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.     

Interface characterization of duplex metal-coated SiC fiber-reinforced Ti-15-3 matrix composites

The interfacial reaction behavior of duplex metal (Cu/Mo and Cu/W)-coated SiC (SCS-6) fiber-reinforced Ti-15-3 composites, before and after thermal exposure, has been studied. The effect of thermal exposure on the shear sliding resistance of these composites was also obtained using a thin-specimen push-out test. The results are compared to those of an original SiC (SCS-6) fiber-reinforced Ti-15-3 composite. The interfacial reaction behavior is strongly affected by the existence of a coating layer. Both the Cu/Mo and Cu/W coating layers prevent the growth of a reaction layer. However, the coatings could not effectively prevent diffusion of alloying elements; only the W layer exists after the thermal exposure. On the other hand, the interface shear sliding stress minimally depends on the duplex metal coating layers prior to the thermal exposure, and this sliding stress in both the SiC/Cu/Mo/Ti-15-3 and SiC/Cu/W/Ti-15-3 composites decreases slightly relative to that in the SiC/Ti-15-3 composite. After thermal exposure, the interface shear sliding stress increases for the SiC/Ti-15-3 composite. In distinction, the interface shear sliding stress significantly decreases after thermal exposure in both the SiC/Cu/Mo/Ti-15-3 and SiC/Cu/W/Ti-15-3 composites. Theses behaviors are attributed to the decrease of radial clamping stress, which originates from a volume expansion associated with the β → α phase transformation.     

Fabrication of fiber-reinforced metal-matrix composites by variable pressure infiltration

A model for variable pressure infiltration of fibrous preforms by molten metal has been developed. The mechanism of the infiltration and the effects of fiber distribution and wettability on infiltration resistance and composite microstructure have been studied. It is shown that the model is in good agreement with the experimental data on infiltration of carbon-fiber preforms by Al-Si eutectic. The solution of the resulting equation shows that the rate of infiltration is only a function of the rate of change of pressure Ф, by which the infiltration processing is controlled precisely. Two kinds of infiltration modes have been found. A critical fiber volume fractionV _c exists, which is the turning point of the infiltration modes as well as permeability. As fiber volume fraction exceedsV _c, the infiltration mode changes from nonuniform to uniform, resulting in a sharp decrease in permeability. The permeability and resistances of these two infiltration modes are well predicted by the variable pressure infiltration theory. If fibers are wetted by molten metal, the preforms can be completely infiltrated at low applied pressure. In the case of nonwetting, poor infiltration of the preforms up toV _cresults, though high pressure is applied, but quality composites are formed at a low applied pressure if the fibers in the bundles are fixed relative to each other. A novel process, variable pressure infiltration technique, has been generated, which offers the advantages of low applied pressure, easy control of the pro-cessing, and no requirement of wetting. Quality C/A356 composites have been fabricated by this technique with the investment precision casting molds at a pressure of 0.6 MPa. Also, the mechanical properties of the composites are studied. The composites have high strength with a special fracture mechanism.     

低温模压法制备的C/C-SiC复合材料的摩擦磨损性能

为深入了解低成本法制备的C/C-SiC复合材料的摩擦磨损规律,以短炭纤维、Si粉、炭粉和粘结剂为原料,通过均匀混合、模压成形、1 600℃反应烧结制备了C/C-SiC复合材料,研究了孔隙度、SiC含量及环境湿度对该复合材料摩擦磨损性能的影响,并用光学显微镜及X射线衍射仪对磨屑进行观测分析,对不同状况下的摩擦磨损机理进行研究。结果表明:C/C-SiC复合材料的致密度决定其磨损方式;SiC在摩擦过程中作为硬质支撑点,其含量对摩擦系数及其稳定性具有关键性影响;湿态时的摩擦系数与线磨损均略有下降,但仍能保持其良好的摩擦磨损性能。     

反应熔渗法制备C/C-ZrC复合材料的微观结构及烧蚀性能

采用反应熔渗法(reactive melt infiltration,RMI)制备ZrC改性多孔C/C复合材料,研究不同孔隙度的C/C多孔体在熔渗过程中的增密行为和渗Zr后的相组成及微观形貌,探寻具有最佳熔渗效果的C/C多孔体,并研究所得C/C-ZrC复合材料在不同温度下的氧乙炔焰烧蚀行为。结果表明,随C/C多孔体密度增加,C/C-ZrC复合材料的密度降低;其中密度为1.40 g/cm3的多孔体熔渗效果最佳,开孔隙率由熔渗前的28.2%降低到6.6%。;熔渗的Zr液易与网胎层处的炭纤维和基体炭反应,生成的ZrC陶瓷相主要分布在原网胎层位置。择取原始密度为1.40 g/cm3的C/C多孔体熔渗后进行60 s的氧乙炔焰烧蚀实验,在3 000℃下的线烧蚀率和质量烧蚀率分别为0.003 3 mm/s和0.004 2 g/s,在2 500℃下的线烧蚀率和质量烧蚀率分别为0.008 0 mm/s和0.009 0 g/s,C/C-ZrC复合材料在3 000℃下的抗烧蚀性能明显优于2 500℃下的抗烧蚀性能。     

Fatigue and fracture behavior of Nb fiber-reinforced MoSi2 composites

Fatigue and fracture mechanisms in Nb fiber-reinforced MoSi₂ composites are elucidated in this article. The effects of fiber diameter on fracture and crack-tip shielding mechanisms are discussed after a review of micromechanical models which are applied to the prediction of residual stress levels, toughening, and microcracking phenomena. Toughening is shown to occur by a combination of crack bridging and crack-tip blunting under monotonic and cyclic loading. However, the observed failure mechanisms are different under monotonic and cyclic loading. Composites with smaller (250-μm) fiber diameters are shown to have better fatigue resistance and lower fracture toughness than composites with larger (750-μm) fiber diameters. The occurrence of slower fatigue crack growth rates in the composites reinforced with smaller diameter Nb fibers is rationalized by assessing the combined effects of fiber spacing and interfacial crack growth on the average crack growth rates within the composites.     

Creep rupture life prediction of short fiber-reinforced metal matrix composites

The creep rupture life of an Al/Al₂O₃ composite and its creep behavior were studied. The metal matrix composite was produced by using a squeeze casting technique. High-temperature tensile tests and creep experiments were conducted on a 15 vol pct alumina fiber-reinforced AC2B Al alloy metal matrix composite (MMC). The high-temperature tensile strength of Al/Al₂O₃ composite is 14 pct higher than that of an AC2B Al alloy. The steady-state creep rate and the creep life were measured. The stress exponent in Norton’s equation and the activation energy were computed. The stress exponents of the AC2B and Al/Al₂O₃ composites were found to be 4 and 12.3, respectively. The activation energy of the AC2B and Al/Al₂O₃ composites was found to be 242.74 and 465.35 kJ/mol, respectively. A new equation for predicting creep life was established, which was based on the conservation of the creep strain energy. The theoretical predictions were compared with those of the experiment results, and a good agreement was obtained. It was found that the creep life is inversely proportional to the (n + 1)th power of the applied stress and strain failure energy of creep is conserved. The creep fracture surface, examined by scanning electron microscopy (SEM), showed that the MMC specimen failed in a brittle manner.     

Microstructure and mechanical properties of a welded (α + β) ti alloy

The microstructure and the mechanical properties were studied in bead-on-plate welds in a Ti-6Al-2V-1Mo alloy. The heat affected zone (HAZ) and the fusion zone (FZ) consisted of very large primaryβ grains with theβ-phase transformed to martensite. A special bead-on-plate welding technique allowed independent measurement of the mechanical properties of the HAZ and the FZ. Compared to the as-received (AR) material, the strength and ductility decreased in the weld. The highest fatigue strength was found for the AR material followed by the HAZ and the FZ, whereas the ranking for fatigue crack growth was opposite.     

Thermal conductivity and thermal expansion of graphite fiber-reinforced copper matrix composites

The high specific conductivity of graphite fiber/copper matrix (Gr/Cu) composites offers great potential for high heat flux structures operating at elevated temperatures. To determine the feasibility of applying Gr/Cu composites to high heat flux structures, composite plates were fabricated using unidirectional and cross-plied pitch-based P-100 graphite fibers in a pure copper matrix. Thermal conductivity of the composites was measured from room temperature to 1073 K, and thermal expansion was measured from room temperature to 1050 K. The longitudinal thermal conductivity, parallel to the fiber direction, was comparable to pure copper. The transverse thermal conductivity, normal to the fiber direction, was less than that of pure copper and decreased with increasing fiber content. The longitudinal thermal expansion decreased with increasing fiber content. The transverse thermal expansion was greater than pure copper and nearly independent of fiber content. formerly with NASA Lewis Research Center, is retired David L. McDanels, This article is based on a presentation made in the symposium “High Performance Copper-Base Materials” as part of the 1991 TMS Annual Meeting, February 17–21, 1991, New Orleans, LA, under the auspices of the TMS Structural Materials Committee.     

燃烧合成Cf/TiC-TiB2陶瓷基复合材料的致密化因素探讨

用燃烧合成的方法合成出不同碳纤维含量的Cf/TiC-TiB2陶瓷基复合材料.简述了试验过程,详细研究了影响产品致密度的主要因素.通过理论和测试结果分析,预制坯的致密度、延迟时间td、高压压力P2、高压保压时间tp以及碳纤维的含量等都直接影响产品致密度,进而影响产品的性能.     

Anisotropic yielding behavior of a fiber-reinforced,directionally solidified eutectic alloy

A simple experimental procedure is outlined which is capable of providing information re-quired to describe multiaxial yielding and flow behavior anisotropic materials that have a rotational symmetry. By applying the anisotropic plasticity theory that has been de-veloped recently, it is shown that a total of four simple tension and compression tests is sufficient to describe the yield surface of the highly anisotropic material. The experi-mental data obtained with a directionally solidified (DS), nickel-based, tantalum carbide reinforced eutectic alloy is shown to be in excellent agreement with the theory. Calcu-lated internal parameters based on the plasticity theory are used to interpret the role of microstructural features that contribute to anisotropic yielding behavior.     

Optimization of surface roughness and hardness for CoCrMo alloy dental crown manufactured by selective laser melting

To ameliorate the surface roughness and hardness of CoCrMo alloy dental crown fabricated by selective laser melting (SLM) for enhancing the matching suitability between SLM parts and natural teeth, electrolytic polishing and solution treatment were carried out respectively. The surface roughness of the dental crown polished by different electrolytes and electrolytic parameters, and the hardness of the dental crown solution- treated by various parameters were measured. The surface morphology of the dental crown before and after the polishing was observed by three- dimension profiler. Meanwhile, the microstructure was observed by optical microscope and scanning electron microscope. The results show that the polishing effect of the dental crown polished by HNO3 and H3PO4 solution system reaches the maximum, where the roughness decreases by 70% compared to original state. When the dental crown is solution- treated at 1503K for 10h, the hardness decreases from 470HV for no solution treatment state to 350HV, which is equivalent to the value of human natural teeth. Overall performance of CoCrMo alloy dental crown improves a lot.     

Processing and mechanical properties of AI2O3 fiber-reinforced NiAl composites

The mechanical properties of NiAl-matrix composites reinforced with 125-μm diameter single-crystal A1₂O₃ (sapphire) fibers have been examined over the temperature range of 300 to 1200 K. Composites were fabricated with either a strong or weak fiber-matrix interfacial bond strength. During fabrication, a fiber-matrix interaction occurred such that fibers extracted from the NiAl matrix were fragmented and significantly weaker than the as-received fibers. Tensile results of the weakly bonded composite demonstrated that the composite stiffness was greater than the monolithic at both 300 and 1200 K in spite of the weak bond. Room-temperature strengths of the composite were greater than that of the monolithic but below rule-of-mixture predictions (even when the degraded fiber strengths were accounted for). At 1200 K, the ultimate strength of the composite was inferior to that of the monolithic primarily because of the poor fiber properties. No tensile data was obtained on the strongly bonded material because of the occurrence of matrix cracking during fabrication. Primarily because of the fiber strength loss, sapphire-NiAl composite mechanical properties are inferior to conventional high-temperature materials such as superalloys and are currently unsuitable for structural applications.     

An evaluation of fiber-reinforced titanium matrix composites for advanced high-temperature aerospace applications

The current capabilities of continuous silicon-carbide fiber-reinforced titanium matrix composites (TMCs) are reviewed with respect to application needs and compared to the capabilities of conventional high-temperature monolithic alloys and aluminides. In particular, the properties of a firstgeneration titanium aluminide composite, SCS-6/Ti-24Al-11Nb, and a second-generation metastable beta alloy composite, SCS-6/TIMETAL 21S, are compared with the nickel-base superalloy IN100, the high-temperature titanium alloy Ti-1100, and a relatively new titanium aluminide alloy. Emphasis is given to life-limiting cyclic and monotonie properties and to the influence of time-dependent deformation and environmental effects on these properties. The composite materials offer a wide range of performance capabilities, depending on laminate architecture. In many instances, unidirectional composites exhibit outstanding properties, although the same materials loaded transverse to the fiber direction typically exhibit very poor properties, primarily due to the weak fiber/matrix interface. Depending on the specific mechanical property under consideration, composite cross-ply laminates often show no improvement over the capability of conventional monolithic materials. Thus, it is essential that these composite materials be tailored to achieve a balance of properties suitable to the specific application needs if these materials are to be attractive candidates to replace more conventional materials. 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.