Intersections of deformation twins in TiAl II. Models and analyses

Abstract

In TiAl with the L1₀-ordered structure there are two basic crystallographic configurations in which twin bands from two systems intersect each other, both of which have been investigated in this work. Transmission 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 is accommodated. It is shown that in L1₀ many crystallographic relations exist in which the displacement continuity at the matrix/twin interface is maintained in shear transmissions. Most of them, however, are found to be inconsistent with the experimental observations. The present 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 involved is not sufficient to acount for the occurrence/absence of a concerned 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 satisfying the crystallographic conditions are opposed by the stress concentration 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 to lead to limited shear transmission in some mechanisms. Type-I intersection in TiAl deformed at room temperature has been interpreted in terms of slip on the {115) plane in the barrier twin. At high temperatures, accommodation shear in type-I intersection occurs via ½〈110]{001) slip. The observed type-II intersection has been explained on the basis of subsidiary twinning in the barrier twin.