SLIP SYSTEMS AND DISLOCATION EMISSION FROM CRACK TIPS IN SINGLE-CRYSTAL TIC AT LOW-TEMPERATURES

Low temperature (20-900 degrees C) plastic deformation in TiC0.91 sing le crystals has been studied using microindentations on (111), (001) a nd (110) surfaces, the dislocation structures around microindents bein g characterized by transmission electron microscopy (TEM). Deformation occurs primarily by {111}[1(1) over bar0$] and {110}[1(1) over bar0$] slip, with the favored slip system determined by crystal orientation. Indentations below 300 degrees C produced distinct dislocation half l oops-hexagonal loops arising from {111} slip and elongated loops from {110} slip. At 500 degrees C, much more extensive plastic deformation occurred, accomplished mainly by the motion of edge dislocations from these same systems. The dislocation configurations suggest a relativel y 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 degrees C. Cleavag e cracks around indents on {111} and {110} surfaces introduced below 5 00 degrees C, but not those on {001}, arrested with dislocation emissi on at the crack lips. The emitted dislocations were coplanar dislocati on 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 acti vate {001} slip, even through this slip system has not, to date, been reported in macroscopic tests.