Atomic configuration in hexagonal-close-packed TiC zH x phase estimated by statistical thermodynamics

Abstract

Through statistical thermodynamic analysis of equilibrium PTC (pressure-temperature-composition) relationships obtained experimentally for an alloy system containing interstitial constituents, information concerning the configuration of the interstitial atoms in the alloy lattice as well as the extent of the stability of the interstitial atoms can be derived. This line of approach was taken for analysis of PTC data reported by Rexer and Peterson for h.c.p. (hexagonal close packed) TiC z H x phase ( z = 0.26; 0.4 < x < 0.8) in the temperature range 700–850 °C and for a hydrogen partial pressure of no more than 1 atm, with the aim of elucidating the configurations of the C and H atoms in the h.c.p. Ti lattice. The following picture was derived for the h.c.p. TiC z H x phase through this analysis: (1) C atoms occupy O-sites (octahedral interstices) randomly; (2) two of the four T-sites (tetrahedral interstices) around an O-site occupied by a C atom are preferentially occupied by H atoms; these two T-sites must lie at opposite ends of the diagonal line of the rectangle formed by the four T-sites at the corners; (3) the rest of the H atoms distribute over the other two free T-sites around the O-site occupied by the C atom.