First-principles investigations of planar defects

University dissertation from Stockholm : KTH Royal Institute of Technology

Author: Song Lu; Kth.; [2012]

Keywords: Natural Sciences; Naturvetenskap;

Abstract: Two types of planar defects, phase interface and stacking fault, are addressed in thisthesis. The first-principles exact-muffin orbitals method in combination with thecoherent-potential approximation is the main density functional theory (DFT) toolfor our studies. The investigation is mainly carried out for stainless steels which arefundamental materials in modern society. Ferritic and austenitic stainless steels arethe two largest subcategories of stainless steels.In ferritic stainless steels, the interface between Fe-rich α and Cr-rich α′ phasesformed during spinodal phase decomposition is studied. This decomposition isknow to increase the hardness of ferrites, making them brittle (also called the "475◦ Cembrittlement"). We calculate the interfacial energies between the Cr-rich α′ -Fex Cr1−xand Fe-rich α-Fe1−y Cry phases (0 < x, y < 0.35) and show that the formation energyis between ∼0.02 and ∼0.33 J m−2 for the ferromagnetic state and between ∼0.02and ∼0.27 J m−2 for 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 Feand Cr atoms near the interface. The theoretical results are applied to estimate thecritical grain size for nucleation and growth in Fe-Cr stainless steel alloys.In close-packed alloys possessing the face centered cubic crystallographic lattice ,stacking faults are very common planar defects. The formation energy of a stackingfault, named stacking fault energy, is related to a series of mechanical properties.Intrinsic stacking fault energy for binary Pd-Ag, Pd-Cu, Pt-Cu and Ni-Cu solid so-lutions are calculated using the axial interaction model and the supercell model. Bycomparing with experimental data, we show that the two models yield consistentformation energies. For Pd-Ag, Pd-Cu and Ni-Cu, the theoretical SFEs agree wellwith those from the experimental measurements. For Pt-Cu no experimental resultsare 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 plasticdeformation experiments and show that the dmin values can be evaluated from firstprinciples 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 withvarious 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 Mndecreases the SFE at 0 K, but at room temperature it increases the SFE in high-Ni (cNi16%) alloys. The SFE always decreases with increasing Co. Niobiumincreases the SFE significantly in low-Ni alloys, however this effect is strongly di-minished in high-Ni alloys. The SFE-enhancing effect of Ni usually observed inFe-Cr-Ni alloys is inverted to SFE-decreasing effect in the hypothetical alloys con-taining more than 3% Nb in solid solution. The revealed nonlinear compositionivdependencies are explained in terms of the peculiar magnetic contributions to thetotal SFE.

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