Changes in the deformation mode in 6 types of γ domain (AM–CT) and α2 plates in TiAl polysynthetically-twinned crystals fatigued along a loading axis parallel to lamellar planes with stress amplitude, Δσ, of 420 to 450MPa was examined by transmission electron microscopy which focussed upon the continuity of macroscopic strains and slip/twinning planes at lamellar boundaries. At Δσ = 420 and 450MPa, the strain continuity was always maintained               at lamellar boundaries by activation of one of               the

symmetrical twinning systems in A-type domain and selection of the predominant deformation mode between ordinary dislocations and twins in (B and C)-type γ domains. The (B and C)-type γ domains of BM, BT, CM and CT behaved as 2 sets of (BM,CT) and (BT,CM) because each set selected either the deformation mode of ordinary dislocations or twins as the predominant system in order to maintain macroscopic strain continuity. The set (BT,CM), which accounted for a larger volume fraction than the set (BM,CT) in TiAl polysynthetically-twinned crystals used here, selected a twinning system at Δσ = 450MPa. Ordinary dislocations were selected at Δσ = 420MPa. At Δσ = 450MPa, twinning deformation prevented the further motion of ordinary dislocations with a Burgers vector parallel to lamellar boundaries, and rapid fatigue hardening occurred, accompanied by a reduction in the accumulated plastic strain energy. An anomalous change in strain energy during fatigue was influenced by the volume fraction of a set of (B and C)-type domain and the anomalous behavior in fatigued TiAl- polysynthetically-twinned crystals could disappear when each type of γ domain was equally distributed.

Selection and Change in Deformation Mode to Maintain Continuity of Strains and Slip/Twinning Planes at Lamellar Boundaries in Fatigued TiAl Polysynthetically Twinned Crystals. T.Nakano, Y.Nakai, Y.