Atomic Size Effect Research Articles

Effect of Co doping partial of Ga on the structure and magnetic properties of Ga2MnNi

The influence of Co on the crystal structure and magnetic properties of the Ga2−xMnNiCox (x = 0,0.3,0.5,0.7) are investigated in this study. It has been found that the doping of Co does not change the cubic Heusler structure of the parent compound Ga2MnNi. The introduction of a small amount of Co reduces the p-d covalent hybridization effect and decreases the degree of atomic ordering, resulting in an abnormal increase in the lattice parameter when x = 0.3 (from 5.8387 ± 0.0020 Å to 5.8665 ± 0.0012 Å). The atomic size effect plays the main role when x > 0.3, leading to the decrease of lattice parameter. Due to the influence of the lattice parameter and the atomic site occupation on the magnetic interaction, the Curie temperature (TC) and the saturation magnetization are found to decrease at first and then increase with the chemical substitution of Co for Ga. A reentrant spin glass (RSG) below Tf = 65.8 K is confirmed in Ga1.5MnNiCo0.5 alloy due to the competition of ferromagnetic and antiferromagnetic interactions.

Understanding alloying behaviors of Sc, Ni and Zn additions on Al/TiB2 interfaces based on interfacial characteristics and solute properties

In this study, the alloying effects of Sc, Ni and Zn elements on both Al(001)/TiB2(0001) and Al(111)/TiB2(0001) interfaces were assessed using first-principles calculations. The most thermodynamically stable Al/TiB2 interface models with Ti terminal and center stacking sequences were selected. To describe interfacial stability and wettability, the interface energy and the work of adhesion were calculated, respectively. For interfacial stability, the alloyed Al(001)/TiB2(0001) interfaces followed the sequence of Ni > Zn > Sc, while an inverse order was found on the alloyed Al(111)/TiB2(0001) interfaces. For interfacial wettability, Sc- and Ni-alloyed Al(111)/TiB2(0001) interfaces displayed an obvious improvement, and the others showed a slight change. The analysis results of chemical bonding and strain distribution illustrated that the alloying effect of Sc and Ni addition mainly relied on the interfacial structure, and the Zn doping interface largely depended on atomic properties. Consequently, the alloying behavior of solute atoms on the Al/TiB2 interface can be explained by the atomic size effect and electronic interaction.

Unusual solute segregation phenomenon in coherent twin boundaries

Interface segregation of solute atoms has a profound effect on properties of engineering alloys. The occurrence of solute segregation in coherent twin boundaries (CTBs) in Mg alloys is commonly considered to be induced by atomic size effect where solute atoms larger than Mg take extension sites and those smaller ones take compression sites in CTBs. Here we report an unusual solute segregation phenomenon in a group of Mg alloys—solute atoms larger than Mg unexpectedly segregate to compression sites of {10overline 11} fully coherent twin boundary and do not segregate to the extension or compression site of {10overline 12} fully coherent twin boundary. We propose that such segregation is dominated by chemical bonding (coordination and solute electronic configuration) rather than elastic strain minimization. We further demonstrate that the chemical bonding factor can also predict the solute segregation phenomena reported previously. Our findings advance the atomic-level understanding of the role of electronic structure in solute segregation in fully coherent twin boundaries, and more broadly grain boundaries, in Mg alloys. They are likely to provide insights into interface boundaries in other metals and alloys of different structures.

Thermal Vibration of Zinc Oxide Nanowires by using Nonlocal Finite Element Method

Zinc oxide nanowires (ZnO NWs) can be used in some NEMS applications due to their remarkable chemical, physical, mechanical and thermal resistance properties. In terms of the suitability of such NEMS organizations, a correct mechanical model and design of ZnO NWs should also be established under different effects. In this study, thermal vibration analyses of elastic beam models of ZnO NWs are examined based on Eringen's nonlocal elasticity theory. The resulting equation of motion is solved with a finite element formulation developed for the atomic size-effect and thermal environment. The vibration frequencies of ZnO NWs with different boundary conditions are calculated under nonlocal parameter and temperature change values ​​and numerical results were discussed.

A new proposed parameter related with atomic size effect for predicting hardness of HEA coatings

The new parameter ε we proposed in this study is to be more conducive to the hardness design of high-entropy alloy coatings. Here, the parameter ε is defined as the ratio of the atomic size mismatch to the atomic stacking mismatch. This parameter can be used as the criterion for measuring the magnitude of lattice distortion caused by the atomic size effect, which reveals the influence of the atomic size effect on the hardness of the alloy. We collected nearly 100 groups of on high-entropy alloy coatings in order to verify the feasibility of parameter ε, and to analyze the relevant data from available literatures by using the parameter ε. We found that the hardness value of the high-entropy alloy coatings showed monotonicity with the ε increased. When ε < 6.4, the phase structure of the coating is mainly fcc phase, the increase in hardness of the coating is smaller with the increments of ε. The opposite of bcc phase as the main phase for ε > 6.4, hardness value of the coating escalates with the increase of parameter ε. Based on this finding, it is suggested that the parameter ε can then be used to design a high-entropy alloy coatings with the desired hardness.

Structure and chemistry investigations of Ni3InAs thin film on InAs substrate

In situ X-Ray diffraction, Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) were used to investigate the structure and the chemistry of the first phase formed after solid-state diffusive reaction between a Ni thin film (f) and an InAs substrate (s) at low temperature. The results show the formation of a single phase for which the Ni3InAs stoichiometry and the P63/mmc-hexagonal (a = b = 4.00 Å and c = 5.10 Å) structure were evidenced. Investigations in Electron Diffraction are in agreement with a disordered B8~1.5 structure in which additional Ni atoms are randomly distributed with In and As on the equivalent interstitial sites. The epitaxy relationship was also found, namely the [001]s//[210]f zone axes as well as the (220)s//(-120)f and (002)s//(100)f atomic planes. These findings were discussed taking into account atomic size effects.

Application of Machine Learning to Predict Grain Boundary Embrittlement in Metals by Combining Bonding-Breaking and Atomic Size Effects.

The strengthening energy or embrittling potency of an alloying element is a fundamental energetics of the grain boundary (GB) embrittlement that control the mechanical properties of metallic materials. A data-driven machine learning approach has recently been used to develop prediction models to uncover the physical mechanisms and design novel materials with enhanced properties. In this work, to accurately predict and uncover the key features in determining the strengthening energies, three machine learning methods were used to model and predict strengthening energies of solutes in different metallic GBs. In addition, 142 strengthening energies from previous density functional theory calculations served as our dataset to train three machine learning models: support vector machine (SVM) with linear kernel, SVM with radial basis function (RBF) kernel, and artificial neural network (ANN). Considering both the bond-breaking effect and atomic size effect, the nonlinear kernel based SVR model was found to perform the best with a correlation of r2 ~ 0.889. The size effect feature shows a significant improvement to prediction performance with respect to using bond-breaking effect only. Moreover, the mean impact value analysis was conducted to quantitatively explore the relative significance of each input feature for improving the effective prediction.

Effect of solute redistribution on twinnability of twins at a crack tip in Al alloys

The twinnability of twins at a crack tip in Al-X (X = Zn, Li, Ge, Mg, Au, Ag Si, Cu, Sc, Sr, Y, Rb, Zr and La) have been investigated within the framework of the generalized planar fault energy curves by using first-principles calculations. It is found that concentration distribution of solute atom is a key factor affecting the twinnability. Solute atoms under Fermi–Dirac distribution would lead to a greater influence in twinnability than uniform distribution. For most Al alloys, twinnability almost keeps stable, which directly explains the rarity of twins produced in Al alloys. It is significant that the twinnabilities of Al-X (X = Sr, Rb and La) increase continuously with the increasing concentration of Sr, Rb and La atoms. Our results indicate that the atomic size effect plays an important role in the change of twinnability, and atoms whose radius is more than 1.3 times of Al would have more obvious effects on twinnability.

The stability of deformation twins in aluminum enhanced by alloying elements

Introducing and stabilizing twins in aluminum is a challenge for metals research due to their high formation energy. Employing first-principles calculations, we investigated the twin boundary segregation of alloying elements and their impact on the twin boundary energy in aluminum. Alloying elements with small solubilities but strong interaction with twin boundary would significantly reduce twin boundary energies in aluminum at low temperatures. With increasing temperature, their segregation near twin boundary weakens, leading to their influence on twin boundary energies reduced. Some elements with large solubilities may greatly reduce the twin energies not only at low temperatures but also at high temperatures. Based on careful analysis of charge density and atomic radius, it has been found that chemical difference has little influence on twin boundary energy whereas the atomic size effect plays a leading role in causing the change of twin boundary energy.

Electronic structure tuning of the anomalous thermoelastic behavior in Nb-X (X = Zr, V, Mo) solid solutions

Nb exhibits an anomalous temperature dependency of the elastic constant c44, which increases from around 500 to 2500 K. This anomaly can be affected by alloying. To study the effect of atomic size and electronic structure on the thermoelastic behavior in bcc Nb-X (X = Zr, V, Mo) solid solutions, the shear thermoelastic constants c44(T) and c′(T) are investigated theoretically using a density functional theory based model in which electronic and thermal expansion effects are treated separately. For all binary solid solutions, an anomalous thermoelastic behavior is predicted, which can be attributed to the electronic entropy induced by a high population of electronic states at the Fermi level (&amp;gt;0.8 states/eV atom). The onset of the increase in c44 remains unchanged for isoelectronic Nb-V indicating the absence of a size effect on the anomalous thermoelastic behavior. In contrast, the anomalous thermoelastic behavior can be tuned by alloying with Zr or Mo, due to the valence electron concentration induced change in the density of states in the vicinity of the Fermi level, leading to a shift in the anomalous trend of c44 to lower temperatures. An anomalous temperature behavior is also predicted for the shear elastic constant c′ for Nb-Mo solid solutions with Mo concentrations between 24 and 33 at. %. With increasing Mo concentrations, the anomaly in both elastic constants is suppressed due to the continuous reduction in electronic states at the Fermi level.

Atomic packing and size effect on the Hume-Rothery rule

Atomic packing problems have been widely investigated by physicists, mathematicians, material scientists as a long-standing scientific issue. While it is widely known that the monodisperse particles are closely packed into the face-centered cubic structure, the increase of polydispersity will suppress the crystallization. It is still unsolved when and how the crystalline packing collapses as the particle-size difference increases over a critical value. On the other hand, the atomic-size factor of the solution has gone deeply into the concept of applied physicists and physical metallurgy scientists since the proposal of Hume-Rothery rules in 1934. Although great efforts have been devoted to understanding this simple but important empirical rule of the atomic size effect, it remains a question without a complete answer even in binary alloy systems. Here, the two outstanding issues were solved in a rigorous geometric packing instability model even in the multicomponent system. The physical scenario presented here are helpful for understanding the close packing instability and the atomic-size effect in the Hume-Rothery rules.

Atomic size effect of alloying elements on the formation, evolution and strengthening of γ′-Ni3Al precipitates in Ni-based superalloys

Mechanical properties of Ni-based superalloys strongly depend on phase and site preferences of alloying elements which influence bonding strength within γ′-Ni3Al precipitates and microstructural characteristics of these unique class of materials. In the current work, therefore, besides disclosure of the phase partitioning behaviours of alloying X elements (i.e. X = Co, Cr, Nb, Ta or Ti) among γ′ and γ phases, their site occupancy tendencies in γ′ precipitates (determined via first-principles ab initio calculations at 0 K) and effects on the microstructural evolution of Ni80Al15X5 alloy systems (exposed to aging at 800 °C for 4, 16, 64 and 256 h, respectively) have been examined. Bonding features of Ni-Al, Ni-X and Al-X atomic pairs within Ni3Al-X intermetallics have been simulated by utilizing charge density difference (CDD) method, which reveals site preferences of alloying X elements as well. Present theoretical and experimental investigations have shown that mechanical strength of Ni-based superalloys is predominantly affected by bonding properties within γ′ precipitates. As atomic radii of alloying X elements become closer to that of Al atom, energy change parameter, ENi→AlX, values decrease and more Al sublattice sites are preferentially occupied in γ′ precipitates. Correspondingly, bonding strength of Ni-X atomic pairs along <110> directions of Ni3Al-X phases and micro-hardness properties of both as-cast and pre-aged Ni80Al15X5 alloy systems enhance in the order of X = Co < Cr < Ti < Nb < Ta additions. Nevertheless, with increasing aging time, mechanical strength of alloys weakens in parallel with increasing size of γ′ precipitates simultaneously evolved from near-spherical to irregular forms.

Is Heusler alloy Ti2NiAl a half-metal

The stable structure of Heusler alloy Ti2NiAl and its influence on the electronic structure and magnetic properties were investigated theoretically. It is found that in Ti2NiAl, the L21 and XA structures have similar stabilities, this is related to the competition between the “number of valence electrons” and atomic size effect. L21B type structure can lower the total energy of Ti2NiAl further by 0.04eV/f.u. and has the highest stability, in which Ti and Ni atoms occupy the A, C sites randomly. The energy difference between L21/XA and L21B structures is about 0.04eV/f.u. Different atomic ordering influence the electronic structure obviously. XA type Ti2NiAl is a half-metal with an integral total moment of 3.00 μB/f.u., as has been reported in literature. But this character is destroyed in both L21 and L21B structures. In the stable L21B structure, Ti2NiAl is a normal ferromagnetic metal with a total magnetic moment of 0.84 μB/f.u.

Effects of calcium on planar fault energies in ternary magnesium alloys

We are motivated by the need to design magnesium alloys that are free of rare-earth additions, but, nevertheless, forgeable and free of strong basal texture. It has become recently clear that calcium is a promising candidate to replace the economically strategic rare-earth elements. To this end, we focus on the planar faults that typically bound partial glide dislocations of the hcp lattice. We have made first-principles calculations to examine the generalized stacking fault energy (SFE) of the basal, first- and second-order pyramidal planes. We examine the changes in fault energy and anisotropy for increasing alloy concentrations and the effect of low concentrations of calcium. For the calculation of the SFEs from first principles, we use the non-self-consistent Harris-Foulkes approximation to the local density functional theory. We demonstrate that while this approximation leads to high computational efficiency, there is no significant loss of precision compared to the self-consistent Hohenberg-Kohn functional. We go beyond all previous work in which the alloying element is assumed to reside within the fault and instead address the more realistic situation in which the SFE is modified by the remote presence of the impurity. This allows us to determine whether segregation is expected and we find that while elements do segregate to the basal fault they do not to pyramidal faults. Nevertheless, in either case, the fault energy is strongly modified by alloying. This argues that either a long-ranged electronic structure effect is in play, or the fault energy modification is affected by the atomic size difference---particularly large in the case of Ca. We find that Mg-Li-Ca and Mg-Zn-Ca alloys show a remarkable decrease in anisotropy, which is consistent with their known high strength and formability. In favorable cases, this comes about by strengthening basal slip rather than weakening nonbasal slip. The Ca contribution increases inversely with the atomic size of the alloying element, allowing us to speculate that alloying effects are generally atomic size effects.

Universal description of heating-induced reshaping preference of core-shell bimetallic nanoparticles.

To achieve universal description of the reshaping process of core-shell bimetallic nanoparticles, we combined the tight-binding Ising Hamiltonian model with molecular dynamic simulations to propose a general theoretical model at the atomic scale while considering the temperature, bond energy, atomic size, and surface energy effects. Based on this model, we can quantitatively analyze the tendency of core-shell structured bimetallic nanoparticles toward the reshaping phenomenon upon heating. By rapidly screening 196 types of bimetallic nanoparticles (containing transition metal elements from VIII to IIB groups in the fourth, fifth, and sixth rows of the periodic table), we identified forty-four kinds of bimetallic nanoparticles with reshaping tendency upon heating, which was validated by molecular dynamic simulations and available experimental results. With increasing temperature, the bimetallic nanoparticles with reshaping preference were transformed from an icosahedron to a star-like shape. In contrast, the structure of bimetallic nanoparticles without reshaping preference was transformed from an icosahedron to a sphere shape, which is usually considered to be the normal pre-melting phenomenon. Further structural analysis indicated that the reshaping of bimetallic nanoparticles could be ascribed to different diffusion mechanisms, where a dominant unidirectional mechanism leads to reshaped bimetallic nanoparticles and a bidirectional diffusion mechanism results in no-reshaped bimetallic nanoparticles. This study provides a deep insight into the origin of reshaping in bimetallic nanoparticles, and it may stimulate extensive studies on engineering bimetallic nanoparticles to switch on/off reshaping upon heating, for example, by modifying the structures, atomic arrangement or composites of bimetallic systems in future.

First-principles study of phase stability of bccXZn(X=Cu, Ag, and Au) alloys

First-principles density-functional theory is used to study the phase stability/instability and anomalies in the formation of the high-temperature bcc phases of $X\mathrm{Zn}$ ($X$ = Cu, Ag, and Au) alloys. Although from perhaps a naive point of view, their properties are expected to monotonically depend on the noble-metal ($X$) column position in the periodic table, this is not the case. For example, the middle-column AgZn alloy has a lower bcc order-disorder (critical) temperature than the CuZn and AuZn alloys above and below in the column. It is shown that this and other nonmonotonic behaviors can be explained in terms of a competition between atomic-size effects and $X$-atom $d$-orbital spatial extent. For example, charge-density studies and pair-potential modeling of $X\mathrm{Zn}$ alloys show that the effective Ag-Zn bond is significantly weaker than either the Cu-Zn or Au-Zn bond at their respective equilibrium lattice constants. We find that an increased atomic-core size effect initially weakens the $X$-Zn bonding as one goes from CuZn to AgZn, but then the larger $d$-orbital spatial extent for higher principal quantum numbers becomes a more dominant effect and increases the bonding from AgZn to AuZn. This study is focused on the highly symmetric cubic high-temperature phases, where relative bond-strength magnitudes should be far more important than any bond-directionality effects; the lattice parameters, bulk moduli, elastic constants, Debye temperatures, heats of formation, and order-disorder temperatures for the bcc phases of the three $X\mathrm{Zn}$ alloys are calculated and compared with experiment.

General trends between solute segregation tendency and grain boundary character in aluminum - An ab inito study

Quantum mechanical calculations have been performed to establish general trends in propensity for the segregation of solutes across the periodic table at or in the neighborhood of an extended set of commonly observed special grain boundaries in face centered cubic aluminium. To this end, Al has been considered as the matrix and elements from 3d and 4d transition metals as well as those from group II, III and IV have been selected as solute atoms. For transition metal solutes, we find a concave-up parabolic-like dependency of segregation energy as a function of atomic number that is argued to be caused by the competition between chemical bonding and atomic size effects. The analysis is corroborated quantitatively by the computation of crystal orbital Hamiltonian population for solute-Al and Al-Al pairs as well as the Voronoi polyhedral surrounding solutes at a sample GB. The parabolic-like (concave-down trend) dependency of the cohesiveness of grain boundaries is explained by an equivalent trend in the bonding strength of Al-Al pairs at the segregated GBs. We extend this investigation to examine the stability of the solid solution polycrystalline state by comparing the calculated segregation energy against the combined energetic cost of grain boundary and intermetallic precipitate formation. The results may serve as a design tool for tailoring polycrystalline alloys with desired properties.

Phase stability and mechanical properties of AlHfNbTiZr high-entropy alloys

Refractory high-entropy alloys (HEAs) have potential to be high temperature structural materials, but poor ductility and phase stability have been remaining the key issues for their application. In this paper, the phase compositions, phase stability and mechanical properties of Alx(HfNbTiZr)100-x (x = 0, 3, 5, 7, 10 and 12 in atomic percent) HEAs have been systematically studied. It has been shown that after cold-worked and heat-treated at 1273 K for 0.5 h, all the HEAs formed a single solid solution (SS) phase with BCC structure. For the HEAs with more than 7% Al, a (Al, Zr)-rich phase with hexagonal structure was precipitated when aged at 873 K. The yield strength increases linearly with the content of Al for the SS treated HEAs. Typically, Al5(HfNbTiZr)95 exhibits the fracture strength and elongation of 915.2 MPa and 31.5%, respectively, resulting in the product of strength and elongation up to 28828 MPa%, which is comparable to that of FCC HEAs with excellent ductility. The strengthening mechanism for these HEAs has been discussed based on the solid solution strengthening effects. Atomic size effect and electron concentration are considered to attribute to the rapid solid solution strengthening. And dislocation substructure evolution was also evaluated for current HEAs.

Effect of certain alkaline metals on Pr doped glasses to investigate spectroscopic studies

Incorporation of different Alkaline earth metal like Barium, Calcium and strontium in sodium lead borate glass doped with Pr3+ is studied. Physical parameters such as density, molar volume, molar refractivity etc have been evaluated. Effect of different atomic size of alkaline metal using optical and physical parameters is analysed. XRD and FTIR were carried out to know the structural behaviour of the glasses. Absorption and Emission spectra are recorded at room temperature and the results were discussed.

Parallels in Structural Chemistry between the Molecular and Metallic Realms Revealed by Complex Intermetallic Phases.

The structural diversity of intermetallic phases poses a great challenge to chemical theory and materials design. In this Account, two examples are used to illustrate how a focus on the most complex of these structures (and their relationships to simpler ones) can reveal how chemical principles underlie structure for broad families of compounds. First, we show how experimental investigations into the Fe-Al-Si system, inspired by host-guest like features in the structure of Fe25Al78Si20, led to a theoretical approach to deriving isolobal analogies between molecular and intermetallic compounds and a more general electron counting rule. Specifically, the Fe8Al17.6Si7.4 compound obtained in these syntheses was traced to a fragmentation of the fluorite-type structure (as adopted by NiSi2), driven by the maintenance of 18-electron configurations on the transition metal centers. The desire to quickly generalize these conclusions to a broader range of phases motivated the formulation of the reversed approximation Molecular Orbital (raMO) approach. The application of raMO to a diverse series of compounds allowed us recognize the prevalence of electron pair sharing in multicenter functions isolobal to classical covalent bonds and to propose the 18 - n electron rule for transition metal-main group (T-E) intermetallic compounds. These approaches provided a framework for understanding the 14-electron rule of the Nowotny Chimney Ladder phases, a temperature-driven phase transition in GdCoSi2, and the bcc-structure of group VI transition metals. In the second story, we recount the development of the chemical pressure approach to analyzing atomic size and packing effects in intermetallic structures. We begin with how the stability of the Yb2Ag7-type structure of Ca2Ag7 over the more common CaCu5 type highlights the pressing need for approaches to assessing the role of atomic size in crystal structures, and inspired the development of the DFT-Chemical Pressure (CP) method. Examples of structural phenomena elucidated by this approach are then given, including the Y/Co2 dumbbell substitution in the Th2Zn17-type phase Y2Co17, and local icosahedral order in the Tsai-type quasicrystal approximant CaCd6. We next discuss how deriving relationships between the CP features of a structure and its phonon modes provided a way of both validating the method and visualizing how local arrangements can give rise to soft vibrational modes. The themes of structural mechanisms for CP relief and soft atomic motions merge in the discovery and elucidation of the incommensurately modulated phase CaPd5. In the conclusion of this Account, we propose combining raMO and CP methods for focused predictions of structural phenomena.