| 1. |
- Li, Xiaojie, et al.
(författare)
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Ab initio calculations of mechanical properties of bcc W-Re-Os random alloys : effects of transmutation of W
- 2016
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 28:29
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Tidskriftsartikel (refereegranskat)abstract
- To examine the effect of neutron transmutation on tungsten as the first wall material of fusion reactors, the elastic properties of W1-x-yRexOsy (0 <= x, y <= 6%) random alloys in body centered cubic (bcc) structure are investigated systematically using the all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA). The calculated lattice constant and elastic properties of pure W are consistent with available experiments. Both Os and Re additions reduce the lattice constant and increase the bulk modulus of W, with Os having the stronger effect. The polycrystalline shear modulus, Young's modulus and the Debye temperature increase (decrease) with the addition of Re (Os). Except for C-11, the other elastic parameters including C-12, C-44, Cauchy pressure, Poisson ratio, B/G, increase as a function of Re and Os concentration. The variations of the latter three parameters and the trend in the ratio of cleavage energy to shear modulus for the most dominant slip system indicate that the ductility of the alloy enhances with increasing Re and Os content. The calculated elastic anisotropy of bcc W slightly increases with the concentration of both alloying elements. The estimated melting temperatures of the W-Re-Os alloy suggest that Re or Os addition will reduce the melting temperature of pure W solid. The classical Labusch-Nabarro model for solid-solution hardening predicts larger strengthening effects in W1-yOsy than in W1-xRex. A strong correlation between C' and the fcc-bcc structural energy difference for W1-x-yRexOsy is revealed demonstrating that canonical band structure dictates the alloying effect on C'. The structural energy difference is exploited to estimate the alloying effect on the ideal tensile strength in the [0 0 1] direction.
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| 2. |
- Li, Xiaoqing, et al.
(författare)
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Anomalous ideal tensile strength of ferromagnetic Fe and Fe-rich alloys
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:2
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Tidskriftsartikel (refereegranskat)abstract
- Within the same failure mode, iron has the lowest ideal tensile strength among the transition metals crystallizing in the body-centered cubic structure. Here, we demonstrate that this anomalously low strength of Fe originates partly from magnetism and is reflected in unexpected alloying effects in dilute Fe(M) (M = Al, V, Cr, Mn, Co, Ni) binaries. We employ the structural energy difference and the magnetic pressure to disentangle the magnetic effect on the ideal tensile strength from the chemical effect. We find that the investigated solutes strongly alter the magnetic response of the Fe host from the weak towards a stronger ferromagnetic behavior, which is explained based on single-particle band energies.
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| 3. |
- Li, Xiaoqing, et al.
(författare)
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Elastic anharmonicity of bcc Fe and Fe-based random alloys from first-principles calculations
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 95:2
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Tidskriftsartikel (refereegranskat)abstract
- We systematically investigate elastic anharmonic behavior in ferromagnetic body-centered cubic (bcc) Fe and Fe1-xMx (M = Al, V, Cr, Co, or Ni) random alloys by means of density-functional simulations. To benchmark computational accuracy, three ab initio codes are used to obtain the complete set of second-and third-order elastic constants (TOECs) for bcc Fe. The TOECs of Fe1-xMx alloys are studied employing the first-principles alloy theory formulated within the exact muffin-tin orbital method in combination with the coherent-potential approximation. It is found that the alloying effects on C-111, C-112, and C-123, which are governed by normal strains only, are more pronounced than those on C-144, C-166, and C-456, which involve shear strains. Remarkably, the magnitudes of all TOECs but C-123 decrease upon alloying with Al, V, Cr, Co, or Ni. Using the computed TOECs, we study compositional effects on the pressure derivatives of the effective elastic constants (dB(ij)/dP), bulk (dK/dP), and shear moduli (dG/dP) and derive longitudinal acoustic nonlinearity parameters (beta). Our predictions show that the pressure derivatives of K and G decrease with x for all solute elements and reveal a strong correlation between the compositional trends on dK/dP and dG/dP arising from the fact that alloying predominantly altersdB(11)/dP. The sensitivity of dB(11)/dP to composition is attributed to intrinsic alloying effects as opposed to lattice parameter changes accompanying solute addition. For Fe and the considered Fe-based alloys, beta along high-symmetry directions orders as beta[111] > beta[100] > beta[110], and alloying increases the directional anisotropy of beta but reduces its magnitude.
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| 4. |
- Li, Xiaoqing, et al.
(författare)
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Elastic properties of vanadium-based alloys from first-principles theory
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:1, s. 014105-
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Tidskriftsartikel (refereegranskat)abstract
- The effect of Cr and Ti on the fundamental mechanical properties of V-Cr-Ti alloys has been investigated using the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The static lattice constant and elastic parameters have been calculated for the body-centered-cubic V1-x-yCrxTiy (0 <= x,y <= 0.1) random solid solution as a function of composition. Our theoretical predictions are in good agreement with the available experimental data. Alloys along the equicomposition region are found to exhibit the largest shear and Young's modulus as a result of the opposite alloying effects obtained for the two cubic shear elastic constants. The classical solid-solution hardening (SSH) model predicts larger strengthening effect in V1-yTiy than in V1-xCrx. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, it is shown that the present theoretical results can account for the variations of DBTT with composition.
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| 5. |
- Li, Xiaojie, et al.
(författare)
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First-principles study of crystal-face specificity in surface properties of Fe-rich Fe-Cr alloys
- 2019
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Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 3:3
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Tidskriftsartikel (refereegranskat)abstract
- A density-functional theory investigation of the (100) and (110) surfaces of the body-centered cubic (bcc) Fe1-xbCrxb binary alloys, x(b) <= 15 at.%, is reported. The energies and segregation energies of these surfaces were calculated for chemically homogeneous concentration profiles and for Cr surface contents deviating from the nominal one of the bulk. The implications of these results for the surface alloy phase diagram are discussed. The surface chemistry of Fe-Cr(100) is characterized by a transition from Cr depletion to Cr enrichment in a critical bulk Cr composition window of 6 < x(b) < 9 at.%. In contrast, such threshold behavior of the surface Cr content is absent for Fe-Cr(110) and a nearly homogeneous Cr concentration profile is energetically favorable. The strongly suppressed surface-layer relaxation at both surfaces is shown to be of magnetic origin. The compressive, magnetic contribution to the surface relaxation stress is found to correlate well with the surface magnetic moment squared at both surface terminations. The stability of the Cr surface magnetic moments against bulk Cr content is clarified based on the surface electronic structure.
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| 6. |
- Li, Xiaoqing, et al.
(författare)
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Ideal strength of random alloys from first principles
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:21, s. 214203-
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Tidskriftsartikel (refereegranskat)abstract
- The all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation was employed to investigate the ideal tensile strengths of elemental V and Mo solids, and V-and Mo-based random solid solutions. Under uniaxial [001] tensile loading, the ideal tensile strength of V is 11.6 GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ideal tensile strengths are 26.7 and 37.6 GPa for V in the [111] and [110] directions, respectively. The ideal strength of Mo is 26.7 GPa in the [001] direction and decreases when a few percent of Tc is introduced in Mo. For the V-based alloys, Cr increases and Ti decreases the ideal tensile strength in all principal directions. Adding the same concentration of Cr and Ti to V leads to ternary alloys with similar ideal strength values as that of pure V. The alloying effects on the ideal strength are explained using the electronic band structure.
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| 7. |
- Li, Xiaoqing, et al.
(författare)
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Tensile strain-induced softening of iron at high temperature
- 2015
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- In weakly ferromagnetic materials, already small changes in the atomic configuration triggered by temperature or chemistry can alter the magnetic interactions responsible for the non-random atomicspin orientation. Different magnetic states, in turn, can give rise to substantially different macroscopic properties. A classical example is iron, which exhibits a great variety of properties as one gradually removes the magnetic long-range order by raising the temperature towards its Curie point of T-C(degrees) = 1043 K. Using first-principles theory, here we demonstrate that uniaxial tensile strain can also destabilise the magnetic order in iron and eventually lead to a ferromagnetic to paramagnetic transition at temperatures far below T-C(degrees). In consequence, the intrinsic strength of the ideal single-crystal body-centred cubic iron dramatically weakens above a critical temperature of similar to 500 K. The discovered strain-induced magneto-mechanical softening provides a plausible atomic-level mechanism behind the observed drop of the measured strength of Fe whiskers around 300-500 K. Alloying additions which have the capability to partially restore the magnetic order in the strained Fe lattice, push the critical temperature for the strength-softening scenario towards the magnetic transition temperature of the undeformed lattice. This can result in a surprisingly large alloying-driven strengthening effect at high temperature as illustrated here in the case of Fe-Co alloy.
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| 8. |
- Tian, Li-Yun, et al.
(författare)
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Elastic constants of random solid solutions by SQS and CPA approaches : the case of fcc Ti-Al
- 2015
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:31
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Tidskriftsartikel (refereegranskat)abstract
- Special quasi-random structure (SQS) and coherent potential approximation (CPA) are techniques widely employed in the first-principles calculations of random alloys. Here we scrutinize these approaches by focusing on the local lattice distortion (LLD) and the crystal symmetry effects. We compare the elastic parameters obtained from SQS and CPA calculations, taking the random face-centered cubic (fcc) Ti1-xAlx (0 <= x <= 1) alloy as an example of systems with components showing different electronic structures and bonding characteristics. For the CPA and SQS calculations, we employ the Exact Muffin-Tin Orbitals (EMTO) method and the pseudopotential method as implemented in the Vienna Ab initio Simulation Package (VASP), respectively. We show that the predicted trends of the VASP-SQS and EMTO-CPA parameters against composition are in good agreement with each other. The energy associated with the LLD increases with x up to x = 0.625 similar to 0.750 and drops drastically thereafter. The influence of the LLD on the lattice constants and C12 elastic constant is negligible. C-11 and C-44 decrease after atomic relaxation for alloys with large LLD, however, the trends of C-11 and C-44 are not significantly affected. In general, the uncertainties in the elastic parameters associated with the symmetry lowering turn out to be superior to the differences between the two techniques including the effect of LLD.
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| 9. |
- Fu, Jie, et al.
(författare)
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Improved Finnis-Sinclair potential for vanadium-rich V-Ti-Cr ternary alloys
- 2017
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 705, s. 369-375
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Tidskriftsartikel (refereegranskat)abstract
- We have developed an improved Finnis-Sinclair (IFS) potential for vanadium-rich V-Ti-Cr random alloys with body-centred cubic structure. An extra exponential term is added to the original FS potential to enhance the repulsive interaction. The IFS potential is fitted to experimental crystal structure, cohesive energy and elastic constants of pure metals (V, Ti and Cr) and theoretical data of binary alloys (V15Ti, V15Cr and Ti8Cr8). The good agreement of the predicted formation energies of mono-vacancy and selfinterstitial of octahedral interstitial site, tetrahedral interstitial site, < 111>-dumbbell, < 110>-dumbbell, and < 100>-dumbbell with available experimental and theoretical data confirms the validity of our IFS potential in pure V. Furthermore, the agreement of elastic properties and defect properties of typical alloy (V-4-Ti-4-Cr) with experimental or DFT data also support the applicability of the IFS potential in Vrich ternary V-Ti-Cr alloys. Finally, this work also provides a reference to develop empirical potentials for other ternary alloys.
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| 10. |
- Li, Xiaoqing, et al.
(författare)
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Mechanical properties and defective effects of bcc V-4Cr-4Ti and V-5Cr-5Ti alloys by first-principles simulations
- 2011
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Ingår i: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 50:9, s. 2727-2731
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Tidskriftsartikel (refereegranskat)abstract
- V-(4-5) wt.% Cr-(4-5) wt.% Ti alloys are important candidate structural materials for the first-wall and blanket in future fusion reactor. Thus it is necessary to study the fundamental mechanical properties and the irradiation effects of the V-based alloys. Within a random solid solution model, the elastic constants and ideal strength of the V-4Cr-4Ti and the V-5Cr-5Ti alloys were calculated and compared with those of pure V solid. According to the theoretical Cauchy pressure and the ratio of bulk modulus and shear modulus, both alloys exhibit good ductility. Within the 250-atom supercell, inclusion of one vacancy defect or one interstitial H (He) atom will further enhance the ductility of these alloys.
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