STEM investigation of the chemistry and bonding changes associated with the grain boundary embrittlement of Cu by Bi

Abstract

It has long been known that trace amounts of Bi can embrittle Cu after appropriate heat treatments. The Bi segregates to the grain boundaries and weakens them such that failure occurs through intergranular fracture without plastic deformation. This behavior is demonstrated in the scanning electron micrograph of a typical Cu-Bi fracture surface in Figure 1. It is known that the Bi extends for only a few atomic layers into the grains on either side of the grain boundary. This narrow segregation width was been confirmed using Energy Dispersive X-ray Spectroscopy (EDS) on a VG HB603 STEM. Figure 2 shows the ratio of Bi to Cu as the probe is stepped across the grain boundary.The segregation behavior is well understood, however it is not yet properly understood how the Bi causes embrittlement once it is at the grain boundaries. The Bi must change the bonding at the boundaries so that the boundaries become weak and hence the most likely fracture path. The Electron Energy Loss Near Edge Structure (ELNES) coupled with the small probes and high current density available in a field emission STEM can provide information about the localized electronic structure and hence bonding at grain boundaries. Previous investigations indicated that the near edge structure of Cu was altered at the grain boundaries due to the presence of Bi.