicating a local ordering of FCC phase. The changes of fracture surfaces after the tensile deformation are also corresponding to the variations in mechanical properties.     

Microstructure,mechanical properties and shape memory effect of Ni–Mn–Ga–B high-temperature shape memory alloy

The effects of boron addition on the microstructure, transformation temperature, mechanical properties and shape memory effect of (Ni₅₄Mn₂₅Ga₂₁)_100−xB_x alloys were investigated. The results showed that the martensitic transformation start temperatures M_s decreased monotonically from 465 K for x = 0–278 K for x = 3. Boron addition refined the grain and significantly enhanced the mechanical properties. The compressive fracture strain of 22.3% and reversible strain of 6.8% were obtained in (Ni₅₄Mn₂₅Ga₂₁)_99.5B_0.5 alloy.     

Revealing the Microstates of Body-Centered-Cubic (BCC) Equiatomic High Entropy Alloys

Attributing to the attractive mechanical properties, e.g., high yield strength and fracture toughness, the atomic and electronic basis for high entropy alloys (HEAs) are under extensive studies. In the present work, the local atomic arrangement of body-centered-cubic (BCC) equiatomic HEAs are revealed by the CN14 cluster-plus-glue-atom model and the 32 atoms special quasirandom structures. Moreover, the cluster-plus-glue-atom model is utilized to generate ordered and disordered configurations. The bonding lengths among the same and different alloying elements are comprehensively compared in term of their partial pair correlation function (PCF). According to the specific (well-defined) position of each partial PCF of the BCC structure, the order–disorder/random configurational transitions are revealed by the absence of partial PCF peaks. Here, the WMoTM₁TM₂ (TM = Ta, Nb, and V) BCC equiatomic refractory HEAs are selected as a case study. Through mixing various groups of alloying elements, the atomic-size differences not only result in the lattice mismatch/distortion but also yield the formation of weak spots. Their bonding-charge density captures the electron redistributions caused by the coupling effect of the lattice distortion and valance electron differences among various elements, which also presents the physical nature of the loosely-bonded weak spots and the tightly-bonded clusters. It is worth mentioning that both the PCF and the negative enthalpy of mixing can be utilized to characterize the clusters or the short range ordering in the HEAs. The microstates revealed by the cluster-plus-glue-atom model are in line with the novel small set of the ordered structures method reported in the literature.     

Effect of Cr on Microstructure and Properties of a Series of AlTiCr x FeCoNiCu High-Entropy Alloys

A series of AlTiCr _x FeCoNiCu (x: molar ratio, x = 0.5, 1.0, 1.5, 2.0, 2.5) high-entropy alloys (HEAs) were prepared by vacuum arc furnace. These alloys consist of α-phase, β-phase, and γ-phase. These phases are solid solutions. The structure of α-phase and γ-phase is face-centered cubic structure and that of β-phase is body-centered cubic (BCC) structure. There are four typical cast organizations in these alloys such as petal organization (α-phase), chrysanthemum organization (α-phase + β-phase), dendrite (β-phase), and inter-dendrite (γ-phase). The solidification mode of these alloys is affected by Chromium. If γ-phase is not considered, AlTiCr_0.5FeCoNiCu and AlTiCrFeCoNiCu belong to hypoeutectic alloys; AlTiCr_1.5FeCoNiCu, AlTiCr_2.0FeCoNiCu, and AlTiCr_2.5FeCoNiCu belong to hypereutectic alloys. The cast organizations of these alloys consist of pro-eutectic phase and eutectic structure (α + β). Compact eutectic structure and a certain amount of fine β-phase with uniform distribution are useful to improve the microhardness of the HEAs. More γ-phase and the microstructure with similar volume ratio values of α-phase and β-phase improve the compressive strength and toughness of these alloys. The compressive fracture of the series of AlTiCr _x FeCoNiCu HEAs shows brittle characteristics, suggesting that these HEAs are brittle materials.     

Effect of Sn content on microstructure and tensile properties of as-cast and as-extruded Mg−8Li−3Al−(1,2,3)Sn alloys

The effects of Sn content on microstructure and tensile properties of as-cast and as-extruded Mg−8Li−3Al− (1,2,3)Sn (wt.%) alloys were investigated by X-ray diffractometry (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and tensile test. It is found that, as-cast Mg−8Li−3Al−(1,2,3)Sn alloys consist of α-Mg+β-Li duplex matrix, MgLiAl₂ and Li₂MgSn phases. Increasing Sn content leads to grain refinement of α-Mg dendrites and increase in content of Li₂MgSn phase. During hot extrusion, complete dynamic recrystallization (DRX) takes place in β-Li phase while incomplete DRX takes place in α-Mg phase. As Sn content is increased, the volume fraction of DRXed α-Mg grains is increased and the average grain size of DRXed α-Mg grains is decreased. Increasing Sn content is beneficial to strength but harmful to ductility for as-cast Mg−8Li−3Al−(1,2,3)Sn alloys. Tensile properties of Mg−8Li−3Al− (1,2,3)Sn alloys are improved significantly via hot extrusion and Mg−8Li−3Al−2Sn alloy exhibits the best tensile properties.     

Microstructure and martensitic transformation of TiNiNbB shape memory alloys

In present work, microstructure, martensitic transformation and mechanical properties of Ti₄₄Ni_47−xNb₉B_x (x = 0, 0.5, 1, 5 at.%) alloys were investigated as a function of B content. The results show that the addition of B significantly influences the microstructure of the alloys. The microstructure of Ti₄₄Ni₄₇Nb₉ alloy consists of B2 parent phase matrix and β-Nb phase. When the B content is 0.5 at.%, Nb₃B₂ phase presents. With further increasing B content to above 1 at.%, TiB and NbB phases present instead of Nb₃B₂ phase. With increasing B content, the transformation temperatures increase due to the reduced Ni/Ti ratio and Nb content in the matrix. The mechanical properties can be optimized by the addition of 1 at.% B.