Effect of creep strain on microstructural stability and creep resistance of a TiAl/Ti3Al lamellar alloy

Creep of a TiAl/Ti3Al alloy with a lamellar microstructure causes progressive spheroidization of the lamellar microstructure. Microstructural observations reveal that deformation-induced spheroidization (DIS) occurs by deformation and fragmentation of lamellae in localized shear zones at interpacket boundaries and within lamellar packets. Deformation-induced spheroidization substantially increases the interphase interfacial area per unit volume, demonstrating that DIS is not a coarsening process driven by reduction of interfacial energy per unit volume. Creep experiments reveal that DIS increases the minimum creep rate (epsilon(min)) during creep at constant stress and temperature; the activation energy (Q(c)) and stress exponent (n) for creep are both reduced as a result of DIS. Values of n and Q(c) for the lamellar microstructure are typical of a dislocation creep mechanism, while estimated values of n and Q(c) for the completely spheroidized microstructure are characteristic of a diffusional creep mechanism. The increase in epsilon(min) associated with DIS is thus attributed primarily to a change of creep mechanism resulting from microstructural refinement.