INTERSECTIONS OF DEFORMATION TWINS IN TIAL .1. EXPERIMENTAL-OBSERVATIONS

In TiAl with the L1(0)-ordered structure there are two basic crystallo graphic configurations in which deformation twin bands from two system s intersect each other, both of which are investigated in this paper. The basic crystallography for twinning and twin intersection in the L1 (0) structure is discussed. Transmission electron microscopy (TEM) obs ervations have shown that, in both configurations, shears carried by t he incident twins are able to transmit through the barrier twin. Defle ctions in the intersecting twin bands have been observed in TEM by ima ging the twin bands edge-on. Structural changes in and around the inte rsection region have been examined using a combination of dark-field i maging and micro-diffraction. For intersections occurring along [110] directions (type-I), two types of structural changes have been observe d in specimens deformed at low and high (about 800-degrees-C) temperat ure. At room temperature, type-I intersection has been found to leave one twin band deflected with the other remaining rectilinear. The latt ice in the intersection region, which remains in the L1(0) structure, is congruent with the lattice of the deflected twin. At high temperatu res deflections are found in both twin bands, one of which can be desc ribed 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 c onsists of lamellae of a subsidiary twin which alternate with lamellae of the crossed twin band. Type-II intersection has been found to be a ssociated with 1/2[110] dislocations in the twin-matrix interface. Mod els for the intersection of deformation twins are analysed in part II of this work in the following paper.