Polycrystalline elastic constants of in situ cementite (Fe3C)

By extrapolation, we estimated cementite's elastic constants. Altogether, we studied seven Fe-C binary alloys varying from 0 to 17.3 at% carbon. Thus, we measured hypoeutectoid, near-eutectoid, and hypereutectoid alloys. Using the hypereutectoid compositions, for cementite, we report a complete set of quasiisotropic (polycrystal) elastic constants. To correct for texture, we used a Voigt-Reuss-Hill averaging method. Surprisingly, for the dilatational modes, cementite's elastic constants differ little from those of b.c.c. iron and are about 10% higher for a shear mode such as the Young modulus. This stiffening indicates a tendency toward Fe-C covalent bonding. The decrease in Poisson ratio supports covalent bonding, but the small decrease indicates smaller covalent-bonding effects than expected by Pauling. Against ten previous Young modulus reports, we find cementite stiffer than alpha-iron, almost 10% in the Young modulus. Our results support Pauling's suggested Fe-C bonding over Petch's suggested Fe-Fe bonding. We compare our results with those from recent ab Mina calculations, which make very different predictions, which suggests that cementite occurs in at least two crystal structures, a suggestion made at least twice previously. Our derived Debye temperature T-D = 501 K differs considerably from previous reports. Within measurement errors, our bulk modulus agrees with three recent geophysical measurements. (C) 2010 Elsevier B.V. All rights reserved.