Microstructure of AI2O3 fiber-reinforced superalloy (INCONEL 718) composites

Composites of INCONEL 718 alloy reinforced with either single-crystal (SAPHIKON) or polycrys-talline (Du Pont's FP) A1₂O₃ fiber were fabricated by pressure casting. Optical and transmission electron microscopy were used to characterize the microstructure of the composites and to determine the nature of the fiber/matrix reaction. The widely dispersed fibers in the SAPHIKON-fiber-reinforced composite had no influence on the solidification of the matrix. Six phases, γ-Ni₃Al, γ'-Ni₃Nb, δ-Ni₃Nb, TiC, NbC, and Laves, were present in the matrix of the composite. The last three phases were formed during solidification and the others precipitated during subsequent cooling. The high density of fibers in the FP-fiber-reinforced composite led to a more uniform microstructure within the matrix. Only three phases,γ″-Ni₃Nb, NbC, and Laves, were identified. Diffusion of Ti into the A1₂O₃ fiber resulted in preferential grain growth in the FP fiber in areas adjacent to the fiber/matrix interface. The fiber/matrix bond strength in shear in the SAPHIKON-fiber-reinforced composite was in excess of 150 MPa.     

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.     

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.     

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     

Influence of particle size and volume percent of flaky mo particles on the mechanical properties of AI2O3/Mo composites

The influence of particle size and volume percent of Mo particles on flake-forming behavior of Mo powders during a ball milling process and three-point flexural strength and fracture toughness of A1₂O₃ composites reinforced with flaky Mo particles have been investigated. The flake-forming behavior of Mo powders mixed with A1₂O₃ powders becomes prominent with increasing Mo particle size, while remaining almost independent of Mo volume percent. The microstructure of the composites reinforced with flaky Mo particles is anisotropic, depending on the arrangement of these Mo particles in the A1₂O₃ matrix. The microdispersion of flaky Mo particles contributes remarkably to an increase in both flexural strength and fracture toughness. The flexural strength increases with decreasing Mo particle size, while the fracture toughness increases with increasing Mo particle size, which corresponds to an increase of the flake-forming tendency of these particles. Furthermore, the flexural strength and fracture toughness can be simultaneously improved by increasing the volume fraction of flaky Mo particles. The microstructural observations indicate that the improvement in strength may be attributed to a grain-refining effect due to inhibition of grain growth of the matrix by the presence of Mo particles. In addition, the improvement in fracture toughness may be due to plastic deformation of Mo particles at a crack tip, which is accelerated more by the flaky rather than the small spherical shape.     

Influence of Er2O3 content on microstructure and mechanical properties of ZTA-TiO2 composites

The influence of Er_2 O_3 addition on the phase evolution and mechanical properties of sintered(1600 ℃,4 h) ZTA(yttria stabilized zirconia toughened alumina)-TiO_2 composites was investigated. The SEM and XRD results reveal the formation of a new erbium zirconium oxide,Zr_3 Er_4 O_(12),with a granulate morphology when Er_2 O_3 content is higher than 1 wt%. The grain sizes of both Al_2 O_3 and yttria-stabilized zirconia phases decrease with an increase in the Er_2 O_3 content. The relative density, Vickers hardness and fracture toughness of the composites are found to be strongly dependent on their grain sizes, relative densities and the formation of the Zr_3 Er_4 O_(12) secondary phases. The composite with 5 wt% Er_2 O_3 shows the highest relative density(99.93%), Vickers hardness(1752 HV) and fracture toughness(7.92 MPa·m~(1/2)).     

添加剂CeO2对WC-Al2O3复合材料微观组织和力学性能的影响

为了进一步提高WC-Al2O3复合材料的力学性能,研究少量添加剂CeO2对热压烧结WC-Al2O3复合材料的影响。研究结果表明,在WC-Al2O3复合材料中添加0.1%CeO2后,与不含添加剂相比,组织细化且力学性能有所提高;少量添加剂CeO2具有抑制WC氧化脱碳、细化晶粒、改善基体WC和第二相Al2O3颗粒之间界面结合状况的作用。WC-Al2O3-0.1%CeO2复合材料的致密度达到98.82%,维氏硬度、断裂韧性和抗弯强度分别达到16.89GPa、9.85MPa·m1/2和1024.05MPa。     

Effects of CeO2 additives on the microstructure and mechanical properties of WC-Al2O3 composites

To improve the mechanical properties of WC-Al2O3 composites,the effects of trace amount of CeO2 additives on the microstructure and mechanical properties of the WC-Al2O3 composites prepared by hot pressing were investigated. The results revealed that the WC-Al2O3 composites doped with 0.1% CeO2 possessed refined microstructure and enhanced mechanical properties compared with that of the undoped WC-Al2O3 composites.Trace CeO2 suppressed the decarburization of WC,promoted the microstructural refinement,and improved the interface coherence of the WC matrix and Al2O3. When 0.1% Ce O2 was added to the WC-Al2O3 composites,the effect of CeO2 resulted in the achievement of a relative density of 98.82% with an excellent Vickers hardness of 16.89 GPa,combining a fracture toughness of 9.85 M Pa·m1 /2with an acceptable flexural strength of1 024.05 MPa.     

NiAl/TiC改性C/C复合材料的微观结构及力学性能

采用熔渗法对C/C多孔坯体进行预熔渗Ti处理,再用NiAl对预熔渗Ti后的C/C多孔坯体进行金属基体改性,制备出NiAl/TiC金属陶瓷改性C/C复合材料,并初步探讨C/C复合材料中NiAl/TiC金属陶瓷复合结构的形成机理及其对改善复合材料力学性能的作用机理。研究结果表明:预熔渗Ti后,Ti与基体炭反应生成TiC。由于NiAl与TiC润湿性好,生成的TiC可有效改善NiAl在C/C多孔坯体中的熔渗深度。NiAl在C/C多孔坯体中的熔渗深度为3~5 mm,同时,NiAl金属相与TiC陶瓷相在材料中呈镶嵌结构复合生长且分布无规则。经NiAl/TiC金属陶瓷熔渗后,复合材料的密度达到2.39 g/cm3,开孔率为13.44%,抗压强度为85.3 MPa,抗弯强度为67.2 MPa。     

Anelastic relaxation in AI-4 Wt Pct Cu-AI2O3 fiber-reinforced composites

In many industrial applications, like high precision weighing and positioning, the elastic and dimensional stability of materials is required at a nanometric scale. High-resolution laser interferometry and mechanical spectroscopy have been employed to measure low-temperature anelastic creep of the short-fiber-reinforced composite Al-4 wt pct Cu-Al₂O₃. The typical strain resolution of the laser interferometer is 10^-10. Fiber reinforcement has been found to increase the dislocation density in the metal matrix; in parallel, damping and anelastic creep are enhanced. This behavior has been explained on the basis of the structure of interparticle dislocations and θ′ relaxation.     

Processing and mechanical properties of magnesium-lithium composites containing steel fibers

Deformation-processed metal-metal composites (DMMC) of Mg-Li alloys containing steel reinforcing fibers were prepared by infiltrating a preform of steel wool with the molten matrix. The Li content was varied to control the crystal structure of the matrix; Mg-4 wt pct Li is hexagonal close packed (hcp), while Mg-12 wt pct Li is body-centered cubic (bcc). The low carbon steel used as the reinforcing fiber is essentially bcc. The hcp/bcc and bcc/bcc composites were subsequently deformed by rolling and by extrusion/swaging and mechanically tested to relate the tensile strength of the composites to true deformation strain. The hcp/bcc composites had limited formability at temperatures up to 400 °C, while the bcc/bcc composites had excellent formability during sheet rolling at room temperature but limited formability during swaging at room temperature. The tensile strengths of the hcp/bcc composite rod and the bcc/bcc composite sheet and rod increased moderately with deformation, though less than predicted from rule-of-mixtures (ROM) calculations. This article presents the experimental data for these DMMC materials and comments on the possible effect of texture development in the matrix and fiber phases on the deformation characteristics of the composite material.     

纳米NiAl2O4/Al2O3复合粉末的制备新工艺

本研究采用高温氧化的方法制备出纳米NiAl2O4／Al2O3粉体。在纳米Al2O3粉体表面包覆一层金属Ni，在1350℃高温下焙烧Ni／Al2O3复合粉体得到纳米NiAl204／Al2O3粉体。利用TEM对Ni／Al2O3复合粉体进行观察，发现Ni／Al2O3复合粉体颗粒成球形，大小为50～60nm；通过对Ni／Al2O3复合粉体的DTA析，显示Ni／Al2O3复合粉体在900℃和1300℃下有新相生成，经XRD检测，新相分别为NiO和NiAl2O4。     

Failure characteristics of 6061/AI2O3/15ρ and 2014/AI2O33/15ρ composites as a function of loading rate

The effects of loading rate on the toughness and fracture mechanisms of two cast 6061/Al₂O₃/15p and 2014/Al₂O₃/15p composites under the as-worked (AW) and AW + T6 conditions have been examined. The quasistatic bending and high-rate impact tests were conducted over strain rates from 5 X 10^-4 to 1 X 10³ s^-1 using screw-driven or servohydraulic high-rate systems. The results showed that the peak loadP _max, specimen deflectiond, specimen lateral expansion fraction Δw, crack initiation energyE _i, propagation energyE _p, total fracture energyE _t and deformation zone all tended to increase with increasing strain rate. Under quasistatic loading, the composites failed predominantly by ma-trix/reinforcement interface decohesion. As the loading rate increased, reinforcement failure became the major failure mechanism. Differences in the effect of matrix microstructure and stress state on the fracture properties also are discussed. In comparing the fracture modes in the AW and AW + T6 specimens, the latter showed a higher tendency toward particle cracking. Based on mechanical data, the degree of specimen deflection and expansion and fracture modes, the AW composites exhibited a higher strain-rate dependence. The T6 specimens, due to their intrinsicly more brittle nature, appeared to be less influenced by loading rate over the strain-rate range examined.     

Interfacial reactions in Al-Mg (5083)/AI2O3p composites during fabrication and remelting

The interface reactions between a-Al₂O₃ particles and 5083 Al-Mg alloys during fabrication by compocasting and subsequent remelting at 800 °C for 30 minutes have been studied using analytical transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Experimental results show that MgO is the main reaction compound produced by the reaction 3Mg (1) + A1₂O₃ (s) = 3MgO (s) + 2A1 (1). The MgO crystals formed by interfacial reaction are very small (about 5 to 20 run in diameter), and the reaction zone is about 50 to 80-nm wide for as-cast materials and about 100 to 150 nm after remelting. The reaction kinetics is controlled by seepage of Mg and release of Al (1) along the newly formed MgAl₂O₄ or MgO grain boundaries. Because of the large volume expansion during the formation of MgO from the reaction between Mg (1) and A1₂O₃ and because of the low diffusivity in MgO crystals, the reaction rate is very low.     

Triclinic Ni2AI phase in 63.1 atomic percent NiAl

Transmission electron microscopy (TEM) studies on the 63.1 at pct NiAl specimens aged in the Ll₀ phase indicate the presence of localized regions consisting of very high-density twins along with precipitates of Ni₂Al stoichiometry. In regions representative of the 7R phase, it is observed that the ordered Ni₂Al phase exists in lattice correspondence with the 7R phase. Based on the electron-diffraction results, the crystal structure of this Ni₂Al phase is determined to be triclinic, belonging to the space group . This new Ni₂Al phase alternatively may be viewed as a periodically microtwinned monoclinic Ni₂Al phase, existing in coherence with the matrix Ll₀ phase that is also periodically microtwinned.