The Bogoliubov inequality approach in the pair approximation and Monte Carlo simulations are used in a site-disordered Ising model in order to obtain the critical temperature of the ${\mathrm{Fe}}_{1\ensuremath{-}q}{\mathrm{Al}}_{q}$ system. A similar study is also made for a classical Heisenberg model using Monte Carlo simulations. The results are compared with experimental data and the best fit shows a nonlinear relationship between the effective exchange parameter ${J}_{\mathrm{FeFe}}$ and Al concentration $q$. The results obtained for the critical temperature by Monte Carlo simulations on both models (Ising and Heisenberg), despite being better than those from the pair approximation, still give the same general overview of the experimental phase diagram.