Polycrystalline elastic moduli of a high-entropy alloy at cryogenic temperatures

CrMnCoFeNi is a FCC high-entropy alloy (HEA) that exhibits strong temperature dependence of strength at low homologous temperatures in sharp contrast to pure FCC metals like Ni that show weak temperature dependence. To understand this behavior, elastic constants were determined as a function of temperature. From 300 K down to 55 K, the shear modulus (G) of the HEA changes by only 8%, increasing from 80 to 86 GPa. This temperature dependence is weaker than that of FCC Ni, whose G increases by 12% (81–91 GPa). Therefore, the uncharacteristic temperature-dependence of the strength of the HEA is not due to the temperature dependence of its shear modulus.     

Rapid solidification and liquid-phase separation of undercooled CoCrCuFexNi high-entropy alloys

Fluxing and cyclic superheating technique was used to investigate the rapid solidification behavior of CoCrCuFe_xNi (x = 1.0, 1.5, 2.0, molar concentration) high-entropy alloys in this study. The microstructures of CoCrCuFe_xNi (x = 1.0, 1.5, 2.0) high-entropy alloys solidified at different undercoolings (ΔT) were investigated. Liquid-phase separation leading to Cu-rich and Cu-depleted regions, were obtained in the solidified microstructure from highly undercooled melt. This occurs when the melt undercooling exceeds a critical undercooling (ΔT_crit) of 160 K for CoCrCuFeNi, 190 K for CoCrCuFe_1.5Ni and 293 K for CoCrCuFe₂Ni alloy. However, typical dendrites and interdendritic regions were observed in rapid-solidified CoCrCuFe_xNi alloys prepared from melts with a small undercooling (ΔT < ΔT_crit). Conversely, a large amount of Cu-rich spheres and even egg-type structures were observed in alloys solidified from melts with large degree of undercooling, ΔT in excess of the critical value, ΔT_crit. A large amount of Cu-rich nano-phases were found in the matrix, possibly, due to the precipitation of Cu-rich phase from the supersaturated solid solution obtained during solidification. The positive enthalpies of mixing between Cu and the other elements in the multi-component alloys resulted in the occurrence of liquid-phase separation prior to the liquid–solid transformation starts. The sluggish diffusion effect of high-entropy alloys and rapid solidification play an important part in the precipitation of nanophase during the solid-state transformation in the Cu-based matrix. Similar to other immiscible alloys, liquid-phase separation occurred when a critical undercooling was exceeded. Differently, nanophases were found in the microstructures of multi-component CoCrCuFe_xNi (x = 1.0, 1.5, 2.0) alloys.     

In-situ TEM observation of structural changes in nano-crystalline CoCrCuFeNi multicomponent high-entropy alloy (HEA) under fast electron irradiation by high voltage electron microscopy (HVEM)

The structural changes induced in a CoCrCuFeNi multicomponent nano-crystalline high-entropy alloy (HEA) under fast electron irradiation were investigated by in-situ transmission electron microscopy (TEM) using a high voltage electron microscope (HVEM). A fine-grained face centered cubic (fcc) single phase was obtained in the sputtered specimens. The fcc solid solution showed high phase stability against irradiation over a wide temperature range from 298 to 773 K, and remained as the main constituent phase even when the samples were irradiated up to 40 displacement per atom (dpa). Moreover, the irradiation did not seem to induce grain coarsening. This is the first report on the irradiation damage in 5-component HEA under MeV electron irradiation.     

A new strategy to design eutectic high-entropy alloys using mixing enthalpy

Although eutectic high entropy alloys (EHEAs) display homogeneously fine microstructure, excellent castability and promising industrial application potential, how to design eutectic compositions in high entropy alloys (HEAs) remains to be challenging. Here, a novel strategy, specifically, through calculation of mixing enthalpy, was used to locate eutectic compositions in HEAs. As a proof of this concept, a series of EHEAs were located and prepared following the mixing enthalpy method. Using this new strategy, eutectic compositions can be designed conveniently, once one can classify elements into two different groups. This new alloy design strategy can be readily adapted to locate new EHEAs.     

Enhanced mechanical properties of HfMoTaTiZr and HfMoNbTaTiZr refractory high-entropy alloys

Although refractory high-entropy alloys have exceptional strength at high temperatures, they are often brittle at room temperature. One exception is the HfNbTaTiZr alloy, which has a plasticity of over 50% at room temperature. However, the strength of HfNbTaTiZr at high temperature is insufficient. In this study, the composition of HfNbTaTiZr is modified with an aim to improve its strength at high temperature, while retaining reasonable toughness at room temperature. Two new alloys with simple BCC structure, HfMoTaTiZr and HfMoNbTaTiZr, were designed and synthesized. The results show that the yield strengths of the new alloys are apparently higher than that of HfNbTaTiZr. Moreover, a fracture strain of 12% is successfully retained in the HfMoNbTaTiZr alloy at room temperature.     

Microstructural evolution after thermomechanical processing in an equiatomic,single-phase CoCrFeMnNi high-entropy alloy with special focus on twin boundaries

The FCC-structured equiatomic CoCrFeMnNi high-entropy alloy was produced by arc melting and drop casting. After homogenization, the drop-cast ingots were cold rolled to sheets with six different final thicknesses (thickness reductions of 21, 41, 61, 84, 92 and 96%). Samples were cut from the rolled sheets and annealed for 1 h at temperatures between 400 and 1000 °C. The recrystallization temperature was then determined as a function of cold work by means of scanning electron microscopy and electron backscatter diffraction measurements. Additionally, Vickers indentation was performed on these samples. It was found that the microhardness first tends to increase slightly upon annealing below the recrystallization temperature but then drops steeply for higher annealing temperatures due to the onset of recrystallization. To study grain growth kinetics, samples that underwent 96% cold rolling were first recrystallized for 1 h at 800 °C, which is the lowest temperature at which complete recrystallization occurs, and then annealed at temperatures between 800 and 1150 °C for various times. The grain growth exponent was determined to be approximately n = 3, and the activation energy Q = 325 kJ/mol, both of which agree well with published values for this alloy. EBSD measurements were made in the as-recrystallized and grain growth samples to analyze the annealing twins. The density of annealing twins in the grain growth samples was found to depend only on grain size, i.e., it was independent of annealing temperature and time. No such correlation could be found for the as-recrystallized samples. These observations are discussed in the framework of existing theories for the formation of annealing twins.     

Effect of cryo-deformation on structure and properties of CoCrFeNiMn high-entropy alloy

Microstructure evolution in high-entropy alloy CoCrFeNiMn during plane-strain multipass rolling to a thickness strain of 80% at 293 and 77 K was studied. Deformation at both temperatures was found to be accompanied by twinning. At 77 K, twinning was more extensive in terms of the fraction of twinned grains and the length of the twinning stage thereby providing faster kinetics of the microstructure evolution. Micro-shear bands formed in the microstructure of the alloy at the late stages of rolling (at ε ≈ 80% at 293 K and ε ≈ 40% at 77 K). The ultimate tensile strength of specimens rolled at 77 K or 293 K was found to be 1500 or 1200 MPa, respectively while the strength in the initial homogenized condition was 440 MPa. The contribution of various mechanisms to the hardening of the alloy following rolling at 77 K and 293 K was analyzed quantitatively.     

Corrosion behavior and surface film characterization of TaNbHfZrTi high entropy alloy in aggressive nitric acid medium

Corrosion behavior of TaNbHfZrTi high-entropy alloy (HEA) was investigated in nitric and fluorinated nitric acid at ambient (27 °C) and boiling (120 °C) conditions. The alloy passivated spontaneously during potentiodynamic polarization in 11.5 M HNO₃ at ambient condition. The corrosion rate was negligible in boiling 11.5 M HNO₃, exposed for 240 h. Scanning electron microscopic (SEM) studies did not show any significant corrosion attack. The high corrosion resistance of TaNbHfZrTi HEA was attributed to its single phase bcc structure. X-ray photoelectron spectroscopic (XPS) analysis revealed that the protective passive film formed in boiling nitric acid was predominantly composed of Ta₂O₅, in contrast to the presence of ZrO₂ and HfO₂ in air-formed native film. Potentiodynamic polarization studies indicated a pseudo-passivation behavior of the HEA in 11.5 M HNO₃ + 0.05 M NaF at ambient condition. In boiling fluorinated nitric acid, SEM images of TaNbHfZrTi HEA displayed a severely corroded morphology indicating the instability of the metal-oxides of the alloying elements. XPS investigations confirmed the presence of ZrF₄, ZrOF₂ and HfF₄ along with un-protective oxides of Ta, Nb and Ti on the film, resulting in decreased corrosion resistance of TaNbHfZrTi HEA in fluorinated nitric acid.     

原位合成多元硼化物对Al0.5CoCrCuFeNiBx高熵合金硬度与耐磨性能的影响

本文研究了Al0.5CoCrCuFeNiBx (x=0-1)的组织、相组成、硬度及耐磨性能。并预测了Al0.5CoCrCuFeNiBx (x=0-1)中简单固溶体形成规律。未添加硼元素的合金具有简单FCC固溶体结构。添加硼元素后,合金由简单FCC固溶体及多元硼化物组成。硼以硼化物形式析出,没有固溶到FCC固溶体中,因而添加硼对FCC固溶体的晶格常数无影响。硼化物的析出使合金的硬度提高,并且硬度随着硼含量的增加而呈线性增加。当硼含量x?0.4时,合多的磨耗阻变化不明显,但当硼含量x?0.6时,合金的磨耗阻抗随着硼含量增加而呈线性增加。 随着硼含量的增加,合金的磨损机制由粘着磨损转变为氧化磨损。合金硬度与耐磨性能的提高是高硬度的粗大硼化物与韧性的FCC固溶体基体共同作用的结果。     

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.     

Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering

An equiatomic CoCrFeNiMn high-entropy alloy was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). During MA, a solid solution with refined microstructure of 10 nm which consists of a FCC phase and a BCC phase was formed. After SPS consolidation, only one FCC phase can be detected in the HEA bulks. The as-sintered bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition associated with the structure coarsening and phase transformation was observed during SPS process.     

Corrosion studies of single crystals of iron-gallium alloys in aqueous environments

Iron-gallium (Fe-Ga) alloys have attracted lot of attention for use in sensor and actuator applications due to an excellent combination of large low-field magnetostriction, high mechanical strength, good ductility and low cost. This paper reports the very first measurement of the corrosion behavior of single crystals of Fe-Ga alloys. The corrosion behaviors of single crystals of Fe-15 at.% Ga, Fe-20 at.% Ga and Fe-27.5 at.% Ga alloys were studied in acidic (0.1 M HCl), basic (0.1 M NaOH) and simulated seawater (3.5 wt.% NaCl) environments. Dependence of corrosion rates on the crystal orientation is also examined. The polarization resistance technique was used for measuring the corrosion potentials and corrosion rates. The corrosion rate of single crystals of Fe-Ga alloys in 0.1 M HCl aqueous solution is higher than that in 3.5 wt.% NaCl aqueous solutions. The corrosion rate of single crystals of Fe-Ga alloys is least in 0.1 M NaOH aqueous solutions.     

Alloy design for high-entropy alloys based on Pettifor map for binary compounds with 1:1 stoichiometry

Pettifor map for binary compounds with 1:1 stoichiometry was utilized as an alloy design for high-entropy alloys (HEAs) with exact or near equi-atomicity in multicomponent systems. Experiments started with selecting GuGd binary compound with CsCl structure from Pettifor map, followed by its extensions by selecting the binary compounds with the same CsCl structure to CuDyGdTbY equi-atomic quinary alloy and to Cu₄GdTbDyY and Ag₄GdTbDyY quinary alloys and Cu₂Ag₂GdTbDyY senary alloy in sequence. X-ray diffraction revealed that CuDyGdTbY alloy was formed into a HEA with mixture of bcc, fcc and hcp structures, whereas the Cu₂Ag₂GdTbDyY HEA was a single CsCl phase. The results suggest a potential of Pettifor map for the development of HEAs by utilizing its information of crystallographic structures. The further analysis was performed for composition diagrams of multicomponent systems corresponding to simplices in a high dimensional space. The present results revealed that a strategy of equi-mole of compounds instead of conventional equi-atomicity also works for the development of HEAs.     

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.     

The microstructure and phase equilibrium of new high performance high-entropy alloys

In recent years, series of high-entropy alloy have been well developed with high hardness and high temperature stability. These properties could apply in hard surface welding technology. Several AlCrFeMnNi high-entropy alloys have been developed and made as welding rods to apply in the hardface welding on carbon steel using nickel-based alloy as bond coating layer. The hardness of hardface can reach Hv 900 after aging at 700 °C for 4 h. One of alloys performed resistant with the thermal effect during hardfacing operation and maintain better hardness in the multiple overlays.An electron probe microanalyzer (EPMA) was used to verify the phase compositions of hardfacing microstructure. There is nanoprecipitate phase formed within the matrix grain and might contribute the high hardness. A primitive phase diagram calculation was done by using the Calphad method. The calculated results show that the hardening mechanism is due to bcc order/disorder coherent strengthening within the matrix grain. Two high-entropy alloys were equilibrated at 700 °C for 10 days. The equilibrium phases are consistent with the calculated isopleth. The new phase may relate with FCC phase structure and still under investigation.     

Nanocrystalline Al–Fe intermetallics – light weight alloys with high hardness

Nanocrystalline Al–Fe alloys containing 60–85 at.% Al were produced by consolidation of mechanically alloyed nanocrystalline or amorphous (Al85Fe15 composition) powders at 1000 °C under a pressure of 7.7 GPa. The hardness of the alloys varied between 5.8 and 9.5 GPa, depending on the Al content. The specific strength, calculated using an approximation of the yield strength according to the Tabor relation, was between 544 and 714 kNm/kg. Based on the results obtained, we infer that application of high pressure affected crystallisation of amorphous Al₈₅Fe₁₅ alloy, influencing the phase composition of the crystallisation product, and phase changes in nanocrystalline Al₈₀Fe₂₀ alloy, inhibiting them.     

FeSiBPNbCu alloys with high glass-forming ability and good soft magnetic properties

Effects of Cu addition on the glass-forming ability (GFA), thermal stability, magnetic properties and crystallization process of (Fe_0.76Si_0.09B_0.1P_0.05)_99−xNb₁Cu_x (x = 0, 0.25, 0.5, 0.75, 1) alloys were investigated. The introduction of Cu effectively stimulates the precipitation of the α-Fe(Si) without obvious deterioration of the GFA, and successfully modifies the simultaneous precipitation of α-Fe(Si), Fe₂B and Fe₃(B,P) phases in (Fe_0.76Si_0.09B_0.1P_0.05)₉₉Nb₁ alloy into separable precipitation of each phase at different temperatures during annealing, leading to the enhancement of soft magnetic properties. The saturation magnetic flux density of the representative (Fe_0.76Si_0.09B_0.1P_0.05)_98.25Nb₁Cu_0.75 alloy could be enhanced from 1.43 to 1.51 T after annealing at 530 °C for 10 min due to the precipitation of α-Fe(Si) nanoparticles with a diameter of about 22 nm dispersing randomly in the amorphous matrix. The integration of high GFA and excellent soft magnetic properties makes the FeSiBPNbCu alloys promising soft magnetic materials for industrial applications.     

Magnesium secondary alloys: Alloy design for magnesium alloys with improved tolerance limits against impurities

The development of secondary magnesium alloys requires a completely different concept compared with standard alloys which obtain their corrosion resistance by reducing the levels of impurities below certain alloy and process depending limits. The present approach suitable for Mg-Al based cast and wrought alloys uses a new concept replacing the β-phase by τ-phase, which is able to incorporate more impurities while being electro-chemically less detrimental to the matrix. The overall experimental effort correlating composition, microstructure and corrosion resistance was reduced by using thermodynamic calculations to optimise the alloy composition. The outcome is a new, more impurity tolerant alloy class with a composition between the standard AZ and ZC systems having sufficient ductility and corrosion properties comparable to the high purity standard alloys.