Steady-state creep behavior in an isotropic functionally graded material rotating disc of Al-SiC composite
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I. INTRODUCTION
FUNCTIONALLY graded materials (FGMs) are the composites in which the components (or their contents) vary in some direction enabling these materials to provide unique performance. The idea of FGM was conceived to provide a material that may withstand severe thermomechanical loadings requiring heat-resistant ceramics on the high-temperature side and tough metals with high thermal conductivity on the lower temperature side. This was achieved by a gradual variation in composition from ceramic to metal, as shown schematically in Figure 1. Jackson et al.[1] have shown an approach to model and design functionally graded material components for fabrication with local composition control. Their approach is based on subdividing the solid model into subregions and associating analytic composition blending functions with each region. These blending functions defined the composition throughout the model as mixtures of the primary materials available to the solid freeform fabrication machine. The role of design rules restricting maximum and minimum concentrations has also been discussed in their analysis. Fukui et al.[2] manufactured Al-Al3Ni FGMs of metal intermetallics by the centrifugal casting method and then conducted experiments to evaluate strength gradients determined by a number of three-point bending tests. Zhu and Miller[3] have examined creep behavior of FGM, having S.B. SINGH, Lecturer, is with the School of Basic and Applied Sciences, Thapar Institute of Engineering and Technology, Patiala-147 004, India. S. RAY, Professor and Head, is with the Department of Metallurgical and Materials Engineering, University of Roorkee, Roorkee (Uttaranchal)-147 667, India. Manuscript submitted April 26, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
a thermal barrier coating of zirconium-8 wt pct yttria on a bond coat of Fe-25Cr-5Al-0.5Y plasma sprayed on 4140 steel. Creep was investigated in this material under a thermal gradient produced by laser heating on the ceramic surface. In the ceramic layer, the creep was primary. But time-, temperature-, and stress-dependent deformation resulted in coating shrinkage in the loading direction leading to stress relaxation. Williamson et al.[4] have carried out a finite element study on the effects of interlayer and creep on the residual stress in nickel-alumina metal ceramic joints. The power-law relation suggested by Ashby[5] has been used to describe the steady-state creep behavior of both nickel and alumina. It has been concluded that above 700 K, creep strain is significant to influence residual stress and the joint behavior. In earlier studies,[6,7,8] it was found that in a rotating isotropic disc containing 20 vol pct silicon carbide undergoing isothermal creep, tangential stress is maximum at the inner radius and it results in higher creep strain there compared to that at the outer radius. These results indicate that by incorporating a relatively higher amount of particles near the inner radius as compared to that near the outer radius, it may be possible to lowe
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