铸态和退火态Al_(0.5)CoCrFeNiTi_(0.5)高熵合金的组织与性能

采用真空电弧熔炼法熔炼出Al_(0.5)CoCrFeNiTi_(0.5)高熵合金,并在600、800和1000℃下进行真空退火热处理。利用X射线衍射仪(XRD)、光学显微镜(OM)、电子探针(EPMA)、硬度计、万能试验机以及电化学工作站对合金铸态和不同温度退火态的微观组织、硬度、压缩力学性能以及在3.5%NaCl溶液中的耐蚀性进行研究。组织分析表明:铸态和退火态的Al_(0.5)CoCrFeNiTi_(0.5)合金均由富(Cr,Fe)的FCC、富(Al,Ni,Ti)的BCC和σ三相组成,但退火处理使合金的组织形貌和各相的相对含量发生了改变,铸态下的粗大白色FCC柱状晶转变为细小的FCC+BCC+σ的混合组织;随着退火温度的升高,BCC和σ相含量增加。800℃退火态合金成分均匀性最好,1000℃退火态合金由于退火温度过高,组织粗大,元素偏析重新加剧。硬度试验和压缩试验结果表明:合金在铸态和3种温度退火态下的硬度都较高,表现出良好的抗回火软化能力;800℃退火态合金中由于BCC和σ相的增加,其硬度和屈服强度最高,但塑性最差。1000℃退火态合金由于大量σ相的析出以及组织粗大,其屈服强度、断裂强度和压缩形变率都急剧降低。600℃退火态合金具有理想的FCC、BCC和σ相的组成含量,其综合力学性能最好。电化学腐蚀试验表明:铸态和3种温度退火态的合金在3.5%NaCl溶液中都表现出良好的耐蚀性,800℃退火态合金由于其成分均匀性最好,耐蚀性最好。     

AlCoCrFeNi高熵合金铸态与退火态的耐蚀性研究

采用真空电弧熔炼法熔炼出AlCoCrFeNi高熵合金,并在600℃、800℃、1000℃下进行了真空退火处理。利用XRD、SEM、EDS和电化学腐蚀试验对合金退火前后的微观组织以及在3.5%的NaCl溶液、0.5mol/L的NaOH溶液、0.5mol/L的H2SO4溶液中的耐蚀性进行了研究。组织分析表明合金在铸态和三种温度的退火态下均没有复杂结构的脆性金属间化合物生成;在铸态和600℃、800℃退火处理后合金均由简单的BCC结构构成,成树枝晶形态;在1000℃退火处理后,树枝晶消失,微观组织转变为短棒状的FCC析出相和BCC基体交替排列;在铸态及三种温度的退火态下,Co、Fe、Ni分布较均匀,Cr、Al存在一定的偏析,Al在铸态偏析最严重,Cr在800℃退火态偏析最严重。电化学腐蚀试验结果表明铸态和三种温度退火态的AlCoCrFeNi 合金在 3.5%的 NaCl 溶液和0.5mol/L 的 NaOH 溶液中的耐蚀性优良;在 3.5%的 NaCl 溶液中1000℃退火态的耐蚀性最好;在0.5mol/L 的 NaOH 溶液中,4种状态的合金耐蚀性相差不大;在 0.5mol/L H2SO4 溶液中,4种状态的合金都发生了钝化现象,1000℃退火态维钝电流密度最小,破钝电位最高,耐蚀性最好。     

退火处理对CoFeNiVTi高熵合金组织和性能的影响

采用真空电弧炉熔炼和铜模吸铸法制备了CoFeNiVTi高熵合金柱状试样，并对其在氩气保护条件下进行了800℃退火20 h的处理。利用X射线衍射仪(XRD)、扫描电镜(SEM)、能谱仪(EDS)和压缩实验等研究了铸态和退火态CoFeNiVTi合金的显微组织和力学性能。结果表明：铸态合金为单相BCC固溶体结构，显微组织呈现典型的柱状晶特征，合金元素均匀分布；退火态合金由BCC基体和金属间化合物Ni₂V₃型的σ析出相构成，σ析出相具有明显的取向特征，呈粗大板条状和细小针状。与铸态合金相比，由于σ析出相的存在，退火后合金的断裂强度有所下降，但仍高达2.5 GPa,硬度则显著提高至800 HV0.2。     

退火处理对AlCoCuFeNi0.2高熵合金组织与性能的影响

利用电弧熔炼技术制备得到AlCoCuFeNi_(0.2)高熵合金,研究了铸态与900℃退火态高熵合金的组织、力学性能、磁学性能之间的差异。研究发现,铸态及900℃退火态合金都是BCC+FCC+有序BCC共存结构,BCC相是主相,组织都是典型的树枝晶组织,都具有优良软磁性能。900℃退火后,BCC相向FCC相转变,合金塑性显著改善,强度和硬度有所下降,饱和磁化强度得到提高。     

退火对AlCuFeNiTiCrx高熵合金硬度的影响

采用铜模吸铸法制备AlCuFeNiTiCr_x(x=0.5, 1.0, 1.5, 2.0)高熵合金,并在1000℃下进行3 h退火处理。通过X射线衍射仪(XRD)、光学显微镜、维氏硬度计,分别测试了铸态和退火态AlCuFeNiTiCr_x(x=0.5, 1.0, 1.5, 2.0)高熵合金的微观结构演变及维氏硬度值。发现铸态和退火态试样仅由简单的体心立方(BCC)和面心立方(FCC)固溶体相组成,而退火态试样中的FCC相与铸态试样中的FCC相相比有所增加。根据金相显微照片,两种状态下的高熵合金仅存在枝晶和晶间相,且退火后的显微组织变得更加均匀。两种状态下高熵合金的硬度均随Cr含量的增加而增加,且退火态试样的硬度值远大于铸态试样。     

退火温度对CoCrFeNiAl高熵合金组织与性能的影响

用传统铸造法制备了CoCrFeNiAl高熵合金,研究了不同退火温度对合金组织及性能的影响。结果表明,铸态的CoCrFeNiAl高熵合金为面心立方晶体结构(FCC)和体心立方晶体结构(BCC)的混合结构;随退火温度的升高,合金的晶体结构转变为FCC+BCC+Ordered BCC的混合结构;当合金温度达到800℃时,在枝晶之间开始析出AlNi_3、CrFe金属间化合物,合金的晶体结构转变为FCC+BCC+Ordered BCC+AlNi_3+CrFe的混合结构。CoCrFeNiAl高熵合金具有较高的硬度,并随着退火温度的升高,合金的硬度逐渐增大,硬度最高为463 HV0.2。CoCrFeNiAl高熵合金具有较好的室温压缩性能,铸态合金的压缩断裂强度、压缩率分别可达2275 MPa、18.8%。随着退火温度的升高,塑性逐渐降低,但强度逐渐增加,经600℃退火后,具有最佳的室温压缩性能,压缩断裂强度、压缩率分别为2631 MPa、12.5%。     

AlCrMoTiNi(Zr)高熵合金的组织与性能

采用粉末冶金工艺制备了AlCrMoTiNi和AlCrMoTiZr两种合金,采用X射线衍射仪、扫描电镜和数显硬度计对铸态和退火态合金的组织与硬度进行了分析。结果表明,AlCrMoTiNi合金由FCC和BCC固溶体组成,而AlCrMoTiZr合金由BCC相和金属间化合物组成,两种合金经700~1 000℃退火后,其物相和组织结构并未发生明显改变。AlCrMoTiNi和AlCrMoTiZr铸态合金的硬度(HV)分别为447和623,退火后最高硬度(HV)分别达到615和656。     

热处理温度对AlxCoCrFeNi系高熵合金组织和硬度的影响

采用光学显微镜、X射线衍射仪及硬度计分析了Al_xCoCrFeNi系(x=0. 25、0. 55、0. 75及1. 25)高熵合金在不同温度(800、1 000及1 200℃)退火后的组织及硬度演变规律。结果表明:随着Al元素含量的增加,Al_xCoCrFeNi高熵合金的显微组织由FCC单相逐渐转变为FCC+BCC双相和BCC单相,其硬度也逐渐升高; Al_(0. 25)CoCrFeNi合金的组织热稳定性较好; 800和1 000℃退火对Al_(0. 55)CoCrFeNi和Al_(0. 75)CoCrFeNi合金的组织影响较小,硬度也未发生明显变化; 1 200℃退火后,合金组织粗化,硬度下降;退火温度对BCC结构的Al_(1. 25)CoCrFeNi高熵合金的组织和性能有重要影响,800℃退火时有σ相生成; 1 000℃退火,σ相含量减少,1 200℃退火,σ相再次形成,因此对应的合金硬度先升高后降低再升高。     

退火对FeCoNi2-xMnGax高熵合金微观组织、力学性能和磁性能的影响

利用非自耗真空电弧熔炼炉制备FeCoNi_2-xMnGa_x(x=0, 0.5, 0.75, 1.0, 1.25)高熵合金铸锭,然后在800℃和1000℃退火8 h,研究了退火对高熵合金微观组织、力学性能和磁性能的影响。结果表明,随着Ga含量的增加,铸态合金组织由未添加Ga时(x=0)的FCC单相过渡到FCC+BCC双相,最终在Ga含量为25at%时(即x=1.25)转变为BCC单相。经过800℃和1000℃退火后,在FCC相内产生了棒状的析出物。双相组织的FeCoNi_2-xMnGa_x合金具有较好的磁性能和压缩性能,退火后合金的力学性能和磁性能均有所提高。FeCoNi_1.25MnGa_0.75合金1000℃退火后的力学性能最优,在形变量约为20%时的抗压强度为1852.5 MPa。FeCoNi_1.0MnGa_1.0合金1000℃退火后的饱和磁化强度最大,为121.8 emu/g。     

退火对CrCuFeMnTi高熵合金组织结构和力学性能的影响

为了研究退火处理对CrCuFeMnTi高熵合金组织结构和力学性能的影响,通过真空电弧熔炼法在氩气保护下制备了铸态合金,在不同温度下对合金进行退火处理,观察其组织结构并进行力学性能测试。结果表明:铸态CrCuFeMnTi高熵合金由1个密排六方和1个面心立方结构固溶体构成,形成由枝晶相和枝晶间相组成的典型枝晶组织形貌;合金在750℃以下退火时,主要以元素的扩散和组织的均匀化为主,经过900℃等温退火处理后,合金中密排六方结构的固溶体逐渐转变成体心立方结构固溶体,该相变过程是由元素扩散引起及控制的;经过750℃以下的退火处理后,合金的硬度和断裂强度均有所提高,但断裂行为均表现出脆性特征。     

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CrO-Cr2O3-SiO2 and CrO-Cr2O3-SiO2-Al2O3 systems

Available thermodynamic and phase diagram data have been critically assessed for all phases in the CrO-Cr₂O₃-SiO₂ and CrO-Cr₂O₃-SiO₂-Al₂O₃ systems from 298 K to above the liquidus temperatures and for oxygen partial pressures ranging from equilibrium with metallic Cr to equilibrium with air. All reliable data have been simultaneously optimized to obtain one set of model equations for the Gibbs energy of the liquid slag and all solid phases as functions of composition and temperature. The modified quasi-chemical model was used for the slag. The models permit phase equilibria to be calculated for regions of composition, temperature, and oxygen potential where data are not available.     

Crystal structures of the Ag4HgGe2S7 and Ag4CdGe2S7 compounds

The crystal structures of the Ag₄HgGe₂S₇ and Ag₄CdGe₂S₇ compounds were investigated using X-ray powder diffraction. These compounds crystallize in the monoclinic Cc space group with the lattice parameters a=1.74546(8), b=0.68093(2), c=1.05342(3) nm, β=93.398(3)° for Ag₄HgGe₂S₇ and a=1.74364(8), b=0.68334(3), c=1.05350(4) nm, β=93.589(3)° for Ag₄CdGe₂S₇. Atomic parameters were refined in the isotropic approximation (R_I=0.0761 and R_I=0.0727, respectively).     

X-ray diffraction study of Ba0.985Na0.015Ti0.985Nb0.015O3, Ba0.6Na0.4Ti0.6Nb0.4O3 and Ba0.3Na0.7Ti0.3Nb0.7O3 compositions

The Ba_0.985Na_0.015Ti_0.985Nb_0.015O₃, Ba_0.6Na_0.4Ti_0.6Nb_0.4O₃ and Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ compositions of the (1 − x) BaTiO₃–xNaNbO₃ (BTNNx) system have been studied by X-ray diffraction and by measurements of dielectric properties. The specimens with composition BTNN (x = 0.015, 0.40 and 0.70) have been refined by the JANA program from X-ray powder diffraction data. Ceramic samples with composition (1 − x) BaTiO₃ + xNaNbO₃ (where x = 0.015, 0.40 and 0.70) were prepared by calcinations from appropriate mixture of BaCO₃, TiO₂, Na₂CO₃ and Nb₂O₅. The calcined powder was sintered at temperature range 1200–1400 °C. As the composition x increased from 0.015 (and 0.70), the ferroelectric ceramics (x = 0.015, FE) with tetragonal phase changed to the ferroelectric relaxors (RFE, x = 0.40). RFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to 180 K) due to a frustration between RFE and FE state. These ceramics present the classical ferroelectric character when 0 ≤ x < 0.075 and 0.55 < x ≤ 1 and relaxor character when 0.075 ≤ x ≤ 0.55.     

Magnetic, electrical and plastic properties of Fe76Nb2Si13B9, Fe75Ag1Nb2Si13B9 and Fe75Cu1Nb2Si13B9 amorphous alloys

Influence of 1 h annealing in vacuum on magnetic, electrical and plastic properties of Fe₇₆Nb₂Si₁₃B₉, Fe₇₅Ag₁Nb₂Si₁₃B₉ and Fe₇₅Cu₁Nb₂Si₁₃B₉ melt spun ribbons were carefully investigated. It was shown that in all cases soft magnetic properties can be significantly enhanced by applying 1-h annealing at characteristic temperatures T_op. This optimization annealing causes that permeability increases more than 15-times and magnetic losses (tangent of loss angle) achieves a minimum in relation to the as quenched state. Using structural examinations (X-ray and HRTEM) it was shown that for the Fe₇₅Cu₁Nb₂Si₁₃B₉ alloy the optimized microstructure corresponds to a nanocrystalline αFe(Si) phase whereas in other alloys to a relaxed amorphous phase free of iron nanograins. As a consequence of this fact the Fe₇₆Nb₂Si₁₃B₉ and Fe₇₅Ag₁Nb₂Si₁₃B₉ alloys show higher plasticity in comparison to the nanocrystalline Fe₇₅Cu₁Nb₂Si₁₃B₉ alloy. Temperatures of the first stage of crystallization, and related diffusion parameters were determined using measurements of resistivity versus temperature with different heating rates.     

Thermodynamics and phase diagrams in the binary systems Na2O-SiO2, Na2O-GeO2, Na2O-B2O3, Li2O-B2O3, CaO-B2O3, SrO-B2O3, and BaO-B2O3

We applied our model to the enthalpy of mixing data of the binary systems Na₂O-SiO₂, Na₂O-GeO₂, Na₂O-B₂O₃, Li₂O-B₂O₃, CaO-B₂O₃, SrO-B₂O₃, and BaO-B₂O₃. The most stable composition in the liquid, that is where the enthalpy of mixing is most negative, is with a metal-oxygen ratio of 4 to 3, for monovalent metals (Na and Li) and 3 to 4 for divalent metals (Ba and Ca) in liquid silicates or borates. The same applies to the CaO-SiO₂, CaO-Al₂O₃, PbO-B₂O₃, PbO-SiO₂, ZnO-B₂O₃, and ZnO-SiO₂ systems. The oxygen to metal ratio, its constant value in various types of systems, reflects and describes the structure of the liquid. Using the analyzed enthalpies of mixing data and the available phase diagrams, we calculated the enthalpies of formation of the various binary compounds. The results are in excellent agreement with data in the literature that were obtained from direct solid-solid calorimetry.     

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CrO-Cr2O3, CrO-Cr2O3-Al2O3, and CrO-Cr2O3-CaO systems

Available thermodynamic and phase diagram data have been critically assessed for all phases in the CrO-Cr₂O₃, CrO-Cr₂O₂-Al₂O₃, and CrO-Cr₂O₂-CaO systems from 298 K to above the liquidus temperatures and for oxygen partial pressures ranging from equilibrium with metallic Cr to equilibrium with air in the case of the first two systems and toP _{O 2} = 10⁻³ atm for the CrO-Cr₂O₃-CaO system. All reliable data have been simultaneously optimized to obtain one set of model equations for the Gibbs energy of the liquid slag and all solid phases as functions of composition and temperature. The modified quasichemical model was used for the slag. The models permit phase equilibria to be calculated for regions of composition, temperature, and oxygen potential where data are not available.     

Fine structure and the mechanism of the low-temperature decomposition of the nonstoichiometric compounds YBa2Cu3O6.8 and YBa2Cu3O6.8 doped with Ce

The resistance of the nonstoichiometric compounds YBa₂Cu₃O_6.8 (single crystals, ceramics) and YBa₂Cu₃O_6.8 (Ce) (single crystal) to low-temperature decomposition (at 200°C) in air and an argon atmosphere is studied by X-ray diffraction and electron microscopy. At the first stage, the compounds are found to undergo oxygen separation into two phases, namely, oxygen-rich and oxygen-lean phases as compared to the initial state, that have different lattice parameters. After 20-to 35-h annealing, the disordering of the heavy Y, Ba, Ba, Y atoms along the c axis in ceramic pellets begins mainly because of the action of elastic stresses. This process occurs via the formation of the packets of numerous stacking faults on (001) planes. The disordering of the ceramic matrix ends in the formation of a CsCl-type cubic phase upon 100-h annealing, which is accompanied by a significant decrease in the diamagnetic response and an increase in T _c from 75 K in the initial state to 90 K. The retained superconductivity and the increase in T _c are caused by the presence of a large number of oxygenrich ortho-phase particles in a nonsuperconducting matrix; these particles are correlated with each other and form a multicoupled system of superconducting filaments. Original Russian Text ? S.V. Sudareva, E.P. Romanov, T.P. Krinitsina, E.I. Kuznetsova, Yu.V. Blinova, I.B. Bobylev, N.A. Zyuzeva, A.M. Burkhanov, 2008, published in Fizika Metallov i Metallovedenie, 2008, Vol. 106, No. 4, pp. 378–387.     

Thermodynamic properties and phase diagrams of the binary systems B2O3–Ga2O3, B2O3–Al2O3 and B2O3–In2O3

We calculated the binary phase diagrams B₂O₃–Ga₂O₃, B₂O₃–In₂O₃ and B₂O₃–Al₂O₃, and the Gibbs energy of formation of the binary compounds, using experimental liquidus data. The B₂O₃–Ga₂O₃ system is of industrial importance, because liquid B₂O₃, in which Ga₂O₃ is not very soluble, is used to protect GaAs during growth of single crystals of GaAs. During recovery of noble metals B₂O₃ is added to slags containing Al₂O₃ to lower the melting point and the viscosity. The B₂O₃–In₂O₃ system is of much less importance to industry. In all three systems we have a liquid miscibility gap, and also solid binary compounds, none of which melt congruently. The miscibility gaps are not surprising, because even in the B₂O₃–Bi₂O₃ system where four congruently melting compounds are present, a liquid miscibility gap exists close to B₂O₃.