Microstructural and failure characteristics of metal-lntermetallic layered sheet composites
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INTRODUCTION
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M E T A L - I N T E R M E T A L L I C composites are unique structures, as they offer an attractive combination of properties from both component phases, i.e., high stiffness, high modulus, and low density of the intermetallic and high toughness of the metal. Previously, intermetallic matrices have been reinforced with ductile metals in the form of particles, fibers, tubes, and layersJ~ ~ol These composites have been fabricated via traditional solidification techniques, tL2j pressure-aided consolidation of prealloyed intermetallic powders mixed with the ductile reinforcement, 13 81 and a reaction between elemental powders (to form the intermetallic) mixed with metal fibers or foils. [8'9'~~ This last method is attractive because it offers the potential for lower processing temperatures and pressures than conventional powder-metallurgical techniques, as the transient liquid phase that commonly forms during reactive processing (particularly for aluminides) aids in densification, l~ ~51 The process of forming intermetallic compounds from reactions between elemental powders has been variously termed self-propagating, high-temperature synthesis (SHS), reaction synthesis, and combustion synthesis. This technique has been used to produce intermetallic and ceramic powders, dense monoliths, and in situ two-phase compositesJ ~-~Sj Recently, it was shown that an SHS reaction could be initiated at the interface between elemental Ni and AI foils. 1~6jResearchers at the United States Bureau of Mines have modified this technique to produce in situ layered sheet composites from a reaction between elemental foils. Ij7-2~ Early experiments have demonstrated the feasibility of producing composites using S H S I17'181 and confirmed that the formation of the composites proceeds via an SHS reaction at the interface between the foils.l~91 The primary purpose of this study was first to characterize the unique microstructures and then to investigate the failure behavior of these layered composite structures.
D.E. ALMAN, Materials Research Engineer, J.C. RAWERS, Materials Scientist, and J.A. HAWK, Supervisory Materials Engineer, are with the Materials Science Division, United States Bureau of Mines, Albany Research Center, Albany, OR 97321-2198. Manuscript submitted March 21, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
EXPERIMENTAL
Detailed information on the formation of the twodimensional layered sheet composites has been published previously, t~7-2~ In this study, five layers of aluminum foil (0.15-ram thick) were alternately stacked between six layers of metal foil (Ni, Ti, or Fe; 0.15-mm thick). The foil "sandwiches" (roughly 50 x 50 mm in area) were placed between the cylindrical graphite platens of an induction-heated vacuum hot press (Figure 1). The stacked foils were heated to 900 K and held for 60 minutes with no pressure applied on the composites other than the weight of the graphite platens (roughly 1 kg). The samples were then heated to 1000 K, which is above the melting temperature of A1 (i.e., 9
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