DETERMINATION OF FREE-ENERGY OF DELTA-FERRITE GAMMA-AUSTENITE INTERPHASE BOUNDARY OF LOW-CARBON STEELS BY IN-SITU OBSERVATION/

Phase transformations of delta-ferrite/gamma-austenite in low carbon s teels containing 0.017 mass% phosphorus are observed in-situ by using a combination of confocal scanning laser microscope with infrared imag e furnace. Dihedral angles at triple points of one high energy delta/d elta grain boundary and two incoherent delta/gamma interphase boundari es are measured by holding the sample at temperatures where delta- and gamma-phases co-exist in quasi-equilibrium. From the average value of observed dihedral angles, free energy of gamma/delta incoherent bound ary for a steel with low sulfur content (0.007 mass%) is estimated to be 0.450 J/m(2) at ca. 1700 K. The effects of temperature and sulfur c ontent on the dihedral angle and free energy of incoherent delta/gamma boundary are also discussed.