INTERSECTIONS OF DEFORMATION TWINS IN TIAL .2. MODELS AND ANALYSES

In TiAl with the L1(0)-ordered structure there are two basic crystallo graphic configurations in which twin bands from two systems intersect each other, both of which have been investigated in this work. Transmi ssion electron microscope observations described in part I have shown the structural changes associated with twin intersections in these two configurations. The aim of the present paper is to examine mechanisms in which the shear imposed by the incident twin on the barrier twin i s accommodated. It is shown that in L1(0) many crystallographic relati ons exist in which the displacement continuity at the matrix/twin inte rface is maintained in shear transmissions. Most of them, however, are found to be inconsistent with the experimental observations. The pres ent paper aims to investigate factors that have limited the occurrence of certain reaction schemes. It is found that the energetics measured in terms of the balance in the self energies of the dislocations invo lved is not sufficient to acount for the occurrence/absence of a conce rned transmission scheme. Of particular importance is the distribution of shear stresses ahead of the pile-up of incident twinning Shockleys which has been evaluated here using the well-developed pile-up model. It is shown that many of the dislocation emission mechanisms satisfyi ng the crystallographic conditions are opposed by the stress concentra tion of the pile-up and agreements with the experimental observations have been found in dislocation mechanisms which are driven most by the stress concentration of the pile-up. The effect of the orientation of the applied uniaxial load has also been analysed and has been shown t o lead to limited shear transmission in some mechanisms. Type-I inters ection in TiAl deformed at room temperature has been interpreted in te rms of slip on the {115BAR} plane in the barrier twin. At high tempera tures, accommodation shear in type-I intersection occurs via 1/2[110]{ 001) slip. The observed type-II intersection has been explained on the basis of subsidiary twinning in the barrier twin.