An electron microscopic investigation was made of twin shapes in the YBa2Cu3O7-δ matrix of melt-textured pellets that contained dispersed particles of Y2BaCuO5. The recorded images were compared with the shapes predicted by thin twin-dislocation theory for three regions (twin tip, root, intermediate regions). These twin-shape investigations yielded twin-boundary energies of 60.0mJ/m2 that agreed with the values of 46.5mJ/m2 obtained by relating the twin spacing to local twin-colony size, as determined from the spacings between Y2BaCuO5 particles in the same samples. The addition of Pt decreased the twin boundary energy to 26.8mJ/m2, as evaluated using the twin shape method and to the 18.3mJ/m2 found using the interparticle twin-spacing method. The lattice frictional force resisting twining dislocation motion was also estimated from the minimum twin thickness and the twin shape. The estimated frictional force agreed with the limit set by the reported de-twinning stress. The frictional force and the de-twinning stress were expected to increase sharply as the temperature decreased. A critical length scale of about 250nm was estimated for the point where it was energetically favourable to create twins.

Shape of a Twin as Related to the Inelastic Forces Acting on Twinning Dislocations in YBa2Cu3O7-δ. V.S.Boyko, S.W.Chan, M.Chopra: Physical Review B, 2001, 63[22], 224521