Fe(100)面上へのイオウの偏析と2次元化合物の析出

Abstract

The segregation behavior of sulfur has been investigated on Fe(100) surface by AES and LEED in the temperature range from 873 to 1123 K. The bulk concentration of sulfur in specimens was varied from 17 to 100 wt ppm, which covered the areas both below and above the solubility limit of sulfur in α-Fe. In the area below the solubility limit, the equilibrium surface concentration of sulfur was about 30 at%, which was independent of the annealing temperature and the bulk concentration of sulfur. The surface structure c(2×2) was observed on the sulfur-segregating surface for all specimens. In the area above the solubility limit, the surface concentration of sulfur amounted to about 40 at% for some specimens, but the scattering of data between specimens was larger. From these results, it was concluded that the two-dimensional surface phase of Fe2S precipitates on Fe(100) surface below the solubility limit of sulfur and that the additional precipitation of bulk phase FeS occurs on the surface above the solubility limit. The thermodynamical consideration was given on the two-dimensional surface phases, and the attempt was made to construct the equilibrium phase diagram containing the surface phases. The qualitative representation of Fe(100)-S system has been given: (1) When the bulk concentration of sulfur is very small, the surface phase is in the state of solid solution, and the surface concentration increases with an increase in bulk concentration; McLean-type segregation. (2) If the bulk concentration exceeds a critical value (less than 1 wt ppm), the two-dimensional surface phase Fe2S precipitates, and the surface concentration of sulfur remains almost constant until the bulk concentration arrives at the bulk solubility limit (100 wt ppm at 1073 K). (3) Above this concentration, the three-dimensional FeS precipitates and coexists with the surface phase Fe2S on the surface.