EFFECT OF V AND NH ADDITION ON THE FATIGUE BEHAVIOR OF TIAL POLYSYNTHETICALLY TWINNED CRYSTALS

Effect of V and Nb addition on the fatigue behaviour of TiAl alloys wa s investigated using polysynthetically twinned (PST) crystals. The cyc lic hardening and fatigue life of TiAl PST crystals depended strongly on the orientation and alloying elements. When the stress was applied parallel to the lamellar boundaries at phi=0 degrees where phi is the angle between the loading axis and lamellar planes, slip occurred acro ss the lamellar boundaries showing strong hardening. When slip occurre d in the gamma matrix parallel to the lamellar boundaries at phi=45 de grees, cyclic behaviour and fatigue life were strongly influenced by t he V and Nb addition: the fatigue life was prolonged by Nb addition, w hile V addition reduced the fatigue life. In the fatigued TiAl PST cry stals, numerous deformation twins and 1/2[1 (1) over bar 0] ordinary d islocations were observed in the gamma phase depending on the type of domain. Activation of twins and the frequency of cross-slip events for the 1/2 [1 (1) over bar 0] ordinary dislocations were very sensitive to additional elements of V and Nb. Since V and Nb addition were expec ted to decrease and increase the stacking fault energy, respectively, V-doping accelerated twinning and suppressed the cross-slip of the ord inary dislocations. In contrast, a small number of twins were observed and the cross slip of the ordinary dislocations frequently occurred i n Nb-doped crystals. Since deformation twins induced in an early stage of cyclic deformation interrupted the motion of dislocations through gamma/gamma domain boundaries under cyclic loading and formed highly r esidual stress, the addition of V reduced the fatigue life.