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.     

复合材料Al/Al4C3/Al2O3的组织结构与力学性能

采用机械合金化（ＭＡ）球磨和热压烧结工艺制备了复合材料Ａｌ／Ａｌ４Ｃ３／Ａｌ２Ｏ３，对其组织结构和力学性能进行了研究。结果表明，发育良好的Ａｌ４Ｃ３棒状单晶体和等轴状γＡｌ２Ｏ３均匀分布在铝晶界或晶粒内部，二者体积含量约为６６ｖ％。Ａｌ／Ａｌ４Ｃ３和Ａｌ／Ａｌ２Ｏ３界面洁净，为直接的原子结合，但不存在确定的位向关系。复合材料的室温、高温强度及刚度比粉末冶金纯铝（Ｐ／ＭＡｌ）显著提高。     

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.     

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     

Effect of magnesium on the aging behavior of Al-Zn-Mg-Cu/Al2O3 metal matrix composites

The effect of magnesium content on the aging behavior of Al-Zn-Mg-Cu alloy reinforced with alumina (A1₂O₃) was studied by using the differential scanning calorimetry (DSC) technique and hardness measurement. The magnesium contents were studied in the range from 1.23 to 2.97 wt pct. The addition of magnesium was found to increase the coherent Guinier-Preston (GP) zones in com-posites. The apparent formation enthalpy of GP zones of composites (0.1V _f) was 0.932 cal/g for 1.23 wt pct magnesium content and 1.375 cal/g for 2.97 wt pct magnesium content. The precipitation time to achieve the maximum hardness in the composites depends on the magnesium content. The time changed from 12 to 48 hours as the magnesium content increased from 1.23 to 2.97 wt pct. Both Vickers microhardness and Rockwell hardness increased with increasing magnesium content. The maximum hardness occurred in the composites that contained maximum amounts of GP zones and η′ precipitates. However, the microhardness of the composites was always lower than that of monolithic alloys due to the alumina fibers which caused the suppression of GP zones and η′ for-mation in the composites.     

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     

Tensile behavior of AI2O3/FeAI + B and AI2O3/FeCrAIY composites

The feasibility of Al₂O₃/FeAl + B and Al₂O₃/FeCrAlY composites for high-temperature applications was assessed. The major emphasis was on tensile behavior of both the monolithics and composites from 298 to 1100 K. However, the study also included determining the chemical compatibility of the composites, measuring the interfacial shear strengths, and investigating the effect of processing on the strength of the single-crystal A12O3 fibers. The interfacial shear strengths were low for Al₂O₃/FeAl + B and moderate to high for Al₂O₃/FeCrAlY. The difference in interfacial bond strengths between the two systems affected the tensile behavior of the composites. The strength of the A1₂O₃ fiber was significantly degraded after composite processing for both composite systems and resulted in poor composite tensile properties. The ultimate tensile strength (UTS) values of the composites could generally be predicted with either rule of mixtures (ROM) calculations or existing models when using the strength of the etched-out fiber. The Al₂O₃/FeAl + B composite system was determined to be unfeasible due to poor interfacial shear strengths and a large mismatch in coefficient of thermal expansion (CTE). Development of the Al₂O₃/FeCrAlY system would require an effective diffusion barrier to minimize the fiber strength degradation during processing and elevated temperature service.     

原位还原法制备Al/Al2O3复合材料的力学性质及反应动力学

以石英为先驱体,以液态铝为还原剂,在1073～1523 K的温度范围内对原位生成铝/氧化铝复合材料进行了研究,对获得的复合材料的物理和机械性能做了测定,并对材料显微结构进行了观测和分析.在1473K制备的铝/氧化铝复合材料密度为2.95g/cm3,最大弹性模量为130 GPa,最大三点抗弯强度为580 MPa,最大拉伸强度为268 MPa,洛氏硬度为86.产物铝/氧化铝复合材料的形状与作为先驱体的二氧化硅的形状几乎一致.讨论了反应过程的动力学.     

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.     

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。     

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.     

热处理对SiCp/Al复合材料拉伸变形行为的影响

采用压力浸渗法制备SiC体积分数为55%的SiCp/Al-10Si-0.7Mg复合材料.通过拉伸试验和扫描电镜研究不同热处理工艺对该复合材料拉伸变形行为及显微组织的影响.结果表明,热处理对高体积分数SiCp/Al-10Si-0.7Mg复合材料的弹性模量影响很小,但T6热处理可显著提高其在拉伸时的弹性极限,而冷热循环处理可以在T6热处理的基础上进一步提高拉伸弹性极限.高体积分数SiCp/Al-10Si-0.7Mg复合材料的拉伸弹性极限的提高主要是热处理过程中基体铝合金中的固溶和时效强化、残余内应力的消除、位错强化和加工硬化综合作用的结果.     

SiCp/2014Al复合材料的热变形行为及本构模型

在Gleeble-3180热模拟机上对碳化硅颗粒增强铝基(SiCp/2014Al)复合材料进行热压缩试验,研究其在变形温度为350,400,450 ℃和500 ℃,应变速率为0.001,0.01,0.1s-1和1.0 s-1条件下的热变形行为。根据热压缩实验的真应变-真应力数据,在考虑应变、应变速率和变形温度对流动应力的耦合影响下构建修正的Johnson-Cook(JC)本构模型,同时建立人工神经网络模型(ANN)。结果表明:SiCp/2014Al复合材料的流变应力随应变速率的增加和温度的降低而增大。与修正的JC模型相比,ANN模型具有较低的均方根误差(0.51 MPa)和平均绝对误差(1.43%),以及较高的相关系数(0.999 7),表明其对SiCp/2014Al复合材料热变形流变应力的预测具有更高的预测精度和可靠性。      

Al2O3/AlTiC/ZrO2/透辉石陶瓷材料的力学性能及微观结构

将金属Al、Al3Ti和TiC以AlTiC中间合金的形式以及ZrO2颗粒共同引入Al2O3基体材料中,热压制备了Al2O3/TiC/ZrO2/AlN复合材料.在此基础上,添加(体积分数)1%透辉石作为烧结助剂,以实现复合材料的液相烧结并促进其致密化程度.复合材料在烧结过程中有新相AlN生成;同时Al、TiC以及Al3Ti释放的Ti原子发生原子重组生成Al2Ti4C.对热压后材料的硬度、断裂韧度和抗弯强度进行了测试和分析;探讨了透辉石对材料致密化程度及力学性能的影响效果;研究了复合材料断面断裂方式的变化对其力学性能的影响;并对AlTiC中间合金的细化特性进行了分析.     

Mechanical behavior of Al-Li/SiC composites: Part II. Cyclic deformation

The deformation and failure mechanisms under cyclic deformation in an 8090 Al-Li alloy reinforced with 15 vol pct SiC particles were studied and compared to those of the unreinforced alloy. The materials were tested under fully reversed cyclic deformation in the peak-aged and naturally aged conditions to obtain the cyclic response and the cyclic stress-strain curve. The peak-aged materials remained stable or showed slight cyclic softening, and the deformation mechanisms were not modified by the presence of the ceramic reinforcements: dislocations were trapped by the S′ precipitates and the stable response was produced by the mobile dislocations shuttling between the precipitates to accommodate the plastic strain without further hardening. The naturally aged materials exhibited cyclic hardening until failure, which was attributed to the interactions among dislocations. Strain localization and slip-band formation were observed in the naturally aged alloy at high cyclic strain amplitudes, whereas the corresponding composite presented homogeneous deformation. Fracture was initiated by grain-boundary delamination in the unreinforced materials, while progressive reinforcement fracture under cyclic deformation was the main damage mechanism in the composites. The influence of these deformation and damage processes in low-cycle fatigue life is discussed.     

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.     

Diffusional reactions during processing of TIMETAL 21S/Al2O3 composites

The stability of an Al₂O₃ reinforcement in TIMETAL 21S has been investigated by annealing diffusion couples and consolidated fiber composites at 1100 °C, 900 °C, and 750 °C. Diffusion couple studies indicate that γ-TiAl, α ₂-Ti₃Al, and α-Ti(Al,O) phases can form upon annealing above the β transus of TIMETAL 21S, but γ-TiAl, α ₂-Ti₃Al, and a ternary T phase form during annealing below the β transus. The phases developed during diffusional interaction define a diffusion path between TIMETAL 21S and Al₂O₃. A coating of Nb, Mo, or Ta between TIMETAL 21S and Al₂O₃ acts as a diffusion barrier, but the coatings can diffuse into TIMETAL 21S at high temperature. In agreement with a kinetics analysis, a 2-μm-thick interface coating of Nb, Mo, or Ta in the TIMETAL 21S/Al₂O₃ composite can prevent the reaction during processing (2 hours at 850 °C or 900 °C) with no detectable diffusion into the matrix. If there are imperfections such as pinholes or cracks present in the diffusion barrier, the reaction quickly starts at the interface and does not remain confined at the imperfection; rather, it progresses along the interface. The mechanism for progressive development of interface reaction at a discontinuity in the diffusion barrier has been proposed. The analysis of the diffusional interface reactions in this work has identified some of the governing design concepts for development of robust high-temperature titanium-based composites.     

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.