We present the results of ab initio modeling of structure of dilute Ti-Fe, a typical representative of quenched Ti-based transition-metal alloys. We have demonstrated that beyond the solubility limit this alloy cannot be described in common terms of substitutional and interstitial alloys. Instead, very stable local clusters are formed in both low-temperature hcp and high-temperature bcc phases of alloys, with almost identical local structures. This gives an example of geometrically frustrated state and explains unusual concentration behavior of Mössbauer spectra discovered long ago for this system.
An extensive set of first-principles density functional theory calculations have been performed to study the behaviour of He, C and N solutes in austenite, dilute Fe-Cr-Ni austenitic alloys and Ni in order to investigate their influence on the microstructural evolution of austenitic steel alloys under irradiation.
We present results of an extensive set of first-principles density functional theory calculations of point defect formation, binding and clustering energies in austenitic Fe with dilute concentrations of Cr and Ni solutes.
Mn doping in dilute III-V alloys has been examined as a route to enhance ferromagnetic stability. Strong valence band bowing is expected at the dilute limit, implying a strong modification of the ferromagnetic stability upon alloying, with even an increase in some cases. Using first principle electronic structure calculations we show that while codoping with a group V anion enhances the ferromagnetic stability in some cases when the effects of relaxation of the lattice are not considered, strong impurity scattering in the relaxed structure result in a reduction of the ferromagnetic stability.
For powder samples of CuAl$_{1-x}$Fe$_x$O$_2$ ($x =$ 0, 0.01, 0.05, and 0.1), measured optical properties are compared with model simulations and phonon spectra are compared with simulations based on weighted dynamical matrix approach.
The present work examines the effect of alloying elements (denoted X) on the ideal shear strength for 26 dilute Ni-based alloys, Ni$_{11}$X, as determined by first-principles calculations of pure alias shear deformations. The variations in ideal shear strength are quantitatively explored with correlational analysis techniques, showing the importance of atomic properties such as size and electronegativity. The shear moduli of the alloys are affirmed to show a strong linear relationship with their ideal shear strengths, while the shear moduli of the individual alloying elements were not indicati…
The addition of dilute concentrations of bismuth (Bi) into GaAs to form GaBiAs alloys results in a large reduction of the band gap energy Eg accompanied by a significant increase of the spin-orbit-splitting energy (delta_SO), leading to an Eg < delta_SO regime for ~10% Bi composition which is technologically relevant for the design of highly efficient photonic devices. The quaternary alloy GaBiNAs offers further flexibility for band gap tuning, because both nitrogen and bismuth can independently induce band gap reduction. This work reports sp3s* tight binding and 14-band k.p models for the stu…
A multiple scattering formulation for the electromigration wind force on atoms in dilute alloys is developed. The theory describes electromigration via a vacancy mechanism. The method is used to calculate the wind valence for electromigration in various host metals having a close-packed lattice structure, namely aluminum, the noble metals copper, silver and gold and the $4d$ transition metals. The self-electromigration results for aluminum and the noble metals compare well with experimental data. For the $4d$ metals small wind valences are found, which make these metals attractive candidates f…
The vacancy concentration at finite temperatures is studied for a series of (CoCrFeMn)$_{1-x_\mathrm{Ni}}$Ni$_{x_\mathrm{Ni}}$ alloys by grand-canonical Monte-Carlo (MC) simulations. The vacancy formation energies are calculated from a classical interatomic potential and exhibit a distribution due to the different chemical environments of the vacated sites. In dilute alloys, this distribution features multiple discrete peaks, while concentrated alloys exhibit an unimodal distribution as there are many different chemical environments of similar vacancy formation energy. MC simulations using a n…
Using an asymptotic solution of the M-impurity thermodynamics of a dilute s-d system, the impurity energy and impurity heat capacity DeltaC(T) are derived for dilute magnetic alloys with spin 1/2 and spin 3/2 impurities. The parameters which enter DeltaC are adjusted to fit experimental data on impurity heat capacity of CuCr and LaCeAl_2. Agreement is satisfactory for CuCr, at temperatures below 1K, and good for LaCeAl_2. The magnitude of theoretical DeltaC(T) agrees with experiment and does not require scaling as in previous s-d theories. Nonlinear dependence of DeltaC(T) on impurity concentr…
The challenging problem of skew scattering for Hall effects in dilute ferromagnetic alloys, with intertwined effects of spin-orbit coupling, magnetism and impurity scattering, is studied here from first principles. Our main aim is to identify chemical trends and work out simple rules for large skew scattering in terms of the impurity and host states at the Fermi surface, with particular emphasis on the interplay of the spin and anomalous Hall effects in one and the same system. The predicted trends are benchmarked by referring to three different \emph{ab initio} methods based on different appr…
The occurrence of segregation in highly dilute alloys under irradiation is an unusual phenomenon that has so far eluded theoretical explanation. Using ab initio calculations, we are able to explain the origin of radiation-induced rhenium segregation in dilute tungsten-rhenium alloys.
Incorporation of bismuth (Bi) in dilute quantities in (In)GaAs has been shown to lead to unique electronic properties that can in principle be exploited for the design of high efficiency telecomm lasers. This motivates the development of simple models of the electronic structure of these dilute bismide alloys, which can be used to evaluate their potential as a candidate material system for optical applications. Here, we begin by using detailed calculations based on an $sp^{3}s^{*}$ tight-binding model of (In)GaBi$_{x}$As$_{1-x}$ to verify the presence of a valence band-anticrossing interaction…
A series of structural polytypes formed in an Mg-1at.%Zn-2at.%Y alloy has been identified, which are reasonably viewed as long-period stacking derivatives of the hcp Mg structure with alternate AB stacking of the close-packed atomic layers. Atomic-resolution Z-contrast imaging clearly revealed that the structures are long-period chemical-ordered as well as stacking-ordered; unique chemical order along the stacking direction occurs as being synchronized with a local faulted stacking of AB'C'A, where B' and C' layers are commonly enriched by Zn/Y atoms.
We present a theoretical analysis of electronic structure evolution in the highly-mismatched dilute carbide group-IV alloy Ge$_{1-x}$C$_{x}$. For ordered alloy supercells, we demonstrate that C incorporation strongly perturbs the conduction band (CB) structure by driving hybridisation of $A_{1}$-symmetric linear combinations of Ge states lying close in energy to the CB edge. This leads, in the ultra-dilute limit, to the alloy CB edge being formed primarily of an $A_{1}$-symmetric linear combination of the L-point CB edge states of the Ge host matrix semiconductor. Our calculations describe the…
The magnetic properties of trilayers consisting of a diluted magnetic alloy, CuMn (Cu0.99Mn0.01), a soft ferromagnet, Py(Ni0.8Fe0.2), and an antiferromagnet, alpha-Fe2O3, were investigated. The samples, grown by UHV magnetron sputtering, were magnetically characterized in the temperature range T = 3-100 K. Typical exchange bias features, namely clear hysteresis cycle shifts and coercivity enhancements, were observed. Moreover the presence of an inverse bias, which had been already reported for spin glass-based structures, was also obtained in a well defined range of temperatures and CuMn thick…
Starting from a phase-field description of the isothermal solidification of a dilute binary alloy, we establish a model where capillary waves of the solidification front interact with the diffusive concentration field of the solute. The model does not rely on the sharp-interface assumption, and includes non-equilibrium effects, relevant in the rapid-growth regime. In many applications it can be evaluated analytically, culminating in the appearance of an instability which, interfering with the Mullins-Sekerka instability, is similar to that, found by Cahn in grain-boundary motion.
We present the first resonant x-ray reflectivity measurements from a liquid surface. The surface structure of the liquid Hg-Au alloy system just beyond the solubility limit of 0.14at% Au in Hg had previously been shown to exhibit a unique surface phase characterized by a low-density surface region with a complicated temperature dependence. In this paper we present reflectivity measurements near the Au LIII edge, for 0.2at% Au in Hg at room temperature. The data are consistent with a concentration of Au in the surface region that can be no larger than about 30at%. These results rule out previou…
Impurity diffusion in Zr is potentially important for many applications of Zr alloys, and in particular for their use of nuclear reactor cladding. However, significant uncertainty presently exists about which elements are vacancy vs. interstitial diffusers, which can inhibit understanding and prediction of their behavior under different temperature, irradiation, and alloying conditions. Therefore, first-principles calculations based on density functional theory (DFT) have been employed to predict the temperature-dependent dilute impurity diffusion coefficients for 14 substitutional alloying el…
An argument is given showing that Coulomb attraction between conduction electrons and impurity ions in a dilute magnetic alloy (DMA) can be disregarded, provided the system's inverse temperature beta is replaced by an effective inverse temperature t < beta. This replacement allows to remove the singularity in Kondo's expression for DMA impurity resistivity and extend his theory to 0 K. The extended Kondo formula agrees with experimental data on resistivity of CuFe in the range of low temperatures and in the neighbourhood of the resistivity minimum. Using an asymptotic solution of the thermodyn…
Planar solidification from an undercooled melt has been considered using the phase-field model. The solute and the phase fields have been found in the limit of small impurity concentration. These solutions in the limit of vanishing velocity of the interface motion give the equilibrium partition coefficient and the liquidus slope. Asymptotic expansions for the solute and for the phase fields, and the relation between the diffusive speed and the parameters of the phase field model have been found at high growth velocity. A comparison with numerical calculations is presented.
The thermodynamic activities of all components in Ga-In-Tl system have been predicted at 1073, 1173 and 1273 K, using Molecular interaction volume model (MIVM). The infinite dilute activity coefficients for Ga-In and Ga-Tl binary subsystems, which were needed for the determination of thermodynamic activities of all components in Ga-In-Tl, have been predicted by using a method that is based on Complex formation model for liquid alloys. The computed thermodynamic activities of Ga in Ga-In-Tl were observed to satisfactorily agree with the available experimental data when the newly computed coeffi…
We use current-perpendicular-to-plane (CPP) exchange-biased spin-valves to directly measure spin diffusion lengths lsf for N = Cu(2.1 at.%Ge) and Ag(3.6 at.%Sn) alloys. We find lsf(Cu2%Ge)) = 117+10-6 nm and lsf(Ag4%Sn)= 39 +/- 3 nm. The good agreement of this lsf(Cu2%Ge) with the value lsf(Cu2%Ge) = 121 +/- 10 nm derived from an independent spin-orbit cross-section measurement for Ge in Cu, quantitatively validates the use of Valet-Fert theory for CPP-MR data analysis to layer thicknesses several times larger than had been done before. From the value of lsf(Ag4%Sn), we predict the ESR spin-or…
In the present work, a thin film was deposited on quartz substrate by reactive RF magnetron sputtering of high purity (99.999%) aluminium target using ultra-high pure (Ar + N2) gas mixture. The percentage ratio of Ar and N2 in the gas mixture was 95% and 5%, respectively. Chemical characterization using x-ray photoelectron spectroscopy (XPS) and energy-dispersive xray (EDX) spectroscopy reveals that in the presence of dilute nitrogen, Al prefers to react with residual oxygen to form Al2O3 while the nitrogen is incorporated in it. The stoichiometry of bulk film is Al2N0.38O3.1. Magnetic and ele…
Using an ab initio approach, we explore the stability of small vacancy and vacancy-chromium clusters in dilute body-centred cubic Fe-Cr alloys. To explain experimental observations described in C.D. Hardie et al., J. Nucl. Mater. 439, 33 (2013) and showing the occurrence of Cr segregation in low-Cr alloys, we investigate if chromium can form stable bound configurations with vacancies in alloys with chromium concentration below the low-temperature chromium solubility limit of 10-11 at. %. We find that a single vacancy can attract up to four Cr atoms in the most energetically favourable cluster …
We develop an atomistic, nearest-neighbor sp3s* tight-binding Hamiltonian to investigate the electronic structure of dilute bismide alloys of GaP and GaAs. Using this model we calculate that the incorporation of dilute concentrations of Bi in GaP introduces Bi-related defect states in the band gap, which interact with the host matrix valence band edge via a Bi composition dependent band anti-crossing (BAC) interaction. By extending this analysis to GaBiAs we demonstrate that the observed strong variation of the band gap Eg and spin-orbit-splitting (SO) energy with Bi composition can be well ex…
An archetypical spin-glass metallic alloy, Cu0.83Mn0.17, is studied by means of an ab-initio based approach. First-principles calculations are employed to obtain effective chemical, strain-induced and magnetic exchange interactions, as well as static atomic displacements, and the interactions are subsequently used in thermodynamic simulations. It is shown that the calculated atomic and magnetic short-range order accurately reproduces the results of neutron-scattering experiments. In particular, it is confirmed that the alloy exhibits a tendency toward ordering and the corresponding ordered pha…
The mobility of vacancies in alloys may limit dislocation climb. Using a combined density functional theory and kinetic Monte Carlo approach we investigate vacancy diffusion in Ni-Re, Ni-W, and Ni-Ta binary alloys up to 10 at.% solute concentration. We introduce an interaction model that takes into account the chemical environment close to the diffusing atom to capture the effect of solute-host and solute-solute interactions on the diffusion barriers. In contrast to an ideal solid solution it is not only the diffusion barrier of the solute atom that influences the vacancy mobility, but primari…
Investigation of ideal shear strength of dilute binary and ternary Ni-based alloys using first-principles calculations, CALPHAD modeling and correlation analysis
Kinetics of homogeneous nucleation and growth of copper precipitates under electron irradiation of Fe_{1-x}Cu_x alloys at concentrations x from 0.06 at.% to 0.4 at.% and temperatures T from 290 to 450C is studied using the kinetic Monte Carlo (KMC) simulations and the statistical theory of nucleation (STN). The conventional assumption about the similarity of mechanisms of precipitation under electron irradiation and during thermal aging is adopted. The earlier-developed ab initio model of interactions in Fe-Cu alloys is used for both the KMC simulations and the STN calculations. Values of the …
We investigate thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys to form a unique solute-enriched stacking-fault (SESF), which is an intrinsic-II type stacking-fault (I2-SF) enriched by the Zn and Y atoms and represents the structural-unit of the long-period stacking/order (LPSO) phase. SESF in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, and hence our thermodynamic analysis is based on the Gibbs energy comparison between hcp and fcc phases over the Mg-Zn-Y ternary composition ranges, using the calculation of phase diagrams (CALPHAD) me…
The variation of TO zone-center vibration spectra with concentration in mixed zincblende-type semiconductors can be understood within a paradigm of unified "one bond - two modes" approach, which has been recently outlined as a rather general concept, and emerges from a number of previous experimental and theoretical studies. The crucial issue is that the vibration frequency, associated with a certain cation-anion bond, depends on the length of the latter, and the bond length, in its turn, depends not only on the average alloy concentration, but on local variations of it. In an (A,B)C substitut…
Dilute bismuth (Bi) III-V alloys have recently attracted great attention, due to their properties of band-gap reduction and spin-orbit splitting. The incorporation of Bi into antimonide based III-V semiconductors is very attractive for the development of new optoelectronic devices working in the mid-infrared range (2-5 $μ$m). However, due to its large size, Bi does not readily incorporate into III-V alloys and the epitaxy of III-V dilute bismides is thus very challenging. This book chapter presents the most recent developments in the epitaxy and characterization of GaSbBi alloys and heterostru…
In this work, we investigate the radiation-induced segregation (RIS) resulting from the coupling between the atomic and point defect (PD) fluxes towards the structural defects of the microstructure. This flux coupling depends on the migration mechanisms of PDs and atoms, including thermal diffusion mechanisms and forced atomic relocations (FAR) occurring in displacement cascades. We derive an analytic model of the PD and solute RIS profiles accounting for PD production and mutual recombination, the FAR mechanism, and the overall sink strength of the microstructure controlling the elimination o…
Using spectroscopic ellipsometry measurements on GaP$_{1-x}$Bi$_{x}$/GaP epitaxial layers up to $x = 3.7$% we observe a giant bowing of the direct band gap ($E_{g}^Γ$) and valence band spin-orbit splitting energy ($Δ_{\textrm{SO}}$). $E_{g}^Γ$ ($Δ_{\textrm{SO}}$) is measured to decrease (increase) by approximately 200 meV (240 meV) with the incorporation of 1% Bi, corresponding to a greater than fourfold increase in $Δ_{\textrm{SO}}$ in going from GaP to GaP$_{0.99}$Bi$_{0.01}$. The evolution of $E_{g}^Γ$ and $Δ_{\textrm{SO}}$ with $x$ is characterised by strong, composition-dependent bowing. …
Anion substitution with bismuth (Bi) in III-V semiconductors is an effective method for experimental engineering of the band gap Eg at low Bi concentrations, in particular in gallium arsenide (GaAs). The inverse Bi-concentration dependence of Eg has been found to be linear at low concentrations x and dominated by a valence band-defect level anticrossing between As and Bi occupied p levels. This dependence breaks down at high concentrations where empirical models accounting only for the As-Bi interaction are not applicable. Predictive models for the valence band hybridization require a first-pr…
Dilute-N GaPAsN alloys have great potential for optoelectronics lattice-matched to Si. However, there is a lack of systematic calculation of the optical response of these alloys. The present paper uses the sp^3d^5s^*s_N tight-binding model to calculate the fullband electronic structure of dilute-N GaPAsN, and then calculate the optical response functions considering direct transitions within the electric dipole approximation. Good agreement is obtained for the dielectric function in comparison to available optical data for dilute nitrides. To achieve this, the sp^3d^5s^* parameters for GaP and…
We demonstrate automated generation of diffusion databases from high-throughput density functional theory (DFT) calculations. A total of more than 230 dilute solute diffusion systems in Mg, Al, Cu, Ni, Pd, and Pt host lattices have been determined using multi-frequency diffusion models. We apply a correction method for solute diffusion in alloys using experimental and simulated values of host self-diffusivity. We find good agreement with experimental solute diffusion data, obtaining a weighted activation barrier RMS error of 0.176 eV when excluding magnetic solutes in non-magnetic alloys. The …
Kinetics of niobium and titanium carbide precipitates in iron has been simulated with cluster dynamics. The simulations, carried out in austenite and ferrite for niobium carbides, respectively in austenite for titanium carbide, were analyzed for dependency on temperature, solute concentration, and initial cluster distribution. The results are presented for different temperatures and solute concentrations and compared to available experimental data. They show little impact of initial cluster distribution beyond a certain relaxation time and that highly dilute alloys with only monomers present a…
Deconvoluting Mechanisms of Radiation-Induced Grain Boundary Segregation in Dilute Ni-X (X = Fe, Cr, Mn) Alloys via Ion Irradiation and Phase-field Modeling
Idaho National Laboratory（爱达荷国家实验室）；Texas A&M University（德克萨斯农工大学）
The body centered cubic (BCC) high entropy alloys MoNbTaW and MoNbTaVW show exceptional strength retention up to 1900K. The mechanistic origin of the retained strength is unknown yet is crucial for finding the best alloys across the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a theory of strengthening of edge dislocations in BCC alloys. Unlike pure BCC metals and dilute alloys that are controlled by screw dislocation motion at low temperatures, the strength of BCC HEAs can be controlled by edg…
Highly mismatched semiconductor alloys such as GaNAs and GaBiAs have several novel electronic properties, including a rapid reduction in energy gap with increasing x and also, for GaBiAs, a strong increase in spin orbit- splitting energy with increasing Bi composition. We review here the electronic structure of such alloys and their consequences for ideal lasers. We then describe the substantial progress made in the demonstration of actual GaInNAs telecomm lasers. These have characteristics comparable to conventional InP-based devices. This includes a strong Auger contribution to the threshold…
We simulate dendritic growth in directional solidification in dilute binary alloys using a phase-field model solved with an adaptive-mesh refinement. The spacing of primary branches is examined for a range of thermal gradients and alloy compositions and is found to undergo a maximum as a function of pulling velocity, in agreement with experimental observations. We demonstrate that wavelength selection is unambiguously described by a non-trivial crossover scaling function from the emergence of cellular growth to the onset of dendritic fingers, a result validated using published experimental dat…
The spontaneous magnetization as a function of temperature is investigated for a number of disordered Ni-Fe-Mo and Ni-Fe-W alloys using superconducting quantum interference device magnetometry, with a focus on the low-T behavior as well as the critical exponents associated with the magnetic phase transition. While the low-T magnetization is found to be well described by Bloch's T^{3/2} law, an extraordinary enhancement of the spin-wave parameter B and the reduced coefficient B_{3/2}=BT_C ^{3/2} are observed with increasing Fe dilution as compared to conventional 3d ferromagnets, whereas the cr…
Sustained external forces acting on a material provide additional mechanisms to evolve the state of the system, and these mechanisms do not necessarily obey the microscopic detailed balance. Therefore, standard methods to compute the thermodynamic and kinetic properties do not apply in such driven systems. The competition between these mechanisms and the standard thermally activated jumps leads to non-equilibrium steady states. We extend the Self-Consistent Mean Field theory to take into account forced atomic relocations (FARs) as a model of these additional kinetic mechanisms. The theory is a…
Using an $sp^{3}s^{*}$ tight-binding model we demonstrate how the observed strong bowing of the band gap and spin-orbit-splitting with increasing Bi composition in the dilute bismide alloy GaBi$_{x}$As$_{1-x}$ can be described in terms of a band-anticrossing interaction between the extended states of the GaAs valence band edge and highly localised Bi-related resonant states lying below the GaAs valence band edge. We derive a 12-band $\textbf{k}\cdot\textbf{p}$ Hamiltonian to describe the band structure of GaBi$_{x}$As$_{1-x}$ and show that this model is in excellent agreement with full tight-b…
The transitional state of ordering in the long-period stacking ordered (LPSO) structure in Mg-Al-Gd alloy has been investigated by scanning transmission electron microscopy. It is found that the domains are the origin of observed transitional state. Three new patterns of clusters at this state can be fully explained by the superimposition of L12 type clusters located in different domains. The domains are further verified by different tilted views. Growth of preferred domains during ageing process will result in final ordered LPSO structure. Based on the idea of domains, less ordered LPSO struc…
Tuning the opto-electronic properties through alloying is essential for semiconductor technology. Currently, mostly isovalent and isostructural alloys are used (e.g., group-IV and III-V), but a vast and unexplored space of novel functional materials is conceivable when considering more complex alloys by mixing aliovalent and heterostructural constituents. The real challenge lies in the quantitative property prediction for such complex alloys to guide their experimental exploration. We developed an approach to predict compositional dependence of both band-structure and electrical properties fro…
In the fcc lattice of Pd, hydrogen occupies octahedral interstitial positions with cubic symmetry. One could therefore expect that in dilute solid H solutions in Pd the fundamental band of optical H vibrations should be reduced to a narrow line of 3-fold degenerate, non-interacting isotropic local oscillators. A few attempts to test this assumption by inelastic neutron scattering gave ambiguous results due to the very small limiting hydrogen concentration, x<0.001, of the PdH$_x$ solutions at low temperatures. We increased maximum x by alloying Pd with 20 at% Au and studied disordered Pd$_{0.8…
We present results of a numerical mean field treatment of interacting spins and carriers in doped diluted magnetic semiconductors, which takes into account the positional disorder present in these alloy systems. Disorder is found to enhance the ferromagnetic transition temperature significantly for metallic densities not too far from the metal-insulator transition. Concurrently, the ferromagnetic phase is found to have very unusual temperature dependence of the magnetization as well as specific heat as a result of disorder. Unusual spin and charge transport is implied.
Palladium-iron alloys produced by high-dose implantation of iron ions into epitaxial palladium films were investigated with the ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) techniques. The samples reveal distinct multiple FMR responses depending on the dose of iron ion implantation. The post-implantation annealing at 770 K does not bring the implanted films to a homogeneous solid solution state, as might be expected from the Pd-Fe phase diagram. On the contrary, the system approaches a stable state composed of several magnetic phases. FMR spectra exhibit an angular beh…
We present an approach to calculate the atomic bulk solubility in binary alloys based on the statistical-thermodynamic theory of dilute lattice gas. The model considers all the appropriate ground states of the alloy and results in a simple Arrhenius-type temperature dependence determined by a {\it "low-solubility formation enthalpy"}. This quantity, directly obtainable from first-principle calculations, is defined as the composition derivative of the compound formation enthalpy with respect to nearby ground states. We apply the framework and calculate the solubility of the A specie in A-Ti all…
