Vaporization of Mn from surface of Fe-Cr-Mn alloy

In order to develop a new surface modification utilizing vapor pressure differences among alloy elements, the vaporization of Mn, which has high vapor pressure, from Fe-Cr-Mn alloy was studied. Fe-20%Cr-20%Mn alloy was heated in various atmospheres at temperatures ranging from 1173 to 1373 K. Mass changes for the samples during heat treatment and vaporized materials from the samples were examined. Concentration profiles of the alloy elements were measured before and after heat treatment. The decreases in sample mass during heat treatments were measured. The mass loss was larger at a higher temperature and for a longer time. The materials vaporized from a sample contained Mn more than 98%. The Mn concentration on the sample surface, heat treated in a closed tube, was 13-15%. On the other hand, the Mn concentration on the sample surface, heat treated under a constant pressure using a vacuum pump, was less than 1%. The mass loss for a specimen heat treated under a constant vacuum pressure increased with the square root of heat time and the surface Mn concentration did not depend on the vacuum conditions, temperatures or time. A concentration gradient under a surface was produced by Mn vaporization. The concentration profiles split into two regions. The Mn concentration gradient was lower at the surface site and higher at the base metal side. The decrease of the Mn concentration is considered to cause the phase change from gamma to alpha. Under the heat condition for a constant vapor pressure, the activation energy for Mn vaporization is 248 kJ/mol and is consistent with that for Mn diffusion in alpha iron. On the assumption that the vaporization behavior is controlled by the diffusion of Mn, the Mn concentration profiles were calculated. The results agreed with the measured profiles for Mn concentration using EPMA.