Boundary phase stability and critical phenomena in higher order solid solution systems II

The thermodynamic treatment of double-pseudobinary solutions of the type (A xBy) r o(M uNv) s o presented in a preceding publication was extended to include the conditions defining the critical points for the asymmetric case (r o≠s o) and approximations for the spinodal and binodal surface near the critical solution point. Closed solutions for the coordinates of the critical points were obtained only for systems with ideal mixing behavior, and the isothermal binodal and spinodal near the critical solution point in such systems are adequately approximated by circles and ellipses, respectively. An axes ratio of $$\sqrt 3$$ is nearly independent of the relative sublattice abundance and the major effect of changes in the ratios o/r o is a rotation of the binodal by an angle tg $$\alpha = \sqrt {\frac{{s_0 }}{{r_0 }}} .$$ . The principal features of nonideal regular systems for temperatures close to the critical solution point are described by expressions derived from small term expansions of the conditional equations, but generalizations are not possible to the same extent as for the case with ideal boundary systems. The results are discussed and the application of the equations demonstrated on model examples.