Classification of gamma-gamma and gamma-alpha(2) lamellar boundaries on the basis of continuity of strains and slip-twinning planes in fatigued TiAl polysynthetically twinned crystals

Effects of lamellar boundaries on deformation in two-phase TiAl(gamma)-Ti3A l(alpha (2)) alloy were geometrically and experimentally examined and the t ypes of the boundaries were classified focusing on the continuity of macros copic strain components and slip-twinning planes. TiAl polysynthetically tw inned crystals were fatigued at 420 MPa with the loading axis parallel to t he lamellar boundaries and [11 (2) over bar] in gamma domains. The lamellar boundaries were classified into 13 groups composed of three gamma - gamma true-twin, three gamma - gamma pseudotwin, three gamma - gamma 120 degrees rotation boundaries and four gamma - alpha (2) interfaces. They were furthe r subdivided into two groups with continuous and discontinuous geometry. Tr ansmission electron microscopy observations showed that deformation modes u nexpected from Schmid fact or consideration were activated in some gamma do mains to maintain the macroscopic strain continuity through the lamellar bo undary and that the boundary acted as an effective barrier to the transfer of deformation.