FGM Research Activities in Europe
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Graded Mo-Si 3N4 shingles for hightemperature applications were produced by Hulsmann2 by a powder-stacking technology. Temperature and stress gradients were calculated with the finite-element method (FEM) code MSC-Nastran. Similar
Figure 1. Graded transition between PSZ ceramic (gray) and an NiCr alloy (white) in a thermal-barrier coating specimen prepared by centrifugal P/M (Dollmeier, Cherradi, llschner 1994).
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experiments were performed by Y.C. Ahn3 with graded Mo-Al2O3 shingles. Composite powders were milled, achieving average powder sizes of less than one micron, and consolidated by powder-stacking technology and hot pressing. FGM design and a variant of reactive P/M was also used to combine ductile Nb-fiber reinforcement and an oxygen-resistant TijAlySi2 outer coating to improve both mechanical and environmental properties of TiAl-based alloys.4 Microwave heating has proven to offer unique potential for effectively heating a strongly microwave-absorbing material within a nonabsorbing matrix, provided that the mixture is homogeneous and the absorbing phase is diluted enough to retain a penetration depth sufficient for volumetric heating.5 An initial compositional gradient, as well as a compositional gradient derived by microwave heating of an initially homogeneous mixture with a macroscopic thermal gradient, were used to produce ceramic FGMs, as well as metal-ceramic FGMs. In monolithic ceramics, components with coarser grains in the interior and extremely fine grains close to the outer surface can be generated. Other Bulk FGM Applications TiAl powders with various concentrations of ternary a-stabilizing alloying elements were worked by P/M methods to form microstructural gradients within one component, so that requirements on ductility (e.g., in the root block of a turbine blade) and creep strength (e.g., in the airfoil section) can be satisfied at the same time. The fully lamellar microstructures in the airfoil section, obtained after annealing in the a field, exhibit high creep strength but a low ductility of 0.5% plastic elongation. At the same time, duplex structures resulting from compositions in the root, heat-treated at the same temperatures in the a + y field, yield acceptable ductility levels of 2% but combine with a reduced creep strength.6 Also by P/M, graded Cu-Ni and Cu-Ni-Sn specimens with localized hardening by spinodal decomposition (Figure 2) were analyzed with respect to a constant rate of fatigue crack propagation (da/dN) in 3-point bending, where a is the crack length and N is the number of fatigue cycles.7 Inhomogeneous plastic flow as a function of concen
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