Novel (CoFe_2NiV_(0.5)Mo_(0.2))_(100-x)Nb_x Eutectic High-Entropy Alloys with Excellent Combination of Mechanical and Corrosion Properties

The emergence of eutectic high-entropy alloys(EHEAs) offers new insights into the design of next generation structural alloys,which is due to their stable dual-phase microstructure and outstanding mechanical properties from room to elevated temperatures.In this work,a series of(CoFe_2 NiV_(0.5)Mo_(0.2))_(100-x)Nb_x(0≤x ≤12) EHEAs were designed and prepared via vacuum arc-melting.Typical eutectic microstructure composing lamellar face-centered cubic solid solution phase and C14 Laves phase appears in the as-cast EHEA when x=9.The microstructure turns to hypoeutectic or hypereutectic when x is below or beyond that critical value accordingly.The volume fraction of the hard Laves phase is proportional to the Nb addition,leading to the strength increment yet at the expense of ductility at room temperature.In particular,the EHEA having4 at% Nb shows a compressive strength of 2.1 GPa with an elongation to fracture of 45%,while EHEAs containing 9 and10 at% Nb exhibit ultrahigh yield strengths of over 1.4 GPa.The effect of Nb addition on the corrosion resistance of this Crfree EHEA system was also studied.The EHEA containing 9 at% Nb has the best anti-corrosion performance in the 3.5 wt%NaCl solution at 298±1 K,indicating a good combination of mechanical and corrosion properties.     

Preparation of Ti–Nb–Ta–Zr alloys for load-bearing biomedical applications

Ti–Nb–Ta–Zr alloys for biomedical applications were successfully fabricated by arc melting(AM) and diffusion bonding.The microstructure, mechanical properties and electrochemistry behavior in a simulated body fluid(SBF) were studied.It shows that melted Ti–Nb–Ta–Zr alloy mainly contains β phase although there are a few Ti-rich phases and micropores, the number of which is lower than that in sintered sample with a few Ti-rich and Ta-rich phases.The melted alloys present higher strength(1224 MPa), Young's modulus(15.3 GPa) and corrosion potential(-0.34 V) in SBF, while total recovery strain ratio(67.5%) and pseudoelastic strain ratio(8.4%) of sintered Ti–Nb–Ta–Zr alloy keep higher value than 35.7%and 5.0% for melted Ti–Nb–Ta–Zr.The reasons were discussed based on the microstructure of the Ti–Nb–Ta–Zr alloys.     

Microstructure and mechanical properties of laser melting deposited Ti2Ni3Si/NiTi Laves alloys

Two Ti2Ni3Si/NiTi Laves phase alloys with chemical compositions ofNi-39Ti-11Si and Ni-42Ti-8Si （%, mole fraction, the same below）, respectively, were fabricated by the laser melting deposition manufacturing process, aiming at studying the effect of Ti, Si contents on microstructure and mechanical properties of the alloys. The Ni-39Ti-llSi alloy consisting of Ti2Ni3Si primary dendrites and Ti2Ni3Si/NiTi eutectic matrix is a conventional hypereutectic Laves phase alloy while the Ni-42Ti-8Si alloy being made up of NiTi primary dendrites uniformly distributed in Ti2Ni3Si/NiTi eutectic is a new hypoeutectic alloy. Mechanical properties of the alloys were investigated by nano-indentation test. The results show that the decrease of Si and the increase of Ti contents change the microstructures of the alloys from hypereutectic to hypoeutectic, which influences the mechanical properties of the alloys remarkably. Corrosion behaviors of the alloys were also evaluated by potentiodynamic anodic polarization curves.     

Hot-press sintering of MA Fe-based nanocrystalline／amorphous soft magnetic powder

Microstructures and magnetic properties of Fe84 Nb7 B9, Fe80 Ti8 B12 and Fe32 Ni36 (Nb/Ⅴ) 7 Si8 B17 powders and their bulk alloys prepared by mechanical alloying(MA) method and hot-press sintering were studied. The results show that: 1) After MA for 20 h, nanocrystalline bcc single phase supersaturated solid solution forms in Fe84-Nb7 B9 and Fe8o Ti8 B12 alloys, amorphous structure forms in Fe32 Ni36 Nb7 Si8 B17 alloy, duplex microstructure composed of nanocrystalline γ-(FeNi) supersaturated solid solution and trace content of Fe2B phase forms in Fe32 Ni36-V7 Si8 B17 alloy. 2) The decomposition process of supersaturated solid solution phases in Fe84 Nb7 B9 and Fe80 Ti8 B12alloys happens at 710 -780 ℃, crystallization reaction in Fe32 Ni36 Nb7 Si8 B17 alloy happens at 530 ℃ (the temperature of peak value) and residual amorphous crystallized further happens at 760 ℃ (the temperature of peak value), phase decomposition process of supersaturated solid solution at 780 ℃ (the temperature of peak value) and crystallization reaction at 431 ℃ (the temperature of peak value) happens in Fe32 Ni36 V7Si8B17 alloy. 3) under 900 ℃, 30 MPa,0.5 h hot-press sintering conditions, bulk alloys with high relative density(94.7%- 95.8%) can be obtained. Except that the grain size of Fe84 Nb7B9 bulk alloy is large, superfine grains (grain size 50 - 200 nm) are obtained in other alloys. Except that single phase microstructure is obtained in Fe80 Ti8B12 bulk alloy, multi-phase microstructures are obtained in other alloys. 4) The magnetic properties of Fe80 Ti8 B12 bulk alloy(Bs = 1.74 T, Hc = 4.35 kA/m) are significantly superior to those of other bulk alloys, which is related to the different phases of nanocrystalline or amorphous powder formed during hot-press sintering process and grain size.     

Microstructure Characteristics and Mechanical Properties of Nb-17Si-23Ti Ternary Alloys Fabricated by In Situ Reaction Laser Melting Deposition

Fabrication of ternary Nb–17 Si–23 Ti alloys was attempted by in situ reaction laser melting deposition(LMD)with dual powder feeding method from Nb-28 at.% Ti powder mixture and pure Si powder. The microstructures of the asdeposited alloys were examined with scanning electronic microscope, and the phase constituents were analyzed by X-ray energy-dispersive spectrometer and X-ray diffraction. Furthermore, the effect of laser power on microstructure characteristics, microhardness and indentation fracture toughness was also investigated. The in situ reaction LMD process resulted in remarkable refinement of the microstructure. The as-deposited samples mainly consisted of NbSS,metastable(Nb Ti)3 Si and Ti-rich NbSS. With the increase in the laser power from 1000 to 2000 W, the NbSSmorphology changed from discontinuous dendritic to near equiaxed, but the Ti-rich NbSSphase tended to vanish. Furthermore, with the increase in the laser power, the microhardness of as-deposited samples increased from 822 to 951 HV, while the indentation fracture toughness was improved from 12.3 to 14.1 MPa m1/2. The corresponding mechanism is also discussed.     

Development of High Strength and Toughness Non-Heated Al–Mg–Si Alloys for High-Pressure Die-Casting

Based on the 3 factors and 3 levels orthogonal experiment method, compositional effects of Mg, Si, and Ti addition on the microstructures, tensile properties, and fracture behaviors of the high-pressure die-casting Al-x Mg-y Si-z Ti alloys have been investigated. The analysis of variance shows that both Mg and Si apparently infl uence the tensile properties of the alloys, while Ti does not. The tensile mechanical properties are comprehensively infl uenced by the amount of eutectic phase(α-Al + Mg_2Si), the average grain size, and the content of Mg dissolved into α-Al matrix. The optimized alloy is Al-7.49 Mg-3.08 Si-0.01 Ti(wt%), which exhibits tensile yield strength of 219 MPa, ultimate tensile strength of 401 MPa, and elongation of 10.5%. Furthermore, contour maps, showing the relationship among compositions, microstructure characteristics, and the tensile properties are constructed, which provide guidelines for developing high strength and toughness Al–Mg–Si–Ti alloys for high-pressure die-casting.     

Mechanical behaviors of NiAl-Cr(Mo)-based near eutectic alloy with Ti, Hf, Nb and W additions

Effects of Ti, Hf, Nb and W alloying elements addition on the microstructure and the mechanical behaviors of NiAl-Cr(Mo) intermetallic alloy were investigated by means of XRD, SEM, EDX and compression tests. The results show that Ni-31Al-30Cr-4Mo-2(Ti, Hf, Nb, W) alloy consists of four phases: NiAl, α-Cr solid solution, Cr2Nb and Ni2Al(Ti, Hf). The mechanical properties are improved significantly compared with the base alloy. The compression yield strength at 1 373 K is 467 MPa and the room temperature compression ductility is 17.87% under the strain rate of 5.56×10-3 s-1, due to the existence of Cr2Nb and Ni2Al(Ti, Hf) phases for strengthening and Ti solid solution in NiAl matrix and coarse Cr(Mo, W) solid solution phase at cellular boundaries for ductility. The elevated temperature compression deformation behavior of the alloy can be properly described by power-law equation: ε =0.898 σ8.47exp[-615/(RT)].     

Thermodynamic Characteristic and Phase Evolution in Immiscible Cr–Mo Binary Alloys

This paper systematically reports the thermodynamic characteristic and phase evolution of immiscible Cr–Mo binary alloy during mechanical alloying(MA) process. The Cr–35Mo(in at%) powder mixture was milled at 243 and258 K, respectively, for different time. For comparative study, Cr–15Mo and Cr–62Mo powder mixtures were milled at 243 K for 18 h. Solid solution Cr(Mo) with body-centered cubic(bcc) crystal structure and amorphous Cr(Mo) alloy was obtained during MA process caused by high-energy ball milling. Based on the Miedema's model, the free-energy change for forming either a solid solution or an amorphous in Cr–Mo alloy system is positive but small at a temperature range between 200 and 300 K. The thermodynamical barrier for forming alloy in Cr–Mo system can be overcome when MA occurs at 243 K, and the supersaturated solid solution crystal nuclei with bcc structure form continually, and three supersaturated solid solutions of Cr–62Mo, Cr–35Mo and Cr–15Mo formed. Milling the Cr–35Mo powder mixture at 258 K, the solid solution Cr(Mo) forms firstly, and then the solid solution Cr(Mo) transforms into the amorphous Cr(Mo)alloy with a few of nanocrystallines when milling is prolonged. At higher milling temperature, it is favorable for the formation of the amorphous phase, as indicated by the thermodynamical calculation for immiscible Cr–Mo alloy system.     

Microstructure Evolution and Phase Transformation of Ti–6.5Al–2Zr–Mo–V Alloy During Thermohydrogen Treatment

Thermohydrogen treatment(THT) is an effective way to refine microstructure and improve the mechanical properties of the titanium alloys.In the current work,as-cast Ti–6.5Al–2Zr–Mo–V alloy was hydrogenated with different hydrogen contents and processed solution aging.Accordingly,the microstructure evolution and phase transformation were analyzed.Results show that during solution aging,eutectoid decomposition occurs and the product is a mixture of coarse primary a,fine eutectoid product and undecomposed bH.The size of primary a is closely dependent on the hydrogen content,and large primary a can be obtained at medium hydrogen content.Further,the influence of hydrogen content on the growth of primary a phase was revealed.The primary a is much fine,and the eutectoid product is relatively homogeneous with 0.984 wt% H.After THT,the ultimate strength is beyond 1,100 MPa that has increased by 23.15% compared with that in as-cast state.     

Electrochemical properties of TiV-based hydrogen storage alloys

The electrochemical properties of the super-stoichiometric TiV-based hydrogen storage electrode alloys(Ti0.8Zr0.2)(V0.533Mn0.107Cr0.16Ni0.2)x(x=2,3,4,5,6)were studied.It is found by XRD analysis that all the alloys mainly consist of a C14 Laves phase with hexagonal structure and a V-based solid solution phase with BCC structure.The lattice parameters and the unit cell volumes of the two phases decrease with increasing x.The cycle life.the linear polarization.the anode polarization and the electrochemical impedance spectra of the alloy electrodes were investigated systematically.The overall electrochemical properties of the alloy electrode are found improved greatly as the result of super-stoicfhiometry and get to the best when x=5.     

Element Segregation and Solidification Behavior of a Nb,Ti, Al Co-Strengthened Superalloy ЭК151

The as-cast microstructure, element segregation and solidification behavior of a multi-alloyed superalloy ЭК151 have been investigated. The results show that the severe element segregation leads to the complicated precipitations at the inter-dendritic region, including η-Ni_3(Ti, Nb), eutectic(γ + γ') and Laves, which shows the characteristics of both Ti, Al-strengthened and Nb-strengthened alloys. Differential thermal analysis, heating and quenching tests reveal the solidification sequence as follows: Liquids →γ matrix →(Nb, Ti)C →η-Ni 3(Ti, Nb) →eutectic( γ+γ') → Laves. The melting points are between 1250 and 1260 °C for(Nb, Ti)C, between 1200 and 1210 °C for η phase, between 1180 and 1190 °C for eutectic(γ+γ') and Laves. γ' initially precipitates from matrix at 1150 °C and achieves the maximum precipitation at 1130 °C. According to the microstructure evolution captured during solidification and composition analysis by an energy dispersive spectrometer and electron probe microanalyzer,(Nb, Ti)/Al ratio is put forward to explain the formation of η-Ni_3(Ti, Nb) and eutectic( γ+γ'). The solidification of γ matrix increased the Nb, Ti concentration in the residual liquids, so the high(Nb, Ti)/Al ratio would result in the formation of η-Ni_3(Ti, Nb); the precipitation of the phase consumed Nb and Ti and decreased the(Nb, Ti)/Al ratio in the liquid, which led to the precipitation of eutectic(γ + γ'). Laves formed by the sides of η-Ni_3(Ti, Nb) and in front of the eutectic( γ + γ') after Al, Ti were further depleted by the two phases and Cr, Co, Mo were rejected to liquids.     

电弧熔炼Cr-40Ti-20Nb三元合金的组织与压缩性能

采用真空非自耗电弧熔炼的方法制备了Cr-40Ti-20Nb(原子分数)合金.利用光学显微镜(OM),X射线衍射(XRD),扫描电镜(SEM)分析了合金不同凝固位置处的相组成和组织形态.结果表明,合金的凝固组织主要是由Laves相Cr2(Nb,Ti)枝晶组织及丁i的固溶体组成.同时对合金的显微硬度和压缩性能进行了测试,显微硬度值在6 112～7 350MPa之间,比Laves相Cr2Nb的显微硬度低20％.另外室温压缩屈服强度可达1 334-1 524 MPa,与采用多种合金化元素的Cr-12Nb-4Re-2Al合金相当,这表明Ti元素的添加不仅可以改变Laves相Cr2Nhb合金的凝固组织,而且对其塑性也有很大程度的改善.     

硅含量对一种耐蚀合金组织与耐蚀性能的影响

设计并熔炼了不同硅含量的三种耐磨耐蚀合金30Ni-35Cr-6.5Mo-1Nb-0.25N-xSi(x=1、2、3)-Fe,分别命名为1Si、2Si、3Si合金,利用SEM、XRD、硬度测试以及电化学测试研究了三种合金的组织和性能。结果表明,三种合金均为双相结构,两相分别为γ相和σ-Cr₁₃Ni₅Si₂相,且随着硅含量的增加,σ-Cr₁₃Ni₅Si₂相所占的比例逐渐增加,合金的洛氏硬度也逐渐增大。动电位极化曲线以及电化学阻抗谱结果表明,3Si合金在10%H₂SO₄溶液中的耐蚀效果最优,而1Si合金的耐蚀性最差。合金硬度和耐蚀能力的提高主要得益于合金中σ相比例的增大。     

Effect of Heat Treatment on the Microstructure and Mechanical Properties of the Modified 718 Alloy

M718 alloy with an extra high Mo content of 7.50 wt% which reduced Nb addition and increased Al and Ti additions within the composition specifications of 718 alloy has been designed to increase the service temperature of 718 alloy. And the effect of the heat treatment on the microstructure and mechanical properties of M718 alloy has been investigated in this study. The results showed that Laves phase precipitated on the grain boundaries of M718 alloy instead of d-Ni_3 Nb phase in718 alloy, and y'and y'phases precipitated in the matrix of M718 alloy as that in 718 alloy. Increasing the solution temperature from 960 to 1050 ℃ noticeably reduced the intergranular precipitation of Laves phase. The precipitation of Laves phase was appropriate at 1020 ℃ for improving the grain boundary cohesion. Increasing the two-stage aging temperatures markedly increased the sizes of y' andy'phases. As a result, the strength of M718 alloy increased.     

Ti对Zr-Nb二元合金β结构稳定性和力学性能的影响

利用团簇成分式方法设计了[Zr-Zr₁₄](Zr,Nb)3系列二元合金,进而采用Ti替代部分Zr形成三元合金;采用真空铜模吸铸快冷技术制备合金棒材,利用XRD、OM、TEM等对其进行微观组织表征和力学性能测试。结果表明,随着Nb含量增加,Zr-Nb二元合金基体结构从hcp-α向bcc-β转变,β结构稳定的同时伴随有ω相析出。适量Ti添加可显著抑制ω相的析出,进一步改善了合金的β结构稳定性。其中,单相bcc-β合金[Ti-Zr₁₄]Nb3(Zr-17.37Nb-2.98Ti,质量分数,%)表现出低弹性模量(E=57 GPa)的同时具有最佳的力学性能(屈服强度σYS=557 MPa、延伸率δ=15.5%)。     

Ti对Nb-20Si-5Al超高温合金性能的影响

采用放电等离子烧结法(SPS)制备了Nb-20Si-5Al-xTi(x=0,18,20,22,摩尔分数)超高温合金,研究了Ti加入量对Nb-20Si-5Al合金的室温断裂韧度和高温抗氧化性的影响。结果表明,随着Ti加入量的增加,超高温合金的相组成由Nbss、Nb₅Si₃和Al3Nb相转变成为Nbss、(Nb,Ti)5Si3和Ti相。Ti能明显改善Nb-20Si-5Al超高温合金的断裂韧度和高温抗氧化性能,随着Ti加入量的增加均先提高然后降低,在Ti加入量为20%时,合金断裂韧度最大,为7.41 MPa·m^1/2,相比未加Ti时提高了约56.9%,其高温氧化速率最低,为0.72×10^-4g²/(cm⁴·h)。添加Ti元素后,其氧化产物中出现Ti₂Nb₁₀O₂₉、TiNb₂O₇、TiO₂等,可以提高其氧化膜的致密性,从而提高高温抗氧化性能。     

Tb0.3Dy0.7Fe1.95-xTix（x=0,0.03,0.06,0.09）合金的微观组织与磁致伸缩性能

利用高真空非自耗电弧炉制备了Tb_0.3Dy_0.7Fe_1.95-xTi_x（x=0,0.03,0.06,0.09）合金,系统研究了不同Ti含量Tb_0.3Dy_0.7Fe_1.95-xTi_x合金的晶体结构、微观组织、磁致伸缩性能及它们之间的关系.结果表明:添加Ti后的Tb_0.3Dy_0.7Fe_1.95-xTi_x合金基体相仍为MgCu₂型Laves相结构,Ti取代了Tb_0.3Dy_0.7Fe_1.95合金中比其自身半径大的稀土原子Tb和Dy而使晶格常数减小.添加Ti后,初生相TiFe₂的形成使得凝固液体中富含R（R=Tb,Dy）从而抑制了有害相RFe₃的生成,Ti在基体相RFe₂中和富R相中都可溶解,分别形成了（R,Ti）Fe₂基体相和富（R,Ti）相.Ti的添加量对磁致伸缩性能的影响很大,当x=0.03时,Ti的添加使磁致伸缩性能较Tb_0.3Dy_0.7Fe_1.95母合金提高幅度最大,但当x=0.09时,由于顺磁相TiFe₂和富（R,Ti）相的析出对磁致伸缩性能不利,但相对于Tb_0.3Dy_0.7Fe_1.95母合金也有少量提高.     

Design Fe-based Eutectic Medium-Entropy Alloys Fe_2NiCrNb_x

High-entropy alloys have attracted broad research interests due to their unique and intriguing mechanical properties. As a category of high-entropy alloys, eutectic high-entropy alloys combine the advantages of eutectic and high-entropy alloys, with excellent mechanical properties and casting properties. Some eutectic high-entropy alloys have been developed and shown exciting properties. In this paper, based on the physical metallurgy of eutectic high-entropy alloy, medium-entropy alloy Fe_2NiCrNb_x was designed. The as-cast alloy is composed of FCC and Laves phases, Nb element promotes the formation of primary Laves phase, and the hardness of the alloy increases with the increase in Nb element. Among the four alloys, the eutectic chemical composition at eutectic point is Fe_2NiCrNb_(0.34); the alloy has a good strength and plastic balance. The ultimate comprehensive strength is 2267 MPa, and the fracture strain is 30.8%. The experiment data and analyses identified the eutectic points and the excellent mechanical behavior. Moreover, the expensive Co element was replaced by Fe element. This cheap medium-entropy alloy has promising prospect in the consideration of the cost performance ratio.