The effect of nitrogen on the cosegregation of molybdenum in a Fe-3.5wt%Mo-N (100) single crystal

Abstract

In this study the cosegregation of molybdenum and nitrogen to the (100) plane of an iron single crystal was investigated. Ternary systems are considerably more complex than binary systems in that there are seven segregation parameters to determine, as opposed to three. However, a novel approach was undertaken to minimize the amount of variables, by first analysing a similar binary system that was exposed to a nitrogen ambient. Two single crystal were selected for this purpose, i.e. a Fe- 3.5wt%Mo(100) binary system and a Fe-3.5wt%Mo-N ternary system. By exposing the binary crystal to a nitrogen ambient at high temperatures it was observed that molybdenum segregated to the surface. The segregation profiles of the two systems were acquired at constant temperatures from 797 K - 888 K and Auger Electron Spectroscopy was used to monitor the surface concentrations of the relevant species. Since accurate surface temperature measurements are essential to segregation studies, a calibrated infrared thermometer was used. The segregation profiles were generated by measuring time and the Auger signal simultaneously. From the segregation profiles, initial estimates for the diffusion coefficients of Mo were first determined for the binary system by applying Fick’s equation to the segregation profiles. From these values the pre-exponential factor, D0, was determined to be 1.2x10−4±2 m2/s and the activation energy, E, as 258±33 kJ/mol. The diffusion coefficients determined thus, were used as estimates for obtaining the Darken seggregation profiles. In this case the D0 value was found to be 2.4x100±1 m2/s and the E value, 323±16 kJ/mol. The segregation energy, G, of Mo was calculated as -38 kJ/mol. In both cases it was observed that the diffusion coefficient of Mo deviated from the expected value at high temperatures due to the desorption of nitrogen from the surface. Using thermodynamic theory, an expression for the segregation energy of Mo in terms of the nitrogen surface concentration was derived. The Darken fits were repeated and it was found that the high temperature diffusion coefficient values fell on the the Arrhenius linear regression lines. For this special case, the D0 value was calculated as 5.5x101±1 m2/s, the E value as 345±18 kJ/mol. The segregation parameters determined for the binary system were then used as initial values for fitting the experimental data of the ternary system. Using Fick’s equation, the diffusion coefficients of Mo and N in Fe were determined. From the Arrhenius linear regression, the pre-exponential factor for Mo was calculated as 3.6x10−2±1 m2/s and that of N as 4.1x10−1±2 m2/s. The activation energies were 308±20 kJ/mol and 210±40 kJ/mol for Mo and N, respectively. The segregation parameters of the ternary system were then determined via the Darken method. In this case the pre-exponential factors were 1.9x10−4±1 m2/s for Mo and 2.8x100±3 m2/s for N. The activation energies were 271±11 kJ/mol and 323±43 kJ/mol. The segregation energy of Mo was calculated as -32 kJ/mol and for N, -19 kJ/mol. The interaction coefficient between Mo and N was calculated as -19 kJ/mol.