Experimental observations of dislocation core structures and their relation to mechanical behavior

Dislocations are an ideal subject for investigating the interrelationships among atomic-level core structure, microscopic deformation mechanisms, and the macroscopic mechanical behavior of structural materials. Dislocation core structure controls mobility and can therefore govern a material's ability to plastically deform. Two classes of materials are discussed in this dissertation: the yield strength anomaly in intermetallic alloys is characterized in terms of superdislocation core geometry, and experimental observations of dislocations in pure gold and iridium provide comparisons for ab initio calculations.; Ni₃Ge-Fe₃Ge has been found to be a model system for relating the macroscopic mechanical behavior of L1₂ intermetallic alloys with atomic level dislocation core structures. Increases in iron content cause a gradual transition from anomalous to normal behavior, and remarkable low temperature strengthening. Transmission electron microscopy (TEM) and image simulations have been combined to determine the operative deformation mechanisms and to perform detailed measurements of superdislocation dissociations. The transition in yield strength behavior has been correlated with a transition from octahedral glide and cross-slip locking to cube glide, while low temperature strengthening coincides with enhanced cross-slip. The propensity for cross-slip has been related to a significant drop in cube plane antiphase boundary energy. It has also been noted that low temperature strengthening and the transition to cube glide are both consistent with an increase in complex stacking fault energy.; Ab initio computational techniques are finding increasing application in the study of dislocations. These first-principles predictions must, however, be verified through comparison with accurate experimental results. High resolution TEM has permitted the experimental observation of atomic columns within dislocation cores. For screw dislocations, the displacements of atomic columns relative to a perfect lattice revealed the extent of in-plane lattice distortion and yielded a dissociation width of 0.8 nm in both gold and iridium. Displacements around 60° dislocations were also quantified, and mixed character dislocations in iridium were studied with weak-beam TEM. High resolution and weak-beam measurements show excellent agreement for both metals, yielding stacking fault energies of 33 mJ/m2 (gold) and 420 mJ/m 2 (iridium). Moreover, all observations agree with anisotropic elasticity predictions, from screw through edge character.