Effects of ductile phase volume fraction on the mechanical properties of titanium-aluminum titanium alloy metal-intermetallic laminate (MIL) composites

The mechanical properties of Ti-Al3Ti metal-intermetallic laminate (MIL) composites have been investigated with an emphasis on the effect of residual Al at the intermetallic centerline on failure mechanisms in order to ascertain the possible success or failure of using Ti-6Al-4V instead of Ti-3Al-2.5V. MIL composites consisting of alternating layers of Ti, Al, and the intermetallic phase Al3Ti were synthesized by reactive foil sintering in open air. Six initially identical stacks of alternating Ti-3Al-2.5V and 1100-Al foils were processed for different lengths of time, yielding specimens with different metal and intermetallic compositions. Samples were cut from each composite plate (in layer orientations parallel and perpendicular to the intended load direction) for mechanical testing in compression and four point bending, under quasi-static and high strain-rate loading conditions. Examination of the specimens and their fracture surfaces by optical and scanning electron microscopy was performed to establish a correlation between the failure mechanisms present, composite strength, and composition. Results indicated that regardless of loading direction, cracks always initiated in the intermetallic region, and crack propagation and failure were heavily influenced by the thickness of the residual aluminum layers. There is an ideal residual aluminum volume fraction that represents the amount of ductile reinforcement able to most effectively contain the oxides and impurities between intermetallic layers and thereby improve the mechanical properties of the MIL composite.