Beam-induced atomic migration at Ag-containing nanofacets at an asymmetric Cu grain boundary
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Besides the high spatial resolution achieved in aberration-corrected scanning transmission microscopy, beam-induced dynamic effects have to be considered for quantitative chemical characterization on the level of single atomic columns. The present study investigates the influence of imaging conditions in an aberration-corrected scanning transmission electron microscope on the beam-induced atomic migration at a complex Ag-segregated, nanofaceted Cu grain boundary. Three distinct imaging conditions including static single image and serial image acquisition have been utilized. Chemical information on the Ag column occupation of single atomic columns at the grain boundary was extracted by the evolution of peak intensity ratios and compared to idealized scanning transmission electron microscopy image simulations. The atomic column occupation is underestimated when using conventional single frame acquisition due to an averaging of Ag atomic migration events during acquisition. Possible migration paths for the beam-induced atomic motion at a complex Cu grain boundary are presented. I. INTRODUCTION
The majority of structural materials are typically based on crystalline metals, due to their high strength and toughness. A wide range of methods has been developed to tailor the mechanical properties of metals, generally being focused on either alloying additional elements to a matrix element and/or on making use of grain refinement, i.e., increasing the volume fraction of grain boundaries. In addition, grain boundaries are prone to segregation of alloying additions, which not only changes the local chemistry but also their structure,1 bond strength2 and according to recent findings may also lead to grain boundary phase transformations3—sometimes termed “complexions.”4 Two very prominent examples for grain boundary segregation are bismuth and boron additions to copper (Cu). Since both elements are almost immiscible in Cu according to their respective equilibrium phase diagrams,5,6 these elements show pronounced grain boundary segregation. Enrichment of these segregate atoms at the boundary—even at low concentrations—was shown to significantly change the macroscopic mechanical behavior of the material. Bismuth strongly embrittles polycrystalline copper,7 while boron increases the cohesion of a copper boundary significantly.8 Aberration-corrected scanning transmission electron microscopy (STEM) provides sub-Ångstrom spatial resolution at pm precision, thereby being capable to characterize the structure of crystalline materials on the Contributing Editor: Rafal E. Dunin-Borkowski a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.398
atomic level.9 In combination with analytical methods such as energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), chemical information can even be obtained from complex material systems and their interfaces with atomic resolution.10 However, these techniques usually require acquisition times on the order of minutes translating into
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