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First-principles investigations of planar defects
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , viii, 45 p.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:kth:diva-93759ISBN: 978-91-7501-313-8 (print)OAI: oai:DiVA.org:kth-93759DiVA: diva2:523694
Presentation
2012-05-08, Sal N111, Hall 1, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20120426Available from: 2012-04-26 Created: 2012-04-25 Last updated: 2012-05-02Bibliographically approved
List of papers
1. First-principles determination of the alpha-alpha ' interfacial energy in Fe-Cr alloys
Open this publication in new window or tab >>First-principles determination of the alpha-alpha ' interfacial energy in Fe-Cr alloys
Show others...
2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 19, 195103- p.Article in journal (Refereed) Published
Abstract [en]

The interfacial energies (gamma) between the Cr-rich alpha'-FexCr1-x and Fe-rich alpha-Fe1-yCry phases (0 < x, y < 0.35) are calculated to be between similar to 0.02 and similar to 0.33 J m(-2) for the ferromagnetic state and between similar to 0.02 and similar to 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 gamma(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.

Keyword
DUPLEX STAINLESS-STEELS, SPINODAL DECOMPOSITION, PHASE-SEPARATION, ATOM-PROBE, MOSSBAUER, SYSTEM, IRON, TEMPERATURE, SCATTERING, MAGNETISM
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-27054 (URN)10.1103/PhysRevB.82.195103 (DOI)000283709100002 ()2-s2.0-78649743969 (Scopus ID)
Note
QC 20101213Available from: 2010-12-13 Created: 2010-12-06 Last updated: 2017-12-11Bibliographically approved
2. Composition and orientation dependence of the interfacial energy in Fe-Cr stainless steel alloys
Open this publication in new window or tab >>Composition and orientation dependence of the interfacial energy in Fe-Cr stainless steel alloys
2011 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 248, no 9, 2087-2090 p.Article in journal (Refereed) Published
Abstract [en]

Using a first-principles quantum mechanical method, we calculated the (001) and (110) interfacial energies between the low temperature alpha and alpha' phases of Fe-Cr alloys as functions of chemical composition. Weshow that the interfacial energies and the interfacial energy anisotropy are highly composition dependent. In particular, the increasing interfacial energy anisotropy with decreasing compositional gap may induce different morphology of the decomposed phases for different compositions of the host alloys.

Keyword
anisotropy, interfacial energy, stainless steels, ab initio
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-46198 (URN)10.1002/pssb.201147123 (DOI)000295367500015 ()2-s2.0-80052082179 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20111102Available from: 2011-11-02 Created: 2011-11-02 Last updated: 2017-12-08Bibliographically approved
3. Determining the minimum grain size in severe plastic deformation process via first-principles calculations
Open this publication in new window or tab >>Determining the minimum grain size in severe plastic deformation process via first-principles calculations
Show others...
2012 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 60, no 11, 4506-4513 p.Article in journal (Refereed) Published
Abstract [en]

Although the stacking fault energy (SFE) is a fundamental variable determining the minimum grain size (d(min)) obtainable in severe plastic deformation (SPD) processes, its accurate measurement is difficult. Here we establish the SFEs of binary Pd-Ag, Pd-Cu, Pt-Cu and Ni-Cu solid solutions using the axial interaction model and the supercell model in combination with first-principles theory. The two models yield consistent formation energies. For Pd-Ag, Pd-Cu and Ni-Cu, the theoretical SFEs agree well with those from the experimental measurements. For Pt-Cu no experimental results are available, and thus our calculated SFEs represent the first reasonable predictions. We discuss the correlation of the SFE and d(min), in SPD experiments and show that the d(min) values can be evaluated from first-principles calculations.

Keyword
Minimum grain size, Stacking fault energy, First-principles theory, Severe plastic deformation
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-93764 (URN)10.1016/j.actamat.2012.04.024 (DOI)000306621300015 ()2-s2.0-84861853886 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council
Note

QC 20120824

Available from: 2012-04-26 Created: 2012-04-26 Last updated: 2017-12-07Bibliographically approved
4. Stacking fault energies of Mn, Co and Nb alloyed austenitic stainless steels
Open this publication in new window or tab >>Stacking fault energies of Mn, Co and Nb alloyed austenitic stainless steels
2011 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 14, 5728-5734 p.Article in journal (Refereed) Published
Abstract [en]

The alloying effects of Mn, Co and Nb on the stacking fault energy (SFE) of austenitic stainless steels, Fe-Cr-Ni with various Ni contents, are investigated via quantum-mechanical first-principles calculations. In the composition range (c(Cr) = 20%, 8 <= c(Ni) <= 20%, 0 <= c(Mn), c(Co), c(Nb) <= 8%, balance Fe) studied here, it is found that Mn always decreases the SFE at 0 K but increases it at room temperature in high-Ni (c(Ni) greater than or similar to 16%) alloys. The SFE always decreases with increasing Co content. 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 an SFE-decreasing effect by Nb for c(Nb) greater than or similar to 3%. The revealed nonlinear composition dependencies are explained in terms of the peculiar magnetic contributions to the total SFE.

Keyword
Stacking fault energy, First-principles electron theory, Austenitic stainless steels
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-39520 (URN)10.1016/j.actamat.2011.05.049 (DOI)000294091400034 ()2-s2.0-79960412098 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council
Available from: 2011-09-20 Created: 2011-09-12 Last updated: 2017-12-08Bibliographically approved

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