Study On The Elastic Anisotropy Of Cementite

Cold drawn pearlitic steel wires under large strain, which have the world’s highest strength, cause wide attention of scholars both at home and abroad owing to its mechanism of deformation and strength. As the most important strengthening phase of pearlitic steels, the performance, shape, distribution and orientation change under cold drawn of cementite have a great influence on the performance of pearlitic steel wires. Traditional theory holds that cementite belongs to brittle phase. However, a lot of research results show that plastic deformation occur on cementite under cold drawn. Therefore, the properties, especially mechanical properties, have become research focus. In the present work, mechanical properties of cementite and alloy elements effects had been studied by the first-principles calculations, which was also verified rationality of calculation results by the corresponding experimental designs.The first-principles calculations revealed that single-crystal elastic moduli C44 of cementite was small, which was one seventh of C55 or C66 and cementite had the extreme elastic anisotropy. Elastic modulus with the different crystal orientations of cementite had been measured by electron back-scattered diffraction （EBSD） and nanoindentation, which were also compared with the first-principles calculation results. The comparison results showed that elastic modulus changing with crystallographic orientation calculated by the energy method were almost same with measured results, which calculated by the stress vs. strain method had a difference with measured results. The elastic anisotropy of calculated results was larger than measured results.The cohesive energy and formation energy calculation results of alloyed cementite indicated that Mn in 8d occupancies of cementite had the strongest chemical bonds and formed alloyed cementite easier, which were consistent with XRD results of the bulk alloyed cementite prepared by mechanical alloying （MA） and spark plasma sintering （SPS）. With the increase of Cr contents, Cr had a tendency to form alloyed cementite from 4c occupancies to 8d occupancies.Elastic modulus calculations of Fe₂MnC and Fe₂CrC revealed that adding alloy elements increased elastic modulus of cementite and elastic modulus of Fe₂MnC were larger than Fe₂CrC.