The effect of processing parameters on the formation of titanium-titanium aluminide laminated metal composites
In the present investigation, the feasibility of processing a titanium-titanium aluminide laminated metal composite structure has been assessed via two mechanisms of formation: diffusion bonding and self propagating high temperature synthesis (SHS). Alternating layers of commercial purity aluminum and titanium foils were diffused or reacted at temperatures ranging from 575C to 640C to form a layer of titanium aluminide intermetallic. The product material was then evaluated on the basis of morphology and chemistry using SEM EDS, and X-ray diffraction techniques. Specific emphasis was placed on the degree of consolidation of the aluminide, phase chemistry, and quality of the metal-intermetallic bond. Subsequent examination showed that despite the mechanism of formation (diffusion bonding or SHS), the resulting aluminide layer was composed completely of TiAl3. This was confirmed by both EDS and X-ray diffraction analysis, where the presence of other stable equilibrium phases (Ti3 Al and TiAl) was not detected. It was observed in diffusion bonding experiments that formation of titanium aluminide began with the initiation of an SHS reaction at the interface of the elemental metal foils via the grain boundaries. Following this, reaction proceeded by normal diffusion mechanisms, obeying a typical parabolic growth rate with respect to annealing time, yielding a consolidated, homogeneous layer. SHS reactions were found to result in a drastically different morphology than that observed in diffusion bonding experiments. Specimens were characterized by a band of spheroidal particles, composing a loosely consolidated layer at the titanium-aluminum foil interface. Diameter of the particles averaged 1-5[mu]m, with total layer thickness of approximately 10 [mu]m.