The formation of core–rim structures in Fe40Al/(TiC–TiN–WC) cermets produced by pressureless melt infiltration

Ti(CN)-based cermets were produced by pressureless melt infiltration of TiC, TiN, and WC mixed powders by Fe–40 at.% Al iron aluminide intermetallic (Fe40Al). Before infiltration, the powder compacts were pre-sintered at different temperatures for different times in order to form hard Ti(CN)-based skeletons with given values of porosity. A core–rim morphology of the solid particles evidenced under SEM characterizes the microstructure of cermets made from pre-sintered Ti(CN) skeletons infiltrated by Fe40Al. The mechanism of the core–rim structure formation was studied using XRD, SEM, TEM, and EDS. This core–rim structure is composed of an almost pure Ti(CN) core surrounded by a bi-layer rim of (Ti,W)(CN) solid solution. However, direct infiltration of as-pressed TiC, TiN, and WC powder mixtures by Fe40Al led to mostly Ti(CN) cermets without core–rim structures. The formation of a (Ti,W)(CN) phase during the pre-sintering step determines the existence of the core–rim microstructure. The Ti(CN) core and the (Ti,W)(CN) inner rim formed at the pre-sintering stage by solid state reaction, while the outer rim was formed during the infiltration stage by the dissolution of (Ti,W)(CN) material from the inner rim and precipitation as a solid solution of low W content.