Abstract

The segregation of impurity atoms to grain boundaries can have significant influence on the cohesive properties, atomic arrangements and the mechanical properties of such interfaces. Therefore, it strongly impacts the macroscopic behavior of materials and understanding the atomic structure and related segregation behavior at grain boundaries is crucial to tailor materials with optimized physical properties ^[1]. Copper (Cu) is an attractive material for electronic applications, because of its good electrical and thermal conductivity. The effect of Sulphur on grain boundaries in Cu is of particular interest since it is improving the electromigration resistance but can also lead to grain boundary embrittlement ^[2].

In this study the atomic structure and chemistry of grain boundaries in polycrystalline Cu with variable Sulphur content (7 – 4000 ppm) are analyzed. In order to determine the distribution, size and orientation of the grains, Electron Backscatter Diffraction (EBSD) measurements were performed, as shown in Figure 1. The measurement reveals a grain size of 1-4 mm and an orientation of the grain normal close to the [001] direction. Transmission electron microscopy (TEM) specimens are prepared conventionally by grinding, electro polishing and Ar⁺ ion beam milling at specific regions from the EBSD scan in order to select special grain boundaries. This is shown exemplarily by the black circle in Figure 1.

TEM methods (bright-field imaging, selected area diffraction and high resolution TEM) are used to investigate the structure of selected grain boundaries. A preliminary example represented in Figure 2 reveals a high angle grain boundary. The specimen is tilted so that the upper grain is oriented in [001] zone axis while the resulting orientation of the lower grain is close to the [013] zone axis. The tilt component between both grains was determined to  by measuring the misorientation between the (200) reflexes of both grains which is in good agreement with the EBSD measurement. The twist component of this random grain boundary is not yet fully determined and is part of current investigations.

The atomic structure and segregation of Sulphur are characterized by aberration-corrected scanning transmission electron microscopy (STEM) in combination with analytical techniques including energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). Hence, information on the atomic arrangement of impurity atoms (interstitial and substitutional), the respective bonding and possible grain boundary precipitates will be obtained with sub-nanometer resolution.

References:
[1] M. Rajagopalan, M. A. Tschopp, and K.N. Solanki, JOM1, 1(2014)
[2] A.Wimmer, M. Smolka, W.Heinz, T. Detzel, W.Robl, C. Motz, V.Eyert, E. Wimmer, F. Jahnel, R. Treichler, G. Dehm; Mater. Sci. Eng. A, 618(2014), pp. 398-405

Figures:

Figure 1: EBSD measurement of polycrystalline copper with 7ppm Sulphur showing the grain size and orientation. The black circle indicates the area the TEM specimen was taken from and the black line the grain boundary which was investigated.

Figure 2: (A) Bright-field TEM image showing a random high angle grain boundary (prepared from the region shown in Figure 1) with the upper grain oriented in [001] zone axis (B) and the lower grain close to the [013] zone axiy (C). The red rectangle in (A) displays the position of the HRTEM image of the grain boundary in (D).

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