Grain boundary structure and its interaction with dislocations in copper
Authors: Peter Nicolas J , Liebscher Christian H , Kirchlechner Christoph , Dehm Gerhard 
Published: December 2016

Most of the nowadays used structural materials are of polycrystalline nature since single crystal fabrication is very cost intensive and sometimes not even desired. For instance, increasing the internal interface fraction – like grain boundaries – is known to strengthen metals significantly according to the Hall‐Petch relation, thereby reducing the amount of load bearing material needed and hence the overall cost. Unfortunately, internal interfaces are often found to be weakest links within a failed material, contradicting the benefit from increased interface fractions. Consequently, understanding the dislocation interaction with distinct grain boundaries has the potential to tune a structural material's performance. However, profound knowledge does not only include tracking of dislocation movement but it is crucial to understand grain boundaries from the atomistic structural and chemical level including aspects like faceting, grain boundary phase transformations and/or segregation of impurities. Thus, the present study aims on comprehensively understanding the interplay between atomistic grain boundary structure and its mechanical behavior (i.e. interaction with dislocations) using a ∑5 36.9° {310} tilt grain boundary in copper as a model system.

Macroscopic copper bicrystals were grown using the Bridgeman method. The seed crystals were arranged to form a symmetric ∑5 36.9° {310} tilt grain boundary. Grain boundary orientation and structure where studied by aberration‐corrected high‐resolution STEM as well as conventional TEM methods and compared to findings from SEM‐EBSD measurements. For mechanical testing, tailored square‐shaped nanocompression pillars were FIB machined on top of TEM lamellae to ensure a defined testing geometry and stress state. Defects introduced into the compression samples upon FIB machining were annihilated during an annealing step. Finally,
in situ
nanomechanical testing was performed using conventional TEM as well as STEM mode to study the dislocation behavior at the boundary. In addition,
in situ
microcompression experiments were performed inside the SEM to establish the mechanical size effect of the chosen boundary system.

It is shown by EBSD that the desired ∑5 grain boundary was successfully grown within the entire macroscopic bicrystal. However, structural investigations at the atomic level reveal a range of deviations from the ideal symmetrical case along the course of the boundary throughout the bicrystal. The observed grain boundary structures include symmetrical and asymmetrical segments as well as segments showing grain boundary facets (Figure 1). The overall 36.9° misorientation is found to remain within the deviation predicted by Brandon's criterion in all of the structures.
In situ
SEM deformation along the [100]‐compression direction reveals activation of multiple major slip systems in single crystals that were not found in the ∑5 bicrystal compression samples (Figure 2). Preliminary TEM compression experiments indicate a cross slip mechanism as the prevalent dislocation‐boundary interaction rather than the formation of pile ups. This observation is also supported by the measured stress vs. strain curves as boundary containing pillars do not show increased hardening behavior relative to single crystalline reference samples.