This overview article addresses a unified micromechanical approach to the analysis of overall thermomechanical behavior of metal matrix composites. Emphasis is on their behavior beyond the linear elastic regime. First, the mean-field model for a heterogeneous medium with misfitting ellipsoidal inhomogeneities, on which the present approach is based, is briefly described. Then, the general formulas for estimating the effective values of material properties, including elastic constants, coefficients of thermal expansion, heat capacities and thermal conductivities, are given with some explicit expressions derived for simple binary systems. The remainder of this article highlights an analysis of post-yield inelastic behavior. On the assumption that a quasi-homogeneous plastic strain is induced over the entire matrix, a numerical analysis procedure based on energy principles is formulated for solving general multiaxial elastoplasticity problems. It is also shown that the simple case of axisymmetric triaxial loading can be treated by an alternative method using the yield condition expressed in terms of the matrix mean stress.
Fundamental researches were carried out to improve thermal stability of NiCrAlY/PSZ FGM in uniform and gradient temperature fields. NiCrAlY/PSZ NFGMs, NiCrAlY/PSZ FGMs and NiCr/PSZ FGMs were prepared on SUS310S substrates by plasma twin torches method under atmospheric or low pressure. Designed compositional gradient of FGMs could be sprayed. Porosity in NiCrAlY/PSZ FGMs was increased with volume fraction on PSZ. When the FGMs were heat treated in a uniform temperature field of 1473 K, PSZ part was sintered and vertical cracks were formed by shrinkage. At the same time, metal which contained a number of Fe atoms went into the cracks, and climbed up to the surface. The Fe atoms were migrated from the substrate through NiCrAlY part. Therefore, it is most important to decrease porosity in PSZ area as well as to control the distribution of pore to restrain crack formation. Coating metal with the same composition as the substrate is desirable for the thermally stable FGM. When the FGM was heat treated in a gradient temperature field of 1473-873 K, there was little change in the FGM structure. The FGM showed a high potential for protection coatings of turbine blades.
Room-temperature yield strength of powder-metallurgy Al-ceramic particle composite alloys produced by mechanical alloying was analyzed from a viewpoint of microstructure which was characterized by several features: high dislocation density (in the order of 1014 m–2), uniform dispersion of fine Al4C3 and Al2O3 particles (35 nm in size) and coarse ceramic particles (0.4-1.0 μm), and small grain size (0.5 μm). A large portion (more than 80%) of yield strength was concluded to be contributed through dispersion hardening by the fine particles and particle reinforcing by the coarse particles; the contribution was greater through the former mechanism than through the latter one for a given volume fraction of particles. The former mechanism was based on detaching pinned-down dislocations at the fine particles from them, and the latter one on restricting matrix deformation by mechanical constraint around the coarse particles. Impurities in the Al matrix made a modest contribution to strength, probably through solution hardening. These mechanisms raised the strength additively. Work hardening due to a high density of dislocations introduced during processing and grain boundary strengthening due to small grain size were considered not to be principal mechanisms for determining the yield strength.
The titanium-based metal matrix composites (MMC) reinforced with relatively large-sized ceramic particulates have been produced by the blended elemental (BE) P/M route. The extra low chlorine titanium powder, Al3Ti master alloy powder, and FeB and Cr3C2 ceramic powders were used as starting materials. The dispersion of either TiB or TiC particulate reinforcements in the matrix were therefore made in situ during the sintering process. The distribution of particulate reinforcements in the matrix was reasonably random. Tensile tests were done at temperatures up to 923 K. It has been found that the tensile strength of these composites are superior to those of the unreinforced matrix alloys in the whole temperature ranged examined. The elastic modulus was also increased by the presence of these ceramic reinforcements. No ductility was found for both composites at room temperature, but some ductility was observed at temperatures above 623 K. These observations were interpreted in terms of the failure mechanisms observed using scanning electron microscopy.
Reaction is caused at the interface between fiber and metal-matrix in a composite at an elevated temperature. The reaction affects greatly on the strength of the composite. In this work, SiC fiber reinforced Ti alloy composites were fabricated by diffusion bonding. The reaction products were examined after heat-treatment of the composites. Effect of the reaction products on the strength of the composites was studied. SiC fiber was used as the reinforcement material. Ti, Ti-6A-4V or Ti-8Mo/Ti-6A-4V was used as the matrix. The heat-treatment was carried out at a temperature 1073 or 1123 K for 32.4-360 ks. The strengths of heat-treated composites were measured at a room temperature. In the case of Ti matrix, the strength of composites decreased to 60% with increasing temperature and time of the heat-treatment. The decreasing of the strength of composites was little in the cases of the alloy matrices except at 1123 K for 360 ks. The reaction products were TiC and Ti5Si3 in every matrices of this work. TiC was formed on the surface of the SiC fiber and Ti5Si3 was formed outer for TiC. From these results, the alloying elements were effective to decrease the reaction at the interface. The abrupt decrease of the strength of the composites was due to increasing of defects between SiC and the reaction products. A small pit was often observed at the periphery of the fiber in fracture surface. The morphology of the fracture surface showed the pit was the initiation of the crack propagation. This means the deterioration of the fiber depends not on the thickness of the reaction zone but on the pit, especially in the case of serious degradation. Then the deterioration of the fiber was often rather serious even though the thickness of the reaction zone was small.
Microstructure and mechanical properties of carbon fiber/SiC composites prepared by chemical vapor infiltration (CVI) using ethyl-trichloro-silane (ETS) and methyl-trichloro-silane (MTS) as sources of SiC have been examined. The CVI was conducted under non-isothermal condition keeping a pressure of 13.3 kPa and temperature of 1273-1573 K at the downstream side. The highest degree of infiltration with a density higher than 80% was obtained. Main matrix formed was β SiC for both reactants. However silicon also deposited in SiC matrix for MTS. Preferential wettability of SiC to the carbon fiber was observed and graphite phase was detected in the interface between the matrix and the carbon fiber by TEM. Mechanical properties were evaluated by bend tests at room temperature. High strength of around 800 MPa was obtained for ETS. However the fracture strength strongly depended on the thickness of SiC layer covering the composite as well as porosity. The strength increased with decreasing both surface coated layer and porosity. Apparent fracture toughness of the present carbon fiber/SiC composite was 6-10 MPam1/2 at room temperature.
The change of model I fracture toughness with microcrack growth was investigated using in-plane random-oriented short carbon-fiber reinforced carbon matrix composite (2DRSF-C/C). Two types of the specimens (tapered double cantilever beam and single edge notched beam) were used in this study. In-situ observation of the crack growth behavior was carried out by using a special optical microscope equipped with a loading device. The initial fracture toughness of 2DRSF-C/C was almost the same as that of monolithic carbon. Both the stress and energy criteria were compared to evaluate the initial values of the fracture toughness. The energy criterion gave better estimation. The values of the fracture toughness increased rapidly with crack growth. The influence of the specimen design was almost negligible both for initial values and propagation values. Microcrack observation showed bridged fibers. It is considered that the crack closure force which hinders the crack from propagating was caused by the pulled-out fibers. Tests at an elevated temperature indicated the increase of both the initial and propagation values of the fracture toughness.
A simple Dagdale model for fiber bridging processes and mechanisms of unidirectionally fiber-reinforced brittle matrix composites is addressed. The model, the most primitive and the simplest one, has great advantages in analytical solutions and practical considerations, although it is not enough for quantitatively explaining experimental results. It is emphasized that the details of crack-face constitutive relation (crack bridging tractions vs. crack opening displacement relationship) are not essential for describing the toughness increment of fiber bridging systems, but that the averaged bridging traction and the bridging zone length are the most important in the considerations of crack-bridge toughening.
SiC based ceramics have excellent properties in strength, hardness and chemical stability at high temperatures. Such attractive characteristics make them promising condidates for high temperature structural or wear and corrosion-resistant materials. On the other hand, carbides, nitrides or borides of transition metals, such as TiC, NbC, ZrC, TiN, TiB2 and ZrB2, have prominent refractriness with good electric conductivity. Sintered SiC and refractory compounds, however, have low fracture toughness. In order to improve the fracture toughness of ceramic materials, use of refractory compounds as a filler has been attempted. In this article fabrication and mechano-physical properties of SiC-X (X; carbides, nitrides or borides of transition metals) composites and their microstructure-macro properties relation are reviewed. Refractory compounds particles/SiC matrix composites exhibited improved mechanical properties compared to the monolithic SiC or the compounds. The electric conductivity of the composites can be varied by changing the amount of carbides, nitrides or borides of transition metals. Use of refractory compounds/SiC composites may solve two problems: low fracture toughness of ceramic materials and difficulty of machining them by using an electric discharge method.