1997 Volume 37 Issue 12 Pages 1210-1217
Effect of V and Nb addition on the fatigue behaviour of TiAl alloys was investigated using polysynthetically twinned (PST) crystals. The cyclic 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 ø=0° where ø is the angle between the loading axis and lamellar planes, slip occurred across the lamellar boundaries showing strong hardening. When slip occurred in the γ matrix parallel to the lamellar boundaries at ø=45°, cyclic behaviour and fatigue life were strongly influenced by the V and Nb addition: the fatigue life was prolonged by Nb addition, while V addition reduced the fatigue life.
In the fatigued TiAl PST crystals, numerous deformation twins and 1/2  ordinary dislocations were observed in the γ phase depending on the type of domain. Activation of twins and the frequency of cross-slip events for the 1/2  ordinary dislocations were very sensitive to additional elements of V and Nb. Since V and Nb addition were expected to decrease and increase the stacking fault energy, respectively, V-doping accelerated twinning and suppressed the cross-slip of the ordinary dislocations. In contrast, a small number of twins were observed and the cross-slip of the ordinary dislocations frequently occurred in Nb-doped crystals.
Since deformation twins induced in an early stage of cyclic deformation interrupted the motion of dislocations through γ/γ domain boundaries under cyclic loading and formed highly residual stress, the addition of V reduced the fatigue life.