THE GEOMETRY AND NATURE OF PINNING POINTS OF 1 2 (110] UNIT DISLOCATIONS IN BINARY TIAL ALLOYS/

The b = 1/2[110] unit dislocations in deformed TiAl alloys exhibit a u nique morphology, consisting of numerous pinning points along the disl ocation line aligned roughly along the screw dislocation direction, an d bowed-out segments between the pinning points. The three-dimensional arrangement of these dislocations has been characterized in detail, b ased on post-mortem weak-beam transmission electron microscopy observa tions in deformed binary Ti-50 at.% Al and Ti-52 at.% Al alloys. The b owed segments glide on parallel (111) primary planes, and the pinning points are jogs with a range of heights, up to a maximum of about 40nm . The substructure evolution is consistent with dislocation glide invo lving frequent double cross-slip and consequent jog formation. The dis locations experience a large glide resistance during the forward (nonc onservative) motion of these jogs. Pinning of unit dislocations is an intrinsic process in these alloys and is not related to the presence o f interstitial-containing precipitates in the matrix. The temperature- dependent increase: in the linear pinning point density is not very se nsitive to alloy composition. An outline of a flow-stress model is pre sented, based on a single dislocation experiencing a spectrum of resis ting forces resulting from a range of jog heights; the shorter jogs co ntribute to glide resistance via friction, and the taller jogs via a d ipole-dragging mechanism. Estimates of the resisting force due to both these processes are shown to account reasonably well for the measured flow-stress.