Microstructural observations of pressure cast Ni3Al/Al2O3 and Ni/Al2O3 composites

Pressure castings of Ni₃Al(IC218)/Al₂O₃ and Ni/Al₂O₃ composites, made with continuous DuPont FP α-Al₂O₃ and DuPont PRD166 α-Al₂O₃+20 wt pct partially stabilized ZrO₂ 20 μm diameter fibers, were examined by optical, scanning electron microscope (SEM), and transmission electron microscope (TEM) techniques. According to optical magnifications, excellent infiltration took place. However, in SEM and TEM magnifications, small gaps were found adjacent to regions where bonding had taken place between fibers. On the basis of available evidence, the gap formation was attributed to trapped gases and microshrinkage. Titanium was added to the metal to promote infiltration. Diffusion of Ti into the fibers of the Ni/Al₂O₃ composites occurred, but similar diffusion into the fibers of the IC218/Al₂O₃ composites did not take place. The qualitatively higher bond strength of the interfaces of the Ni/Al₂O₃ composites was ascribed to the diffusion of Ti into Al₂O₃. No interface reaction layer was found in any of the composites. Very little grain growth was found to take place in either the FP or PRD 166 fibers after casting and after a subsequent ten day anneal at 1150 °C.     

The interface phase in Al−Mg/Al2O3 composites

Auger and electron diffraction studies of Al₂O₃ isolated from a composite prepared by introducing fibers into a vigorously agitated Al−Mg melt indicated the presence of MgAl₂O₄ on the fiber surface. The evidence suggested that the spinel was present as discrete crystals. The thickness and coverage of the spinel is likely to vary with processing conditions. R. MEHRABIAN formerly with the University of Illinois     

Interfaces in continuous filament-reinforced Al2O3/NiAl composites

Interfacial structures in a continuous Al₂O₃ filament-reinforced NiAl composite were investigated by transmission electron microscopy (TEM). A graphite phase, which is an artifact of the composite fabrication procedure, decorates the interfacial region of the composite. The presence of the graphite is believed to play a role in both the low interfacial bond strength in the as-fabricated composite and the further reduction in bond strength after 10 thermal cycles in the temperature range of 373 to 1373 K. In regions where the graphite phase was not present, there appeared to be an intimate bond between the NiAl matrix and the A1₂O₃ filaments. Simulation of TEM diffraction contrast images based upon a three-dimensional (3D) finite element analysis was employed to investigate the nature of the residual strains in regions along the interface. The simulations suggested that radial residual strains within the Al₂O₃ filaments were randomly distributed along the interface. These strains are believed to be related to dislocation nucleation in the NiAl, which results from the relaxation of the thermally generated residual stresses. L. WANG, formerly Graduate Student, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115     

Al2O3/Cu复合材料内氧化粉末的制备

通过引入高能球害，为Al2O3／Cu复合材料的内氧化工艺提供了新的粉末制备途径。高能球窘可制备亚稳态的Cu-Al预合金粉末，使用该粉末进行内氧化，既可避免复杂的雾化制粉装置，又可使Al的脱溶氧化变得容易，缩短内氧化的周期；Cu2O粉末与Cu-Al预合金粉末一起进行球磨。一方面改善了粉末内氧化的动力学条件，另一方面避免了Cu2O分解后产生的富铜相。球磨过程适宜的酒精加入量为15mL／100g粉料。Cu-Al系粉末在经过96h的球窘后合金已经形成，合金粉末向层片结构发展，此时粒子已经细化。X射线衍射图谱中Al的衍射峰的消失是Cu-Al系粉末形成合金的标志，并不是晶粒细化所致。Cu-2．0％Al比Cu-0．8％Al的合金更容易细化，层片结构更突出。     

Mechanical behavior and failure micromechanisms of Al/Al2O3 composites under cyclic deformation

The mechanical behavior under fully reversed cyclic deformation was determined through the incremental step method for two Al alloys reinforced with 15 vol pct A1₂O₃ particulates in the naturally aged and peak-aged conditions. The composites exhibited cyclic strain hardening in all cases, but the hardening was more pronounced in the naturally aged condition. This behavior was reflected by the stress-strain curves in monotonie tension and in fatigue, and the cyclic strain-hardening coefficient was about twice the monotonie one for both materials and tempers. The tensile and cyclic strengths of the materials were very similar, and the dominant failure mechanism under both loading conditions was paniculate fracture, which was very localized around the fracture region in fatigue, but was spread along the specimen length in monotonie tension. In addition, a few A1₂O₃ particulates were broken in compression during cyclic deformation. The final fracture micromechanism was the growth and coalescence of voids in the matrix from broken ceramic particulates. This last stage in the fracture process was fast and started when a critical volume fraction of broken reinforcements (between 30 and 45 pct) was reached in a given section of the specimen. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Wettability of silicon carbide ceramic by Al2O3/Dy2O3 and Al2O3/Yb2O3 systems

Wettability is an important phenomenon in the liquid phase sintering of silicon carbide (SiC) ceramics. This work involved a study of the wetting of SiC ceramics by two oxide systems, Al₂O₃ /Dy₂O₃ and Al₂O₃ /Yb₂O₃, which have so far not been studied for application in the sintering of SiC ceramics. Five mixtures of each system were prepared, with different compositions close to their respective eutectic ones. Samples of the mixtures were pressed into cylindrical specimens, which were placed on a SiC plate and subjected to temperatures above their melting points using a graphite resistance furnace. The behavior of the melted mixtures on the SiC plate was observed by means of an imaging system using a CCD camera and the sessile drop method was employed to determine the contact angle, the parameter that measures the degree of wettability. The results of variation in the contact angle as a function of temperature were plotted in graphic form which showed that the curves displayed a fast decline and good spreading. All the samples of the two systems presented final contact angles of 40° to 10° indicating their good wetting on SiC in the argon atmosphere. The melted/solidified area and interface between SiC and melted/solidified phase were evaluated by scanning electron microscopy (SEM) and their crystalline phases were identified by X-ray diffraction (DRX). The DRX analysis showed that Al₂O₃ and RE₂O₃ reacted and formed the Dy₃Al₅O₁₂ (DyAg) and Yb₃Al₅O₁₂ (YbAg) phases. The results indicated that the two systems had a promising potential as additives for the sintering of SiC ceramics.     

Microstructure of Mullite/ZrO2 and Mullite/Al2O3/ZrO2 tough ceramic composites

The microstructural evolution in mullite/zirconia and mullite/alumina/zirconia composites after different duration heat treatments at 1570°C has been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray microanalysis. The effect of free alumina on microstructural and mechanical parameters is also reported.     

Dislocations in continuous filament reinforced W/NiAl and Al2O3/NiAl composites

Continuous filament reinforced W/NiAl and Al₂O₃/NiAl composites (as-processed, annealed, and thermally cycled) have much higher dislocation densities than that of monolithic NiAl. These higher dislocation densities resulted from the relaxation of thermal residual stress, which developed during the cooling of the sample from elevated temperatures and was caused by the difference in the coefficients of thermal expansion between the matrix and the reinforcement. The dislocation density in the region adjacent to the matrix-filament interface was high and decreased only slightly with distance from the interface in the 30 vol pct composites. The as-processed and annealed composites exhibited a rather homogeneous dislocation density in the matrix. After thermal cycling, these composites showed no large difference in the dislocation density and morphology. However, there were local regions of lower dislocation densities. This difference was examined in relationship to filament fracture, surface matrix cracking, and degree of bonding.     

Titanium preconditioning of Al2O3 for liquid-state processing of Al-Al2O3 composite materials

Chemical vapor deposition (CVD) of titanium onto the surface of alumina substrates was used to improve the wetting by molten aluminum prior to infiltration. The CVD titanium coatings on alumina substrates were characterized by X-ray diffraction and energy-dispersive X-ray spectroscopy (EDS) as dual-phase, elemental titanium and titanium aluminide, Ti₃Al. The kinetics of the coating process were quantified and analyzed using established reaction kinetics models. An overall activation energy of 219 kJ/mole was calculated for the formation of the titanium coatings on planar alumina substrates, for the temperature range of 610 °C to 870 °C. The CVD-coated planar and porous alumina substrates were immersed in molten aluminum, and wetting of planar substrates and infiltration of porous bodies were documented. Formation of both aluminum-rich and titanium-rich reaction products was observed through EDS cross-sectional analysis.     

工艺参数对Fe3Al/Al2O3复合材料力学性能的影响研究

以自制的亚微米Fe3Al为增强相、Al2O3为基体相,通过常压烧结制备出Fe3Al/Al2O3复合材料,研究了Fe3Al含量、烧结温度及保温时间对复合材料力学性能的影响.结果表明:增加Fe3Al含量、提高烧结温度及延长保温时间都可以不同程度的提高复合材料力学性能.最佳工艺参数为:Fe3Al含量(质量分数)为15%,成形压力为2488MPa,烧结温度为1380℃.此条件下制备的复合材料的各项力学性能较好:相对密度为93%,维氏硬度为9.3GPa,断裂韧度为7.51MPa·m1/2.烧结温度对提高复合材料力学性能的影响较大.     

Oxidation behavior of single-crystal Al2O3-fiber-reinforced Ni3Al-based composites

A series of single-crystal Al₂O₃-fiber-reinforced Ni3Al-based intermetallic matrix composites were fabricated by pressure casting. The matrices employed were binary Ni₃Al, Ni₃Al-0.5 at. pct Cr, and Ni₃Al-0.34 at. pct Zr. The development of microstructure upon oxidation in air at either 1100 °C or 1200 °C was investigated by optical, scanning, and transmission electron microscopy. In air-oxidized binary Ni₃Al, some of the fibers were fully or partially covered with a layer of oxide. A weak fiber/matrix bond in this system, which led to fiber debonding during composite processing, is believed to be responsible for the ingress of O into the composite and oxidation of the matrix in the debonded regions at the fiber/matrix interface. Addition of Cr to Ni₃Al resulted in an almost threefold increase in fiber/matrix bond strength. No oxidation of the interface was observed. A thick layer of oxide was formed around all the fibers when the composite was thermally cycled prior to isothermal annealing. Addition of Zr to Ni₃Al resulted in the formation of a layer of ZrO₂ on the surface of the fibers during composite processing. The ZrO₂ layer provided a fast path for the diffusion of O, which led to the formation of a rootlike oxide structure around the fibers. The rootlike structure consisted of a network of Al₂O₃-covered ZrO₂.     

原位自生Al2O3颗粒增强Al基复合材料的研究

本研究是在Al基材料中加入了Cu3O颗粒,原位反应生成Al2O3颗粒,从而增强Al基材料.研究采用粉末冶金的方法,先冷压成型,再热压,在温度680~C压℃力1MPa时保温10分钟,成功制备了Al2O3/Al基复合材料.研究了Cu2O含量对该复合材料的密度、硬度、抗弯强度等性能的影响,结果表明:Al-8Ni-3Cu-2Cu2O复合材料的综合性能最好,硬度达到78.66HRF,抗弯强度达到254.35MPa.利用扫描电镜观察复合材料的表面形貌(SEM图像),并对试样成分进行分析(BSE图像),发现试样的成分分布比较均匀.通过XRD图谱和热力学分析表明:经热压后,该复合材料新生成物相主要为Al2O3.     

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.     

Fabrication of Al2O3-reinforced Ni3Al composites by a novel in situ route

A novel in situ technique has been used to fabricate an Al₂O₃-reinforced Ni₃Al matrix composite. The composite was prepared by first incorporating a low volume fraction of continuous Al₂O₃ fibers in a Ni₃Al alloy containing 0.34 at. pct Zr. Pressure casting was used to embed the fibers. Casting resulted in partial reduction of the Al₂O₃ fiber by the Zr present in the matrix and the formation of a layer of ZrO₂ on the surface of the fibers. The final composite was then prepared by air annealing the precursor composite at 1100 °C for 10 days. Air annealing led to the formation of networks of Al₂O₃ around the fibers. The matrix in the immediate vicinity of the networks consisted of Ni₃Al particles in a matrix of disordered α-Ni(Al). The Al₂O₃ networks raised the yield and tensile strength of the material by 35 and 18 pct, respectively. The composite had a tensile ductility of 14 pct.     

Synthesis and characterisation of Al2O3/Ni-type composites obtained by spark plasma sintering

This paper presents the influence of sintering on the structure, morphology and compressing strength of alumina/nickel composite compacts obtained by spark plasma sintering (SPS). Al₂O₃/Ni composites were prepared by SPS in argon atmosphere at temperatures in the range of 1000–1200 –C with a holding time of 2, 5 and 10?minutes. The heating rate was 200 C?min^?1. These composites have been characterised by X-ray diffraction, SEM and EDX. The relative density and compressive strength of the as-obtained compacts were determined. The results showed that the alumina particles are uniformly dispersed in a quasi-continuous Ni network, and there was no sign of phase changes during sintering. The maximum strength of the alumina/nickel composite with a content of 75 vol. ? Al₂O₃ and 25 vol. ? Ni was about 240?MPa for the samples sintered at 1200?C for 10?minutes.

Special block from the conference RoPM2017 guest edited by Ionel Chicinas, Technical University, Cluj-Napoca.     

Catalytic Synthesis of Tert-Butyl Acetate by Nd2O3/Al2O3-Nd2O3/ZnO

Nd2O3 was used to support Al2O3 and ZnO to prepare a supported solid base catalyst and investigate the effect of catalyst and reaction conditions on the synthesis of tert-butyl acetate. The composited oxide of Nd2O3/Al2O3-Nd2O3/ZnO exhibited excellent catalytic activity for the synthsis of tert-butyl acetate. The molar ratio of tert-butanol to acetic anhydride is 31, the catalyst in total amount of reactant nearly 0.5%, and reaction time 6 h. With the above conditions, yield of the reaction could reach to 65%. The structure of product were verified by the FT-IR, Element analysis, and MS, which proved that the product was tert-butyl acetate.     

Probing the initial stage of synthesis of Al2O3/Al composites by directed oxidation of Al-Mg alloys

In the directed oxidation of Al-Mg alloys, the amount of MgO that forms in the initial stage prior to the incubation period affects the rate of oxidation of Al to Al₂O₃ in the composite growth stage. The mechanism of formation of MgO and the duration of the initial stage were investigated experimentally and theoretically. The variables studied were total pressure in the reaction chamber, partial pressure of oxygen, and the nature of the diluent gas which affects the diffusion coefficients of magnesium vapor and oxygen in the gas phase. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and the diffusivity of oxygen. The duration of the initial stage decreased with the increase in oxygen pressure. To understand the role of magnesium evaporation in the oxidation behavior of the alloy, the velocity, temperature, and concentration fields in the gas phase were simulated numerically. The calculated concentration profiles of magnesium vapor and oxygen as a function of time were consistent with the experimentally measured oxidation rates and confirm reaction-enhanced gaseous diffusion-limited vaporization of magnesium in the initial stage of oxidation of Al-Mg alloys. The region where the magnesium vapor is oxidized in the gas phase moved progressively closer to the alloy surface during the initial stage of oxidation. The end of the initial stage and the start of the incubation period corresponded to the arrival of the oxygen front close to the surface when the spinel formation occurred.