Reactivity and interface characteristics of titanium-alumina composites

Abstract

The reaction kinetics between α-Ti alloys and single crystal sapphire, the phase composition and morphology of the reaction-zone, and the phase compatibility in the system Ti-Al-O were investigated as part of a study to determine the feasibility of fabricating useful Al₂O₃-reinforced titanium matrix composites. In the temperature range 650 to 1000° C titanium reduces Al₂O₃ to form a complex reaction layer consisting of two distinct zones; an inner zone adjacent to the Al₂O₃ of a TiO phase containing isolated particles of (Ti, Al)₂O₃, presumably, and an outer zone of a Ti₃Al phase adjacent to the Ti matrix. The isothermal growth of the reaction layer follows a parabolic rate law. The temperature dependence of the rate constants fits an Arrhenius equation yielding activation energies of 50 to 52 kcal/mol. The high Al alloys, except Ti-6Al-2Sn-4Mo-2Zr, reacted more rapidly than pure Ti indicating that Al diffusion through the reaction zone may be the rate-limiting step.