Considerations on the Kaufman approach to binary phase diagram calculation

The Kaufman approach to phase equilibria involving primarily the fcc, bcc and hcp phases was examined outside the Group Nos. 4 to 10 range where it is customarily employed. The “stability parameters” (ΔH and ΔS of transformation) for most elements in the Group Nos. 1 to 3 region were found to fit satisfactorily the correlation curves of stability parametervs group no.; some of the parameters for Al, Be, Mg and Ti, however, did not. The rare earth parameters fit well in the Group No. 3.5 position they were expected to occupy. A sample phase diagram calculated between two Group 1 elements was in good agreement with experiment. Phase boundaries of fcc +bcc regions adjacent to terminal solid solutions in several Group 1/Group 2 and Group 1/Group 3 systems, on the other hand, were quite unsatisfactory. This difficulty was traced to the high, positive regular solution constants calculated for both phases. Such constants were shown to result from the downward concavity of a plot of enthalpy of vaporizationvs Group No. in the Nos. 1 to 3 region; in the Nos. 4 to 10 range this plot is concave upward.