Theory Prediction Of Structure And Properties For Mn/Cr Doping Alloyed Cementite

Cementite having a orthogonal structure is one of the most important phase in thesteel. Elements Mn and Cr as the representatives of a carbide-forming elements undercertain conditions can be able to enter the cementite to form the alloyed cementite. Insubsequent steel handling and working of the process (especially the tempering of the steel,aging, carburizing, hot working, cold working and a high pressure process), theperformance of the steel will change as the changes in the structure, composition andproperties of the alloyed cementite. However, in the process, what changes occur on thestructure and composition of the doping alloyed cementites, whether the formation ofalloyed cementite is stable or not and other issues have not been clear understanded,especially from the electronic and atomic level. And the solution to these problems are aguide to making process and performance improvements of alloyed steel.In this thesis, lattice parameter, geometry density, energy stability and mechanicalstability, elastic constants, bulk modulus, magnetic moments, densities of states of alloyedcementite added Mn/Cr are calculated using first-principles density-functional methods, so,the rule of alloy elements impact on electronic configuration and phase stability in atomicscale are studied. The main results are as follows:(1) When Mn/Cr atoms replace one Fe atom to form alloyed cementites, there are sixkinds of alloyed cementites;(2) Since the difference between the radii of Mn/Cr and Fe atoms, attice constantsand lattice type of doped alloyed cementite after the lattice distortion are changed;(3) From the point of formation energy, Cr alloyed cementites formation energy is thehighest and lest stable, but cementite formation energy is the lest and most stable;(4) Mn/Cr alloyed cementites have lower mechanical stability. Two kinds of alloyatoms all decrease shear modulus of cementites;(5) Adding Mn/Cr to cementite, alloyed cementite magnetic property is lower thancementite, and Cr alloyed cementite magnetic property is lower than Mn alloyedcementite.