This study discusses microstructure evolution, diffusion behavior and bonding strength of a couple comprising of an iron aluminium alloy (Fe-Al) and high carbon-steel (FeCMn) during diffusion bonding. A columnar microstructure evolves from the joint interface toward FeCMn and disappears in couples bonded for a long period. Aluminium diffusion from Fe-Al to FeCMn and columnar microstructure evolution are retarded as compared to a couple consisting of an Fe-Al and ferrite steel. The carbide in the FeCMn impedes the aluminium diffusion and retards the columnar grain evolution. When the carbide is dissolved in the ferrite during the aluminium diffsuion from Fe-Al, coarse grains evolve due to the coalescence of the columnar grains and a high-bonding strength is obtained. The hardness variation is minimum in the FeCMn of a couple bonded for a short period, which is explained by the microstructural changes in the columnar grain evolution and carbide dissociation. (c) 2006 Springer Science + Business Media, Inc.