Abstract

JORDAAN, W.A.; TERBLANS, J.J.  and  SWART, H.C.. The effect of nitrogen on the co-segregation with molybdenum in a Fe-3.5wt%Mo-N(100) single crystal. S. Afr. j. sci. [online]. 2008, vol.104, n.9-10, pp.393-397. ISSN 1996-7489.

In this study, the co-segregation of molybdenum and nitrogen to the (100) surface of an iron single crystal was investigated by studying a Fe-3.5wt%Mo-N single crystal. Ternary segregation systems are considerably more complex than binary systems in the sense that there are seven unknown segregation parameters to determine, as opposed to three for a binary system. A novel approach was undertaken to minimize the number of unknown variables and thus reduce the calculation time needed for the ternary system, by first analysing the segregation behaviour of molybdenum in a binary system [Fe-3.5 wt%Mo(100)] that was exposed to a nitrogen ambient. Exposing the binary crystal to nitrogen at high temperatures, the molybdenum segregated to the surface. The segregation profiles of the two systems were acquired at constant temperatures in the range 797-888 K using Auger electron spectroscopy (AES). The segregation profiles were fitted with the modified Darken model and the segregation parameters, namely, the pre-exponential factor (D₀), activation energy (E), segregation energy (ΔG) and the interaction energies (Ω_Fe-Mo, Ω_Fe-N, and Ω_Mo-N) for molybdenum and nitrogen, were determined. For the binary system the segregation parameters for molybdenum were: D_0,Mo = 2.4 ×10^0±1 m² s^-1, E_Mo = 323 ± 16kJ mol^-1, and ΔG_Mo = -38 ± 5 kJ mol^-1. These segregation parameters were then used as initial values to fit the experimental data of the ternary system. The segregation parameters for molybdenum in the ternary system were determined as: D_0,Mo = 1.9 ×10^-4±1 m² s^-1, E_Mo =271 ± 11kJ mol^-1 and ΔG_Mo= -32 ± 5 kJ mol^-1. The segregation parameters for nitrogen in the ternary system were D_0,N = 2.8 ×10^-0±3 m² s^-1, E_N = 323 ± 43 kJ mol^-1 and ΔG_N = -19 ± 3 kJ mol^-1. The interaction energy between molybdenum and nitrogen was ΔΩ_Mo-N = -19 ±3 kJ mol^-1.

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