The specific mode by which strain is accommodated at a grain boundary depends ultimately on the defects that are admissible at the interface. Understanding the structure and topology of such defects is, therefore, vital to connecting atomistic and mesoscale descriptions of grain boundary behavior. In this presentation we discuss the structure and function of interfacial disconnections (i.e. line defects possessing both step and dislocation character [1]) at a 90° <110> tilt boundary in gold. An

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... example of one such boundary, which is vicinal to {111}/{112}, is shown in Figure 1 . Here, locally coherent {111}/{112} terraces are separated by interfacial disconnections that join steps of five {242} planes with two {111} planes in the upper and lower crystals, respectively. We designate these defects as ""b 5/2 ". A strain of 5.7% is required to maintain coherency of the {111} planes crossing the horizontal {112} λ /{111} µ terraces at this interface. We have shown that the observed array of defects efficiently accommodates this coherency strain [2] . In some cases the interfacial disconnections are dissociated. For instance, in Figure 2 , a "b 5/2 " defect has dissociated into two components, one matching three {242} planes with one {111} plane (b 3/1 ) and the other matching eight {242} planes with three {111} planes (b -8/-3 ). We also find this same configuration in atomistic simulations of the interface (Figure 3) . By measuring the Burgers vectors of the defects, we have shown that this dissociation is favored by elastic energy criteria. Moreover, our analysis shows that the b 3/1 defect is glissile and is thus able move readily to the center of the {111} terrace in response to repulsive interactions with the b -8/-3 defects that bound the adjacent terraces [3] . The defects we have discovered at this grain boundary are in many ways analogous to those that occur at epitaxial or tranformation interfaces between different crystallographic phases. These have a dual function in both accommodating the interfacial coherency strain and dictating the local boundary inclination. In principle, similar defects should arise at other grain boundaries. Establishing the defect properties of these and related interfacial defects provides a more fundamental understanding of the strain relief mechanisms at grain boundaries.