AZ31镁合金表面单宁酸转化膜的组织结构与耐腐蚀性能

利用单组分单宁酸为成膜剂,在AZ31镁合金表面制备无铬转化膜,采用L9(34)正交试验研究转化处理液中单宁酸的浓度、处理液pH值、温度和处理时间对转化膜形成和耐腐蚀性能的影响,获得最优的转化膜处理工艺。用扫描电镜与X射线光电子能谱对镁合金表面单宁酸转化膜的表面形貌、元素组成及化学价态进行分析,并通过电化学极化曲线和阻抗谱测试,考察镁合金表面单宁酸转化膜的耐腐蚀性能,阐明其形成机制。结果表明,镁合金在温度为50℃,pH值为2.5,单宁酸质量浓度为10 g/L的转化液中浸泡10 min,即可获得耐蚀性良好的单宁酸转化膜。单宁酸转化膜由镁合金表面溶解形成的镁离子Mg2+与单宁酸分子的酚羟基、羧基发生反应生成的镁金属有机螯合物组成,呈网状裂纹结构均匀覆盖于镁合金表面;单宁酸转化膜能有效提高AZ31镁合金的耐腐蚀性能,交流阻抗达到1 250?/cm2,是基体镁合金阻抗(35?/cm2)的300多倍。     

AZ31镁合金表面钼酸盐转化膜的制备与耐蚀性能

以Na2Mo O4为主盐,与氧化剂H2O2、成膜促进剂NaF和Na2Si O3一起组成化学转化液,在AZ31镁合金表面制备钼酸盐转化膜,利用扫描电镜和X线光电子能谱仪分析转化膜的形貌和组成,通过电化学阻抗测试研究转化膜在3.5%Na Cl溶液中的腐蚀行为,并讨论成膜机理,研究转化液中Na2Mo O4浓度与p H以及成膜温度和时间对薄膜结构与耐腐蚀性能的影响。结果表明:转化液的优化组成为0.2 mol/L Na2Mo O4+0.12 mol/L NaF+0.014mol/L Na2Si O3+0.012 mol/L H2O2;优化工艺条件为p H=5,温度60℃,转化时间30 min;转化膜为黄棕色,主要由Mg Mo O4,Mg F2,Mo O2,Mo O3和Mg Si O3组成,转化膜宏观上完整均匀,存在网状微裂纹;钼酸盐转化膜能有效提高AZ31镁合金的耐腐蚀性能,对基体合金有一定的保护作用。     

Desiliconization and decarburization behavior of molten Fe-C-Si(-S) alloy with CO<Subscript>2</Subscript> and O<Subscript>2</Subscript>

One of the most important problems in the steelmaking process is an increase of the disposal slag mainly discharged from the dephosphorization process. In order to reduce the quantity of the disposal slag, the complete removal of silicon from molten pig iron is considered very effective before the dephosphorization in the pretreatment process. From this point of view, the desiliconization and the decarburization behavior of Fe-C-Si alloy with CO₂ and O₂ has been investigated in the present work. It is thermodynamically calculated that silicon should be oxidized in preference to carbon over 0.60 mass pct Si under the condition of s_SiO2=a _C=1 at 1573 K and is experimentally confirmed that silicon is only oxidized under the condition in actual. Even under the competitive region of desiliconizing and decarbonizing, under 0.60 mass pct Si, silicon is found to be oxidized down to about 0.1 mass pct Si in preference. The overall rate constants for the desiliconization and the decarburization are derived, and the value for the desiliconization is one order of magnitude larger than that for the decarburization. The influence of sulfur is also examined, and the retarding effect is not observed on the oxidation reactions.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Concentration on Density and Structure of (CaO-SiO<Subscript>2</Subscript>)-xAl<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag

The effect of Al₂O₃ concentration on the density and structure of CaO-SiO₂-Al₂O₃ slag was investigated at multiple Al₂O₃ mole percentages and at a fixed CaO/SiO₂ ratio of 1. The experiments were conducted in the temperature range of 2154 K to 2423 K (1881 °C to 2150 °C) using the aerodynamic levitation technique. In order to understand the relationship between density and structure, structural analysis of the silicate melts was carried out using Raman spectroscopy. The density of each slag sample investigated in this study decreased linearly with increasing temperature. When the Al₂O₃ content was less than 15 mole pct, density decreased with increasing Al₂O₃ content due to the coupling of Si (Al), whereas above 20 mole pct density of the slag increased due to the role of Al³⁺ ion as a network modifier.     

Mass transfer and adhesion in electrospark alloying of AL9 alloy with AlN - Ti(Zr)B<Subscript>2</Subscript> - Ti(Zr)Si<Subscript>2</Subscript> ceramic electrodes

A study has been made on the adhesion of the new composite ceramic AlN - Ti(Zr)B₂ -Ti(Zr)Si₂ with steel 45 and alloy AL9 in relation to the mass-transfer kinetics in spark working. There is a three-stage mass-transfer mechanism. The materials in the electrode and the substrate affect the electrical erosion of the electrode.     

Mass transfer and adhesion in electrospark alloying of AL9 alloy with AlN - Ti(Zr)B<Subscript>2</Subscript> - Ti(Zr)Si<Subscript>2</Subscript> ceramic electrodes

A study has been made on the adhesion of the new composite ceramic AlN - Ti(Zr)B₂ -Ti(Zr)Si₂ with steel 45 and alloy AL9 in relation to the mass-transfer kinetics in spark working. There is a three-stage mass-transfer mechanism. The materials in the electrode and the substrate affect the electrical erosion of the electrode.     

Effect of Ce<Subscript>2</Subscript>O<Subscript>3</Subscript> on Structure,Viscosity, and Crystalline Phase of CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Li<Subscript>2</Subscript>O-Ce<Subscript>2</Subscript>O<Subscript>3</Subscript> Slags

Aiming at devising new mold flux for Ce-bearing stainless steel, a fundamental investigation on the effect of Ce₂O₃ on properties of the CaO-Al₂O₃-Li₂O-Ce₂O₃ slag was provided by the present work. The results show that adding Ce₂O₃ could decrease the viscosity of the slag due to its effects on decreasing the polymerization of the slag. The crystalline process was restrained by increasing the content of Ce₂O₃, and the crystalline phases also can be influenced by the slag structure. The crystalline phases were transferred from LiAlO₂ and CaO to LiAlO₂ and CaCeAlO₄ with the addition of Ce₂O₃ to the slag, which could be well confirmed by the structure of the unit cell of the crystals.     

Phase-equilibrium data and liquidus for the system “MnO”-CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> at Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> of 0.55 and 0.65

Phase-equilibrium data and liquidus isotherms for the system “MnO”-CaO-(Al₂O₃+SiO₂) at silicomanganese alloy saturation have been determined in the temperature range of 1373 to 1723 K. The results are presented in the form of the pseudoternary sections “MnO”-CaO-(Al₂O₃+SiO₂) with Al₂O₃/SiO₂ weight ratios of 0.55 and 0.65. The primary-phase fields have been identified in this range of conditions.     

The effect of CaF<Subscript>2</Subscript> on thermodynamics of CaO-CaF<Subscript>2</Subscript>-SiO<Subscript>2</Subscript>(-MgO) slags

To address the role of CaF₂ in the CaO-CaF₂-SiO₂(-MgO) slag system employed for the production of low-pressure rotor steels, the thermodynamic aspects of the slag were investigated by equilibrating it with liquid iron at 1873 K in CaO or MgO crucibles. Presaturation of slag with an oxide block piece of CaO or MgO in a Pt crucible and application of a carbon paste to the outside of an oxide crucible were designed to prevent crucible failure during the slag-metal experiments. The liquidus isotherm and phase boundary of the preceding slag system were investigated using the slag-metal equilibria. Also, the effect of CaF₂ on the sulfide capacity and the activity coefficient of Fe _t O were of particular interest in controlling the sulfur level and cleanliness of low-pressure rotor steels.     

Effects of Zr on the eutectoid decomposition behavior of Nb<Subscript>3</Subscript>Si into (Nb)/Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>

The effect of Zr on the formation of Nb/Nb₅Si₃ lamellar microstructure by eutectoid decomposition reaction of Nb₃Si is investigated. It has been shown that the kinetics of the eutectoid decomposition of high-temperature Nb₃Si phase into Nb and Nb₅Si₃ phases are sluggish in the binary Nb-Si system and that they are enhanced by Zr additions. The time-temperature-transformation (TTT) diagram for the decomposition is experimentally determined and the acceleration of the reaction by small Zr addition of 1.5 at. pct is confirmed by comparison with the reported TTT curves of binary and ternary alloys containing Ti. The role of the ternary element on the decomposition kinetics is discussed in terms of crystallographic orientation relationships (ORs) and Zr distribution in the parent Nb₃Si phase during solidification. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Deformation and fracture of a composite material based on a high-strength maraging steel covered with a melt-quenched Co<Subscript>69</Subscript>Fe<Subscript>4</Subscript>Cr<Subscript>4</Subscript>Si<Subscript>12</Subscript>B<Subscript>11</Subscript> alloy layer

Multifractal analysis is used to study the deformation and fracture of a promising composite material consisting of a wire base made of K17N9M14 maraging steel covered with a surface layer made from a Co₆₉Fe₄Cr₄Si₁₂B₁₁ amorphous alloy. As compared to its components, this material has a substantially better set of the mechanical properties.     

Corrosion behavior of plasma-sprayed coatings on a Ni-base superalloy in Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-60 Pct V<Subscript>2</Subscript>O<Subscript>5</Subscript> environment at 900 °C

The shrouded plasma spray process was used to deposit NiCrAlY, Ni-20Cr, Ni₃Al, and Stellite-6 metallic coatings on a Ni-based superalloy (62Ni-23Cr-1.48Al-0.80Mn-0.37Si-0.10Cu-0.025C-bal Fe). NiCrAlY was used as a bond coat in all cases. Hot corrosion studies were conducted on uncoated as well as plasma-spray-coated superalloy specimens after exposure to molten salt at 900 °C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDAX) and electron-probe microanalysis techniques were used to analyze the corrosion products. The uncoated superalloy suffered accelerated corrosion in the form of intense spalling of the scale. The NiCrAlY coated specimen showed a minimum weight gain, whereas the Stellite-6 indicated a maximum weight gain among the coatings studied. All the coatings were found to be successful in developing resistance against hot corrosion, which may be attributed to the formation of oxides, and spinels of nickel, aluminum, chromium, or cobalt.     

Experimental Investigation and Thermodynamic Modeling of the B<Subscript>2</Subscript>O<Subscript>3</Subscript>-FeO-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> System for Recycling of NdFeB Magnet Scrap

NdFeB magnet scrap is an alternative source of neodymium that could have a significantly lower impact on the environment than current mining and extraction processes. Neodymium can be readily oxidized in the presence of oxygen, which makes it easy to recover neodymium in oxide form. Thermochemical data and phase diagrams for neodymium oxide containing systems is, however, very limited. Thermodynamic modeling of the B₂O₃-FeO-Fe₂O₃-Nd₂O₃ system was hence performed to obtain accurate phase diagrams and thermochemical properties of the system. Key phase diagram experiments were also carried out for the FeO-Nd₂O₃ system in saturation with iron to improve the accuracy of the present modeling. The modified quasichemical model was used to describe the Gibbs energy of the liquid oxide phase. The Gibbs energy functions of the liquid phase and the solids were optimized to reproduce all available and reliable phase diagram data, and thermochemical properties of the system. Finally the optimized database was applied to calculate conditions for selective oxidation of neodymium from NdFeB magnet waste.     

Effect of Superficially Applied Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating on High-Temperature Corrosion Behavior of Ni-Base Superalloys

Inhibitors and oxide additives have been investigated with varying success to control high-temperature corrosion. Effect of Y₂O₃ on high-temperature corrosion of Superni 718 and Superni 601 superalloys was investigated in the Na₂SO₄-60 pct V₂O₅ environment at 1173 K (900 °C) for 50 cycles. Y₂O₃ was applied as a coating on the surfaces of the specimens. Superni 601 was found to have better corrosion resistance in comparison with Superni 718 in the Na₂SO₄-60 pct V₂O₅ environment. The Y₂O₃ superficial coating was successful in decreasing the reaction rate for both the superalloys. In the oxide scale of the alloy Superni 601, Y and V were observed to coexist, thereby indicating the formation of a protective YVO₄ phase. There was a distinct presence of a protective Cr₂O₃-rich layer just above the substrate/scale interface in the alloy. Whereas Cr₂O₃ was present with Fe and Ni in the scale of Superni 718. Y₂O₃ seemed to be contributing to better adhesion of the scale, as comparatively lesser spalling was noticed in the presence of Y₂O₃.     

Development of Mo(Si,Al)<Subscript>2</Subscript>-base oxidation-resistant coating on Nb-base structural materials

Mo(Si,Al)₂-base oxidation-resistant coatings for Nb-base structural materials have been studied. The coating is composed of a Mo(Si,Al)₂-base Al reservoir and Al₂O₃ interlayer to suppress interface reactions between the Al reservoir and the substrate. To develop a suitable Al-reservoir material, some Mo(Si_0.6,Al_0.4)₂-HfB₂ composites were prepared. Their oxidation resistance and coefficients of thermal expansion were investigated, in addition to their chemical reactivity with the Nb substrate at high temperatures. As a result, Mo(Si_0.6,Al_0.4)₂-20 vol pct HfB₂ was selected as one of the satisfactory Al reservoirs. The introduction of a stable Al₂O₃ interlayer was attempted using a novel powder metallurgical process to overlay the Nb substrates with the Al reservoir, where the Nb substrates were subjected to a slight surface oxidation prior to the coating process. The Nb specimens, which are thoroughly coated with the Al reservoir and Al₂O₃ interlayer, can be successfully fabricated by this method. The coated Nb specimens are not damaged at all after prolonged exposure in flowing Ar-20 pct O₂ at 1673 K for 120 hours. Furthermore, the Al₂O₃ interlayer is very effective and no reactions occur at the interface. Thus, this Mo(Si,Al)₂-base oxidation-resistant coating is applicable to Nb. The utility of the coating system is also confirmed for a Nb_SS/Nb₅Si₃ composite. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Processing,microstructure, and mechanical properties of (Nb)/Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript> two-phase alloys

It has been shown that a fine lamellar structure composed of Nb solid solution, (Nb), and Nb₅Si₃ is formed through eutectoid decomposition in the Nb-Si binary system and its ternary derivatives. Such alloys would exhibit a high strength at over 1400 K, yet showing room-temperature toughness of over 10 to 20 MPa m^1/2 if a proper lamellar spacing is chosen. In the present work, effects of processing on the microstructure evolution and mechanical properties are investigated on the Nb-18 at. pct Si alloys prepared by hot pressing (HP) and spark-plasma sintering (SPS). The powders used in the present work are of pure Nb and Nb₅Si₃ in order for the fabrication to become possible at temperatures higher than the melting point of Si and to reduce the formation of SiO₂. The results show that the SPS yields more uniform two-phase microstructure but the alloy fabricated through HP tends to provide higher elevated temperature strength. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Electronic structure and ferromagnetic effect in Ni<Subscript>2</Subscript>MnGa alloy

By using the first-principles discrete variational method (DVM), we have investigated the magnetic contribution to the binding energy, the Fermi energy level, the interatomic energy, the bond order (BO), the total density of states (DOS), and the charge density distribution for varying c/a in Ni₂MnGa alloy. The binding energy curves for the ferromagnetic (FM) phase for varying c/a are more complicated than those in the nonferromagnetic (NM) phase whose energy difference displays the magnetic contribution to the binding of the crystal. The interactions between the central atom Mn and the surrounding atoms, including the interatomic energies and the BOs, are different from each other, which leads to crystal anisotropy. The finding suggests that the interatomic energy difference between the FM and NM phases will be the origin of magnetic anisotropy. The difference in electron density on the different distortions for the FM and NM phases varying with c/a shows the different redistribution of the magnetization for the Mn and Ni in the [110] plane. This work was supported by the National Science Foundation of China under Grant No. 50301011.     

Processing,microstructure, and mechanical properties of cast <Emphasis Type="Italic">in-Situ</Emphasis> Al(Mg,Mn)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>(MnO<Subscript>2</Subscript>) composite

Cast particulate composites, containing in-situ generated reinforcing particles of alumina, have been developed by solidification of slurry obtained by dispersion of externally added manganese dioxide particles (MnO₂) in molten aluminum, and alumina is formed by reaction of manganese dioxide with molten aluminum. The chemical reaction also releases manganese into molten aluminum. Magnesium is added to the melt in order to help wetting of alumina particles by molten aluminum and to retain the particles inside the melt. The present work aims to understand the influence of key parameters such as processing temperature, time, and the amount of MnO₂ particles added on the microstructure and mechanical properties of the resulting cast in-situ composites. The sequence of addition of MnO₂ particles and magnesium has significant influence on the microstructure and mechanical properties. Increasing processing temperature and time increases the extent of reduction of MnO₂ particles, generating more alumina particles as well as releasing more manganese to the matrix alloy. Alumina helps to nucleate finer and sometimes blocky MnAl₆ in the matrix of the composite and thereby results in relatively higher ductility and increased strength in the composite as compared to the base alloy of similar composition. Even in the presence of relatively higher porosity of 8 to 9 vol pct, one observes a percent elongation not below 7 to 8 pct, which is considerably higher than those observed in cast Al(Mg)-Al₂O₃ composite synthesized by externally added alumina particles.     

Amphoteric behavior of alumina in viscous flow and structure of CaO-SiO<Subscript>2</Subscript> (-MgO)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> slags

The viscosity of CaO-SiO₂ (-MgO)-Al₂O₃ slags was measured to clarify the effects of Al₂O₃ and MgO on the structure and viscous flow of molten slags at high temperatures. Furthermore, the infrared spectra of the quenched slags were analyzed to understand the structural role of Al₂O₃ in the polymerization or depolymerization of silicate network. The Al₂O₃ behaves as an amphoteric oxide with the composition of slags; that is, the alumina behaves as a network former up to about 10 mass pct Al₂O₃, while it acts as a network modifier, in parts, in the composition greater than 10 mass pct Al₂O₃. This amphoteric role of Al₂O₃ in the viscous flow of molten slags at the Newtonian flow region was diminished by the coexistence of MgO. The effect of Al₂O₃ on the viscosity increase can be understood based on an increase in the degree of polymerization (DOP) by the incorporation of the [AlO₄]-tetrahedra into the [SiO₄]-tetrahedral units, and this was confirmed by the infrared (IR) spectra of the quenched slags. The influence of alumina on the viscosity decrease can be explained on the basis of a decrease in the DOP by the increase in the relative fraction of the [AlO₆]-octahedral units. The relative intensity of the IR bands for the [SiO₄]-tetrahedra with low NBO/Si decreased, while that of the IR bands for [SiO₄]-tetrahedra with high NBO/Si increased with increasing Al₂O₃ content greater than the critical point, i.e., about 10 mass pct in the present systems. The variations of the activity coefficient of slag components with composition indirectly supported those of viscosity and structure of the aluminosilicate melts.