Diffusion bonding is a key solid-state joining process used in the manufacturing of high-performance components, where microstructural evolution at material interfaces plays a critical role in determining the final properties of the assembly. Understanding and controlling these microstructural mechanisms remains challenging due to the complex coupling between atomic diffusion and phase transformations. The objective of this work is to develop a reliable and practical numerical framework capable of capturing the key physical phenomena governing diffusion bonding, with a particular focus on microstructural evolution in the reaction interlayer. A thermodynamically consistent multiphase-field model is formulated based on the CALPHAD approach to describe the system chemical free-energy.