FUNCTIONALLY GRADED METALS AND METAL-CERAMIC COMPOSITES .2. THERMOMECHANICAL BEHAVIOR

Following a review of the processing of functionally graded metals and metal-ceramic composites in Part 1; this Part 2 of the two part serie s focuses on the thermomechanical behaviour. The paper begins with an overview of the fundamentals of thermoelastic and thermoplastic deform ation in metal-ceramic composites. Various approaches, including the r ule of mixture approximations, mean field theories, crystal plasticity models, discrete dislocation models, and continuum finite element for mulations of the constitutive phases of the composites, are discussed, and the significance and limitations of these approaches are highligh ted. Issues specific to the thermomechanical analyses of graded materi als are then addressed. It is reasoned that the introduction of a new length scale to the problem due to compositional gradients inevitably calls for detailed micromechanical analyses of the size, shape, contin uity, and spatial dispersions of the constituent phases of graded meta l-ceramic composites. Models for the thermal, elastic, and plastic def ormation of graded multilayers are then presented within the context o f classic beam and plate theories, in conjunction with strategies for developing 'design diagrams' for thermomechanical performance. Methods to identify the conditions governing the onset of instability and abr upt shape changes due to large deformation in graded multilayers are a lso provided. The macroscopic continuum analyses are followed by discu ssions of micromechanics simulations of the real microstructural dispe rsions by recourse to computational models which invoke von Mises type and crystal plasticity theories. Experimental methods to assess the v alidity of such models are then examined, along with typical results o f processing induced internal stresses and thermal stresses arising fr om temperature excursions in model systems with gradients in metal-cer amic concentrations. It is demonstrated that stepwise or continuously graded metal-ceramic composites can be designed to improve interfacial bonding between dissimilar solids, to minimise and optimally distribu te thermal stresses, to suppress the onset of plastic yielding, to mit igate the deleterious effects of singular fields at free edges of mult ilayers where interfaces intersect free surfaces, to reduce the effect ive driving force for fracture, and to arrest cracks.