Carbon segregation to the low-index surfaces of an Fe–10 at. % Si single crystal

Abstract

Several mechanisms for carbon segregation to iron and iron alloy surfaces, specifically Fe–Si alloy single crystals, have been reported. This paper reports on measurements of carbon segregation to the (111) surface of an Fe–10 at. % Si crystal as measured with Auger electron spectroscopy in the temperature range 573–690 K. Three distinct consecutive processes have been identified in support of reported results on the (100) and (110) surfaces. The first is bulk to surface diffusion until the surface coverage reaches a value of 12–14 at. %. The activation energy for this process is 41±5 kJ/mol. The second process is associated with the growth of two-dimensional graphite islands on the surface until a surface coverage of about 45% is reached. This coverage is described as a ‘‘stable monolayer.’’ Further increase in the carbon concentration is associated with a further two-dimensional growth. The rate constants for the last two processes were determined by a fit of the Avrami equation to the data. The activation energies, determined from the Arrhenius plots, are, respectively, 115±10 and 147±10 kJ/mol. The results also indicate that the segregated graphite layer has some epitaxial relation to the substrate.