Critical Assessment of Activities of Structural Units in Fe–Al Binary Melts Based on the Atom and Molecule Coexistence Theory

Abstract A thermodynamic model for calculating the mass action concentrations{N_i}of structural units in Fe–Al binary melts based on the atom–molecule coexistence theory,i. e., AMCT–{N_i}model, has been developed and verified to be valid by comparing with reported activities{a_{{\rm{R, }}i}}of both Al and Fe relative to pure liquid as standard state in Fe–Al binary melts over a temperature range from 1823 to 1973 K (1550 to 1700 °C). Furthermore, Raoultian activity coefficients\gamma _i^0of both Al and Fe in the Fe-rich corner or Al-rich corner of Fe–Al binary melts as well as the standard molar Gibbs-free energy changes{\Delta _{{\rm{sol}}}}G_{{\rm{m, }}i{\rm{(l)}} \to {{{\rm{[}}i{\rm{]}}}_{{\rm{ [ \% }}i]{\rm{ = 1}}{\rm{. 0}}}}}^{\Theta {\rm{, \% }}}of dissolved Al or Fe for forming [% Al] or [% Fe] as 1.0 in Fe–Al binary melts have also been determined by the developed AMCT–{N_i}model and verified to be accurate. The reported activities{a_{{\rm{R, }}i}}of both Al and Fe from the literature can be well reproduced by calculated mass action concentrations{N_i}of free Al and free Fe in Fe–Al binary melts. A small effect of changing temperature from 1823 to 1973 K (1550 to 1700 °C) on reaction abilities of both Al and Fe from the available literature is also confirmed by calculated mass action concentrations{N_i}of free Al and free Fe in Fe–Al binary melts. The obtained activity coefficients{\gamma _i}of both Al and Fe in Fe–Al binary melts can be described by a quadratic polynomial function and a cubic polynomial function, respectively. Furthermore, accurate expressions of Raoultian activity coefficients\gamma _i^0of both Al and Fe in Fe-rich corner or Al-rich corner of Fe–Al binary melts are also obtained as\ln \gamma _{{\rm{Al}}}^0 = - {\rm{9, 646}}{\rm{. 5}}/T{\rm{ + 2}}{\rm{. 196}}and\ln \gamma _{{\rm{Fe}}}^0 = - {\rm{6, 799}}{\rm{. 1}}/T - {\rm{0}}{\rm{. 01367}}, respectively. In addition, expressions of the first-order activity interaction coefficients\varepsilon _i^iore_i^iorh_i^iof both Al and Fe coupled with three activity coefficients\gamma _i^{}orf_{ \% {\rm{, }}i}^{}orf_{{\rm{H, }}i}^{}relative to three standard states are also obtained from the developed AMCT–{N_i}model for Fe–Al binary melts.