Structure and defects of MBE grown NbAl2O3 interfaces

Single crystal niobium films were grown by Molecular Beam Epitaxy (MBE) on (0001)_S sapphire substrates. Cross-sectional and plan-view specimens of the films were prepared so that the NbAl₂O₃ interface could be investigated by conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The atomistic structure of the interface was identified by HREM. Regions of good matching and regions containing structural defects alternate at the interface. The defects can be described as misfit dislocations with no “stand-off” from the interface. The spacing of the defects corresponds to the 1.9% mismatch between the two lattices. In regions of good matching a fixed translational state between both lattices is established, which could be determined. Image simulations constructed with this translational state were in good agreement with experiment. In plan-view, periodic arrays of fringes were observed. These fringes are formed by a superposition of moire´fringes and dislocation contrast. Weak-beam techniques were used to separate these two contributions.     

Pore removal during diffusion bonding of NbAl2O3 interfaces

Results from a brief study of pore removal during diffusion bonding of interfaces between Nb and Al₂O₃ are presented. Sequential observations of the evolution of interface microstructure during the later stages of diffusion bonding indicates that pore removal occurs by the growth of bonding fronts in particular crystallographic directions dependent on the orientation of Nb, but not on that of the Al₂O₃. Bonding fronts meet, resulting in cylindrical voids which then breakup into uniformly distributed spherical voids. Characterization of the breakup suggests that the process is volume diffusion controlled. Finally, the effect of thermal grooving of Nb grain boundaries on mass transport during diffusion bonding is discussed.     

Deformation modes in NbAl3

The DO₂₂ lattice of the NbAl₃ intermetallic compound shows very limited ductility at room temperature. In this study the slip and twinning systems that are active during the deformation process were investigated. Evaluation of the possible deformation modes was performed and contrast analysis in the transmission electron microscope revealed both expected and unexpected deformation modes. Two types of dislocations were found in the deformed structure, namely thea 〈110〉 superdislocation on the {112} plane and loops of unidentified dislocations on the {010} plane. No evidence of 〈201〉 superdislocations was found, probably due to the fact that this type of dislocation is expected to move in groups of four. Twins of the {112} type were found to play an important role in the deformation process since they supply a component of shear perpendicular to the (001) plane.     

Fracture mechanics of Ti/Al2O3 interfaces

Measurement of mixed mode interfacial fracture toughness in Ti/Al₂O₃ bimaterial couples has been accomplished by four-point bending tests of sandwich specimens with symmetrical cracks for different processing temperatures and thicknesses of Ti interlayers. Fracture surfaces and sample cross sections were analyzed by SEM, XPS, EDAX, EPMA and AES. The interfacial fracture toughness measured with four-point bending tests increases with increasing applied bonding temperature up to 950°C. Above this temperature toughness decreases. The deterioration of toughness is found to be due to an intermetallic phase (Ti₃Al) produced by diffusion at high temperature. The interfacial fracture toughness also increases when the Ti interlayer is thicker. The measured critical strain energy release rate (or interfacial fracture energy) ranges from 10 to 45 J/m². This is much larger than the estimated true work of adhesion which is about 2 J/m². This is because of plastic energy dissipation in the Ti interlayer during the fracture process.     

Nonequilibrium synthesis of NbAl3 and Nb-Al-V

NbAl₃, like many ordered intermetallic compounds, appears to have great potential for application as high-temperature structural material because of the high melting point and stability in high- temperature oxidizing environments. However, the D0₂₂ (tI8) lattice structure shows limited ductility at room temperature. Addition of group IVB, VB, and VIB elements to A1₃X (D0₂₂ type of compounds) as partial replacement of X is a method being applied by various researchers to promote ductility in these materials by activation of numerous dislocations and ordered twin- ning. In this study, V was added in different amounts to improve the ductility of NbAl₃. How- ever, this has an adverse effect on the oxidation response of the system, as is explained in Part II of this article. This article deals with the microstructure evolution, phase identification, and characterization of laser-clad (rapidly solidified) nominal NbAl₃ and also studies the effect of V additions on the final microstructure in this system. Laser cladding of nominal NbAl3 produces two metastable unreported phases [body-centered tetragonal (bct) and base-centered orthorhombic phases]. Addition of V suppresses the primary phase (NbAl₃) in the Nb-Al-V system. on leave from the Department of Metallurgy on leave from the Department of Metallurgy     

Thermodynamic analysis of aluminate formation at Fe/Al2O3 and Cu/Al2O3 interfaces

Subsolidus ternary phase relationships in the systems FeAlO and CuAlO are reviewed in order to provide a basis for thermodynamic analysis of aluminate formation at Fe/α-Al₂O₃ and Cu/α-Al₂O₃ interfaces. Expressions for the critical oxygen activity in the Fe-rich and Cu-rich solid solutions necessary to stabilize the equilibrium aluminate phases (FeAl₂O₄ and CuAlO₂, respectively) in the presence of α-Al₂O₃ are derived using data available in the literature. As in previous work in the NiαAl₂O₃ system, aluminate formation by solid state reaction of Fe and Cu with α-Al₂O₃ was calculated to require a threshold concentration of oxygen in the metal, which is of the order of 1/5 the solubility limit. The sults are presented in stability diagrams and compared with previous results on the Ni/α-Al₂O₃ system in terms of the free energies of formation of the aluminates and the relative free energes of solution of oxygen in the metals. The results are also compared with available experimental observations on Fe/α-Al₂O₃ and Cu/α-Al₂O₃ interfaces.     

The thermodynamics of spinel interphase formation at diffusion-bonded Ni/Al2O3 interfaces

Interface microstructural development during solid state diffusion bonding of Ni to single-crystal α-Al₂O₃ has been studied by electron microscopy. Nickel aluminate spinel (NiAl₂O₄) interphase layers ∼ 1 μm thick formed under high vacuum bonding conditions. Very high vacuum (VHV) annealing caused the spinel to disappear, indicating that its stability depends critically upon the oxygen activity. High vacuum diffusion bonding utilizing initially oxygen-free Ni and oxygen-containing Ni established that spinel formation requires a threshold oxygen activity, and furthermore, that the source of the required oxygen can be oxygen initially dissolved in the Ni. Thermodynamic calculations confirm that the threshold oxygen level necessary to stabilize the spinel increases from 0.006 at.% (60 at.ppm) at 1273 K to 0.025 at.% (250 at. ppm) at 1663 K. Further analysis indicates the spinel exhibits a maximum thickness determined by the difference between the initial and threshold oxygen concentrations and the Ni thickness. Considering the solubility limit of oxygen in solid Ni, the spinel thickness is limited to ∼0.005 times the Ni thickness. The reaction is explored further in the context of diffusion path concepts with a calculated NiAlO phase diagram.     

The influence of solid-state and liquid-phase bonding on fatigue at Al/Al2O3 interfaces

Fracture and fatigue experiments have been conducted on liquid phase bonded (LPB) and solid-state bonded (SSB) aluminum-alumina interfaces. The LPB interfaces contain voids and dendritic FeAl₃ precipitates, whereas SSB interfaces are relatively defect-free. These precipitates result in local embrittlement, yet both interfaces are strong and tough. Upon cyclic loading, mode 1 cracks in both systems grow alternately along the interface and within the Al. The development of a tortuous crack path elevates the apparent fatigue threshold through crack closure. Under mixed mode loading, fatigue cracks approaching SSB interfaces propagate through the Al rather than along the interface. Conversely, for LPB interfaces, mixed mode cyclic crack growth along the interface occurs in preference to propagation in the Al. Correlation between the striation spacing and the crack tip opening displacement suggests a growth mechanism based on crack tip blunting. A.G. EVANS, Professor, formerly with the Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138     

The influence of solid-state and liquid-phase bonding on fatigue at Al/Al2O3 interfaces

Fracture and fatigue experiments have been conducted on liquid phase bonded (LPB) and solid-state bonded (SSB) aluminum-alumina interfaces. The LPB interfaces contain voids and dendritic FeAl₃ precipitates, whereas SSB interfaces are relatively defect-free. These precipitates result in local embrittlement, yet both interfaces are strong and tough. Upon cyclic loading, mode 1 cracks in both systems grow alternately along the interface and within the A1. The development of a tortuous crack path elevates the apparent fatigue threshold through crack closure. Under mixed mode loading, fatigue cracks approaching SSB interfaces propagate through the A1 rather than along the interface. Conversely, for LPB interfaces, mixed mode cyclic crack growth along the interface occurs in preference to propagation in the A1. Correlation between the striation spacing and the crack tip opening displacement suggests a growth mechanism based on crack tip blunting.     

A new phase in a rapidly solidified and consolidated NbAl3-1 Pct TiB2 alloy

Transmission electron microscopy (TEM) investigation was conducted to characterize the microstructure of a rapidly solidified and consolidated NbAl₃-l pct TiB₂ alloy. The study demonstrated that the alloy consisted of a two-phase microstructure with second-phase particles distributed uniformly in a NbAl₃ matrix. The dominant particle was identified to be an intermetallic phase having a chemical stoichiometry of Nb₃Al₂ and a hexagonal crystal structure (a = 0.78 andc = 0.52 nm). The structure was determined to belong to the P6₃/mcm space group. In addition, dispersion of fine NbB particles was also observed. Although the existing Al-Nb binary phase diagrams do not indicate a phase of the type Nb₃Al₂, it is suggested that formation of such a phase is likely based on analogy with Al-Zr and Al-Hf systems.     

制备SiO_2包覆的Fe_3O_4磁性纳米复合粒子

首先用化学共沉淀法制备出柠檬酸三铵改性的Fe3O4磁性纳米粒子,然后用反相微乳液法制备了SiO2包覆的Fe3O4磁性纳米复合粒子。用X射线衍射仪(XRD),透射电子显微镜(TEM),傅立叶-红外光谱仪(FTIR)和振动样品磁强计(VSM)表征SiO2包覆的Fe3O4磁性纳米复合粒子。结果表明,SiO2成功包覆在Fe3O4表面,且饱和磁化强度下降,但矫顽力趋近于零,仍显示超顺磁性。     

镁砂中Fe2O3、Al2O3、P2O5、TiO2的联合测定

镁砂产品检验中,采用碱熔融法共同预处理样品,在同一母液中,联合测定铁、铝、磷、钛的含量,缩短了检测时间。通过分别控制各元素的显色酸度,各元素线性良好,测得的Fe2O3、Al2O3、P2O5、TiO2的相对标准偏差分别小于1.11%、2.78%、4.84%、5.88%。实验结果表明,方法的准确度和精密度都较高。     

CaO-La2O3-B2O3-Eu2O3转光玻璃的合成及荧光性质

X射线衍射研究表明CaO-La2O3-B2O3-Eu2O3体系的玻璃化温度在1025℃附近。荧光光谱和ESR谱研究表明,在CaO-La2O3-B2O3-Eu2O3玻璃体系中存在着Eu2 和Eu3 两种价态离子。316,360,379,394,413,462和532nm锐线激发峰和592,616和650红区发射峰分别对应Eu3 的f-f激发跃迁和5D0-7FJ(J=1,2,3)跃迁发射;351nm和427nm宽带激发峰和蓝区发射分别对应Eu2 的5d-4f激发跃迁和发射。     

Vitrification and Crystallization Behavior of CaO-SiO2-MgO-Al2O3-Fe2O3-Cr2O3 System

Metallurgical and Materials Transactions B - The effect of basicity and Cr2O3 content on vitrification of the CaO-MgO-SiO2-Al2O3-Fe2O3-Cr2O3 system during a melt-quenched process and the...