FeAl–TiC and FeAl–WC composites—melt infiltration processing, microstructure and mechanical properties

TiC-based and WC-based cermets were processed with iron aluminide, an intermetallic, as a binder by pressureless melt infiltration to near full density (>97% TD). Phase equilibria calculations in the quaternary Fe–Al–Ti–C and Fe–Al–W–C systems at 1450°C were performed to determine the solubility of the carbide phases in liquid iron aluminide. This was done by using Thermocalc™ and the results show that molten Fe–40 at.% Al in equilibrium with Ti0.512C0.488 and graphite, dissolves 4.9 at.% carbon and 64 at. ppm titanium. In the Fe–Al–W–C system, liquid Fe–40 at.% Al in equilibrium with graphite dissolves ≈5 at.% carbon and 1 at.% tungsten. Due to the low values for the solubility of the carbide phases in liquid iron aluminide, liquid phase sintering of mixed powders does not yield a dense, homogenous microstructure for carbide volume fractions greater than 0.70. Melt infiltration of molten FeAl into TiC and WC preforms serves as a successful approach to process cermets with carbide contents ranging from 70 to 90 vol.%, to greater than 97% TD. Also, the microstructures of cermets prepared by melt infiltration were very homogenous. Typical properties such as hardness, bend strength and fracture toughness are reported. SEM observations of fracture surfaces suggest the improved fracture toughness to result from the ductility of the intermetallic phase. Preliminary experiments for the evaluation of the oxidation resistance of iron aluminide bonded cermets indicate that they are more resistant than WC–Co cermets.