Accelerated kinetics of surface hardening by diffusion near phase transition temperature: Mechanism of growth of boride layers on titanium

Abstract

Accelerated kinetics of titanium boride (TiB) layer growth on titanium surfaces during solid-state diffusion of boron at temperatures very close to the α → β phase transition temperature (β-transus) has been demonstrated. On the basis of the fact that hcp metals such as Ti show enhanced (anomalous) self-diffusion near the phase transition temperature, the hypothesis of this work has been that the diffusivity enhancement should cause rapid ingress of interstitial/substitutional atoms, thereby accelerating the growth kinetics of the hard boride layer. It is shown in this work that a much deeper growth of TiB into the Ti substrate and an increased coating thickness (∼75 μm) indeed occurs at temperatures very close to the transition temperature, compared with that at temperatures away from the transition. Hardening of the surface layers was also achieved to a significant depth (∼100 μm), with the maximum surface hardness reaching ∼20 GPa (HK), which are desirable for improved surface performance. A diffusion model based on error function solutions to Fick’s second law was developed to quantitatively illustrate the combined effects of normal B diffusion in the TiB phase and anomalous B diffusion in the Ti phase in accelerating TiB layer growth. It has been found that this model adequately explains the accelerated boride layer growth kinetics near the phase transition temperature. The results of the study generally suggest that diffusion near the phase transition temperature can be used as a means of achieving greater diffusion depth of penetration of hard coatings in phase transforming metals.