ON THE COMPOSITION OF CEMENTITE IN EQUILIBRIUM WITH FERRITE AT ROOM-TEMPERATURE

The specific volume (rho(-1) in cm(3)/g) versus wt.% C relationship at 23 degrees C for slow-cooled, high-purity, Fe-C alloys containing fro m 0.09 to 4.17 wt.% C was determined to be 0.12699(07) + wt.% C x 0.00 049(74). The value obtained when this relationship is extrapolated to 6.6895 wt.% C (i.e., to Fe3C) is greater than the specific volume calc ulated for this composition on the basis of lattice parameter measurem ents made on the cementite in these samples, It was concluded that the composition of cementite in equilibrium with ferrite at room temperat ure is slightly off-stoichiometric, Several vacancy models were examin ed in an attempt to calculate its composition, Assuming a C-vacancy de fect structure gives a value of 6.65(3) +/- 0.02(0) wt.% C. This corre sponds to Fe3C0.994+/-0.003 square 0.006+/-0.003 where square represen ts a vacant C site, Room-temperature lattice parameters as well as a c ompletely indexed Cr(K alpha) XRD pattern are given for the cementite.