On the dislocation pole mechanism for twinning in TiAl crystals

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Abstract
A dislocation model is proposed for twin formation in the L1 structure, under a compressive applied stress along the c axis, on the basis of the original pole mechanism for twinning in the fcc structure. Dissociation of a jog on an ordinary \[110]/2 dislocation into a Shockley and a Frank partial in TiAl is energetically favourable according to anisotropic elasticity theory. While a pileup of \[101] superdislocations assists a Shockley loop to spiral around two sessile nodes, the Frank partial expands under mechanical and chemical stresses and mitigates the high repulsive interaction between two Shockley partials. Available data on the apparent critical resolved shear stress for twinning in Ti-56 at.% Al single crystals are explained in terms of reasonable values of a local stress concentration factor and a vacancy supersaturation at elevated temperatures.

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