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 from 0.09 to 4.17 wt.% C was determined to be 0.12699(07) + wt.% C x 0.00049(74). The value obtained when this relationship is extrapolated to 6.6895 wt.% C (i.e., to Fe3C) is greater than the specific volume calculated for this composition on the basis of lattice parameter measurements made on the cementite in these samples, It was concluded that the composition of cementite in equilibrium with ferrite at room temperature is slightly off-stoichiometric, Several vacancy models were examined in an attempt to calculate its composition, Assuming a C-vacancy defect structure gives a value of 6.65(3) +/- 0.02(0) wt.% C. This corresponds to Fe3C0.994+/-0.003 square 0.006+/-0.003 where square represents a vacant C site, Room-temperature lattice parameters as well as a completely indexed Cr(K alpha) XRD pattern are given for the cementite.