Dislocations in shock-loaded titanium diboride
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(Received21 September 1987; accepted21 March 1988) The structure of shock-loaded poly crystalline titanium diboride was examined with transmission electron microscopy. The shock wave from ballistic impact produces prismatic and basal slip in grains favorably oriented with respect to the shock wave. It can be deduced from annealing experiments with the formation of stacking fault hexagons that there is a high concentration of point defects in deformed regions from the motion of dislocation jogs. Weakbeam microscopy shows that the dislocations in TiB2 are dissociated into partial dislocations. The stacking fault energy measured from a screw dislocation in the basal plane was found to be 120 mJ/m 2 . Widely dissociated dislocations in the shocked sample suggest that residual stresses are present in some regions.
I. INTRODUCTION The propagation of shock waves through solids is a subject of interest.x When the shock pressure is reasonably high, there are structural changes in the solid. 23 Dislocations are thought to be generated by the deviatoric stresses of the shock front.4'5 An excess concentration of vacancies present was proposed to be from the nonconservative motion of dislocation jogs.6'7 In some cases, phase transformations occur.2 Titanium diboride is a crystalline solid having a hexagonal AlB2-type crystal structure. 89 It is a stable compound, but can react to form TiB and Ti3B4 under certain conditions. Dislocations in hexagonal materials10 have been characterized by transmission electron microscopy (TEM). Cockayne et a/.11"13 used weak-beam TEM to image dissociated dislocations and evaluate the stacking fault energy. This article is a study of the defects generated in TiB 2 by a shock wave. II. EXPERIMENTAL PROCEDURE The titanium diboride plate from Ceradyne made from hot-pressed powders was ballistically impacted by a tungsten bullet. The back-scattered fragments were collected for this study. Some samples were encapsulated in vacuum tubes and annealed for 1 h at 1450 "C. The samples were then sectioned, cut into disks, and ion-beam thinned for transmission electron microscopy. The TEM was performed on the JEM 200 CX. The dislocations were characterized with trace analysis and Burger's vector analysis.
array in a deformed region. The dislocations either propagate from a grain boundary source or nucleate as loops. Dislocations intersect and form jogs. Figure 2 is a TEM image of a complex jog produced by the intersection of dislocation of Burger's vector b = [ 11.0] in the basal plane with dislocationof Burger's vector b = \ [21.0] oriented along J" = [ IT. 1 ]. Dislocation loops are visible in the matrix and in contact with dislocations. Figure 3 is a micrograph of a dislocation that contains large dislocation loops. The loops have hexagonal geometry in the basal plane with line directions oriented along (ll.O). Some loops are planar with Burger's vector b = [00.1 ], while others are three-dimensional configurations similar to stacking fault tetrahedra. The stacking fault hexagons in Fig. 4 have hexagona
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