This study deals with solid‐state phase transformations occurring at interfaces between a TiN film and an AISI M50 steel substrate during postdeposition annealing at 500 and 1000 °C. Cross‐sectional transmission electron microscopy (XTEM) was used to investigate the nature of phase transformations at these interfaces. The TiN films were ion plated onto M50 steel with an initial Ti underlayer at 100 °C. XTEM and electron energy loss spectroscopy of the interfaces indicated that the ion‐plated Ti underlayers transformed into a TiC phase during annealing at 500 °C and into a Ti(C,N) phase during annealing at 1000 °C. In addition, the density of defects within grains was significantly reduced during annealing and the grains themselves became larger.