Slip systems and dislocation emission from crack tips in single crystal TiC at low temperatures

Abstract

Low temperature (20–900°C) plastic deformation in TiC 0.91 single crystals has been studied using microindentations on (111), (001) and (110) surfaces, the dislocation structures around microindents being characterized by transmission electron microscopy (TEM). Deformation occurs primarily by {111}〈11̄0〉 and {110}〈11̄0〉 slip, with the favored slip system determined by crystal orientation. Indentations below 300°C produced distinct dislocation half loops-hexagonal loops arising from {111} slip and elongated loops from {110} slip. At 500°C, much more extensive plastic deformation occurred, accomplished mainly by the motion of edge dislocations from these same systems. The dislocation configurations suggest a relatively high mobility of edge segments and a large Peierls stress for screw dislocations. Thermal activation apparently increases the mobility of screw segments, and results in dislocation structures containing mixed dislocations with no preferred orientation; this signals the onset of the brittle-ductile transition between 700 and 900°C. Cleavage cracks around indents on {111} and {110} surfaces introduced below 500°C, but not those on {001}, arrested with dislocation emission at the crack tips. The emitted dislocations were coplanar dislocation half loops, arising 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 must have been sufficiently high to activate {001} slip, even through this slip system has not, to date, been reported in macroscopic tests.