No abstract

An elastic-plastic finite element method numerical model has been formulated to study residual stresses developed at graded ceramic-metal interfaces during cooling. The results were compared with those obtained for sharp (nongraded) interfaces to assess the potential for achieving residual stress reductions. Analyses were conducted for various axisymmetric cylindrical specimen geometries relevant to structural joining, coating, and thick film applications. The graded microstructure was treated as a series of perfectly bonded layers, each having slightly different properties. Constitutive relations for the interlayers were estimated using a modified rule-of-mixtures approximation, and strain and stress distributions were calculated for simulated cooling from an assumed fabrication temperature. The results demonstrate the importance of accounting for plasticity when comparing graded and nongraded interfaces. Significant geometrical effects on peak stresses were observed in the graded materials. It is shown that in some cases, optimization of the microstructure is required to achieve reductions in certain critical stress components believed to be important for controlling interface failure.

This issue of the MRS Bulletin provides an up-to-date look at ongoing research activities within the field of functionally gradient materials (FGM). The term FGM, now widely used by the materials community, originated in Japan in the late 1980s as a description for a class of engineering materials exhibiting spatially inhomogeneous microstructures and properties. Of course, gradient materials are not something new. It must be recognized that humans have extensively utilized materials containing microstructural gradients (either those found in nature or those created through processing) since the earliest days of craftsmanship and engineering construction. Indeed, there are examples of graded materials developed long ago, such as case-hardened steel, which are still in common use today. Contemporary examples of these materials serve in technologically significant applications, as, for example, in thermal-barrier coatings for gas turbines. Nevertheless, what is new and exciting about FGMs is the realization that gradients can be designed at the microstructural level to tailor a material for the specific functional and performance requirements of an intended application. In addition, recent advances in processing are opening the possibility for the extension of the gradient materials concept to new materials systems and engineering problems.The recent resurgence of interest in gradient materials has been driven by the need for improved materials, capable of meeting the demanding performance requirements established by emerging technologies such as the aerospace plane, ceramic engines, and nuclear fusion.

No abstract

An elastic-plastic finite element method numerical model previously developed (see Part I of this article) for predicting thermal residual stresses at graded ceramic-metal interfaces has been applied to determine interface conditions favorable for achieving residual stress reductions. Using Al2O3-Ni as a model system, and for a fixed specimen geometry, a study was performed to investigate the effects of different interlayer thicknesses and nonlinear composition profiles on strain and stress distributions established during cooling from an assumed elevated bonding temperature. For each interface condition, relative stress reductions were evaluated by comparing the magnitude of specific stress and strain components important for controlling interface failure with those predicted for a sharp (nongraded) interface. For the geometry considered, stress was reduced by thick graded interlayers and nonlinear composition profiles that distributed the largest property changes over the interlayer region having low elastic modulus and high plasticity. In contrast to the Part I results for a linear composition profile, the optimized interlayer condition effectively reduced the peak near-surface axial stress component.

No abstract

No abstract

No abstract