A thermodynamically consistent phase-field model is formulated for binary-alloy, particularly applicable to the systems which have a significant difference in mobilities of the solute and solvent atoms, e.g., interstitial solutes. The model is based on the mixed-mode of growth which is considered to be the general mode of transformation. As an illustration, the model has been used to simulate austenite to ferrite transformation in one-dimension. The model is made ‘quantitative’ using a different diffusivity-interpolation function without using anti-trapping flux term. The simulation-result is compared with the experimental results, and good agreement with the mixed-mode of growth in the initial stage of the transformation (when the individual nuclei are far apart) has been observed. © 2014, The Minerals, Metals & Materials Society and ASM International.