The lamellar structure of polysynthetically twinned specimens was characterized, before and after creep, in terms of a quantitative description of the microstructural variables such as the lamellar spacing, α2 spacing and α2 volume-fraction. An apparent α2-to-γ phase transformation, which took place when the samples were held at 1150K, was encountered not only in creep-deformed specimens but also in simply heat-treated specimens. The transformation of the α2 phase into the γ phase took place via partial dissolution of α2 lamellae; thus resulting in a substantial increase in the spacing of the α2 lamellae. The driving force for α2 decomposition was identified as being the chemical force to achieve the thermodynamic equilibrium that was not reached in as-grown polysynthetically twinned crystals. Two processes of lamellar refinement during creep were identified. These were mechanical twinning parallel to the lamellar interfaces, plus secondary precipitation of γ-phase in the α2 lamellae. Mechanical twinning parallel to the lamellar interfaces was much more pronounced in soft-oriented polysynthetically twinned specimens than in hard-oriented specimens. Both refinement processes led to an increase in the total number of interfaces, even though some α2 interfaces disappeared as a result of α2 dissolution.

On the Microstructural Stability of TiAl/Ti3Al Polysynthetically Twinned Crystals under Creep Conditions. G.Wegmann, K.Maruyama: Philosophical Magazine A, 2000, 80[10], 2283-98