Abstract
The b = ½ ⟨110] unit dislocations in deformed TiAl alloys exhibit a unique morphology, consisting of numerous pinning points along the dislocation line aligned roughly along the screw dislocation direction, and bowed-out segments between the pinning points. The three-dimensional arrangement of these dislocations has been characterized in detail, based on post-mortem weak-beam transmission electron microscopy observations in deformed binary Ti-50 at.% Al and Ti-52 at.% Al alloys. The bowed segments glide on parallel {111} primary planes, and the pinning points are jogs with a range of heights, up to a maximum of about 40 nm. The substructure evolution is consistent with dislocation glide involving frequent double cross-slip and consequent jog formation. The dislocations experience a large glide resistance during the forward (non-conservative) motion of these jogs. Pinning of unit dislocations is an intrinsic process in these alloys and is not related to the presence of interstitial-containing precipitates in the matrix. The temperature-dependent increase in the linear pinning point density is not very sensitive to alloy composition. An outline of a flow-stress model is presented, based on a single dislocation experiencing a spectrum of resisting forces resulting from a range of jog heights; the shorter jogs contribute to glide resistance via friction, and the taller jogs via a dipoledragging mechanism. Estimates of the resisting force due to both these processes are shown to account reasonably well for the measured flow-stress.