Electronic structure and electronic mechanism of impurity-dislocation interactions in intermetallics

The nature of impurity-dislocation interactions is one of the key physics questions in the solid solution hardening (SSH) problem. The anomalously high increase of the yield stress upon doping with transition metals observed in NiAl and Ni₃Al intermetallics and a number of other metals cannot be explained in the framework of traditional elasticity models and suggests that electronic structure features can play an important role in this phenomenon. Using the first principles real space tight-binding linear-muffin-tin orbital method we performed electronic structure calculations for different types of screw and edge dislocations with <100> and <111> Burgers vectors and {010} and {011} slip planes in NiAl and a <100>{010} edge in a number of bcc transitional metals. For dislocations with <100> Burgers vector, the reconstruction of the electronic states due to dislocation is accompanied by an unusual peculiarity, namely the formation of localized electronic states inside the valence band. By analyzing the nearest-neighbor configurations in dislocation cores, we formulated the conditions necessary for the appearance of these states, as follows: (i) a decrease of the number of nearest neighbors (the number of bonds) around the central atom of the dislocation core; (ii) the contribution of electronic d-states to the formation of these interatomic bonds; and (iii) a specific local symmetry of the atomic arrangement in the region of the dislocation core, when some of the bonds become oriented along one or more of the t_2g orbitals. These conditions can be satisfied for <100> edge dislocations with {010} and {011} slip in all transition metals and alloys with bcc-type structure, where these dislocations exist. To investigate the influence of electronic factors on impurity-dislocation interactions, we calculated the electronic structure and energy of NiAl with a straight <100>{010} edge dislocation and transition metal impurities. We found that localized electronic states in the core of the dislocation are the key factor leading to unusually strong impurity-dislocation interactions via two mechanisms: (i) chemical locking, due to strong hybridization between impurity electronic states and localized dislocation states; and (ii) electrostatic locking, due to long-ranged charge oscillations caused by electron localization. The chemical locking (the strong impurity-dislocation attraction) may influence the macroscopic mechanical properties of NiAl in two ways: (i) for the dislocation glide during plastic deformation, more stress will be necessary to move the dislocation further, hence this mechanism will contribute to SSH; (ii) the segregation of impurities on the dislocation will occur for the dislocation at rest. The electrostatic locking is relatively long-ranged; therefore impurity-dislocation interaction induced by the localized electronic states should give an effective contribution to strengthening. Besides SSH, the additions of transition metal impurities can also improve the high-temperature properties of NiAl, such as creep resistance. The results of our calculations suggest that impurity segregation on edge dislocations due to the strong attractive impurity-dislocation interaction should play an important role in this effect. The results obtained give a new insight into understanding both SSH and the anomalous increase of creep resistance upon alloying in intermetallic alloys. We also suggest how solid solution hardening in NiAl correlates with the electronic structure of impurities rather than with size misfit (as expected according to standard views). These mechanisms can be expected not only in NiAl, but also in other B2 intermetallics where deformations are carried by <100> dislocations (CoTi, CoHf, and CoZr).