Effect of RE on accelerating the kinetics of boride layer growth on titanium alloy

Abstract

In this study, it is found that solid state boriding of titanium alloy with RE additions (REs) can achieve much deeper growth of TiB layer (∼94 μm) and an enhanced total layer (TiB2+TiB) thickness (∼113 μm) than that of conventional one without RE. To illustrate the effect of RE on accelerating the growth kinetics, a diffusion model, using the mass balance equations at the interfaces (TiB2/TiB and TiB/substrate) and considering boriding temperatures (850, 950 and 1000 °C) and times (5min - 10h), was established. Based on these, the corresponding accelerated diffusion-rate constants in TiB2 and TiB layers, are further developed to quantitatively illustrate the effect of RE on accelerating the boron (B) diffusion in boride layers. It is found that this accelerated diffusion-rate constant can adequately explain the effect of RE on accelerating B diffusion through boride layers. And the main reason for the effect of RE on accelerating the kinetics of boride layer growth was demonstrated that RE can cause the concentration improvement of substitutional vacancies by the substitutional mechanism, which would improve the diffusion of B in Ti phase and cause the anomalous B diffusion in TiB layer, thus accelerating the boride layer growth.