arXiv subjects

Nikolai A. Zarkevich

Publications and source records attributed to Nikolai A. Zarkevich.

13 recordsLinked to original sources

arXiv2023

Energy-composition relations in Ni$_3$(Al$_{1-x}$X$_x$) phases

The secondary phase, such as Ni$_3$Al-based $L1_2$ $γ^\prime$, is crucially important for precipitation strengthening of superalloys. Composition-structure-property relations provide useful insights for guided alloy design. Here we use density functional theory combined with the multiple scattering theory to compute dependencies of the structural energies and equilibrium volumes versus composition for ternary Ni$_3$(Al$_{1-x}$X$_x$) alloys with X=(Ti, Zr, Hf; V, Nb, Ta; Cr, Mo, W) in $L1_2$, $D0_{24}$, and $D0_{19}$ phases with a homogeneous chemical disorder on the (Al$_{1-x}$X$_x$) sublattice. Our results provide a better understanding of the physics in Ni$_3$Al-based precipitates and facilitate design of next-generation nickel superalloys with precipitation strengthening.

cond-mat.mtrl-sci↗

arXiv2023

Energy landscape in NiCoCr-based middle-entropy alloys

NiCoCr middle-entropy alloy is known for its exceptional strength at both low and elevated operating temperatures. Mechanical properties of NiCoCr-based alloys are affected by certain features of the energy landscape, such as the energy difference between the hcp and fcc phases (which is known to correlate with the stacking fault energy in the fcc phase) and curvature of the energy surface. We compute formation energies in the Ni-Co-Cr ternary and related quaternary systems and investigate dependences of the relative energies on composition. Such computed composition-structure-property relations can be useful for tuning composition and designing next-generation alloys with improved strength.

cond-mat.mtrl-sci↗

arXiv2020

Half-metallic compositional ranges for selected Heusler alloys

For a material that is a half-metal, there should exist a range of compositions for half-metallicity. This compositional range can be expressed in terms of electron count and computed. We investigate electronic and magnetic properties of doped full- and half-Heusler alloys (stoichiometry XYZ2 and XYZ, respectively) with elements X from groups 13-16 and periods 3-6 of the Periodic Table, Y={Mn, Fe}, and Z={Co, Ni}. Using spin density functional theory, we predict shifts of the Fermi energy in the doped and solid-solution alloys. These predictions can be used for band-gap engineering of multicomponent half-metals and provide the viable range of compositions, such as for a range of n=x+y+z in (Co$_{2-z}Ni_z)$(Mn$_{1-y}$Fe$_{y}$)(Sn$_{1-x}$Sb$_{x}$). This methodology for doped and chemically disordered half-metallic alloys offers a design approach to electronic-structure engineering that can accelerate development of half-metals for novel electronic and spintronic applications.

cond-mat.mtrl-sci↗

arXiv2020

Fifth-degree elastic potential for predictive stress-strain relations and elastic instabilities under large strain and complex loading in Si

Materials under complex loading develop large strains and often transition via an elastic instability, as observed in both simple and complex systems. Here, we present Si I under large strain in terms of Lagrangian strain by an $5^{th}$-order elastic potential found by minimizing error relative to density functional theory (DFT) results. The Cauchy stress-Lagrangian strain curves for arbitrary complex loadings are in excellent correspondence with DFT results, including elastic instability driving Si I$\rightarrow$II phase transformation (PT) and the shear instabilities. PT conditions for Si I$\rightarrow$II under action of cubic axial stresses are linear in Cauchy stresses in agreement with DFT predictions. Such elastic potential permits study of elastic instabilities and orientational dependence leading to different PTs, slip, twinning, or fracture, providing a fundamental basis for continuum simulations of crystal behavior under extreme loading.

cond-mat.mtrl-sci↗

arXiv2019

Parameterization of the Stoner-Wohlfarth model of magnetic hysteresis

The Stoner-Wohlfarth is the most used model of magnetic hysteresis, but its computation is time-consuming. We use machine learning to approximate piecewise this model by easy-to-compute analytic functions. Our parametrization is suitable for fast quantitative evaluations and fitting experimental data, which we exemplify.

cond-mat.mtrl-sci↗

arXiv2019

Reliable thermodynamic estimators for screening multicaloric materials

Reversible, diffusionless, first-order solid-solid phase transitions accompanied by caloric effects are critical for applications in the solid-state cooling and heat-pumping devices. Accelerated discovery of caloric materials requires reliable but faster estimators for predictions and high-throughput screening of system-specific dominant caloric contributions. We assess reliability of the computational methods that provide thermodynamic properties in relevant solid phases at or near a phase transition. We test the methods using the well-studied B2 FeRh alloy as a "fruit fly" in such a materials genome discovery, as it exhibits a metamagnetic transition which generates multicaloric (magneto-, elasto-, and baro-caloric) responses. For lattice entropy contributions, we find that the commonly-used linear-response and small-displacement phonon methods are invalid near instabilities that arise from the anharmonicity of atomic potentials, and we offer a more reliable and precise method for calculating lattice entropy at a fixed temperature. Then, we apply a set of reliable methods and estimators to the metamagnetic transition in FeRh (predicted $346 \pm 12$ K, observed $353 \pm 1$ K) and calculate the associated caloric properties, such as isothermal entropy and isentropic temperature changes.

cond-mat.mtrl-sci↗

arXiv2018

Deformation, lattice instability, and metallization during solid-solid structural transformations under general applied stress tensor: example of Si I -> Si II

Density functional theory (DFT) was employed to study the stress-strain behavior, elastic instabilities, and metallization during a solid-solid phase transformation (PT) between semiconducting Si I (cubic A4) and metallic Si II (tetragonal A5 structure) when subjected to a general stress tensor. With normal stresses ($σ_1$, $σ_2$, $σ_3$) acting along $\left<110 \right>$, $\left<1\bar{1}0 \right>$, and $\left<001\right>$, respectively, dictating the simulation cell, we determine combinations of 6 independent stresses that drive a lattice instability for the Si I$\rightarrow$Si II PT, and a semiconductor-metal electronic transition. Metallization precedes the structural PT, hence, a stressed Si I can be a metal. Surprisingly, a stress-free Si II is metastable in DFT. Notably, the PT for hydrostatic pressures is at 75.81 GPa, while under uniaxial stress it is 11.03 GPa (or 3.68 GPa mean pressure). Our key result: The Si I -> Si II PT is described by a critical value of the modified transformation work, as found with a phase-field method, and the PT criterion has only two parameters for a general applied stress. More generally, our findings are crucial for revealing novel (and more economic) material synthesis routes for new or known high-pressure phases under controlled and predictable non-hydrostatic loading and plastic deformation.

cond-mat.mtrl-sci↗

arXiv2017

FeRh groundstate and martensitic transformation

Cubic B2 FeRh exhibits a metamagnetic transition [(111) antiferromagnet (AFM) to ferromagnet (FM)] around 353 K and remains structurally stable at higher temperatures. However, the calculated zero-Kelvin phonons of AFM FeRh exhibit imaginary modes at M-points in the Brillouin zone, indicating a premartensitic instability, which is a precursor to a martensitic transformation at low temperatures. Combining electronic-structure calculations with ab initio molecular dynamics, conjugate gradient relaxation, and the solid-state nudged-elastic band (SSNEB) methods, we predict that AFM B2 FeRh becomes unstable at ambient pressure and transforms without a barrier to an AFM(111) orthorhombic (martensitic) groundstate below 90K. We also consider competing structures, in particular, a tetragonal AFM(100) phase that is not the global groundstate, as proposed [Phys. Rev. B 94, 180407(R) (2016)], but a constrained solution.

cond-mat.mtrl-sci↗

arXiv2017

Qualitative model of high-$T_c$ superconductivity

We suggest a qualitative model of a high-$T_c$ superconductor, based on considerations of thermodynamics of phase transitions. As an example, we consider the Mott transition and classify 5 solid phases around it. In our model, a combined electronic and structural instability causes segregation into either neutral or charged phases. A charged precipitate with a quantized electric charge is a collective excitation of electrons, stabilized by a collective athermal displacement of ions; this local variation of the charge density, accompanied by a local lattice deformation, can behave as a quasi\-particle. A condensate of charged bosonic quasi\-particles is responsible for the superconductivity.

cond-mat.mtrl-sci↗

arXiv2014

Coexistence pressure for a martensitic transformation from theory and experiment: revisiting the bcc-hcp transition of iron under pressure

The coexistence pressure of two phases is a well-defined point at fixed temperature. In experiment, however, due to non-hydrostatic stresses and a stress-dependent potential energy barrier, different measurements yield different ranges of pressure with a hysteresis. Accounting for these effects, we propose an inequality for comparison of the theoretical value to a plurality of measured intervals. We revisit decades of pressure experiments on the bcc - hcp transformations in iron, which are sensitive to non-hydrostatic conditions and sample size. From electronic-structure calculations, we find a bcc - hcp coexistence pressure of 8.4 GPa. We construct the equation of state for competing phases under hydrostatic pressure, compare to experiments and other calculations, and address the observed pressure hysteresis and range of onset pressures of the nucleating phase.

cond-mat.mtrl-sci↗

arXiv2014

Nudged-elastic band method with two climbing images: finding transition states in complex energy landscapes

The nudged-elastic band (NEB) method is modified with concomitant two climbing images (C2-NEB) to find a transition state (TS) in complex energy landscapes, such as those with serpentine minimal energy path (MEP). If a single climbing image (C1-NEB) successfully finds the TS, C2-NEB finds it with higher stability and accuracy. However, C2-NEB is suitable for more complex cases, where C1-NEB misses the TS because the MEP and NEB directions near the saddle point are different. Generally, C2-NEB not only finds the TS but guarantees that the climbing images approach it from the opposite sides along the MEP, and it estimates accuracy from the three images: the highest-energy one and its climbing neighbors. C2-NEB is suitable for fixed-cell NEB and the generalized solid-state NEB (SS-NEB).

physics.comp-ph↗

arXiv2014

What is the stable atomic structure of NiTi austenite?

Nitinol (NiTi), the most widely used shape-memory alloy, exhibits an austenite phase that has yet to be identified. The usually assumed austenite structure is cubic B2, which has imaginary phonon modes, hence it is unstable. We suggest a stable austenite structure that on average has B2 symmetry (observed by X-ray and neutron diffraction), but exhibits finite atomic displacements from the ideal B2 sites. The proposed structure has a phonon spectrum that agrees with that from neutron scattering, has diffraction spectra in agreement with XRD, and has an energy relative to the ground state that agrees with calorimetry data.

cond-mat.mtrl-sci↗

arXiv2004

Reliable First-Principles Alloy Thermodynamics via Truncated Cluster Expansions

In alloys cluster expansions (CE) are increasingly used to combine first-principles electronic-structure and Monte Carlo methods to predict thermodynamic properties. As a basis-set expansion in terms of lattice geometrical clusters and effective cluster interactions, the CE is exact if infinite, but is tractable only if truncated. Yet until now a truncation procedure was not well-defined and did not guarantee a reliable truncated CE. We present an optimal truncation procedure for CE basis sets that provides reliable thermodynamics. We then exemplify its importance in Ni$_3$V, where the CE has failed unpredictably, and now show agreement to a range of measured values, predict new low-energy structures, and explain the cause of previous failures.

cond-mat.mtrl-sci↗