A tough hypereutectic high chromium white iron has been developed by considering the alloy as a double in-situ fibrous composite and applying the principles of composite theory. The eutectic in the ternary Fe-Cr-C system solidifies as M7C3 carbide rods of irregular hexagonal cross section in a matrix of austenite. The carbide rods are intermittently joined together with no misorientation at the joint. The colonies (eutectic grains) of a 27%Cr alloy solidify with a flat solid – liquid interface. In hypereutectic high chromium white irons the primary carbide solidifies as single crystal rods with no branching. Depending on the thermal conditions in the mould the primary carbides are nucleated only on the mould surface, or repeatedly at the solid – liquid interface. In 27% Cr, 4.5%C hypereutectic alloys, the as-solidified microstructure is one of long parallel aligned primary carbides in a eutectic matrix of carbide rods in austenite, or short randomly orientated primary carbides in a eutectic matrix. In either case the microstructure can be described as a double in-situ fibrous composite. The application of composite theory has resulted in hypereutectic high chromium white iron castings with improved fracture toughness similar to that of heat-treated alloy steel but with superior wear resistance.