AN IN-SITU OBSERVATION OF MECHANICAL TWIN NUCLEATION AND PROPAGATION IN TIAL

A creep-deformed Ti-48 at.% Al-2 at.% Nb-2 at.% Cr specimen was studied using the electron beam illumination method in a transmission electron microscope to investigate mechanical twin nucleation and propagation in situ. The mechanical twins were observed to nucleate by bowing out one or two twinning dislocations at grain boundaries. The mechanical twin nucleus was either an intrinsic stacking fault or an extrinsic stacking fault. The mechanical twin propagated by sequential emission of twinning dislocations from the grain boundaries and homogeneous glide of the twinning dislocations on every adjacent twinning plane. The twinning dislocation is identified to be 1/6[11(2) over bar] and the twinning plane is (111). The twin-matrix interface structure is also identified. A Schmid factor analysis indicates that the observed twin nucleation and propagation resulted from a local stress concentration. A simplified uniaxial tensile stress state necessary to operate (111)[11(2) over bar] twinning system is analysed. Based on the experimental observation, a mechanical twin nucleation and propagation mechanism in TiAl is proposed, and used to characterize mechanical twinning during creep deformation at 765 degrees C.