Weak-beam analysis of dissociated 1/2«112] superdislocations in gamma-TiAl

Abstract

Among the problems encountered in the characterization of 1/2<112] super dislocations in gamma-TiAl intermetallic alloy, one arises from asymmetric or abnormal contrast observed during transmission electron microscopy (TEM) experiments. The diffraction contrast from the core of these superdislocations does not always follow the rules of diffraction contrast established for analyzing partial dislocations bounding stacking faults. In particular, the configuration involving three similar Shockley partials on adjacent planes has often been ruled out owing to the absence of fringes indicating the presence of stacking faults. In order to determine the dissociated configuration, weak-beam TEM observations of edge-oriented 1/2<112]superdislocations have been correlated with computer-simulated images. Dissociation of these superdislocations into similar 1/6<112] Shockley partial dislocations bounding a superlattice extrinsic and intrinsic stacking-fault pair has been consequently determined from these analyses. Comparison of experimental and simulated images confirms that the formation of an antiphase boundary or complex stacking-fault-linked dissociation or locking by stair rod dislocations can be ruled out.