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In the present work, a thermodynamic study was carried out in order to analyze the thermodynamic stability of the and phases in equilibrium with the phase using the calculation of phase diagrams (Calphad) formalism. The two phases and are modelled as substitutional and interstitial solid solutions of boron. The expressions of the chemical potentials ofBandFeare derived in both phases to perform the thermodynamic calculations. A comparison is made between the results provided by the substitutional and interstitial models and good agreement is observed between these two models.

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Abstract. In this current work, the boron diffusion coefficient in Fe 2 B was firstly evaluated using a diffusion model. It considers the effect of the incubation times required to form the Fe 2 B layers by the paste-boriding process on the iron substrate. This model solves the mass balance equation at the (Fe 2 B/substrate) interface under certain assumptions. Afterwards, the effective boron diffusion coefficient in Fe 2 B was evaluated through an application of a simple equation. As a result, the estimated value of boron activation energy in the presence of chemical stresses was found to be equal to 146.5 kJ mol -1 on the basis of experimental data taken from the literature.

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