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.     

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.     

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.     

Combustion synthesis of Ni3Al and Ni3Al-matrix composites

The self-propagating mode of combustion synthesis (SHS) of Ni₃Al starting from compacts of stoichiometrically mixed Ni and Al powders readily forms fully reacted structures with about 3 to 5 pct porosity, if green density of the compacts is greater than about 75 pct of theoretical. SHS-produced Ni₃Al matrix composites with up to 2 wt pct A1₂O₃ whiskers also have relatively low porosity levels. Porosity increases rapidly with lower green densities, higher Al₂O₃, or SiC whisker contents, and the degree of reaction completeness diminishes. The SiC whiskers undergo reaction with the matrix, while Al₂O₃ whiskers are nonreactive. All of these observations correlate well with temperature measurements made during the course of the reaction. The SHS mode can be achieved with agglomerated particle size ratioD _Al/D _Ni ≥ 1, larger than the limit established from studies of the thermal explosion mode of combustion synthesisD _Al/D _Ni ≃ 0.3. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

Oxidation of Al2O3-30%TiCN-0.2%Y2O3 Composite

Thecuttingtoolmaterialshavegoodoxidationre sistance ,buttheyareoxidizedathightemperature(about 10 0 0℃ )duringoperation .Themechanicalpropertyofthecuttingtoolmaterialswillbechange d[1,2 ] .Therefore ,researchonoxidationbehaviorofcuttingtoolsmaterialsathi…     

Fiber strength and fiber/matrix bond strength in single crystal Al2O3 fiber reinforced Ni3Al based composites

A series of single-crystal A1₂O₃ fiber (Saphikon), reinforced Ni₃Al-based composites were fabricated by a liquid metal infiltration technique, pressure casting. Tensile testing and indentation techniques have been employed to measure fiber strength and fiber/matrix interfacial debond shear stress. The Weibull mean strength of the fiber has been observed to decrease drastically upon handling, exposure to high temperature, and casting. Alloying of Ni₃Al with Ti has resulted in a further decrease in fiber strength. Thermal expansion mismatch between the fiber and matrix led to the formation of compressive twins in the fiber. These twins, forming on planes, produced cracks at their intersections, which were parallel to the fiber axis,c-axis. Twin-induced fiber cracking was observed in all cases, but most predominantly when Cr was present. While addition of Cr at the 1 at. pct level had no appreciable effect on the interfacial debond shear stress, addition of 0.5 at. pct Cr resulted in an approximately threefold increase in debond stress, from 19 MPa to about 54.5 MPa. Alloying of Ni₃Al with Cr has also resulted in partial dissolution of the A1₂O₃ fiber. Addition of Ti had a moderate effect on increasing the fiber/matrix bond strength.     

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.     

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     

TiN/TiCN/Al2O3/TiN CVD多层涂层硬质合金的氧化行为

采用高温化学气相沉积(HT-CVD)和中温化学气相沉积(MT-CVD)相结合的复合化学气相沉积新技术在硬质合金基体WC-(W,Ti)C-(Ta,Nb)C-6%Co上分别沉积TiN/TiCN/TiN和TiN/TiCN/Al2O3/TiN涂层,制备TiN/TiCN/TiN和TiN/TiCN/Al2O3/TiN CVD多层涂...     

Interface characterization of fiber-reinforced Ni3Al matrix composites

The interfacial reaction characteristics of SCS-6, Sigma, and B₄C/B fibers with nickel aluminide (Ni₃Al) matrix have been investigated between 780°C to 980°C for times ranging from 1 to 100 hours. The microstructure and elemental compositions across the reaction zone have been analyzed quantitatively using microscopy and electron probe microanalyses, respectively. The results show that Ni₃Al reacts extensively with SCS-6, Sigma, and B₄C/B fibers to form complex reaction products, and Ni is the dominant diffusing species controlling the extent of reaction. In the SiC/Ni₃Al composite, the C-rich layer on the SiC surface can slow down but cannot stop the inward diffusion of Ni into SiC fiber. When the C-rich layer is depleted, a rapid increase in reaction zone thickness occurs. Diffusion barrier coating on the fibers is required to minimize the interfacial reactions.     

Creep behavior of Al-Al3Ni eutectic composites

Specimens of Al-Al₃Ni, directionally solidified at 11 cm per hr, were creep-tested at a single set of conditions to plastic strains in the first, second, and third stages, and to rupture. Specimens from these interrupted tests were sectioned and examined by transmission electron microscopy. Due to the small plastic strains encountered in the first and second stages of deformation(εt < 0.3 pct) relatively little substructure buildup was observed in the matrix of these specimens. In specimens tested to rupture, dense dislocation cells keyed on the whiskers were found. As a result, a time-dependent fracture mechanism was identified which differed from normal tensile fracture, and was related to the substructure buildup. The effects of solidification rate on creep resistance, macrohardness, and compressive yield strength were also investigated and showed that by increasing the solidification rate a more creep-resistant, harder, and stronger material was produced. This has suggested that additional strengthening of this material by a dispersion effect of the whiskers on the matrix is possible. Formerly with United Aircraft Research Laboratories     

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     

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.     

高Al2O3物料对镍电炉渣粘度的影响

论文对Al2O3高达69．46％的含镍物料电炉处理时对炉渣性能的影响进行了研究。研究表明：吉林镍业公司冶炼厂电炉渣1340℃时黏度为0．538P．s；电炉渣在不添加任何其它试剂的情况下，渣中Al2O3含量不宜高于9％；加入添加剂后，炉渣黏度降低，当在配料中加入20％此种含镍物料时，添加剂加入量7％为宜，此时炉渣黏度为0．74P．s（1340℃）。     

Mechanical behavior of rapidly solidified Al-Al2Cu and Al-Al3Ni composites

The eutectic alloys Al-Al₂Cu and Al-Al₃Ni have been unidirectionally solidified at rates from 1.05 to 6.80 in, per min by a semicontinuous casting technique, and then tested in tension at room temperature. In both alloys the flow stress and ultimate tensile strength increased with increasing solidification rate, except for the highest solidification rate. The increases in matrix work-hardening rate with solidification rate were too great to be accounted for by dislocation pileup mechanisms, but were found to correlate with elastic constraint effects of the matrix aluminum phase by the reinforcing phases. In the Al-Al₂Cu eutectic the strength of the Al₂Cu platelets increased as the platelet width decreased with increasing growth rate. Misalignment of the composite caused by either a cellular or a macroscopically concave solid-liquid interface resulted in a decrease in the ultimate strength, especially in the rod-like Al-Al₃Ni alloy. This has been related to the fracture behavior of the composites. The very low fracture toughness of the lamellar Al-Al₂Cu eutectic is consistent with models of composite materials, and seriously limits the alloy’s usefulness for engineering applications.