Intersections of deformation twins in TiAl

Abstract

In TiAl with the L1₀-ordered structure there are two basic crystallographic configurations in which deformation twin bands from two systems intersect each other, both of which are investigated in this paper. The basic crystallography for twinning and twin intersection in the L1₀ structure is discussed. Transmission electron microscopy (TEM) observations have shown that, in both configurations, shears carried by the incident twins are able to transmit through the barrier twin. Deflections in the intersecting twin bands have been observed in TEM by imaging the twin bands edge-on. Structural changes in and around the intersection region have been examined using a combination of dark-field imaging and micro-diffraction. For intersections occurring along <110] directions (type-I), two types of structural changes have been observed in specimens deformed at low and high (about 800°;C) temperature. At room temperature, type-I intersection has been found to leave one twin band deflected with the other remaining rectilinear. The lattice in the intersection region, which remains in the L1₀ structure, is congruent with the lattice of the deflected twin. At high temperatures deflections are found in both twin bands, one of which can be described as resulting from shearing on the (001) basal plane. In type-II intersection, which occurs along <011] directions, both twin bands are found to be deflected, an observation that has been made in specimens deformed at both low and high temperatures. The intersection region consists of lamellae of a subsidiary twin which alternate with lamellae of the crossed twin band. Type-II intersection has been found to be associated with 1/2<110] dislocations in the twin-matrix interface. Models for the intersection of deformation twins are analysed in part II of this work in the following paper.