FeAl-TiC cermets—melt infiltration processing and mechanical properties

Abstract

Abstract TiC-based cermets were processed with iron aluminide, an intermetallic, as a binder by pressureless melt infiltration to near full density (>97% theoretical density). Phase equilibria calculations in the quaternary Fe–Al–Ti–C system at 1723 K by Thermocalc™ were performed to determine the solubility of TiC in liquid iron aluminide. Results show that molten Fe–40at.%Al in equilibrium with Ti 0.512 C 0.488 and graphite, dissolves 4.9 at.% carbon and 64 atomic ppm titanium. Due to the low values for the solubility of the carbide phases in liquid iron aluminide, melt infiltration of molten FeAl into TiC preforms is more successful than conventional liquid phase sintering to process cermets with carbide contents ranging from 70 to 90 vol.%, to greater than 97% of theoretical density. 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. Evaluation of room temperature fracture toughnesses showed that relatively high fracture toughnesses (18 MPa m 1/2 ) can be obtained with iron aluminide as a binder. A systematic correlation of the ligament sizes with the fracture mode was performed. The results clearly show that FeAl ligaments larger than about 1.2 μm fracture by cleavage and those smaller in size fracture predominantly in a ductile manner. Slow crack growth experiments were performed with specimens immersed in water and in an oxygen atmosphere. The extent of environmental embrittlement was evaluated by comparing the work to fracture in water and oxygen, respectively. With additions of more than 20–40 vol.% TiC, the composites showed only small differences in the work of fracture, indicating that environmental embrittlement of iron aluminide is not severe when it is used as a binder in cermets.