Driving force for f.c.c. → b.c.c. martensites in FeX alloys

Information on M s, the starting temperature for formation of martensite, is reviewed and one M s line each for lath and plate martensite are drawn in a number of FeX phase diagrams. A reasonable interpretation of the data indicates the possibility that the distance between the two lines may vary linearly with temperature and be independent of the choice of alloying element. Using thermodynamic descriptions of the binary systems, the driving force for the start of the formation of the two kinds of martensite is calculated from the same interpretation of data. When plotted against temperature the results indicate that the driving force for martensite may not be much affected by solution hardening but may mainly be a function of temperature. For plate martensite it may have a fairly constant value of about 2100 J/mol. For lath martensite it may vary linearly, possibly from 500 J/mol at 800°C to 2100 J/mol at 250°C.