Nonequilibrium Phases in Fe–X–C (X=Cr, Mo or W) Ternary Alloys Quenched Rapidly from Melts

Nonequilibrium phases such as amorphous, chi (χ), epsilon (ε) and austenite have been found in Fe–X–C (X=Cr, Mo or W) ternary alloys quenched rapidly from the melts. The completely amorphous phase is formed in the composition ranges of 27–50 at%Cr and 14–20 at%C for Fe–Cr–C, of 6–27 at%Mo and 13–22 at%C for Fe–Mo–C, and of 10–14 at%W and 18–20 at%C for Fe–W–C. The high amorphous-forming tendency of these alloy systems seems to be due mainly to a remarkable increase of the glass transition temperature by the addition of chromium, molybdenum or tungsten, reflecting in strong-attractive interactions between carbon and alloy elements. The χ and ε phases are solid solutions containing large amounts of carbon and alloy elements, and their formation regions are located near the alloy compositions where the average group number of metallic atoms is in the vicinity of 7 and 7.7, respectively. The χ phase has a complicated bcc structure and its lattice parameter increases from 0.892 to 0.905 nm with increasing carbon and alloy contents. The ε-phase is a hcp structure with a lattice parameter of about a=0.27 nm and c=0.43 nm. The austenite phase has an ultra-fine grain size of about 0.2 μm and appears in the composition ranges of about 5–10 at%C and 4–30 at%Cr, 4–13 at%Mo or 2–8 at%W. The formation of this phase seems to become easy by the combination of three factors of the suppression of phase transformations in the solid state, the increase in solid solubility of carbon and alloy elements and the lowering of the MS point by dissolution of these elements.