On the microstructural stability of TiAl/Ti 3 Al polysynthetically twinned crystals under creep conditions

Abstract

Abstract The lamellar structure of polysynthetically twinned (PST) specimens has been characterized before and after creep testing in terms of a quantitative description of the microstructural variables: lamellar spacing, α2 spacing and α2 volume fraction. An apparent α2 → γ phase transformation taking place when the samples were held at 1150k was encountered not only in creep-deformed specimens but also in a simply heat-treated specimen. The transformation of the α2 phase into the γ phase took place by partial dissolution of α2 lamellae. resulting in a substantial increase in the spacing of the α2 lamellae. The driving force for the α2 decomposition was identified as the chemical force to complete the thermodynamic equilibrium that was not reached in the as-grown PST crystals. Two processes of lamellar refinement during creep deformation were identified, namely mechanical twinning parallel to the lamellar interfaces and secondary precipitation of γ phase in α2 lamellae. Mechanical twinning parallel to the lamellar interfaces was much more pronounced in soft-oriented PST specimens than in hard-oriented PST specimens. Both refinement processes led to an increase in the total number of interfaces even though some α2–γ interfaces disappeared as a consequence of α2 dissolution. Both refinement processes may contribute to the self-strengthening of lamellar α2 + γ alloys in the course of creep and hence the superior creep resistance of the lamellar microstructure.