Deformation mechanisms in the intermetallic compound TiAl

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Abstract
Samples of a polycrystalline Ti-52at.°Al alloy have been deformed in compression at room temperature and 600°C to strains of ≈3°. The deformation microstructures have been characterized using transmission electron microscopy. It is found that, at room temperature, glide of dislocations with b = <101] has occurred, with some contribution from twinning. Dislocations with b=1/2<110] make up a negligible fraction of the dislocation density, and it is argued that these defects are in essence sessile at room temperature. However, in samples deformed at 600°C, the dislocation population is dominated by those with b=1/2<110], whereas those with b = <101] are most often involved in Kear-Wilsdorf configurations. These various results, including the line directions of the dislocations, are interpreted on the basis of the effect of covalent bonding on the anisotropy of the Peierls stresses (following the work of Greenberg, Anisimov, Gornostirev and Taluts) and therefore dislocation mobilities in this compound. The lack of ductility exhibited at room temperature is attributed to a lack of mobility of dislocations with b=1/2<110] caused by covalency, and the formation of sessile configurations by dissociation of dislocations with b = <101]. The increase in ductility of TiAl at higher temperatures is attributed to the operation of the slip systems 1/2<110]{111}, where dislocations with b=1/2<110] become significantly more glissile because of thermal activation, and also to an increased amount of twinning (apparently above 600°C) since the twinning dislocations (with b=1/6<112]) also are expected to be more glissile.