Discovery of a Mechanism for Dislocation Nucleation and Migration Driven by Surface Segregation
Judith C. Yang and her colleagues answered the question of how dislocations nucleate and migrate at heterointerfaces in dissimilar-material systems on their recently published article on Nature Materials. Today most engineered materials are alloys either by design or by then natural incorporation of impurities that remain after processing. Minor compositional changes in these alloys results in big changes at free surfaces and heterophase interfaces. The phenomena called segregation is the manifestation of these effects in the macroscopic world. Segregation can be defined as the enrichment of one element at the surface relative to bulk.
In this study, Judith Yang and her colleagues showed that atomic segregation acts as a source for generating dislocations for the first time. They have used Cu–Au alloy system for studying surface segregation. Real-time transmission electron microscopy (TEM) was used to both spatially and temporally resolve the transition of the coherent, dislocation free interface between a Cu3Au-segregated surface and a Cu(Au) crystal substrate into a semi-coherent structure through the nucleation and subsequent migration of misfit accommodating dislocations. They combined their experimental study with the teory by using density functional theory (DFT) and molecular dynamics (MD) simulations.
They showed both Cu3Au like and Au surface segregation with the TEM measurements. They obtained direct insight into the atomic mechanism of the dislocation formation from in situ TEM observation of the nucleation of a single dislocation out of a coherent Cu3Au/Cu(Au) interface. Their in situ TEM observations showed that alternating pairs of atomic columns diffuse away from the surface and the troughs thus maintain the same depth unless one pair of atomic columns migrates away from the same trough region. Then a deeper trough develops locally resulting in the birth of a unit dislocation at the highly strained location along the Cu3Au/Cu(Au) interface. Their in situ TEM observations indicated that the dislocation nucleation occurs via a surface trapping process. After nucleation at the Cu3Au/Cu(Au) interface, the dislocations can either remain at or glide along the Cu3Au/Cu(Au) interface, or they can migrate into the bulk. Their TEM results showed that dislocations form at the Cu3Au/Cu(Au) interface via the surface trapping process and subsequently migrate into the bulk. They also performed a Burgers circuit analysis and showed that dislocation is more resistant to dissociation. Their TEM images consistently with simulations demonstrated that the dislocations formed from the surface trapping process are stable and can migrate in different ways. They attributed the observed dislocation migration to the interplay of different driving forces and kinetics for dislocation motion. They stated that the driving force for dislocation climb results from the competition between the surface image force that drags the dislocation to the free surface and the force due to the interfacial misfit strain that drags the dislocation away from the outer surface.
In short, they discovered a mechanism for dislocation nucleation and migration driven by surface segregation of solute atoms in a solid solution. Their results show that the surface-segregation-induced composition variations act as the source of strain/stress that drives the nucleation and migration of misfit dislocations, and demonstrate how the surface segregation phenomenon of an alloy constituent can be employed for developing atomistic insight into understanding the formation processes of misfit-accommodating dislocations.
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