First-principles investigations of planar defects

Abstract: Two types of planar defects, phase interface and stacking fault, are addressed in this thesis. The investigation is mainly carried out for stainless steels which are fundamental materials in modern society. For the phase interface, we investigate the metallic bcc/bcc and fcc/bcc phase interfaces.The bcc/bcc phase interface in ferrite steels and the stacking fault in austenite steels are studied, respectively. The interfaces between fcc and bcc phases are studied for the model Fe/Ag system, the methods used in this case may be expanded and adopted for studying the interface between ferrite and austenite in duplex stainless steels in future. Stacking faults in some binary metallic systems are also investigated. The first-principles exact-muffin orbitals method (EMTO) in combination with the coherent-potential approximation (CPA) and the Vienna Ab initio Simulation Package (VASP) are the main density functional theory (DFT) tools for our studies.In ferritic stainless steels, the interface between Fe-richand Cr-rich′phases formed during spinodal phase decomposition is studied. This decomposition is known to increase the hardness of ferrites, making them brittle (also called the ”475◦Cembrittlement”). We calculate the interfacial energies between the Cr-rich α′-FexCr1−x and Fe-rich α-Fe1−yCry phases (0−2for the ferromagnetic state and between∼0.02 and∼0.27 J m−2for the paramagnetic state. Although for both magnetic states, the interfacial energy follows a general decreasing trend with increasing x and y, the fine structures of the γ (x, y)maps exhibit a marked magnetic state dependence. The subtleties are shown to be ascribed to the magnetic interaction between the Fe and Cr atoms near the interface. The theoretical results are applied to estimate the critical grain size for nucleation and growth in Fe-Cr stainless steel alloys.For the fcc/bcc interface, because of the difficulty to model a realistic semicoherent interface with misfit dislocations, alternatively, we perform ab initio calculations to determine the lower and upper bounds of the interfacial energy and work of separation of fcc-Ag/bcc-Fe interface. The strain-free interfacial energy of the coherent interface is taken as the lower bound and the interfacial energy of the commensurate incoherent interface as the upper bound of the interfacial energy of a realistic semicoherent interface. The latter is estimated by applying an averaging scheme based on the interfacial energies obtained for the coherent interfaces. Similar calculations are performed for determining the bounds of the work of separation. We justify the use of the averaging scheme by carrying out large supercell calculations for a semicoherent interface (not realistic either). For a Fe(110)/Ag(111) semicoherent interface, we show that taking either Fe or Ag as the underlying lattice, our averaging scheme can yield a reasonable estimation of the work of separation of the semico-herent interface. However, when taking Ag as the underlying lattice the averaged interfacial energy of the semicoherent interface is significantly underestimated due to the magnetism. The structure and magnetism at the coherent and semicoherent interfaces are discussed.In close-packed alloys possessing the face centered cubic crystallographic lattic ,stacking faults are very common planar defects. The formation energy of a stacking fault, named stacking fault energy (SFE), is related to a series of mechanical properties.Intrinsic stacking fault energy for binary Pd-Ag, Pd-Cu, Pt-Cu and Ni-Cu solid solutions are calculated using the axial interaction model and the supercell model. By comparing with experimental data, we show that the two models yield consistent formation energies. For Pd-Ag, Pd-Cu and Ni-Cu, the theoretical SFEs agree well with those from the experiments. For Pt-Cu no experimental results are available, and thus our calculated SFEs represent the first reasonable predictions. We also discuss the correlation of the SFE and the minimum dmin in severe plastic deformation experiments and show that the dmin values can be evaluated from first principles calculations.After gaining confidence with the axial interaction model, the alloying effects of Mn, Co, and Nb on the stacking fault energy of austenitic stainless alloys, Fe-Cr-Ni with various Ni content, are investigated. In the composition range (cCr= 20%;8≤cNi≤20%;0≤cMn;cCo;cNb≤8%, balance Fe) studied here, it is found that Mn decreases the SFE at 0 K, but at room temperature it increases the SFE in high-Ni (cNi≥16%) alloys. The SFE always decreases with increasing Co. Niobium increases the SFE significantly in low-Ni alloys, however this effect is strongly diminished in high-Ni alloys. The SFE-enhancing effect of Ni usually observed in Fe-Cr-Ni alloys is inverted to SFE-decreasing effect in the hypothetical alloys containing more than 3% Nb in solid solution. The revealed nonlinear composition dependencies are explained in terms of the peculiar magnetic contributions to the total SFE

  CLICK HERE TO DOWNLOAD THE WHOLE DISSERTATION. (in PDF format)