First principles study of C diffusion in WC/W interfaces observed in WC/Co tools after Ti-alloy machining

Abstract

Ti-alloys have many qualities making them ideal for use in aerospace applications, medical implants and chemical industries such as high strength to weight ratio, good high temperature strength and chemical stability. One downside to Ti-alloys is, however, that they are considered difficult to machine. Several investigations have been made in order to understand the wear mechanisms present in machining of Ti-alloys and the most common understanding is a combination of attrition and dissolution-diffusion. Observations by Odelros et al. [1] have shown that there exists a small layer of pure bcc W on top of the outermost WC grains after turning of Ti-6Al-4V. In order for such a layer to form C has to diffuse away from the WC leaving behind only W. In this work Density Functional Theory (DFT) is used together with Harmonic Transition State Theory (HTST) to investigate the pre-factors and barriers for C diffusion into and within two different WC/W interfaces, [0001]/[111] and [101¯0]/[100]. The diffusion into the interfaces show that the barrier for the [0001]/[111] interface is more than twice as high as the barrier for the [101¯0]/[100] interface. Diffusion within the interfaces show, on average, slightly higher barriers for the [0001]/[111] interface.