Macroscopic design and fabrication of functionally gradient materials

Advanced materials were typically developed in the past by improving the physical properties of homogeneous materials or distributing second-phases uniformly in homogeneous matrix materials. Thus, in conventional design, one simply selects materials from a vast range of existing materials based on their physical properties. However, a class of specialty material known as Functionally Gradient Materials (FGM) is emerging, whose properties are tailored by modulating the microstructural features such as the concentration of the second-phase material or the size of the grain structure. For practical applications, FGM poses us with a two-fold challenge: (1) the development of fabrication processes and (2) the development of microstructure-based analysis.;Both aspects of the challenge are addressed but specifically concentrated on the analysis and fabrication of axisymmetric FGM. The gradient properties of the FGM cylinder are attained by spatially varying the volume fraction of the small second-phase within a matrix material. For simplicity, the second-phase volume fraction varies only radially. The volume fraction may range from zero to one within a metal matrix.;Predictions using current micromechanics theories for the effective moduli of composites tend to break down at high volume fractions of the reinforcing phase. The predictions are usually well below experimental measured values of the Young's modulus for volume fractions exceeding about 0.6. The concept of continuity, which is a measure of phase continuity, can be applied to the Mori-Tanaka model. It is shown that contiguity of the second-phase increases with volume fraction, leading eventually to a reversal in the roles of the inclusion and matrix. For the FGM material systems considered in this thesis, neither the contiguity data nor the experimental analysis are available; therefore, the most accepted Mori-Tanaka model is adopted but only their modification is recommended. Inhomogeneous eigenstrain media analysis is performed using the properties predicted by the model. Thermal loading and internal and external pressures are applied to FGM cylinders whose second-phase are unidirectional fibers and spherical inclusions.;In addition, a processing technique for manufacturing FGM is developed using centrifugal casting. A mixture of metal and micron-sized ceramic particles is poured into a crucible. As the crucible rotates, induction heating melts the mixture and the high centrifugal forces cause the denser ceramic second-phases to disperse. By controlling the rotation speed and heat, various distributions of particles can be obtained. Furthermore, the nonwetting quality between ceramic second-phase and molten metal can be improved by the high centrifugal force. Other fabrication methods such as powder metallurgy and directed oxidation are also reviewed and a few recommendations are discussed.