Influence of remelting on microstructure,hardness and corrosion behaviour of AlCoCrFeNiTi high entropy alloy

Abstract

High entropy alloys are a newly developed class of alloys, which tend to form a single solid solution or a mixture of solid solutions with simple crystal structures. These alloys possess excellent mechanical properties, thermal stability and corrosion resistance. In the present paper, an AlCoCrFeNiTi high entropy alloy was obtained by induction melting, and the influence of the remelting process on the mechanical and corrosion resistance characteristics of the alloy was investigated. Thus, optical and scanning electron microscopy revealed less phase segregation and a fine dendritic structure for the remelted alloy, while corrosion tests indicated that present alloy, in remelted state, has better corrosion resistance than as cast alloy and stainless steel. The Vickers microhardness measurements demonstrated an improvement of the alloy microhardness by remelting process due to the decrease in phase segregation and the increase in dendrite refinement level.     

Cr含量对CrMnFeNi系高熵合金腐蚀行为的影响EI北大核心CSCD

利用XRD,SEM/EDS,EBSD,电化学测试等表征手段研究Cr含量对Cr_(x)MnFeNi(x=0.8,1.0,1.2,1.5)高熵合金微观组织与耐蚀性能的影响。结果表明:Cr_(0.8)MnFeNi高熵合金为单相FCC结构,Cr_(x)MnFeNi(x=1.0,1.2,1.5)高熵合金为FCC+BCC双相结构,且BCC相比例随着Cr含量升高而增加。在0.5 mol/L H_(2)SO_(4)溶液中,高熵合金的耐蚀性能随着Cr含量降低而增强,其中,Cr_(0.8)MnFeNi单相高熵合金的耐蚀性能最好,这是因为Cr_(0.8)MnFeNi高熵合金的成分更为均匀。此外,Cr_(x)MnFeNi高熵合金在0.5 mol/L H_(2)SO_(4)溶液中均具有宽泛的钝化区域以及明显的伪钝化区域,表明合金在耐蚀性能上具有较大的研究价值和开发潜力。     

Additive manufacturing of high entropy alloys:A practical review

The novel idea of alloying,which is based on the utilization of multiple principal elements in high concen-trations,has created a novel class of promising materials called high entropy alloys(HEAs).So far,several HEAs with outstanding properties beyond those of conventional alloys have been discovered,and new superior high-entropy alloys are still expected to be developed in the future.However,the fabrication process of HEAs through conventional manufacturing techniques suffers from significant limitations due to the intrinsic requirements of HEAs.Additive manufacturing(AM),on the other hand,has provided new opportunities for fabricating geometrically complex HEAs with the possibility of in situ tailoring of their microstructure features.Considering the growing interest in AM of HEAs during most recent years,this review article aims at providing the state of the art in AM of HEAs.It describes the feedstock requirements for laser based AM techniques.Thereafter,a comprehensive picture of the current state of nearly all HEAs processed by laser metal deposition(LMD),selective laser melting(SLM)and selec-tive electron beam melting(SEBM)is presented.Special attention is paid to the features of AM derived microstructures along with their outstanding properties and underlying mechanisms for various mate-rial processing combinations.The AM of interstitial solute hardening HEAs,HEA matrix composites as well as non-beam based AM of HEAs will also be addressed.The post-AM treatments and the strategies to fabricate defect-free HEAs are summarized.Finally,a conclusion of current state and future prospects of additive manufacturing of HEAs will be presented.     

Microstructures and liquid phase separation in multicomponent CoCrCuFeNi high entropy alloys

The microstructures of high entropy alloys of the system CoCrCuFe_xNi and CoCrCuFeNi_x (where x indicates the molar ratio, which, where not specified, is 1) have been investigated. Many Cu rich spheres were evident in the microstructure of CoCrCuFe_0.5Ni and CoCrCuFeNi_0.5 alloys, which indicates that liquid phase separation had occurred before solidification. During liquid phase separation, the original liquids separated into two liquids: Cu rich and Cu depleted. In contrast, in other alloys (x?=?1.0, 1.5 and 2.0), typical dendritic and interdendritic structures are obtained. Cu and/or Cr rich precipitates, with various morphologies, can be seen in the interdendritic region. Additionally, Cu rich nanoparticles and Cr rich bird shaped structures can be observed in the Cu rich spheres. Sluggish cooperative diffusion causes the element segregation and formation of nanoprecipitates in the microstructures. The calculated positive mixing enthalpies of CoCrCuFe_0.5Ni and CoCrCuFeNi_0.5 alloys are likely reasons for their liquid phase separation.     

Microstructural and mechanical behavior of a CoCrFeNiCu_4 non-equiatomic high entropy alloy

High entropy alloy(HEA)-based alloy design is experiencing a conceptual broadening from equiatomic alloys to non-equiatomic alloys.To provide experimental basis for designing Cu-rich non-equiatomic HEAs,in the current study,a dual phase(Cu-rich and CoCrFeNi-rich phases) face-centered cubic CoCrFeNiCu₄ alloy was systematically investigated.We provided initial and experiment-based understanding of the behavioral change of the alloy during a variety of thermal cycles and thermomechanical processing.The current results indicate that,during heating,preferred precipitation of Cu-rich particles occurs,leading to more pronounced compositional differences between the two constituent FCC phases and increased relative volume fraction of the Cu-rich phase.The Alloy exhibits a continuous melting and discontinuous solidification of the Cu-rich and CoCrFeNi-rich phases.After being cold-rolled to ~90 % thickness reduction,the alloy exhibits a recrystallization temperature higher than 800℃.Annealing at 300 and 500℃ led to strength reduction and/or ductility decrease;further increasing annealing temperature monotonically caused softening and ductilization due to decreased density of pre-existing dislocations.The yield-drop phenomena observed for the 900℃-and 1000℃-annealed specimens are associated with the locking of pre-existing dislocations by some "atmosphere",the nature of which warrants further elucidation.     

Chemical effects on He bubble superlattice formation in high entropy alloys

The probable formation mechanism of He bubble superlattices relies on long range anisotropic diffusion of self-interstitial atoms (SIAs). Here we study He ion irradiation of pure Ni and two equiatomic concentrated solid-solution alloys (CSAs) of FeNi and FeCrNiCo. It is expected from the significantly reduced diffusion of SIAs in CSAs, including high entropy alloys (HEAs), that long range anisotropic SIA migration cannot be active. We report the formation of a He bubble lattice in pure Ni, and for the first time in FeNi and FeCrNiCo systems under 30 keV He ion irradiation at room temperature. The ion dose and flux required to form a bubble superlattice increase with chemical complexity. Comparing to Ni, SIA clusters change directions more frequently due to anisotropic elementally-biased diffusion from the higher degree of chemical non-homogeneity in CSAs. Nevertheless, anisotropic 1-D diffusion of interstitial defects is possible in these complex alloys over incrementally longer time scales and irradiation doses. The sluggish diffusion, characteristic in CSAs, leads to smaller superlattice parameters and smaller bubble diameters. The chemical biased SIA diffusion and its effects on He evolution revealed here have important implications on understanding and improving radiation tolerance over a wide range of extreme conditions.     

NbMoTiVSi_x refractory high entropy alloys strengthened by forming BCC phase and silicide eutectic structure

In order to improve mechanical properties of refractory high entropy alloys,silicide was introduced and NbMoTiVSi_x(x=0,0.1,0.2,0.3,and 0.4,molar ratio) refractory high entropy alloys are prepared by vacuum arc melting.Phase composition,micro structure evolution and mechanical properties were systematically studied.Results show that the silicide phase is formed in the alloys with addition of silicon,and the volume fraction of silicide increases from 0 to 8.3 % with increasing of silicon.Microstructure observation shows that the morphology of dendrite changes from columnar to near equiaxed,eutectic structure is formed at grain boundaries and composed of secondary BCC phase and silicide phase.The average length of the primary and second dendrites decreases with the increasing of silicon.Whereas,the ratio of eutectic structure increases from 0 to 19.8 % with the increment of silicon.The refinement of microstructure is caused by heterogeneous nucleation from the silicide.Compressive tests show that the yield and ultimate strength of the alloys increases from 1141.5 MPa to 2093.1 MPa and from 1700.1 MPa to 2374.7 MPa with increasing silicon content.The fracture strain decreases from 24.7 %-11.0 %.Fracture mechanism is changed from ductile fracture to ductile and brittle mixed fracture.The improvement of the strength is caused by grain bounda ry strengthening,which includes more boundaries around primary BCC phase and eutectic structure in grain boundary,both of them is resulted from the formation of silicide.     

Mechanical,corrosion and magnetic behavior of a CoFeMn1.2NiGa0.8 high entropy alloy

In this study,a magnetic high entropy alloy (HEA) of CoFeMn1.2NiGa0.8 was designed and prepared by arc melting in order to investigate its mechanical,corrosion and magnetic behavior.The results show that the alloy mainly possesses body-centered cubic (BCC) phase and face-centered cubic (FCC) phase.A high compressive strength of 1450 MPa,a strain of 18.5 ％ and a relatively low yield strength of 303 MPa in as-cast condition at room temperature can be achieved in the present alloy.In-situ high-energy X-ray diffraction technique was employed to reveal the deformation mechanism of CoFeMn1.2NiGa0.8 under uniaxial compression and the results show that the competition between BCC phase and FCC phase plays a significant role during the compressive process.The corrosion behavior of CoFeMn1.2NiGa0.8 was investigated in 3.5 wt％ NaCl solution and it turned out that the alloy possessed good corrosion resistance.At last,the magnetic behavior of the CoFeMn1.2NiGa0.8 alloy was studied and it can present a high saturation magnetization of 94.5 emu/g and a coercivity of 26.4Oe at 4 K.This work indicates that the present CoFeMn1.2NiGa0.8 HEA has promising applications as future magnetic functional materials.     

Effect of Ca and Sr additions on high temperature and corrosion properties of Mg-4Al-2Sn based alloys

This study examined the effects of Ca and Sr addition on the creep and corrosion properties of Mg-Al-Sn based alloys with the aim of developing new Mg-Al-Sn-x alloys for automotive powertrain applications. The materials were cast using the squeeze casting process to obtain a dense microstructure without pores. Creep tests were carried out at a constant temperature between 150 °C and 200 °C and a constant applied stress between 50 and 80 MPa until the minimum creep rate had been reached. Potentiodynamic and immersion tests were carried out to evaluate corrosion properties of the alloys. The creep and corrosion resistance were improved by adding Ca and Sr.     

Homogeneous corrosion of high pressure torsion treated Mg-Zn-Ca alloy in simulated body fluid

Magnesium alloys have got extensive attention as biodegradable implant materials due to their biodegradability in the physiological environment and similar elastic modulus to natural bone. But their poor corrosion resistance is a dominant problem that limits their clinical application due to the inhomogeneous distribution of the second phase. Nevertheless, after high pressure torsion (HPT) treatment, the second phase became nano-sized particles and distributed uniformly in grain interiors instead of along grain boundaries. The immersion tests indicated that the HPT-treated sample exhibited homogeneous corrosion resulting from the uniform distribution of the second phase. The results of the potentiodynamic polarization experiments showed that, compared with the as-cast alloy, the corrosion current density of the HPT-treated alloy decreased from 5.3 × 10^− 4 A/cm² to 3.3 × 10^− 6 A/cm².     

Alloying behaviour,thermal stability and phase evolution in quinary AlCoCrFeNi high entropy alloy

An equiatomic quinary AlCoCrFeNi high entropy alloy (HEA) has been synthesized by mechanical alloying. Milled powder after 30?h shows good chemical homogeneity and refined morphology with a mean particle size of ～4?μm. Solid solution phase with BCC crystal structure (a?=?2.89?±?0.02?Å) has been confirmed from XRD and transmission electron microscopy in the as-synthesized high entropy alloy. The milled alloy powder is not thermally stable. Differential scanning calorimetric (DSC) thermogram of 30?h milled powder exhibits the presence of a small peak at ～600?°C (873?K) with a thermal shift near the peak. This thermal shift indicates the diffusive type of phase transformation in this alloy while heating. The analysis of the in-situ heating X-ray diffraction patterns at various temperatures extends support to the diffusive nature of the phase transformation. Upon heat treatment, the disordered BCC solid solution phase partially transforms to Ni₃Al prototype L1₂ phase which precipitates at a lower temperature (350?°C (623?K)) as observed by in-situ XRD experiments. However, at high temperature annealing (575–800?°C (848–1073?K)) the evolution of a partially ordered BCC phase (B2) with lattice parameter (a?=?2.87?±?0.02?Å), and L1₂ phase (a?=?3.58?±?0.05?Å), along with tetragonal σ phase (a?=?8.8?Å and c?=?4.53?Å) are observed. Similar types of phases have also been identified after annealing and microwave sintering at 800?°C (1073?K) & 900?°C (1173?K) respectively. The transformation of ordered BCC phases along with two intermetallics such as L1₂ phase and σ phase suggests that the evolution of the high entropy phase in the milled condition leads to a combination of high entropy and medium entropy phases in the annealed condition.     

Effect of Mo and Ti on the microstructure and microhardness in AlCoFeNiMoTi high entropy alloys prepared by mechanical alloying and conventional sintering

In this investigation, the synthesis of equiatomic AlCoFeNi, AlCoFeNiMo, AlCoFeNiTi, and AlCoFeNiMoTi high entropy alloys, fabricated by mechanical alloying and conventional sintering processes is presented aiming to elucidate the effect of Mo and Ti additions on the properties of the AlCoFeNi base system. X-ray diffraction studies revealed that after 15 h of milling, only BCC and FCC structures were formed. It was also found that by increasing the crystallite size after sintering, phase transformations and composition variations were observed for all the systems studied but BCC and FCC structures prevailed. Further, the addition of the different alloying elements had a significant effect on the microhardness of the HEAs and particularly, the addition of Mo and Ti to form the AlCoFeNiMoTi system presented the highest value of 894 HV0.2. Finally, it was also found that Mo- containing alloys presented considerable porosity.     

高熵合金摩擦磨损性能的研究进展EI北大核心CSCD

近年来,高熵合金成为金属材料领域的研究热点。高熵合金处于相图中心区域,具有广阔的合金成分空间和组织结构形成可能;成分和制备工艺的协同调控,能够获得更丰富的组织结构;非常规的化学结构有望突破传统抗磨、润滑合金的性能极限。本文讨论了耐磨高熵合金的分类,分析了化学活泼金属、软金属、难熔金属的添加对高熵合金抗磨、润滑性能的影响规律;总结了非金属元素和陶瓷相的添加对高熵合金基复合材料摩擦磨损性能的影响;综述了热处理和表面工程技术对高熵合金表面组织结构和摩擦磨损行为的作用;讨论了苛刻工况下抗磨润滑高熵合金的设计方法。对未来高熵合金在摩擦磨损领域的研究和应用进行了展望,高熵合金在解决传统合金的瓶颈问题上具有巨大潜力,如在极端工况下实现稳定润滑抗磨、保证特定功能作用下实现抗磨。     

Composition and phase structure dependence of mechanical and magnetic properties for AlCoCuFeNix high entropy alloys

In this study, the effects of composition and phase constitution on the mechanical properties and magnetic performance of AlCoCuFeNi_x (x = 0.5, 0.8, 1.0, 1.5, 2.0, 3.0 in molar ratio) high entropy alloys (HEAs) were investigated. The results show that Ni element could lead to the evolution from face centered cubic (FCC), body centered cubic (BCC) and ordered BCC coexisting phase structure to a single FCC phase. The change of phase constitution enhances the plasticity but reduces the hardness and strength. One of the interesting points is the excellent soft magnetic properties of AlCoCuFeNi_x HEAs. Soft magnetic performance is dependent on composition and phase transition. AlCoCuFeNi_1.5 alloy, achieving a better balance of mechanical and magnetic properties, could be applied as structure materials and soft magnetic materials (SMMs). High Curie temperature (>900 K) and strong phase stability below 1350 K of AlCoCuFeNi_0.5 alloy confirm its practicability in a high-temperature environment. Atomic size difference (δ) is utilized as the critical parameter to explain the lattice strain and phase transformation induced by Ni addition.     

Microstructure modifications and corrosion behaviors of Cr4Mo4V steel treated by high current pulsed electron beam

In this work, Cr4Mo4V steel was irradiated by high energy current pulsed electron beam (HCPEB) with energy density of 6 J/cm². Morphology and phase composition of the surface layer were analyzed using scanning electron microscopy (SEM) and glancing angle X-ray diffraction (GXRD). The crater-like morphology was observed on surface after HCPEB treatment, and the thickness of melted layer was ∼7 μm. Results from GXRD revealed that HCPEB treatment could suppress martensite transition and the content of retained austenite in the melted layer increased with irradiation number. The corrosion resistance was evaluated by electrochemical polarization tests in neutral 3.5% NaCl solution. Compared with the untreated Cr4Mo4V steel, corrosion potential of the samples treated by HCPEB improved and the corrosion current density decreased. The improved corrosion resistance is attributed to the absence of the carbide, formation of retained austenite and dissolution of alloy elements, particularly of Cr and Mo, into the matrix.     

Characterization and properties of CuZrAlTiNi high entropy alloy coating obtained by mechanical alloying and vacuum hot pressing sintering

CuZrAlTiNi High entropy alloy (HEA) coating was synthesized on T10 substrate using mechanical alloying (MA) and vacuum hot pressing sintering (VHPS) technique. The MA results show that the final product of as-milled powders is amorphous phase. The obtained coating sintered at 950 °C is compact and about 0.9 mm in thickness. It is composed of a couple of face-centered cubic (FCC), one body-centered cubic (BCC) solid solutions and AlNi₂Zr phase. The interface strength between coating and substrate is 355.5 MPa measured by three point bending test. Compared with T10 substrate, the corrosion resistance of CuZrAlTiNi HEA coating is enhanced greatly in the seawater solution, which is indicated by the higher corrosion potential, wider passivation region, and secondary passivation. The average microhardness of the coating reaches 943 HV_0.2, and is about 3.5 times higher than the substrate, which is mainly ascribed to the uniformly dispersed nano-size precipitates, phase boundary strengthening and solid solution strengthening. Moreover, the wear resistance of the coating is slightly improved in comparison with the substrate.     

Formation rules of single phase solid solution in high entropy alloys

A new empirical rule to form single phase solid solution structured high entropy alloys was proposed by integrating three dependent parameters, i.e. atomic size difference δ, mixing enthalpy ΔH_mix and valence electron concentration VEC. It was found that the single phase face centred cubic solid solution will form if δ?<?4.27%, ??7.27?kJ?mol^??1?<?ΔH_mix?<?4?kJ?mol^??1 and VEC>8, while the single phase body centred cubic solid solution will form in the case of δ?<?4.27%, ??7.27?kJ?mol^??1?<?ΔH_mix?<?4?kJ?mol^??1 and VEC?<?6.87. This empirical rule was verified by typical high entropy alloys that have been reported.     

Microstructure and corrosion behavior of electrodeposited nickel prepared from a sulphamate bath

Electrodeposited nickel was prepared from a sulphamate bath at different current densities ranging from 0.01 A cm^− 2 to 0.1 A cm^− 2. Based on the analysis of the microstructure, the corrosion behavior of the electrodeposited nickel in 3.5%NaCl solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). All the electrodeposits display active-passive-transpassive behavior in potentiodynamic polarization process. The electrodeposits with the best corrosion resistance are obtained at 0.05 A cm^− 2. As for other electrodeposits, the corrosion potential and breakdown potential decrease with increasing current density used to prepare electrodeposits. However, the variation of both corrosion current density and passive current density is opposite to that of the corrosion potential. The changes in the charge-transfer resistance determined from the impedance spectra are consistent with the results determined from potentiodynamic measurements.     

Microstructure and mechanical properties of CoCrNi-Mo medium entropy alloys:Experiments and first-principle calculations

The effect of Mo additions on the microstructures and mechanical properties of CoCrNi alloys was investigated,meanwhile,ab initio calculations are performed to quantitatively evaluate the lattice distortion and stacking fault energy(SFE).The yield strength,ultimate tensile strength,and elongation of(CoCrNi)₉₇Mo₃alloy are 475 MPa,983 MPa and 69%,respectively.The yield strength is increased by~30%and high ductility is maintained,in comparison with CoCrNi alloy.Besides the nano-twins and dislocations,the higher density of stacking faults is induced during the tensile deformation for(CoCrNi)₉₇Mo₃alloy.Ab initio calculation results indicate the mean square atomic displacement(MSAD)and SFE value of(CoCrNi)₉₇Mo₃alloy is 42.6 pm²and-40.4 mJ/m²at 0 K,respectively.The relationship between mechanical properties and MSAD,SFE for various multiple principal element alloys is discussed.     

The relationship between thermo-mechanical history,microstructure and mechanical properties in additively manufactured CoCrFeMnNi high entropy alloy

Here,a single-track CoCrFeMnNi high entropy alloy(HEA)was successfully fabricated by laser melting deposition(LMD).Combining the experimental observations and numerical simulation,the microstruc-ture and mechanical properties of the as-deposited parts were systematically studied from the perspective of thermo-mechanical history experienced during the LMD process.The strengthening mech-anisms of the LMDed CoCrFeMnNi HEA parts were clarified.The frictional stress strengthening,grain boundary strengthening and dislocation strengthening contributed the whole yield strength of the parts.Dislocation strengthening dominated the strengthening mechanism.It was expected that the establish-ment of the relationship between thermo-mechanical history,microstructure and mechanical properties of the LMDed CoCrFeMnNi HEA could shed more insights into achieving HEA parts with the desired microstructure and high performance.