The nature of precipitates in boron-doped TiC

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Abstract
A comprehensive transmission electron microscope investigation has been made of two-dimensional defects present in boron-doped TiC. By means of selected area diffraction it has been established that the defects are partially coherent precipitate platelets of TiB2 approximately 10 Å in thickness. Diffraction contrast experiments demonstrate that precipitation takes place at dislocation nodes and that the widely extended nodes are bounded by Shockley partial dislocations. The regions bounded by the partials resemble extrinsic stacking faults but differ from equilibrium faults in that a precipitate is incorporated at each fault. An analysis of extended and contracted nodes which form at twist boundaries indicates that node extension is the result of TiB2 precipitation and is not due to a reduction in overall stacking-fault energy. This result demonstrates that node radius measurements cannot always be related directly to stacking-fault energies and that measurements of this type may not be meaningful when precipitation takes place at nodes. As a result of this study it is concluded that dislocation nodes serve as nucleation sites for the precipitation of TiB2 in (TiC+B) alloys. This observation has significant implications with respect to the high temperature mechanical properties of this material since precipitation and hence precipitation hardening may be controlled by appropriate mechanical pre-treatment.