Research

10 pages, 8692 KiB

Open AccessArticle

Robust Spin-Gapless Behavior in the Newly Discovered Half Heusler Compound MnPK

by Jiaxue You, Jieting Cao, Rabah Khenata, Xiaotian Wang, Xunan Shen and Tie Yang

Materials 2019, 12(19), 3117; https://doi.org/10.3390/ma12193117 - 25 Sep 2019

Cited by 8 | Viewed by 2279

Spin gapless semiconductors have aroused high research interest since their discovery and a lot of effort has been exerted on their exploration, in terms of both theoretical calculation and experimental verification. Among different spin gapless materials, Heusler compounds stand out thanks to their [...] Read more.

Spin gapless semiconductors have aroused high research interest since their discovery and a lot of effort has been exerted on their exploration, in terms of both theoretical calculation and experimental verification. Among different spin gapless materials, Heusler compounds stand out thanks to their high Curie temperature and highly diverse compositions. Especially, both theoretical and experimental studies have reported the presence of spin gapless properties in this kind of material. Recently, a new class of d⁰ − d Dirac half Heusler compound was introduced by Davatolhagh et al. and Dirac, and spin gapless semiconductivity has been successfully predicted in MnPK. To further expand the research in this direction, we conducted a systematical investigation on the spin gapless behavior of MnPK with both generalized gradient approximation (GGA) and GGA + Hubbard U methods under both uniform and tetragonal strain conditions by first principles calculation. Results show the spin gapless behavior in this material as revealed previously. Different Hubbard U values have been considered and they mainly affect the band structure in the spin-down channel while the spin gapless feature in the spin-up direction is maintained. The obtained lattice constant is very well consistent with a previous study. More importantly, it is found that the spin gapless property of MnPK shows good resistance for both uniform and tetragonal strains, and this robustness is very rare in the reported studies and can be extremely interesting and practical for the final end application. This study elaborates the electronic and magnetic properties of the half Heusler compound MnPK under uniform and tetragonal strain conditions, and the obtained results can give a very valuable reference for related research works, or even further motivate the experimental synthesis of the relative material. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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15 pages, 4840 KiB

Open AccessArticle

Half-Metallic Property Induced by Double Exchange Interaction in the Double Perovskite Bi₂BB′O₆ (B, B′ = 3d Transitional Metal) via First-Principles Calculations

by Hong-Zong Lin, Chia-Yang Hu, Po-Han Lee, Albert Zhong-Ze Yan, Wen-Fang Wu, Yang-Fang Chen and Yin-Kuo Wang

Materials 2019, 12(11), 1844; https://doi.org/10.3390/ma12111844 - 06 Jun 2019

Cited by 17 | Viewed by 3870

Abstract

In this paper, we identify three possible candidate series of half-metals (HM) from Bi-based double perovskites Bi₂BB′O₆ (BB′ = transition metal ions) through calculations utilizing the density functional theory (DFT) and full-structural optimization, in which the generalized [...] Read more.

In this paper, we identify three possible candidate series of half-metals (HM) from Bi-based double perovskites Bi₂BB′O₆ (BB′ = transition metal ions) through calculations utilizing the density functional theory (DFT) and full-structural optimization, in which the generalized gradient approximation (GGA) and the strong correlation effect (GGA + U) are considered. After observing the candidate materials under four types of magnetic states, i.e., ferromagnetic (FM), ferrimagnetic (FiM), antiferromagnetic (AF), and nonmagnetic (NM), we found eight promising candidates for half-metallic materials. Under the GGA scheme, there are three ferromagnetic-half-metal (FM-HM) materials, Bi₂CrCoO₆, Bi₂CrNiO₆ and Bi₂FeNiO₆, and three FiM-HM materials, Bi₂FeZnO₆, Bi₂CrZnO₆ and Bi₂CoZnO₆. With implementation of the Coulomb interaction correction (GGA + U), we find two stable half-metallic materials: Bi₂CrNiO₆ and Bi₂CrZnO₆. We determine that the stability of some of these materials are tied to the double exchange interaction, an indirect interaction within the higher powers of localized spin interaction among transition metals via oxygen ions. Found in half-metallic materials, and especially those in the ferromagnetic (FM) state, the double exchange interaction is recognized in the FM-HM materials Bi₂CrCoO₆ and Bi₂FeNiO₆. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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16 pages, 3156 KiB

Open AccessArticle

An Ab Initio Study of Vacancies in Disordered Magnetic Systems: A Case Study of Fe-Rich Fe-Al Phases

by Ivana Miháliková, Martin Friák, Nikola Koutná, David Holec and Mojmír Šob

Materials 2019, 12(9), 1430; https://doi.org/10.3390/ma12091430 - 02 May 2019

Cited by 11 | Viewed by 2868

Abstract

We have performed quantum-mechanical calculations to examine the impact of disorder on thermodynamic, structural and electronic (magnetic) properties of Fe-Al systems with vacancies. A series of supercells was used and their properties were computed employing density-functional theory (DFT) as implemented in the VASP [...] Read more.

We have performed quantum-mechanical calculations to examine the impact of disorder on thermodynamic, structural and electronic (magnetic) properties of Fe-Al systems with vacancies. A series of supercells was used and their properties were computed employing density-functional theory (DFT) as implemented in the VASP package. Our case study is primarily aimed at a disordered solid solution Fe

_{81.25}

Al

_{18.75}

but we have compared our results also with those obtained for the ordered Fe

_{3}

Al intermetallic compound for which experimental data exist in literature. Both phases are found in Fe-Al-based superalloys. The Fe-18.75at.%Al solid solution was simulated using special quasirandom structures (SQS) in three different disordered states with a different distribution of Al atoms. In particular, we have considered a general disordered case (an A2-like variant), the case without the first nearest neighbor Al-Al pairs (a B2-like distribution of atoms) and also the case without both the first and second nearest neighbor Al-Al pairs (the D0

_{3}

-like variant, in fact, an Fe-rich Fe

_{3}

Al phase). The vacancy formation energies as well as the volumes of (fully relaxed) supercells with vacancies showed a large scatter for the disordered systems. The vacancy formation energies decrease with increasing concentration of Al atoms in the first coordination shell around the vacancy (an anti-correlation) for all disordered cases studied. The computed volumes of vacancies were found significantly lower (by 25–60%) when compared with the equilibrium volume of the missing atoms in their elemental states. Lastly, we have analyzed interactions between the vacancies and the Fe atoms and evaluated vacancy-induced changes in local magnetic moments of Fe atoms. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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10 pages, 18359 KiB

Open AccessArticle

Structure–Property Relationships in Shape Memory Metallic Glass Composites

by Daniel Şopu, Xudong Yuan, Franco Moitzi, Mihai Stoica and Jürgen Eckert

Materials 2019, 12(9), 1419; https://doi.org/10.3390/ma12091419 - 01 May 2019

Cited by 22 | Viewed by 3012

Abstract

Metallic glass composites with shape memory crystals show enhanced plasticity and work-hardening capability. We investigate the influence of various critical structural aspects such as, the density of crystalline precipitates, their distribution and size, and the structural features and intrinsic properties of the phase [...] Read more.

Metallic glass composites with shape memory crystals show enhanced plasticity and work-hardening capability. We investigate the influence of various critical structural aspects such as, the density of crystalline precipitates, their distribution and size, and the structural features and intrinsic properties of the phase on the deformation behavior of metallic amorphous Cu

_{64}

Zr

_{36}

composites with B2 CuZr inclusions using molecular dynamics simulations. We find that a low density of small B2 inclusions with spacing smaller than the critical shear band length controls the formation and distribution of plastic zones in the composite and hinders the formation of critical shear bands. When the free path for shearing allows the formation of mature shear bands a high volume fraction of large B2 precipitates is necessary to stabilize the shear flow and avoid runaway instability. Additionally, we also investigate the deformation mechanism of composites with pure copper crystals for comparison, in order to understand the superior mechanical properties of metallic glass composites with shape memory crystals in more detail. The complex and competing mechanisms of deformation occurring in shape memory metallic glass composites allow this class of materials to sustain large tensile deformation, even though only a low-volume fraction of crystalline inclusions is present. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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11 pages, 400 KiB

Open AccessArticle

Thermal Expansion and Other Thermodynamic Properties of α₂-Ti₃Al and γ-TiAl Intermetallic Phases from First Principles Methods

by David Holec, Neda Abdoshahi, Svea Mayer and Helmut Clemens

Materials 2019, 12(8), 1292; https://doi.org/10.3390/ma12081292 - 19 Apr 2019

Cited by 13 | Viewed by 3063

Abstract

Anisotropic thermal expansion coefficients of tetragonal

γ

-TiAl and hexagonal

α_{2}

-Ti₃Al phases were calculated using first principles methods. Two approaches with different computational costs and degrees of freedom were proposed. The predicted values were compared with available experimental data [...] Read more.

Anisotropic thermal expansion coefficients of tetragonal

γ

-TiAl and hexagonal

α_{2}

-Ti₃Al phases were calculated using first principles methods. Two approaches with different computational costs and degrees of freedom were proposed. The predicted values were compared with available experimental data showing that for

γ

-TiAl, the more computational demanding method with decoupled impact of volume and temperature effects on the cell shape leads to significantly better results than that with only ground-state optimised unit cell geometry. In the case of the

α_{2}

-Ti₃Al phase, both approaches yielded comparable results. Additionally, heat capacity and bulk modulus were evaluated as functions of temperature for both phases, and were fitted to provide an analytical formula which can be further used. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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16 pages, 2750 KiB

Open AccessArticle

Ab Initio Study of Elastic and Mechanical Properties in FeCrMn Alloys

by Vsevolod I. Razumovskiy, Carola Hahn, Marina Lukas and Lorenz Romaner

Materials 2019, 12(7), 1129; https://doi.org/10.3390/ma12071129 - 06 Apr 2019

Cited by 8 | Viewed by 3573

Abstract

Mechanical properties of FeCrMn-based steels are of major importance for practical applications. In this work, we investigate mechanical properties of disordered paramagnetic fcc FeCr

_{10 - 16}

Mn

_{12 - 32}

alloys using density functional theory. The effects of composition and temperature changes [...] Read more.

Mechanical properties of FeCrMn-based steels are of major importance for practical applications. In this work, we investigate mechanical properties of disordered paramagnetic fcc FeCr

_{10 - 16}

Mn

_{12 - 32}

alloys using density functional theory. The effects of composition and temperature changes on the magnetic state, elastic properties and stacking fault energies of the alloys are studied. Calculated dependencies of the lattice and elastic constants are used to evaluate the effect of the solid solution strengthening by Mn and Cr using a modified Labusch-Nabarro model and a model for concentrated alloys. The effect of Cr and Mn alloying on the stacking fault energies is calculated and discussed in connection to possible deformation mechanisms. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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11 pages, 4331 KiB

Open AccessArticle

Experimental Chemistry and Structural Stability of AlNb₃ Enabled by Antisite Defects Formation

by Nikola Koutná, Petra Erdely, Siegfried Zöhrer, Robert Franz, Yong Du, Shuhong Liu, Paul H. Mayrhofer and David Holec

Materials 2019, 12(7), 1104; https://doi.org/10.3390/ma12071104 - 03 Apr 2019

Cited by 10 | Viewed by 3629

Abstract

First-principles evolutionary algorithms are employed to shed light on the phase stability of Al–Nb intermetallics. While the tetragonal Al₃Nb and AlNb₂ structures are correctly identified as stable, the experimentally reported Laves phase of AlNb₃ yields soft phonon modes implying [...] Read more.

First-principles evolutionary algorithms are employed to shed light on the phase stability of Al–Nb intermetallics. While the tetragonal Al₃Nb and AlNb₂ structures are correctly identified as stable, the experimentally reported Laves phase of AlNb₃ yields soft phonon modes implying its dynamical instability at 0 K. The soft phonon modes do not disappear even upon elevating the temperature in the simulation up to 1500 K. X-Ray diffraction patterns recorded for our powder-metallurgically produced arc cathodes, however, clearly show that the AlNb₃ phase exists. We propose that AlNb₃ is dynamically stabilised by ordered antisite defects at the Al sublattice, leading also to a shift of the Nb content from 75 to ∼81 at.%. Unlike the defect-free AlNb₃, the antisite-stabilised variant hence falls into the compositional range consistent with our CALPHAD-based phase diagram as well as with the previous reports. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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20 pages, 5633 KiB

Open AccessArticle

Seismic Behavior of Superelastic Shape Memory Alloy Spring in Base Isolation System of Multi-Story Steel Frame

by Yuping Liu, Hongyang Wang, Canxing Qiu and Xingnan Zhao

Materials 2019, 12(6), 997; https://doi.org/10.3390/ma12060997 - 26 Mar 2019

Cited by 29 | Viewed by 5422

Abstract

Owing to excellent re-centering capability and good damping behavior, superelastic shape memory alloys (SMAs) are emerging as a potential new material to enhance the seismic resilience of civil structures. This paper focuses on using base isolation with SMA device for isolated structures. SMA [...] Read more.

Owing to excellent re-centering capability and good damping behavior, superelastic shape memory alloys (SMAs) are emerging as a potential new material to enhance the seismic resilience of civil structures. This paper focuses on using base isolation with SMA device for isolated structures. SMA springs are deemed to be promising candidate as the damper in the base isolation system, due to the compact form, damping contribution, restoring capability and flexible stiffness. This paper reported the concept of an innovative spring which is made of superelastic SMA wire. Then cyclic loading tests were carried out to evaluate the interested cyclic properties. Parametric analyses based on finite element simulations were conducted to reveal the versatile performance of SMA springs. To further examine its seismic behavior in the base isolation system, the SMA spring was later installed at the isolation level of a multi-story steel frame, based on the finite element model built in the earthquake engineering simulation platform OpenSees. An ordinary elastic spring is included for comparison to highlight the features of SMA springs. Both isolated frames were subjected to real earthquakes. The comparisons indicated that using SMA spring is more effective in controlling maximum and residual deformation for the protected superstructures. Thus, this paper well demonstrated the feasibility and merits of using SMA springs in the isolated frames. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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17 pages, 6546 KiB

Open AccessArticle

First-Principles Study on the Mechanical Properties and Electronic Structure of V Doped WCoB and W₂CoB₂ Ternary Borides

by Tong Zhang, Haiqing Yin, Cong Zhang, Ruijie Zhang, Xue Jiang, Qingjun Zheng and Xuanhui Qu

Materials 2019, 12(6), 967; https://doi.org/10.3390/ma12060967 - 22 Mar 2019

Cited by 21 | Viewed by 2935

Abstract

For the purpose of exploring new hard materials and doping methods, the structural, mechanical and electronic properties of WCoB and W₂CoB₂ ternary boride were investigated with 0, 8.33, 16.67, 25 and 33.33 at.% V doping content and W₂CoB [...] Read more.

For the purpose of exploring new hard materials and doping methods, the structural, mechanical and electronic properties of WCoB and W₂CoB₂ ternary boride were investigated with 0, 8.33, 16.67, 25 and 33.33 at.% V doping content and W₂CoB₂ with 0, 5, 10, 15 and 20 at.% V doping content by first-principle calculations. The cohesive energy, impurity formation energy and formation energy indicate the structural stability of V doped WCoB and W₂CoB₂. The elastic constants and mechanical properties imply that V doping leads to the decrement of shear modulus and the increment of ductility. Two different kinds of hardness models verify that V doping contributes to the decrement of hardness, which is closely related to shear modulus. The electronic structure is analyzed by DOS (density of states), PDOS (partial density of states) and charge density difference, which indicate the formation of weaker B–V covalent bonds, W–V and W–W metallic bonds lead to the decrement of mechanical properties. Compared with previous studies of Cr, Mn doped WCoB and W₂CoB₂, V doping leads to worse mechanical properties and hardness, indicating V may not be a suitable choice of doping transition elements. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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26 pages, 12342 KiB

Open AccessArticle

Semi-Empirical Force-Field Model for the Ti_1−xAl_xN (0 ≤ x ≤ 1) System

by G. A. Almyras, D. G. Sangiovanni and K. Sarakinos

Materials 2019, 12(2), 215; https://doi.org/10.3390/ma12020215 - 10 Jan 2019

Cited by 22 | Viewed by 6529

Abstract

We present a modified embedded atom method (MEAM) semi-empirical force-field model for the Ti_1−xAl_xN (0 ≤ x ≤ 1) alloy system. The MEAM parameters, determined via an adaptive simulated-annealing (ASA) minimization scheme, optimize the model’s predictions with respect to [...] Read more.

We present a modified embedded atom method (MEAM) semi-empirical force-field model for the Ti_1−xAl_xN (0 ≤ x ≤ 1) alloy system. The MEAM parameters, determined via an adaptive simulated-annealing (ASA) minimization scheme, optimize the model’s predictions with respect to 0 K equilibrium volumes, elastic constants, cohesive energies, enthalpies of mixing, and point-defect formation energies, for a set of ≈40 elemental, binary, and ternary Ti-Al-N structures and configurations. Subsequently, the reliability of the model is thoroughly verified against known finite-temperature thermodynamic and kinetic properties of key binary Ti-N and Al-N phases, as well as properties of Ti_1−xAl_xN (0 < x < 1) alloys. The successful outcome of the validation underscores the transferability of our model, opening the way for large-scale molecular dynamics simulations of, e.g., phase evolution, interfacial processes, and mechanical response in Ti-Al-N-based alloys, superlattices, and nanostructures. Full article

(This article belongs to the Special Issue Computational Design of Complex Structural Alloys)

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