Nature and dissociation of the dislocations in TiAl deformed at room temperature

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Abstract
TiAl polycrystalline specimens, having the L10 structure, have been deformed at room temperature and examined by transmission electron microscopy. The observations revealed a new type of slip superdislocations a/2⟨112⟩ gliding in the (111) planes, in addition to the a⟨101⟩ and a/2⟨110⟩ dislocations previously described in L10 compounds. Dissociation configurations of the superdislocations a/2⟨112⟩ and a⟨101⟩ have been observed: they explain the numerous extrinsic faulted dipoles limited by 1/6⟨112⟩ partials which are commonly observed in the deformed samples. In particular, the superdislocations dissociate only discontinuously on small segments (about 50 nm) along the dislocation line. It is argued that the superdislocations are pinned by jogs, and the rearrangement of the core structure around the jog leads to a faulted loop surrounded by a sessile 1/6⟨112⟩ partial. Faulted dipoles arise from the trailing of these loops. Extrinsic faults are usually rarely observed since they require the slip of two partials in two consecutive planes. A mechanism of formation of such faults is suggested: it is based on the cross-slip of a partial which becomes screw near the jog.