| As a new class of high-performance structural materials with good strength andtoughness matching, austenitic stainless steel with high content of Cr and Mn haswidely industrial applications, and18Mn-18Cr-0.5N steel used in retaining rings ofgenerators, is a typical representative of it. In actual production, the strength andtoughness of materials and are influenced by crystal defects forms in CrMn austeniticstainless steel due to the thermal motion of the atoms, crystals formation conditionsand other conditions. Studies about the toughening mechanism of CrMn austeniticstainless steels are mostly performed from traditional means of strengthening, such asgrain refinement, dislocation strengthening, solid solution strengthening andprecipitation strengthening. While so far, there are few reports about CrMn austeniticcrystal defects simulation and theoretical prediction from the atomic level using first-principles method.In this thesis, we first calculate the formation energy, binding energy andelectronic properties of three kind of models in pure γ-Fe including vacancy model,solute-solute model and vacancies-solute model by using first-principles method.Single-vacancy, Divacancy and Trivacancy models in the CrMn steel (18Mn-18Cr)without alloying element N are built and optimized based on the basic properties of theγ-Fe point defects. The formation energy, binding energy, electronic properties andmechanical properties of these models are calculated and analyzed. The main resultsobtained are as follows:In the vacancy model of pure γ-Fe, the formation and binding energy three kind ofaustentic structures with single-vacancy, divacancy and trivacancy models increasedgradually with the number of vacancies increasing, indicating that the more thevacancies, the easier the formation of them, and they tend to form vacancy clusters.The second neighbor divacancy and linear trivacanciy can exist in γ-Fe more easilythan the first neighbor divacancy and triangular trivacancies, respectively.Mn is more easily dissolved in pure γ-Fe than Cr, and the structure is stable aftersolid solution. Generally speaking, the ease of formation of the models is: Mn-Mn>Cr-Cr> Cr-Mn, and the structure stability order is: Mn-Mn> Cr-Mn> Cr-Cr. In pureγ-Fe,“Mn-Mn” first nearest structural model most likely to form, followedby second nearest structural model, and the third nearest model is the most difficult toform.“Cr-Cr” and “Cr-Mn” second nearest structural model is the most difficult toform, and the stability is worse. The first and third nearest structural model are likelyto be present in the structural model of γ-Fe.The vacancy-Mn model is more likely to be present in γ-Fe than vacancy-Crmodel. The magnetic moments of the three vacancy-Cr models, namely first, secondand third neareast neighbor models, are2.01μB/cell-32,7.52μB/cell-32and2.99μB/cell-32, respectively. While the magnetic moments of the three vacancy-Mnmodels are2.35μB/cell-32,3.29μB/cell-32and0.73μB/cell-32, respectively.The most stable Fe20Cr6Mn6is obtained in this study. Cr vacancy is the mostlikely to form, and the structure is stable, while Fe vacancy is the least likely to form,and the structure is the most unstable.The magnetic moment of Cr, Fe and Mn vacancyare9.80μB/cell-32，4.78μB/cell-32and0μB/cell-32, respectively.The linear trivacancy structure in Fe20Cr6Mn6is easier to form than thesingle-vacancy and divacancy structure and the structer is stable. The magneticmoment of divacancy and linear trivacancy structures are6.85μB/cell-32and0μB/cell-32, respectively. The linear trivacancy structure has no effect on themagnetism of the stainless steel.The B/G values of Mn, Fe and Cr single-vacancy structure in Fe20Cr6Mn6are1.88,1.57and1.50, respectively. The plastic of the three kind of CrMn austentic vacancystructure changes in decending order. The B/G values of divacancy and lineartrivacancy structure are2.27and2.40, larger than that of perfect crystal. The plastic oflinear trivacancy CrMn austentic structure is the best. |
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