Combustion synthesis and mechanical properties of dense NiTi-TiC intermetallic-ceramic composites
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I. INTRODUCTION
MUCH attention has been afforded the production of porous NiTi alloys via combustion synthesis techniques.[1] The porous nature of these materials affects the desired mechanical properties. Studies have been conducted to determine the relationship between porosity and the desired mechanical properties of porous NiTi alloys.[2,3] Little attention has been given to dense NiTi composites with refractory phases and their corresponding effects on mechanical properties, especially those produced via combustion synthesis. The combustion synthesis process is dependent on a highly exothermic chemical reaction that becomes self-sustaining after only a short energy pulse is applied to initiate the reaction. The effects of adding a carbon reactant to synthesize porous NiTi materials reinforced with a titanium-carbide (TiC) ceramic using combustion synthesis have been investigated. The overall combustion synthesis process and the synthesized product are affected by the addition of the refractory phase, due to the nonstoichiometric TiC formation that in turn influences the type of intermetallic formed, and thus affects the material and mechanical properties.[4,5] Stiff ceramic inclusions introduced into an intermetallic matrix produce internal stresses during thermal excursion that both affect the superelastic nature of the alloy and lead to changes in yield stress and strength.[6] An increase in Ni3Ti particles dispersed within a NiTi matrix increases residual stress within the NiTi grains.[2] An increase of NiTi2 in the intermetallic matrix decreases the shape memory transformation temperature.[7] Both the room-temperature hardness and the recoverable energy of NiTi are affected by the Ni3Ti4 precipitate sizes, due to the relative resistance of the dislocation motion and the recoverable stress-induced martenDOUGLAS E. BURKES, Postdoctoral Fellow, GUGLIELMO GOTTOLI, Graduate Research Assistant, and JOHN J. MOORE are with the Metallurgical and Materials Engineering Department and Center for Commercial Applications of Combustion in Space, Colorado School of Mines, Golden, CO 80401. HU CHUN YI, Director, Advanced Materials and Combustion Laboratory, is with Guigne International Ltd., St. John’s, NL A1L 1C1, Canada. Contact e-mail: [email protected] Manuscript submitted April 23, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
sitic transformation, as well as the changes in the transformation temperatures.[8] The NiTi-TiC composites are mechanically inhomogeneous, due to significantly different modulus values for the NiTi and TiC. Equiatomic NiTi has an approximate modulus of 83 GPa[9] and stoichiometric TiC has an approximate modulus of 440 GPa.[10] The TiC modulus can deviate depending on stoichiometry, ranging from 410 to 510 GPa.[11] The net result of the differing moduli is that during mechanically induced deformation, TiC inclusions deform significantly less than the intermetallic binding material. This results in large internal stresses that vary dramatically throughout the bulk material, leading to premat
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