The low-temperature (20 to 900C) plastic deformation of TiC0.91 monocrystals was studied by micro-indenting (111), (001) and (110) surfaces. The dislocation structures around micro-indentations were characterized by means of transmission electron microscopy. It was found that deformation occurred mainly via {111}<1¯10> and {110}<1¯10> slip, with the favored slip system being determined by the crystal orientation. Indentation at temperatures below 300C produced distinct dislocation half-loops, hexagonal loops which arose from {111} slip and elongated loops which arose from {110} slip. At 500C, much more extensive plastic deformation occurred; mainly via the motion of edge dislocations from these same systems. The dislocation configurations suggested the existence of a relatively high mobility of edge segments, and a large Peierls stress for screw dislocations. Thermal activation appeared to increase the mobility of screw segments and resulted in the creation of dislocation structures that contained mixed dislocations with no preferred orientation. This indicated the onset of a brittle-ductile transition at temperatures of between 700 and 900C. Cleavage cracks around 500C indentations on {111} and {110} surfaces, but not {001} surfaces, were arrested; with dislocation emission at the crack tips. The emitted dislocations were coplanar dislocation half-loops which arose from {001}<110> slip, and resulted from mode-II or mode-III loading of the cleavage crack. The local mode-II and mode-III stress intensity factors had to be sufficiently high to activate {001} slip; even through this slip system had not yet been reported in microscopic tests.

F.R.Chien, X.J.Ning, A.H.Heuer: Acta Materialia, 1996, 44[6], 2265-83