| 1. |
- Punkkinen, Marko Patrick John, et al.
(author)
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Adhesion of the iron-chromium oxide interface from first-principles theory
- 2013
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In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:49, s. 495501-
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Journal article (peer-reviewed)abstract
- We determine the interface energy and the work of separation of the Fe/Cr2O3 interface using first-principles density functional theory. Starting from different structures, we put forward a realistic interface model that is suitable to study the complex metal-oxide interaction. This model has the lowest formation energy and corresponds to an interface between Fe and oxygen terminated Cr2O3. The work of separation is calculated to be smaller than the intrinsic adhesion energy of pure Fe or Cr2O3, suggesting that stainless steel surfaces should preferentially break along the metal-oxide interface. The relative stabilities and magnetic interactions of the different interfaces are discussed. Next we introduce Cr atoms into the Fe matrix at different positions relative to the interface. We find that metallic Cr segregates very strongly to the (FeCr)/Cr2O3 interface, and increases the separation energy of the interface, making the adhesion of the oxide scale mechanically more stable. The Cr segregation is explained by the enthalpy of formation.
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| 2. |
- Lorand, Delczeg, 1980-
(author)
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Density functional study of mono-vancacies in metals and austenitic steel alloys
- 2013
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Doctoral thesis (other academic/artistic)abstract
- Trough the following pages a comprehensive study of open structures will be shown, including mono-vacancy calculations and open surfaces. These are electronic structure calculations using density functional theory within the exact muffin tin method.First I investigate the accuracy of five common density functional approximations for the theoretical description of the formation energy of mono-vacancies in three close packed metals. Besides the local density approximation (LDA), I consider two generalized gradient approximation developed by Perdew and co-workers (PBE and PBEsol) and two gradient-level functionals obtained within the subsystem functional approach (AM05 and LAG). As test cases, I select aluminium, nickel and copper, all of them adopting the face centered cubic crystallographic structure.This investigation is followed by a performance comparison of the three common gradient level exchange-correlation functionals for metallic bulk, surface and vacancy systems. I find that approximations which by construction give similar results for the jellium surface, show large deviations for realistic systems. The particular charge density and density gradient dependence of the exchange-correlation energy densities is shown to be the reason behind the obtained differences. Our findings confirm that both the global (total energy) and the local (energy density) behavior of the exchange-correlation functional should be monitored for a consistent functional design.I also calculate the vacancy formation energies of paramagnetic face centered cubic (fcc) Fe-Cr-Ni alloys as a function of chemical composition. These alloys are well known model systems for low carbon austenitic stainless steels. The theoretical predictions obtained for homogeneous chemistry and relaxed nearest neighbor lattice sites are in line with the experimental observations. In particular, Ni is found to decrease and Cr increase the vacancy formation energy of the ternary system. The results are interpreted in terms of effective chemical potentials. The impact of vacancy on the local magnetic properties of austenitic steel alloys is also investigated.I made a performance comparison of local density and generalized gradient level approach on substitutional defects in five light actinides. This is a complex test for high density calculations to check the weaknesses of the local density approximation against gradient level ones. I believe the existing other gradient level approaches fit our error bar in the obtained data and shows similar trends against the very limited number of experimental data. Based on our ab initio results, I predict that vacancies are more easily formed (more stable) in the fcc(bcc) lattice for U, Np and Pu and in the bcc(fcc) lattice for Th and Pa.
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| 3. |
- Tian, Fuyang, et al.
(author)
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Ab initio investigation of high-entropy alloys of 3d elements
- 2013
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:7, s. 075144-
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Journal article (peer-reviewed)abstract
- Single-phase high-entropy alloys are investigated using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA). Choosing the paramagnetic face-centered-cubic NiCoFeCr alloy as an example, we compare the CPA results with those obtained using the supercell (SC) method. For the equilibrium Wigner-Seitz radius and elastic properties, the single-site mean-field approximation turns out to yield consistent results with the SC approach. Next, we employ the EMTO-CPA method to study the bulk properties of CuNiCoFeCrTix (x = 0.0-0.5,1.0) and NiCoFeCrTi high-entropy alloys. A detailed comparison between the theoretical results and the available experimental data demonstrates that ab initio theory can properly describe the fundamental properties of this important class of engineering alloys. Theory predicts NiCoFeCr and CuNiCoFeCr to be more isotropic and less ductile than the Ti-containing single-phase alloys (CuNiCoFeCrTix with x greater than or similar to 0.4 and NiCoFeCrTi).
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| 4. |
- Tian, Fuyang, et al.
(author)
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Structural stability of NiCoFeCrAlx high-entropy alloy from ab initio theory
- 2013
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:8, s. 085128-
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Journal article (peer-reviewed)abstract
- First-principles alloy theory predicts that at room temperature the paramagnetic NiCoFeCrAlx high entropy alloys adopt the face centered cubic (fcc) structure for x less than or similar to 0.60 and the body centered cubic (bcc) structure for x greater than or similar to 1.23, with an fcc-bcc duplex region in between the two pure phases. The calculated single- and polycrystal elastic parameters exhibit strong composition and crystal structure dependence. Based on the present theoretical findings, it is concluded that alloys around the equimolar NiCoFeCrAl composition have superior mechanical performance as compared to the single-phase regions.
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