The automotive application of discontinuously reinforced TiB-Ti composites

In 1998, Toyota Motor Corporation adopted intake valves and exhaust valves made of titanium-based alloys for the engine of its Altezza. Both valves were manufactured via a newly developed cost-effective powder metallurgy process. The exhaust valve is made of a newly developed titanium metal-matrix composite (MMC). The valve has achieved sufficient durability and reliability with a manufacturing cost acceptable for the mass-produced automobile engine components. For more information, contact Takashi Saito, Toyota Central Research and Development Laboratories, Materials Department, 41-1 Yokomichi Nagakute Aichi, 480-1192 Japan; 011-81-561-63-3090; fax 011-81-561-63-5441; e-mail saito@mosk.tytlabs.co.jp.     

Corrosion behavior of copper-based metal-matrix composites

A research program was conducted to assess the corrosion behavior of copper-based metal-matrix composites. Two categories of composites were studied: commercially available composites and developmental composites. While the anodic polarization curves for the monolithic and composite materials were quite similar, the corrosion behavior varied. The commercially available material was dispersion-strengthened copper, which was found to possess corrosion resistance comparable to that of pure copper. This was due largely to the stability of the protective film in the presence of finely distributed Al₂O₃ particles. The developmental composites were reinforced with graphite in either particulate or fiber form. Graphite content and the presence of dissolved oxygen affected corrosion severity, and all of the developmental composites exhibited uniform corrosion and some localized galvanic corrosion at the reinforcement-copper interface during polarization. Other types of corrosion damage were also observed in the noncommercial materials.     

Thermal fatigue resistance of discontinuously reinforced cast aluminum-matrix composites

The thermal fatigue resistance of AlSi alloys and discontinuously reinforced Al-matrix composites containing graphite, silicon carbide, and fly ash particulates, and short alumina (Saffil) fibers was characterized by measuring the total length of microcracks on gravity-cast and squeeze-cast test specimens as a function of number of thermal cycles (1000–5000 cycles, 270 K amplitude). In each thermal cycle, the test specimens were heated and stabilized in air at 375 °C, water quenched, and air stabilized. In all specimens, the total crack length on a specified region increased with increasing number of thermal cycles. Whereas among monolithic alloys, squeeze-cast Al-12SiCuNiMg alloy exhibited better resistance to thermal cracking than Al-25Si and Al-20SiNi alloys, among the composites, squeeze-cast Al-alumina and Al-fly ash composites exhibited the best thermal fatigue resistance. The theoretical estimates of the thermal fatigue resistance of these composites are consistent with the experimental observations.     

High-temperature discontinuously reinforced aluminum

High-temperature discontinuously reinforced aluminum (HTDRA) composites have been developed for elevated-temperature applications by incorporating SiC particulate reinforcement into a rapidly solidified, high-temperature Al-Fe-V-Si (alloy 8009) matrix. HTDRA combines the superior elevated-temperature strength, stability and corrosion resistance of the 8009 matrix with the excellent specific stiffness and abrasion resistance of the discontinuous SiC particulate reinforcement. On a specific stiffness basis, HTDRA is competitive with Ti-6-Al-4V and 17-4 PH stainless steel to temperatures approaching 480°C. Potential aerospace applications being considered for HTDRA include aircraft wing skins, missile bodies, and miscellaneous engine, spacecraft and hypersonic vehicle components.     

The solidification of metal-matrix particulate composites

The simplicity, economy and flexibility of solidification processes make them attractive methods for the production of particle-reinforced metal-matrix composites. At present, however, there is limited understanding of the phenomena occurring during solidification of these advanced materials. Nucleation and refinement of crystalline phases, physical and chemical interactions between dispersed particles and solidifying interfaces, and buoyancy-driven movement of the particles are areas where a knowledge base is beginning to be formed. Ultimately, the understanding of solidification processes in metal-matrix composites must become complete enough that microstructures can be tailored for specific applications.     

CermeTi® discontinuously reinforced Ti-matrix composites: Manufacturing,properties, and applications

Advanced powder-metallurgy technology has led to the development of the CermeTi® family of titanium metalmatrix composites. Reinforcing the titanium alloy matrix with titanium carbide or titanium boride particles results in superior properties. These discontinuously reinforced titanium composites have excellent room- and elevated-temperature properties and are exceptionally wear resistant. High quality, near-net shape CermeTi composite components are being produced commercially and are being evaluated for potential applications in military vehicles, commercial automotive engines, sporting goods, industrial tooling, and biomedical devices.     

The recycling and reclamation of metal-matrix composites

The development of viable techniques for the recycling and reclamation of metal-matrix composites (MMCs) is critical to the commercialization of these advanced materials. The recycling of both MMC wrought alloy (6061) scrap and foundry alloy (high-silicon) returns has been studied. The MMC extrusion alloy scrap has been recycled back into direct-chill cast logs and the resulting billet has been tested to determine whether the composite properties are degraded by repeated recycling. Similarly, fluxing and degassing techniques have been developed so that MMC foundry alloy gates and risers produced in shape-casting may be recycled back into useful castings. These fluxing and degassing processes have been tested commercially. Ultimately, when either type of MMC scrap can no longer be recycled, the alumina particles in the wrought alloys or the silicon carbide particles in the foundry alloys may be removed by common salt or other fluxing techniques. Rotary salt furnace technology has been shown to be effective for this approach, and the results of large-scale trials are reported here.     

The 3-D analysis of discontinuously reinforced aluminum composite microstructures

Automated serial sectioning is presented as an attractive experimental technique for producing an accurate, large-scale three-dimensional (3-D) digital microstructural model of an extruded discontinuously reinforced aluminum composite material. The model is used as input for an elastic-plastic 3-D finite-element analysis that simulates tensile loading of the microstructure. The 3-D visualization and characterization of the resultant deformation structures within the composite microstructure has allowed some of the critical microstructural features that control the tensile response to be identified.     

The stability of interfaces in high-temperature metal-matrix composites

The interface between the matrix and the reinforcement is an area of great importance in the design of viable high-temperature metal-matrix composites (MMCs). Thorough understanding of the phenomena taking place at the interface is necessary to ensure reliable performance, but the thermodynamic data that would help to predict interfacial reactions are lacking for many systems. However, given certain knowledge, interfaces can be classified according to their stability, providing a tool for composites designers and an impetus for further fundamental work. This article describes the classification system and provides examples of interfacial behavior in a titanium-based material.     

Corrosion behavior of SiC reinforced magnesium composites

The corrosion behavior of two SiC reinforced Mg-based metal matrix composites, Mg-6SiC and Mg-16SiC (in volume percent), has been studied in freely aerated 1 M NaCl solution and compared with that of pure Mg. The presence of SiC particles deteriorated the corrosion resistance of magnesium. Corrosion resistance decreased with increasing SiC volume fraction. The galvanic corrosion current density between pure SiC and pure Mg has been experimentally measured using zero resistance ammeter technique and theoretically determined using mixed potential theory. Galvanic corrosion between Mg matrix and SiC reinforcement in the composites did not contribute significantly to the overall corrosion rate. Electrochemical impedance spectroscopy indicated that the higher corrosion rates for the composites could be related to the defective nature of surface film.     

The oxidation and hot corrosion behavior of tungsten-fiber reinforced composites

The oxidation and hot corrosion behavior of a tungsten-fiber, reinforced Ni~ 20Cr alloy has been examined under the following exposure conditions: (a) pure oxygen at 1 atm pressure; (b) sulfidation in H₂–10 %H₂S; (c) presulfidation in H₂–10 %H₂S followed by oxidation in oxygen; and (d) oxidation in 1 atm oxygen after precoating with approximately 1 mg/cm² of Na₂SO₄. Rapid oxidation of the tungsten fibers causes considerable distortion of the matrix and catastrophic degradation of the matrix follows. Inter diffusion between the matrix and the fibers is also important. During sulfidation, only the matrix forms sulfides, the fibers remaining unaffected. Consequently, presulfidation, although having a dramatic effect on the oxidation of the matrix does not have a damaging effect on the fibres. Equally, the presence of sodium sulfate is not critical, although severe oxidation of the exposed tungsten fibers is still observed.     

Fundamental relationships in metal-matrix composites

“Fundamental Relationships Between Microstructures and Mechanical Properties of Metal Matrix Composites” was a well-attended and technically sound symposium that covered a diversity of topics related to the effects of processing and microstructure on the mechanical properties of metal-matrix composites. In addition to the papers briefly reviewed in this article, many other topics were presented and discussed extensively, including fracture and deformation mechanisms and modeling, fatigue, fatigue crack growth, creep behavior and nondestructive characterization.     

Polycrystalline diamond fibers for metal-matrix composites

This paper describes the development of a novel polycrystalline diamond fiber that has attractive isotropic thermal properties at a moderate cost. The fiber is processed first by chemical vapor deposition (CVD) and then by microwave plasma-enhanced CVD. Using the fiber as a reinforcement for metal-matrix composites offers numerous advantages in terms of engineering properties (e.g., a coefficient of thermal expansion that can be tailored to match the values of semiconductor materials, an enhanced thermal conductivity, and light weight). The resulting composites have a great potential for application in the thermal management of electronic devices.

     

The weld pool region of welded cast metal-matrix ceramic-particulate composites

The successful application of ceramic-particulate reinforced metal matrix composites as engineering materials requires that they be amenable to fabrication by joining processes such as welding. In such a case, the weld pool would undergo intense agitation during welding and the ceramic particulates would tend to segregate, causing deterioration in the mechanical and wear properties of the joined portion. In the present work, a thermo-physical model has been used to predict the velocities of fluid movement in the weld pool as a function of heat input. An attempt has been made to combine analytical and numerical methods in order to ensure the accuracy of the model's predictions and at the same time to make the computation faster. Aluminium-alumina and aluminum-silicon carbide composites were prepared by casting and the resultant plates of standard dimensions and thicknesses were welded using the MIG process. The distribution of the ceramic particulates and their reaction with the matrix in the welded zone were determined using metallographic and SEM techniques. The distribution was then simulated from the fluid flow velocities obtained by the model developed by the authors. A good correlation was seen between the intensity of molten metal velocities and the particle distribution. Particle pushing and engulfment by the solidifying melt interface also appeared to be affecting the particle distribution in the weld zone.     

High-temperature deformation behavior of Ti-TiB<Subscript>w</Subscript> in-situ metal-matrix composites

In developing titanium-based materials for high-performance structural applications, the focus has been on discontinuously reinforced titanium-matrix composites that are based on the in-situ creation of TiB whiskers. This article presents the elevated-temperature compressive flow properties of Ti-TiB_w composites as a function of temperature (from 973 K to 1,273 K), strain rate (from 10⁻⁵ to 10⁻³s⁻¹), and the TiB whisker content (commercially pure titanium and composites with 30 to 86 vol.% TiB_w). In both the commercially pure titanium and the TiB-whisker-containing titanium composites, flow stress increases with increasing strain rate and decreasing test temperature. In this study, flow stress increased with increasing volume fraction of TiB at a given temperature and strain rate. The composite containing 86% TiB_w showed significantly higher elevated-temperature strength when compared to the other composites. The composites exhibit systematic trends in the variation of work-hardening behavior (in terms of work-hardening exponent, n, and work-hardening rate, dσ/dɛ) and the strain-rate dependence. For more information, contact N. Eswara Prasad, Defence Metallurgical Research Laboratory, P.O. Kanchanbagh, Hyderabad 500058, India; e-mail nep@dmrl.ernet.in or neswarap@rediffmail.com.     

Superplasticity and superplastic forming of aluminum metal-matrix composites

Superplasticity hos been observed in many aluminum metal-matrix composites at extremely high strain rates (approximately 0.1–1 S^?1). These materials generally exhibited a strain-rate sensitivity value of about 0.3 and a maximum elongation of about 300%. It is believed thot the presence ofa liquid phose, or in some cases a low-melting-point region, at the reinforcement/matrix interfaces is responsible for the phenomenon. This phenomenon is not observed in all reinforced composites, despite the fact thot they contain fine grain sizes. Thus, a fine matrix grain size is a necessary but insufficient condition for the high-strain-rate superplasticity.     

Determining material properties of metal-matrix composites by NDE

Nondestructive evaluation (NDE) is a promising means of studying silicon carbide particulate (SiC_p)-reinforced aluminum metal-matrix composite (MMC) products at various processing stages. Eddy current techniques are effective in characterizing alloy powders and in evaluating the percentage of reinforcement in Al/SiC_p powder mixtures. Ultrasonic methods can be used to identify SiC_p clusters in large-scale, powder metallurgy processed MMC billets, while eddy current techniques can detect near-surface density variations. Ultrasonic techniques can also be used to determine the anisotropic stiffness constants of composite extrusions; the measured moduli are in good agreement with those determined by tensile testing. These results suggest that NDE can be used to provide on-line, closed-loop control of MMC manufacturing.