EFFECT OF LAMELLAR STRUCTURE AND PLASTIC ANISOTROPY ON THE CYCLIC DEFORMATION AND FATIGUE BEHAVIOR OF TIAL PST CRYSTALS

Cyclic deformation and fatigue properties of TiAl polysynthetically tw inned (PST) crystals were investigated by focusing on the effect of la mellar structure and plastic anisotropy on the cyclic hardening and fa tigue life. Cyclic tension/compression testing at room temperature at total strain-controlled amplitude was carried out on Ti-49.1 at.% Al a nd Ti-50.8 at.% Al PST crystals containing fine and coarse lamellae, r espectively. Strong anisotropic behaviour was observed, depending on t he angle (phi) between the loading axis and the lamellar planes. As th e strain amplitude was increased, the stress amplitude rose with incre asing number of cycles and the cyclic hardening became large at phi=0, while at phi=45 degrees specimens exhibited weak cyclic hardening and broke without significant hardening. Although the lamellar spacing in TiAl PST crystals did not lead to much difference in the cyclic harde ning rate, refinement of lamellae prolonged the fatigue life in both o rientations. The lamellar boundaries act as an effective barrier to th e propagation of microcracks crossing the boundaries at phi=0. The cyc lic hardening and fatigue life of specimens with phi=0 are sensitive t o the deformation substructure, particularly with regard to the number of domains in which 1/2<1(1)over bar0>-type ordinary dislocations for m a densely tangled and piled-up substructure. The cyclic hardening an d fatigue life of crystals with phi=0 also depends upon the rotation a ngle (chi) between the loading axis and the [<(11)over bar2>] directio n on (111) lamellar planes in the gamma matrix.