Elevated temperature mechanical properties and microstructural stability of a two-phase titanium-aluminum/titanium(3)-aluminum lamellar alloy

A two-phase TiAl/Ti{dollar}sb3{dollar}Al alloy with a lamellar microstructure has been shown to exhibit a significantly lower minimum creep rate than the minimum creep rates of the constituent TiAl and Ti{dollar}sb3{dollar}Al single-phase alloys. Fiducial line experiments demonstrate that the creep rates of the constituent phases within the two-phase TiAl/Ti{dollar}sb3{dollar}Al lamellar alloy tested in compression are more than an order of magnitude lower than the creep rates of single phase TiAl and Ti{dollar}sb3{dollar}Al alloys tested at the same stress and temperature. The lower creep rate of the lamellar alloy is attributed to the enhanced work hardening of the constituent phases within the lamellar microstructure compared with the single phase alloys. This proposition is confirmed by TEM observations which reveal that the phases in the two-phase TiAl/Ti{dollar}sb3{dollar}Al lamellar microstructure have a significantly higher dislocation density than the TiAl and Ti{dollar}sb3{dollar}Al single-phase alloys after creep testing. A constitutive model based on composite strengthening has been formulated to predict the minimum creep rate of the lamellar alloy, taking into account the enhanced hardening of the constituent phases within the lamellar microstructure. Excellent agreement is found between model predictions and experimentally determined creep rates. Thermal and thermomechanical exposure result in microstructural evolution, which increases the minimum creep rate of the lamellar alloy. During annealing of the two-phase TiAl/Ti{dollar}sb3{dollar}Al alloy microstructure at 1273K and 1323K, the lamellar microstructure evolves into a coarse, globular microstructure. The resulting increase of the minimum creep rate can be accounted for by consideration of two factors: the decrease in the work hardening rate of the lamellar alloy in response to the overall decrease in interphase interfacial area, and the decreased mechanical strengthening effect associated with the transformation from a lamellar to a globular microstructure. Thermomechanical exposure (i.e. creep) results in deformation-induced spheroidization of the lamellar microstructure. The resulting increase in the minimum creep rate can be explained by the onset of boundary diffusion accommodated creep in the spheroidized regions of the microstructure.