High-temperature interactions of refractory metal matrices with selected ceramic reinforcements

Interdiffusion and reactions occurring at high temperatures between refractory metals (Nb and Ta) and ceramic materials (SiC and A1₂O₃) have been investigated. Diffusion couples were fabricated by depositing Nb and Ta films of ~l-μm thickness onto polished ceramic substrates. These diffusion couples were vacuum annealed at high temperatures for various times. Interfacial reactions were evaluated using optical metallography, Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Kinetic studies in the 800 °C to 1200 °C temperature range for the Nb/SiC system indicated that Nb₂C initially forms, followed by the more stable NbC_xSi_y phase. In some instances, layered structures containing the phases NbC, Nb₂C, and NbC_xSi_y, were observed. The activation energies of formation for the NbC_x and NbC_xSi_y, phases were determined from these measurements. Results from the Ta/SiC system were found to be similar to those from the Nb/SiC system. In both Nb/Al₂O₃ and Ta/Al₂O₃ diffusion couples, annealing for up to 4 hours in the 1100 °C to 1200 °C range did not result in any significant reactions. These results suggest that A1₂O₃ may be a promising diffusion barrier between Nb and Ta metal matrices and SiC ceramic reinforcements. formerly with Lockheed Research and Development Division, is Senior Member of Technical Staff, Sandia National Laboratories, Albuquerque, NM 87185     

Behavior of ceramic particles at the solid- liquid metal interface in metal matrix composites

Directional solidification experiments have been conducted to document SiC particle behavior at the solid-liquid interface in Al-2 pct Mg (cellular interface) and Al-6.1 pct Ni (eutectic interface) alloys. Particle size ranged from 20 to 150 μm diameter. Although predictions based on the thermodynamic approach suggest that no engulfment is possible, it was demonstrated that particles can be entrapped in the solid if adequate solidification rates and temperature gradients are used. The main factors responsible for this behavior are considered to be the difference between the thermal conductivities of particles and metal, the buildup of volume fraction of particles at the interface, and the morphological instability of the interface induced by the particles. A model including the contribution of drag and thermal conductivity has been proposed. Calculation with this model produced numbers for the critical velocity slightly higher than those evaluated experimentally. Various factors which can account for this discrepancy are discussed.     

In situ stress-strain response of small metal particles embedded in a ceramic matrix

The in situ stress-strain response of metal particles embedded in a ceramic matrix was obtained by combining the measurement and the modeling of the crack opening displacement field of a crack in a brittle material bridged by metal particles. The experiments were done on a composite made from platinum particles with a volume fraction of 10% in a magnesium aluminate spinel matrix. The size of the platinum particles was varied from 1 to 12 μm to study the influence of scale on the deformation behavior. Large strain to failure (85%) and ultimate tensile strength of 550 MPa were obtained for the 1 μm particles. But the larger particles failed at a strain of less than 25%; the ultimate tensile strength was also lower. This difference in ductility is explained in terms of debonding at the metal ceramic interface. It is argued that the debonding depends on the length of the dislocation pile up at the interface, and, therefore, on the particle size. The results and the metallographic observations are consistent with a model presented here; in this model the failure condition is given by a combination of the intrinsic yield stress of platinum, and the hydrostatic constraining stress in the metal particle.     

陶瓷颗粒增强铁基复合材料制备技术综述

介绍了常用的陶瓷增强颗粒及其物理、力学性能,重点综述了陶瓷颗粒增强铁基复合材料典型制备技术的优缺点及应用现状。针对陶瓷颗粒增强铁基复合材料错综复杂的影响因素,指出将现有制备技术与计算机模拟技术相结合是今后陶瓷颗粒增强铁基复合材料制备技术的一个重要发展方向。     

钛箔表面二氧化钛纳米管的可控制备研究

采用电化学阳极氧化法在金属钛箔表面制得了二氧化钛纳米管,通过调整阳极氧化电压和时间,控制纳米管的管径及管壁尺寸,采用扫描电镜(SEM)观察了不同制备条件下得到TiO2纳米管阵列的微观形貌,考察了氧化电压及时间对纳米管TiO2纳米管阵列形貌的影响;利用热处理工艺调整二氧化钛纳米管的晶型,通过X射线衍射仪(XRD)对样品晶...     

Nucleation on ceramic particles in cast metal-matrix composites

In order to understand the nucleation on ceramic particles in the melts of metal-matrix composites (MMCs), the effect of segregation of solute on the surface of reinforcement particles in the melt has been analyzed as a function of particle temperature and the surface energy of the particle/liquid melt. The temperature of the particle in the melt, calculated analytically, was found to become close to the melt temperature within a very short time of contact between the particle and the melt. The solute concentration near the particle surface will, therefore, primarily be influenced by the surface energy of the particle and the melt. Based on this, the undercooling due to solute segregation around the particle and the chemical free-energy change due to the formation of the new solid phase on the particle were calculated in selected hypo- and hypereutectic Al-Si alloy melts containing (1) SiC particles or (2) graphite particles. The chemical free-energy change (driving force for nucleation) due to the formation of the new phase on the particle is lower for hypoeutectic compositions than for hypereutectic compositions in the aluminum-silicon alloy systems; this is due to the higher undercooling in the hypereutectic alloys due to solute segregation on the surface of the particle. This suggests that the formation of the primary phase on the surfaces of particles in the melt should be more favorable in the hypereutectic compositions than for hypoeutectic compositions. This also indicates that even when the particle temperature is not significantly lower than the liquidus temperature, nucleation on the particles can take place due to the segregation of the solute on the particles. Experimental observations of the microstructure of several cast metal-matrix composites, including Al-Si-SiC and Al-Si-graphite, show (1) the presence of silicon in contact with the reinforcement particles in hypereutectic alloys, suggesting that nucleation and growth of primary silicon under certain conditions occurs on silicon carbide and graphite particles, possibly due to solute segregation on the surface of the particles, and (2) the presence of reinforcement particles in the last-freezing interdendritic regions of the primary phases in hypoeutectic alloys, suggesting the absence of nucleation of primary phases on the reinforcement surface, as predicted by the analysis.     

Plastic deformation of metal particles in gas-dynamic spraying

The main stage of the gas-dynamic spraying process is the deformation and heating of particles upon impact with the substrate. A solution of the impact deformation problem by use of the finite element method is given. The model developed enables determination of the extent of particle deformation and the time and temperature of impact as functions of the material and particle properties and the initial velocity and temperature of particles. Ukraine State Metallurgical Academy, Dnepropetrovsk. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 10–15, July–August, 1998.     

Identification of thermodynamically stable ceramic reinforcement materials for iron aluminide matrices

Aluminide-base intermetallic matrix composites are currently being considered as potential high-temperature materials. One of the key factors in the selection of a reinforcement material is its chemical stability in the matrix. In this study, chemical interactions between iron alu-minides and several potential reinforcement materials, which include carbides, oxides, borides, and nitrides, are analyzed from thermodynamic considerations. Several chemically compatible reinforcement materials are identified for the iron aluminides with Al concentrations ranging from 40 to 50 at. pct.     

Cracking and damage mechanisms in ceramic/metal multilayers

Investigations of cracking in multilayered ceramic/metal composites are presented. Two aspects are considered: crack renucleation across intact single metal layers and subsequent crack extension. Crack renucleation criteria are determined and compared with predictions. High-resolution strain-mapping techniques are employed to determine the surface strain fields surrounding cracks. Good agreement is found between these experimental measurements and the predictions of a small-scale yielding model. Subsequent crack progression occurs either by the extension of a dominant, nearly planar crack or by the formation of a zone of periodically spaced cracks. Both patterns are analyzed. The dominant cracking behavior is found to depend on the volume fraction and yield strength of the metal.     

Deep filtration of molten aluminum using ceramic foam filters and ceramic particles with active coatings

The inclusions in molten aluminum were removed using the deep filtration of ceramic foam filters and ceramic particles with active coatings. The results of tensile tests showed that the elongation of the filtered tensile specimen S₆ increases by 17.93 pct, but the tensile strength does not. The scanning electron microscope (SEM) examination showed that the secondary cracks and dimples in the filtered tensile specimen S₆ were finer and more homogeneous than those in the unfiltered tensile specimen S₀. In addition, metallographic examination showed that there were only a few inclusions of approximately 6 μm in diameter in the filtered specimen S₆, but more inclusions of approximately 40 μm in diameter were found in the unfiltered specimen S₀. The filtration efficiency of the coated ceramic particles was higher than that of the coated ceramic foam filters. The active coating could effectively capture the inclusions and dissolve Al₂O₃ in them during filtration.     

铝箔样品的制备与火花源原子发射光谱分析

探讨了火花源原子发射光谱法对铝箔分析中样品的制备实验技术,比较各种制备手段、制备方法的可行性及由其所得到的样品检测效果。采用机械压样机破碎样压制方法制备的铝箔样品可在光电直读光谱仪上进行快速分析。在35 T压力,压力保持延时60 s,重复压制7次,预燃时间40 s时,测定铝箔样品中的Zn,Ni,Mg,Si,Fe,Cu,Mn,Cr,Ti 9种元素,测定方法相对标准偏差(RSD)小于3.0%,测定结果与ICP-AES法、化学分析方法基本相符。     

The scattering of light by metal particles

Summary Indicatrices are constructed for the scattering of three monochromatic radiations by iron, nickel, and copper powders in specular and diffuse reflection. Use was made of tables of energy coefficients of reflection from absorbing media [2] and tables of formulas of two diffuse reflection laws [5]. The possibility of discovering which of the scattering laws is in better agreement with experimental results without plotting the whole indicatrix are demonstrated.     

A microstructural study of dislocation substructures formed in metal foil substrates during ultrasonic wire bonding

A study has been conducted on the deformation mechanisms in metal substrates subject to aluminum ultrasonic wire bonding (UWB). Aluminum wires were bonded to copper, nickel, stainless steel, and aluminum bronze foil substrates and then removed in aqueous sodium hydroxide to permit thin sections of bonded areas to be examined in the transmission electron microscope (TEM). The results showed a variety of dislocation substructures formed during bonding, including dislocation cells, subgrains, and planar arrays. Aluminum and copper showed evidence of thermal effects on microstructural evolution during bonding, such as dislocation annihilation at cell walls in copper and complete recrystallization in aluminum. In the nickel and stainless steel substrates, which have higher recrystallization temperatures, thermal effects on microstructure were not observed. In addition, it was found that low stacking-fault energy (SFE) materials, such as aluminum bronze, were less likely to undergo cell formation, and only planar dislocation arrays formed. In general, it is clear that the process of UWB induces cyclic stresses in the substrates, which exceed the yield strength of the metals examined. In addition, there is some heat generated during the bonding process, which can influence the resultant deformation microstructure.     

The role of metal plasticity and interfacial strength in the cracking of metal/ceramic laminates

Two models based on elastic-plastic fracture mechanics and fiber bridging are developed to study the role of plastic yielding in metals and the interfacial strength of metal/ceramic laminates. There are two types of damage observed in metal/ceramic laminates: multiple cracking and macroscopic crack propagation. The former occurs around the macroscopic crack tip and thus distributes the damage and enhances the composite's toughness. The present models establish that there exists a critical metal/ceramic layer thickness ratio above which multiple cracking dominates and that this ratio decreases (hence increasing the possibility of multiple cracking) as the ratios of metal yield stress over ceramic strength, metal modulus over ceramic modulus, and metal/ceramic interfacial strength over ceramic strength increase. Good agreement between the present models and experimental results is observed for both damage modes, i.e. multiple cracking vs macroscopic crack propagation, and for critical stress intensity factors. The elastic-plastic fracture mechanics and fiber-bridging models predict that multiple cracking is ensured if the metal layer thickness is 2.5 times larger than the ceramic layer thickness, regardless of the metal/ceramic properties.