Combining theory with experiments, we study the phase stability, elastic properties, electronic structure and hardness of layered ternary borides AlCr2B2, AlMn2B2, AlFe2B2, AlCo2B2, and AlNi2B2. We ...find that the first three borides of this series are stable phases, while AlCo2B2 and AlNi2B2 are metastable. We show that the elasticity increases in the boride series, and predict that AlCr2B2, AlMn2B2, and AlFe2B2 are more brittle, while AlCo2B2 and AlNi2B2 are more ductile. We propose that the elasticity of AlFe2B2 can be improved by alloying it with cobalt or nickel, or a combination of them. We present evidence that these ternary borides represent nanolaminated systems. Based on SEM measurements, we demonstrate that they exhibit the delamination phenomena, which leads to a reduced hardness compared to transition metal mono- and diborides. We discuss the background of delamination by analyzing chemical bonding and theoretical work of separation in these borides.
•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.
Display omitted
Stainless steel is a good example of a metal that is not easily machined. To explain such behavior an understanding of the fundamental adhesion between the workpiece and the tool is invaluable. It is ...a well-known fact that build-up layers form in the interface, but little attention has been given to the very first layer that adheres to the tool surface. Although this layer rapidly becomes covered by successive material transfer, this layer and its ability to stick to the tool surface control the successive material transfer and influence the cutting properties. In this work, a quick stop test is employed to interrupt the cutting of a 316L stainless steel using a TiN-coated cemented carbide cutting insert. Different analytical techniques, such as transmission electron microscopy, X-ray photoelectron spectroscopy and scanning electron microscopy, as well as theoretical atomistic modeling, were used to study the early adhesion.
Structural stability of β -beryllium Kádas, K.; Vitos, L.; Johansson, B. ...
Physical review. B, Condensed matter and materials physics,
01/2007, Letnik:
75, Številka:
3
Journal Article
Recenzirano
Using density-functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the stability of the body-centered-cubic phase of Be (beta-Be). The elastic ...constants and Debye temperature of beta-Be are calculated over a wide volume range and compared to those obtained for the low-temperature hexagonal phase (alpha-Be). A significant difference in the anisotropy of the bcc and hcp structures is found. In line with experiments, we predict that the hcp -> bcc phase transition occurs at 240 GPa at 0 K and 239 GPa at ambient temperature. We find that the cubic shear constant C '=(C-11-C-12)/2 rapidly decreases for volumes above similar to 1.05V(0), where V-0 is the zero temperature equilibrium volume for beta-Be. At 1.17V(0), the stability condition C-'> 0 is violated and the bcc phase becomes mechanically unstable. We demonstrate that at 0 K, the softening of beta-Be near its experimental volume of 1.063V(0) is related to an electronic topological transition due to the increased number of occupied s states near the Fermi level compared to that at V-0. This softening turns out to be important for the stability of the bcc phase before melting. The disclosed electronic topological transition is found to be present in other analogous hexagonal metals as well.
Using the density functional theory formulated within the framework of the exact muffin–tin orbitals method, we present a systematic study of the top layer relaxation and surface stress of 4d ...transition metals. Our calculations predict layer contractions for most surfaces. We also find that the relaxations of the close packed surfaces decrease with increasing atomic number through the 4d series. We propose that the relaxation is mainly due to the reduction of the number of sp electrons in the surface layer relative to bulk. The surface stress is found to be very sensitive to the relaxation and, therefore, an accurate determination of the layer relaxation is necessary for obtaining reliable values for the surface stress. Comparing the top layer relaxations for the close packed surfaces, we see essential deviations between data derived in different ab initio calculations. At the same time, the overall trend for the present surface stress of 4d metals is in reasonable agreement with recent full-potential data.
Using density functional theory formulated within the framework of the exact muffin-tin orbital method, we investigate the elastic properties of the body-centered cubic Fe
0.85
Ni
0.1
Mg
0.05
alloy ...in the conditions at the Earth's inner core. We demonstrate that in this system, the chemical stabilization effect of Mg is significantly larger than that of Ni. We show that the elastic properties of Fe
0.85
Ni
0.1
Mg
0.05
are in good agreement with those of the Earth's inner core, as given by seismic observations. We find that the excellent mechanical properties of Fe
0.85
Ni
0.1
Mg
0.05
are primarily due to Mg.
Using density functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the thermo-physical properties of body-centered cubic (bcc) iron–magnesium ...alloys, containing 5 and 10 atomic % Mg, under extreme conditions, at high pressure and high temperature. The temperature effect is taken into account via the Fermi–Dirac distribution of the electrons. We find that at high pressures pure bcc iron is dynamically unstable at any temperature, having a negative tetragonal shear modulus (
C
′
). Magnesium alloying significantly increases
C
′
of Fe, and bcc Fe–Mg alloys become dynamically stable at high temperature. The electronic structure origin of the stabilization effect of Mg is discussed in detail. We show that the thermo-physical properties of a bcc Fe–Mg alloy with 5% Mg agree well with those of the Earth’s inner core as provided by seismic observations.
► At the conditions of the Earth’s inner core, Mg stabilizes bcc iron dynamically. ► Fe
0.95Mg
0.05 reproduces the physical properties of the inner core. ► A bcc structured Fe–Mg alloy is a strong candidate model of the inner core.
First-principles calculations have been performed to study the effect of C on the stacking fault energy (SFE) of paramagnetic γ-Fe and FeCrNi austenitic steel. In these systems, the local magnetic ...structure is very sensitive to the volume in both fcc and hcp structures, which emphasizes the importance of the magnetovolume coupling effect on the SFE. The presence of C atom suppresses the local magnetic moments of Fe atoms in the first coordination shell of C. Compared to the hypothetical nonmagnetic case, paramagnetism significantly reduces the effect of C on the SFE. In the scenario of C being depleted from the stacking fault structure or twin boundaries, e.g., due to elevated temperature, where the chemical effect of C is dissipated, we calculate the C-induced volume expansion effect on the SFE. The volume induced change in the SFE corresponds to more than ∼ 50% of the total C effect on the SFE obtained assuming uniform C distribution.
Display omitted
Abstract Intensity measures are generally considered as the fundamental properties of strong ground motion records and are widely utilized in performance-based seismic design methodology to relate ...the seismic hazard levels to the structural damages, or seismic response in general. Recent two decades have exhibited several studies proposing new alternatives that have intended to reduce the variability in seismic demand predictions. Although there exist several simple-to-advanced scalar and vector ground motion intensity measures; the literature is limited in the number of comparative studies investigating the efficiency of these parameters, especially in the entire response range of structures. This study aims at evaluating the correlation of major engineering demand parameters with novel simple-to-advanced intensity parameters following a regression-based approach to calculate efficiency metrics. For a group of low-to relatively high-rise reinforced concrete frames, alternative ground motion record sets have been formed considering different simple scalar intensity measures including peak ground acceleration, peak ground velocity, acceleration spectrum intensity, velocity spectrum intensity and spectral acceleration at the fundamental period of the structure of consideration, and utilizing these record sets, nonlinear time history analyses have been performed using OpenSees software to obtain key engineering demand parameters of the multi-degree-of-freedom systems. The correlation performance of novel scalar and vector intensity parameters have been quantified by evaluating regression models formed in between demand parameters and intensity measures. The regression-based approach has assisted to rank the ground motion intensity parameters according to their efficiency in terms of reducing the variability in response. The results of this comprehensive study display the relative correlation performance of scalar and vector forms together and mark particular scalar parameters as ‘best candidates’ for reliable loss estimation studies, despite their simplicity.
PDF
