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 studie d using the electron beam illumination method in a transmission electr on microscope to investigate mechanical twin nucleation and propagatio n in situ. The mechanical twins were observed to nucleate by bowing ou t one or two twinning dislocations at grain boundaries. The mechanical twin nucleus was either an intrinsic stacking fault or an extrinsic s tacking fault. The mechanical twin propagated by sequential emission o f twinning dislocations from the grain boundaries and homogeneous glid e of the twinning dislocations on every adjacent twinning plane. The t winning dislocation is identified to be 1/6[11(2) over bar] and the tw inning plane is (111). The twin-matrix interface structure is also ide ntified. A Schmid factor analysis indicates that the observed twin nuc leation 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 o bservation, a mechanical twin nucleation and propagation mechanism in TiAl is proposed, and used to characterize mechanical twinning during creep deformation at 765 degrees C.