We investigated the microstructure and mechanical properties of Ti20Zr20Hf20Nb20X20 (X=V or Cr) high-entropy alloys (HEA), produced by induction melting and casting in inert atmosphere. The ...structures of these alloys were studied via X-ray diffractometry and scanning electron microscopy. Results show that Ti20Zr20Hf20Nb20V20 has mainly the body centered cubic (BCC) structure, whereas the BCC matrix of Ti20Zr20Hf20Nb20Cr20 contains small amount of Cr2Nb and Cr2Hf intermetallic compounds. Ti20Zr20Hf20Nb20V20 alloy shows the high strength and the homogeneous deformation under compression at room temperature. The strength and hardness of Ti20Zr20Hf20Nb20Cr20 alloy are further enhanced by the Cr-containing Laves phases segregated during casting. The structural and mechanical properties remained almost unchanged after a short time (10min) heat treatment at 573, 773, 973 and 1173K indicating the resistance to working temperature peaks for these two alloys. Ab initio calculations predict ductile behavior for these and similar refractory HEAs. The theoretically calculated Young's modulus E is in good agreement with the experimental ones.
•Two new high-entropy alloys (Ti20Zr20Hf20Nb20V20 and Ti20Zr20Hf20Nb20Cr20) are investigated.•The microstructural and mechanical properties are studied at different temperatures.•Elastic properties of similar refractory high-entropy alloys are predicted via ab initio alloy theory.
First principles calculations are performed to study the effects of alloying elements (X = Al, Si, Sc, V, Cr, Mn, Cu, Zn, Y, Mo, Ta, W and Re) on the phase stability and elastic properties of ...TiZrHfNb refractory high entropy alloys. Both equimolar and non-equimolar alloys are considered. It is shown that the calculated lattice parameters, phase stability and elastic moduli of equimolar TiZrHfNbX are consistent with the available experimental and theoretical results. The substitutions of alloying elements at Ti, Zr, and Hf sites with various contents show similar effects on the phase stability and elastic properties of the TiZrHfNb-based alloys. The substitutions on Nb site are found to generally decrease the stability of body centered cubic phase. Close connections between the charge densities at the Wigner-Seitz cell boundary and the bulk moduli of TiZrHfNb-based alloys are found. The present results provide a quantitative model for exploring the phase stability and elastic properties of TiZrHfNb-based alloys from the electronic structure viewpoint.
Phase stability and elastic properties of TiZrHfNbX are studied by first principles calculations. Display omitted
•The phase stability of equimolar TiZrHfNbX is closely connected to the d-occupation.•The alloying elements have similar effects on the phase stability and elastic moduli of TiZrHfNb-based HEAs.•The charge density at the Wigner-Seitz cell boundary has strong connections with the bulk moduli of TiZrHfNb-based HEAs.
•Ab initio calculation of the surface relaxation of metals.•Surface energy and surface stress of transition metals.•Surface energy and surface stress of simple metals and light actinides.
We ...investigated surface properties of metals by performing first-principles calculations. A systematic database was established for the surface relaxation, surface energy (γ), and surface stress (τ) for metallic elements in the periodic table. The surfaces were modeled by multi-layered slab structures along the direction of low-index surfaces. The surface energy γ of simple metals decreases as the atomic number increases in a given group, while the surface stress τ has its minimum in the middle. The transition metal series show parabolic trends for both γ and τ with a dip in the middle. The dip occurs at half-band filling due to a long-range Friedel oscillation of the surface charge density, which induces a strong stability to the Peierls-like transition. In addition, due to magnetic effects, the dips in the 3d metal series are shallower and deeper for γ and τ, respectively, than those of the 4d and 5d metals. The surface stress of the transition metals is typically positive, only Cr and Mn have a negative τ for the (100) surface facet, indicating that they are under compression. The light actinides have an increasing γ trend according to the atomic number. The present work provides a useful and consistent database for the theoretical modelling of surface phenomena.
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► We have found a correlation between chemical potentials in surface–bulk atomic exchanges and oxides formed on the surface of Fe–Cr–Al alloys in high temperature ► In Fe–Cr–Al alloys the addition of ...Cr changes the chemical potentials to drive Al to the surface to improve the formation of protective Al-oxide scales ► The described mechanism sheds light on the phenomenon called Third Element Effect (TEE), which does not have a generally accepted explanation so far ► Our research deals with the analysis of the experimental results from the point of view of atomic scale quantum mechanics, which represents a new kind of approach to the problem ► Our first-principles computational investigation covers also an exceptionally wide concentration range of Fe–Cr–Al alloys
Good high-temperature corrosion resistance of Fe–Al alloys in oxidizing environments is due to the
α-Al
2O
3 film which is formed on the surface provided temperature is above 900
°C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe–13Al, Fe–18Al, Fe–23Al and Fe–10Cr–10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe–Cr chemical potential difference and the formation of the protective oxide scales.
Body-Centered Cubic Iron-Nickel Alloy in Earth's Core Dubrovinsky, L; Dubrovinskaia, N; Narygina, O ...
Science (American Association for the Advancement of Science),
06/2007, Letnik:
316, Številka:
5833
Journal Article
Recenzirano
Cosmochemical, geochemical, and geophysical studies provide evidence that Earth's core contains iron with substantial (5 to 15%) amounts of nickel. The iron-nickel alloy Fe₀.₉Ni₀.₁ has been studied ...in situ by means of angle-dispersive x-ray diffraction in internally heated diamond anvil cells (DACs), and its resistance has been measured as a function of pressure and temperature. At pressures above 225 gigapascals and temperatures over 3400 kelvin, Fe₀.₉Ni₀.₁ adopts a body-centered cubic structure. Our experimental and theoretical results not only support the interpretation of shockwave data on pure iron as showing a solid-solid phase transition above about 200 gigapascals, but also suggest that iron alloys with geochemically reasonable compositions (that is, with substantial nickel, sulfur, or silicon content) adopt the bcc structure in Earth's inner core.
Density functional alloy theory is used to accurately describe the three core effects controlling the thermodynamics of random Cu-Au alloys. These three core effects are exchange correlation (XC), ...local lattice relaxations (LLRs), and short-range order (SRO). Within the real-space grid-based projector augmented-wave (GPAW) method based on density functional theory (DFT), we adopt the quasinonuniform XC approximation (QNA), and take into account the LLR and the SRO effects. Our approach allows us to study the importance of all three core effects in a unified way within one DFT code. The results demonstrate the importance of the LLR term and show that going from the classical gradient level approximations to QNA leads to accurate formation energies at various degrees of ordering. The order-disorder transition temperatures for the 25%, 50%, and 75% alloys reach quantitative agreement with the experimental values only when also the SRO effects are considered.
Magnetic phase diagrams of body-centered cubic and face-centered cubic Fe–Ni alloys were constructed using available experimental data and ab initio calculations. The results show that significant ...improvements in the “standard” diagrams (handbooks and CALPHAD databases) are required. The present work demonstrates that the CALPHAD magnetic model is not sophisticated enough to describe the Fe–Ni system. In addition, a new thermodynamic description of the lattice stability for pure Ni is urgently needed, since the recommended magnetic properties for CALPHAD modeling are distinct from the experimental and ab initio results. This work indicates that the construction of magnetic phase diagrams is indispensable during the phase transformation study of magnetic systems.
Within the framework of the exact muffin-tin orbitals (EMTO) theory we have developed a new method to calculate the total energy for random substitutional alloys. The problem of disorder is treated ...within the coherent potential approximation (CPA), and the total energy is obtained using the full charge density (FCD) technique. The FCD-EMTO-CPA method is suitable for determination of energy changes due to anisotropic lattice distortions in random alloys. In particular, we calculate the elastic constants of the Cu-rich face centered cubic Cu-Zn alloys ( alpha-brass) and optimize the c/a ratio for the hexagonal Zn-rich alloys for both the epsilon and eta phases.
The stacking fault energy (SFE) of austenitic stainless steels has been determined using a quantum mechanical first-principles approach. We identify the electronic, magnetic and volume effects ...responsible for the compositional dependence of the SFE. We find that both the alloying element and the composition of the host material are important for understanding the alloying effects. Our results show that no simple and universally valid composition equations exist for the SFE.
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