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1.
  • Al-Zoubi, Noura, et al. (författare)
  • Influence of Magnesium on hydrogenated ScAl1-xMgx alloys : a theoretical study
  • 2011
  • Ingår i: Computational materials science. - 0927-0256. ; 50:10, s. 2848-2853
  • Tidskriftsartikel (refereegranskat)abstract
    • Ab initio total energy calculations, based on the projector augmented wave method and the exact mu±n-tin orbitals method in combination with the coherent-potential approximation, are used to examine the effect of magnesium on hydrogen absorption/desorption temperature and phase stability of hydrogenated ScAl1-xMgx (0 ≤ x ≤ 0:3) alloys. According to the  experiments, ScAl1-xMgx adopts the CsCl structure, and upon hydrogen absorption it decomposes into ScH2 with CaF2 structure and Al-Mg with face centered cubic structure. Here we demonstrate that the stability field of the hydrogenated alloys depends sensitively on Mg content and on the microstructure of the decomposed system. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with Mg concentration.
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2.
  • Al-Zoubi, Noura, et al. (författare)
  • Tetragonality of carbon-doped ferromagnetic iron alloys : A first-principles study
  • 2012
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 85:1, s. 014112
  • Tidskriftsartikel (refereegranskat)abstract
    • Using density-functional theory in combination with the exact muffin-tin orbital (EMTO) method and coherent potential approximation, we investigate the alloying effect on the tetragonality of Fe-C solid solution forming the basis of steels. In order to assess the accuracy of our approach, first we perform a detailed study of the performance of the EMTO method for the Fe(16)C(1) binary system by comparing the EMTO results to those obtained using the projector augmented wave method. In the second step, we introduce different substitutional alloying elements (Al, Cr, Co, Ni) into the Fe matrix and study their impact on the structural parameters. We demonstrate that a small amount of Al, Co, and Ni enhances the tetragonal lattice ratio of Fe(16)C(1) whereas Cr leaves the ratio almost unchanged. The obtained trends are correlated with the single-crystal elastic parameters calculated for carbon-free alloys.
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3.
  • Al-Zoubi, Noura, et al. (författare)
  • The bain path of paramagnetic Fe-Cr based alloys
  • 2011
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 110:1, s. 013708
  • Tidskriftsartikel (refereegranskat)abstract
    • Employing the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, we calculated the total energy and local magnetic moments of paramagnetic Fe-Cr-M (M = Cr, Mn, Fe, Co, Ni) alloys along the tetragonal distortion (Bain) path connecting the body centered cubic (bcc) and the face centered cubic (fcc) structures. The paramagnetic phase is modeled by the disordered local magnetic moment scheme. For all alloys, the local magnetic moments on Fe atoms decrease from the maximum value corresponding to the bcc phase toward the minimum value realized for the fcc phase. Cobalt atoms have non-vanishing local magnetic moments only for tetragonal lattices with c/a < 1.30, whereas the local magnetic moments of Mn show weak crystal structure dependence. We find that Cr stabilizes the bcc lattice and increases the energy barrier as going from the bcc toward the fcc phase. Both Co and Ni favor the fcc lattice and decrease the energy barrier relative to the bcc phase. On the other hand, the tetragonal distortion around the fcc phase is facilitated by Cr and to a somewhat lesser extent also by Ni, but strongly impeded by Co. Manganese has negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. Our findings on the alloying induced softening or hardening of Fe-Cr based alloys against tetragonal distortions are important for understanding the interstitial driven martensitic transformations in alloy steels.
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4.
  • Vitos, Levente, et al. (författare)
  • Stainless Steel Alloys from First-principles Theory
  • 2012
  • Ingår i: La Metallurgia Italiana. - 0026-0843. ; :5, s. 19-27
  • Tidskriftsartikel (refereegranskat)abstract
    • Gaining an accurate description of materials obviously requires the most advanced atomic-scale techniques from both experimental and theoretical areas. In spite of the vast number of available techniques, however; the experimental study of the atomic-scale properties and phenomena even in simple solids is rather difficult. In steels the challenges become more complex due to the interplay between the structural, chemical and magnetic effects. On the other hand, advanced computational methods based on density functional theory ensure a proper platform for studying the fundamental properties of steel materials from first-principles. Our group at the Royal Institute of Technology in Stockholm has an international position in developing and applying computational codes for such applications. Using our ab initio tools, we have presented an insight to the electronic and magnetic structure, and micromechanical properties of austenitic stainless steel alloys. In the present contribution, we review the most important developments within the ab initio quantum-mechanics-aided steel design with special emphasis on the role of magnetism on the fundamental properties of alloy steels.
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5.
  • Zhang, Hualei, et al. (författare)
  • Alloying effects on the elastic parameters of ferromagnetic and paramagnetic Fe from first-principles theory
  • 2011
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 110:7, s. 073707
  • Tidskriftsartikel (refereegranskat)abstract
    • The elastic properties of paramagnetic face-centered-cubic (fcc) Fe(1-x)M(x) (M = Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) random alloys are investigated using the exact muffin-tin orbitals density functional method in combination with the coherent-potential approximation. We find that the theoretical lattice parameter of fcc Fe is strongly enlarged by Al, V, and Rh and slightly reduced by Si, Cr, and Co, while it remains nearly constant with Mn and Ni. Both positive and negative alloying effects appear for the elastic constants C(ij)(x) of fcc Fe. These findings are in contrast to those obtained for ferromagnetic body-centered-cubic (bcc) Fe alloys, where all alloying elements considered here are predicted to enlarge the lattice parameter and decrease the C(11)(x) and C(12)(x) elastic constants of bcc Fe. With some exceptions, alloying has much larger effects on ferromagnetic bcc alloys than on paramagnetic fcc ones. Based on the theoretical elastic parameters of the paramagnetic fcc and ferromagnetic bcc phases, simple parameterizations in terms of chemical composition of the equilibrium lattice constants, single-crystal elastic constants, and polycrystalline elastic moduli of Fe-based alloys are presented.
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6.
  • Vitos, Levente, et al. (författare)
  • Mechanical properties and magnetism: stainless steel alloys from first-principles theory
  • 2011
  • Ingår i: 2010 MRS Fall Meeting. - 978-161839509-2 ; s. 68-79
  • Konferensbidrag (refereegranskat)abstract
    • Stainless steels are among the most important engineering materials, finding their principal scope in industry, specifically in cutlery, food production, storage, architecture, medical equipment, etc. Austenitic stainless steels form the largest sub-category of stainless steels having as the main building blocks the paramagnetic substitutional disordered Fe-Cr-Ni-based alloys. Because of that, austenitic steels represent the primary choice for non-magnetic engineering materials. The presence of the chemical and magnetic disorder hindered any previous attempt to calculate the fundamental electronic, structural and mechanical properties of austenitic stainless steels from first-principles theories. Our ability to reach an ab initio atomistic level approach in this exciting field has become possible by the Exact Muffin-Tin Orbitals (EMTO) method. This method, in combination with the coherent potential approximation, has proved an accurate tool in the description of the concentrated random alloys. Using the EMTO method, we presented an insight to the electronic and magnetic structure, and micromechanical properties of austenitic stainless steel alloys. In the present contribution, we will discuss the role of magnetism on the stacking fault energies and elastic properties of paramagnetic Fe-based alloys.
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