Characterization of the brittle mechanism in a Au-Ge microalloy

In order to improve the hardness of pure gold, alloying with small amounts of germanium (Ge) is of great interest. However, the resulting embrittlement of the new alloy is undesirable for use in jewelry applications. Thus, it is necessary to elucidate the cause of this behavior in this microalloy in order to avoid it in future cases. The Au-Ge alloy was characterized using a combination of inductively coupled plasma mass spectrometry (ICP-MS) to investigate the nominal chemistry, Vickers microhardness to determine the alloy's hardness in comparison to pure Au, X-ray diffraction (XRD) to find the phases/structures, along with light optical microscopy (LOM) and scanning electron microscopy (SEM) to explore the alloy's surface features including grain size, and phase distributions. A heating microscope and differential thermal analyzer (DTA) were also used in order to determine the phase transformation behavior. Trace amount of Ge was found to render Au grains refined to the size of 10 to 160 mu m. The corresponding microhardness was remarkably improved from 31 to 56 HV. The Au-Ge microalloy was found to exhibit Ge enrichment along the grain boundaries, resulting in an interdendritic, eutectic that forms throughout the microstructure. This phase distribution is likely the cause of the brittle mechanical behavior of the Au-Ge alloy.