ORIGIN OF ABNORMALLY LARGE TETRAGONALITY OF MARTENSITE IN HIGH-CARBON IRON-ALLOYS CONTAINING ALUMINUM

The origin of abnormally large tetragonalities of martensite in Fe-Al-C alloys with a high carbon content (about 2 mass%) has been studied from various aspects; i.e., effects of Al content, small amount of Mn addition and quenching rate on the martensite tetragonality (c/a) were examined. First, it is found that the martensite tetragonality depends on the austenitic state. If the austenite contains fine kappa-phase precipitates, large c/a is obtained. Furthermore, the existence of a dumbbell structure of C-Fe-C in the austenite is very effective in enhancing the tetragonality. A possibility of the formation of the L1(2) superlattice structure in the austenite is ruled out. The tetragonality of the "fresh" martensite (the c/a value of the as-formed martensite at low temperature) is decreased with increasing temperature as in the case of previously reported Fe-Ni-C alloys, indicating the change in interstitial positions of carbon atoms.