Atomic Packing Density Research Articles

Soft magnetic properties and corrosion resistance of Fe-Co-B-M (M[dbnd]Nb, Ta and NbNi) metallic glasses

Fe-based metallic glasses (MGs) with good soft magnetic properties, high thermal stability combined with high corrosion resistance should have greatest potential for wide variety of applications. In this work, the effects of Nb, Ta and NbNi additions on the glass forming ability (GFA), thermal stability, magnetic properties and corrosion resistance of the as-spun Fe-Co-B-based amorphous ribbons have been investigated. The (Fe0.7Co0.3)75B21Nb4 and (Fe0.7Co0.3)75B21Ta4 MG possess the larger supercooled liquid region >40 K, presenting a high GFA. Via appropriate adjustment of the Fe/Co content ratio (4:1), the glass transition temperature and onset crystallization temperature of the (Fe0.8Co0.2)75B21Nb4 MG reach up to 800 and 826 K, respectively, indicating the best thermal stability. All the amorphous ribbons show the soft magnetic properties identified by the coercivity <1.3 Oe. Moreover, the (Fe0.8Co0.2)75B21Nb4 MG exhibits the good soft magnetic properties with a high saturation magnetization (Ms) up to 206 emu/g (1.92 T). Besides a positive role of Nb addition, the enhanced Ms can be attributed to the increased magnetic moment with substitution of Co by Fe, and the easy movement of magnetic domains induced by the decreased atomic packing density. In addition, the tested samples are all spontaneously passivated with a large passive region during electrochemical polarization measurement in the seawater solution. Especially, the (Fe0.7Co0.15Ni0.15)75B21Nb4 MG presents the optimal corrosion resistance with the most positive corrosion potential.

Ultrathin Gold Nanowires with the Polytetrahedral Structure of Bulk Manganese.

Despite the intensive interest in thin gold nanowires for a variety of technologically important applications, key details of the mechanism of their formation and atomic-scale structure remain unknown. Here we synthesize highly uniform, very long, and ultrathin gold nanowires in a liquid-phase environment and study their nucleation and growth using in situ high-energy synchrotron X-ray diffraction. By controlling the type of solvents, reducing agents, and gold precursor concentration, it is shown that the nucleation and growth of gold nanowires involve the emergence and self-assembly of transient linear gold complexes, respectively. In sharp contrast with the face-centered-cubic bulk gold, the evolved nanowires are found to possess a tetrahedrally close packed structure incorporating distorted icosahedra and larger size coordination polyhedra of the type observed with the room-temperature phase of bulk manganese. We relate the complexes to synergistic effects between the selected precursor and reducing agents that become appreciable over a narrow range of their molar ratios. We attribute the unusual structural state of gold nanowires to geometrical frustration effects arising from the conflicting tendencies of assemblies of metal atoms to evolve toward attaining high atomic packing density while keeping the atomic-level stresses low, ultimately favoring the growth of cylindrical nanowires with a well-defined diameter and atomically smooth surface. Our work provides a roadmap for comprehensive characterization and, hence, better understanding of 1D metallic nanostructures with an unusual atomic arrangement and may have important implications for their synthesis and performance in practical applications.

Competitive effects of modifier charge and size on mechanical and chemical resistance of aluminoborate glasses

Lithium aluminoborate glasses exhibit high resistance to cracking under contact loading, but low hardness and poor chemical durability in aqueous media. On the other hand, alkaline earth aluminoborate glasses feature improved chemical resistance and hardness, but lower resistance to cracking. In this work, we investigate the possibility to simultaneously improve the mechanical and chemical resistance of aluminoborate glasses by mixing alkali and alkaline earth modifiers. We study the mixed Li/Ba and Li/Mg aluminoborate glasses, since Li+ and Ba2+ have different charge and size but similar modifier field strength (charge to size ratio), while Mg2+ has the highest field strength among these modifiers due to its small size. The two glass series will thus give insights into the competitive effects of modifier charge and size on glass structure, mechanical properties, and dissolution rates in acidic, neutral, and basic solutions. The substitution of barium for lithium at fixed [Al2O3]/[B2O3] ratio does not affect the network structure and properties, such as hardness and dissolution rate. However, the substitution of magnesium for lithium leads to an increase in hardness and chemical durability for all pH solutions (2, 7, and 14), likely as a result of the increase in the fractions of five- and six-fold coordinated aluminum species and atomic packing density. The glass with 5 mol% Li2O and 20 mol% MgO exhibits the best combination of high hardness and low dissolution rate, while maintaining a good crack-resistance comparable to that of the Li-aluminoborate glass. In both mixed glass series, the lowest dissolution rates are measured in neutral solutions, while those in acidic and especially basic media are higher. The structural origins of the trends in chemical and mechanical properties are discussed based on 11B and 27Al nuclear magnetic resonance (NMR) spectroscopy measurements.

An in-depth investigation of Mg-Zn-Ca metallic glasses: A first principles study

The atomic structures, glass forming evolutions, mechanical properties and high pressure behavior of Mg75Zn20Ca5 and Mg60Zn35Ca5 bulk metallic glasses, which are promising candidates for biomedical implants, have been examined by using ab initio molecular dynamics simulations. The pair-distribution function and coordination number analyses show that increasing Zn content in the alloy results in a decrease in several bond distances and an increase in the total coordination number of each species due to the atomic size difference between Mg and Zn atoms. According to the Voronoi tessellation, bond pair and bond angle distribution analyzes, the fivefold geometrical arrangements (pentagonal-bipyramid) are the most predominant in the first coordination shell, indicating the stability of the amorphous states and their dense atomic packing. The most striking result emerged from the calculations of mechanical properties is that an increase of Zn (≥30%) content in the alloy yields embrittlement in the alloys. Under uniaxial compressions, both compositions undergo structural failure between 6 and 8 GPa. Under hydrostatic pressure, a diminishing in fcc/hcp ordering and an enlargement of the ideal icosahedral ordering may indicate a more disordered structure. In our view, these results represent a good step toward understanding the atomic structures Mg-Zn-Ca bulk metallic glasses.

Correlating the five parameter grain boundary character distribution and the intergranular corrosion behaviour of a stainless steel using 3D orientation microscopy based on mechanical polishing serial sectioning

Using 3D-orientation microscopy consisting of electron backscatter diffraction (EBSD) and serial sectioning by mechanical polishing we investigated the correlation between grain boundary (GB) character expressed by its 5-rotational parameters and the GB precipitation and intergranular corrosion (IC) behaviour on an AISI 304L stainless steel. From the measurements, a volume of approximately 4.8 × 106 μm3 containing 451 grains and approximately 1500 GBs was reconstructed. Dominance of {111} planes for Σ3n (n = 1, 2, 3) GBs was found. In general, the IC behaviour of random high angle grain boundaries (RHAGBs) and low-Σ coincidence site lattice (CSL) boundaries (Σ ≤ 29) does not show a large difference. The corrosion behaviour of low angle grain boundaries (LAGBs) is strongly dependent on the misorientation angles, while the crystallographic GB plane becomes dominant for the behaviour of low-Σ CSL GBs and RHAGBs. It was found that the corrosion resistance is related on the atom packing density (APD) of the GB plane. High corrosion resistance appears for GBs with high APD values (in particular for the (111) planes with a maximum APD value).

Structural impact of nitrogen incorporation on properties of alkali germanophosphate glasses

AbstractThe structure, atomic packing density, calorimetric glass transition, and hardness of mixed sodium–lithium germanophosphate oxynitride glasses with varying Ge/P and N/P ratios were investigated. The combined influences of nitridation and mixed network former effect (MNFE) on the glass structure were analyzed using Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and 31P nuclear magnetic resonance (NMR) spectroscopy. Evidence for the existence of germanium in a higher coordination state, i.e., five‐ or sixfold coordination, was obtained by performing XPS analysis of the oxide glasses, with indication of conversion to tetrahedral coordination upon nitridation. Raman spectroscopy measurements implied that the germanate network was modified upon nitridation, including the removal of ring‐like germanate structures and P–O–Ge mixed linkages. The partial anionic N‐for‐O substitution gave rise to the linear dependence of the glass transition temperature (Tg) and hardness (HV) on nitrogen content (expressed as N/P ratio), especially for lower Ge/P ratio. However, nitridation also caused an unexpected increase in liquid fragility and decrease in density. This suggests that the governing structural parameter for property evolution in such LiNaGePON glasses is not only the increased degree of cross‐linking of the phosphate chains, but rather the short‐ and intermediate‐range structural modifications within the germanate component of the oxynitride glasses.

Structural Compromise between High Hardness and Crack Resistance in Aluminoborate Glasses.

Alkali aluminoborate glasses have recently been shown to exhibit a high threshold for indentation cracking compared to other bulk oxide glasses. However, to enable the use of these materials in engineering applications, there is a need to improve their hardness by tuning the chemical composition. In this study, we substitute alkaline earth for alkali network-modifying species at fixed aluminoborate base glass composition and correlate it with changes in the structure, mechanical properties, and densification behavior. We find that the increase in field strength (i.e., the charge-to-size ratio) achieved by substituting alkaline earth oxide from BaO to MgO manifests itself in a monotonic increase in several properties, such as atomic packing density, glass-transition temperature, densification ability, indentation hardness, and crack resistance. Although the use of alkaline earth oxides as modifier enables higher hardness values (increasing from 2.0 GPa for Cs to 5.8 GPa for Mg), their crack resistance is generally lower than that of the corresponding alkali aluminoborate glasses. We discuss the origin of this compromise between hardness and crack resistance in terms of the ability of the glass networks to undergo structural transformations and self-adapt under stress. We show that the extent of volume densification scales linearly with the number of pressure-induced coordination number changes of B and Al.

The Anisotropy of $\Delta E$ Effect of Fe–Ga Single Crystal

An extended model based on the Jiles–Thoekle and Armstrong model is used to model the anisotropy of $\Delta E$ effect of the Fe–Ga single crystal. Some specific crystallographic directions which are [100], [110], [111], and [112] directions are selected to study the magnetomechanical behavior under magnetic field and stress loading. The strain–stress curves and Young’s modulus-stress curves are simulated. Two more cases of direction selection [ $\theta \in (0,\pi)$ , $\varphi = \pi $ /4, and $\theta = \pi $ /2, $\varphi \in (0,2\pi)$ ] are used to study the relationship between $\Delta E$ effect and crystallographic direction, which shows that the directions showing minimum saturated magnetostriction and maximum pure elastic modulus along the two routes have an abnormal change trend of the $\Delta E$ effect. The results show that the $\Delta E$ effect of the Fe–Ga single crystal has a high degree of anisotropy. Both the atomic packing density and the domain motion have great effect on the final magnetomechanical coupling behavior.

Dependence of Crystallographic Orientation on Pitting Corrosion Behavior of Ni-Fe-Cr Alloy 028

The influence of crystallographic orientation on the pitting corrosion behavior of Ni-Fe-Cr alloy 028 was studied using a combination of X-ray diffraction (XRD), electron backscatter diffraction (EBSD), potentiodynamic polarization technique, and atomic force microscopy (AFM). The results show that there is anisotropy of pitting corrosion that strongly depends on crystallographic orientation of the surface plane. The distribution of pit density in a standard stereographic triangle indicates that the crystallographic planes close to {100} are more prone to pitting corrosion compared to planes {110} and {111}. The surface energy calculation of (001) and (111) shows that the plane with a high atomic packing density has a low surface energy with concomitant strong resistance to pitting corrosion. A correlation function between crystallographic orientation and pitting corrosion susceptibility suggests a method that not only predicts the pitting resistance of known textured materials, but also could help to improve corrosion resistance by controlling material texture.

Microstructural evolutions and fractal characteristics in medium range level in AlxNi100-x alloys during rapid solidification process

Recent progress of simulations at the atomic level has led to a better understanding of the structural behaviors of metallic glasses. In this work, molecular dynamic simulations were steered to study the atomic scale structures of AlxNi100-x bulk metallic glasses. To investigate the structural evolution of a wide composition range of Al-Ni compound, we have adopted various analytical techniques: radial distribution function (RDF), coordination numbers and Voronoi Tessellation Analysis. Results of the Voronoi tessellation analysis suggested that the icosahedral-like clusters are identified as the basic local structural motifs in most compositions of Ni-Al alloys and their connection leads to the formation of medium-range order. In addition, Al-poor metallic glasses have the best glass forming ability since they are characterized by more fraction of icosahedral-like clusters. A detailed analysis of the specific Al10Ni90 shows the formation of polyicosahedra built by the intercrossed icosahedral clusters. Interestingly, the power-law scaling behavior between the coordination numbers and the coordination shell radius of AlxNi100-x indicates that the short and medium range order exhibit fractal characteristics related to the formation of icosahedral-like clusters that possesses a high atomic packing density and a low free volume.

Effects of Al addition on atomic structure of Cu-Zr metallic glass

The atomic structures of Cu52Zr48 and Cu45Zr48Al7 metallic glasses (MGs) have been studied by molecular dynamic simulations. The results reveal that the molar volume of the Cu45Zr48Al7 MG is smaller than that of the Cu52Zr48 MG, although the size of the Al atom is larger than that of the Cu atom, implying an enhanced atomic packing density achieved by introducing Al into the ternary MG. Bond shortening in unlike atomic pairs Zr-Al and Cu-Al is observed in the Cu45Zr48Al7 MG, which is attributed to strong interactions between Al and (Zr, Cu) atoms. Meanwhile, the atomic packing efficiency is enhanced by the minor addition of Al. Compared with the Cu52Zr48 binary MG, the potential energy of the ternary MG decreases and the glass transition temperature increases. Structural analyses indicate that more Cu- and Al-centered full icosahedral clusters emerge in the Cu45Zr48Al7 MG as some Cu atoms are substituted by Al. Furthermore, the addition of Al leads to more icosahedral medium-range orders in the ternary MG. The increase of full icosahedral clusters and the enhancement of the packing density are responsible for the improved glass-forming ability of Cu45Zr48Al7.

Models Manufacturing of Crystal Systems: An Introduction to Ceramic Materials

It is presented a teaching methodology that uses models to better understanding and visualization of crystal systems, considering the large number of students with limited vision spatial perception. The understanding of the relationship between the structure of materials with their properties is essential for engineering students in which it’s possible to relate mechanical properties through knowledge of the crystalline structure of the material, such as machining, heat treatment, mechanical forming processes, etc. The parameter identification of: number of atoms, coordination number, atomic packing factor and density is emphasized in the crystalline systems. Students first study the structure of the crystalline solid and the main crystal systems, key issues for further understanding of the remaining content. Knowing the structure of crystalline solids and understanding its applicability in engineering, makes all the difference to motivate students during the course. With the inclusion of this methodology there has been a better understanding of the concepts related to crystalline systems, as the students build their own structures with the most suitable material chosen by them.

First principle study on atomic structure of La65X35(X=Ni, Al) metallic glasses

The atomic-level structure of metallic glasses (MGs) is one of the most fundamental and challenging topics in condensed matter physics. Unlike crystalline metals or alloys, the MGs are lacking in a well-defined description of structure order, which is a major obstruction for relating its structure to physical properties. Obviously, it is vitally important to have an in-depth understanding of the atomic packing scheme in MGs. Due to the limitations of experimental characterization methods, it is hard to obtain the atomic packing scheme of MGs in experiment. Computational simulation on an atomic scale has become an important method of characterizing the atomic structure of MGs. The La-based LaNiAl glass forming system is well-known for its good glass-forming ability, distinctive relaxation peak that is well separated from relaxation, and liquid-liquid transition at a temperature around 1000 K. Many efforts have been made to investigate these novel properties. However, the atomic structure of this system is rarely studied. In this paper, the atomic structure evolution from liquids to glass states in La-based binary MGs La65Ni35 and La65Al35 are studied via ab initio molecular dynamics based on the density functional theory. The local structures are systematically analyzed by the radical distribution function, partial radical distribution function (PRDF), Voronoi tessellation method, and bond-type method in Honeycutt-Andersen. The results indicate that the PRDF of NiNi is much weaker than that of AlAl, which indicates the NiNi avoidance in La65Ni35. The major peaks of PRDFs are always smaller than the sum of efficient radius of the two kinds of atoms, especially for LaNi pairs. Atomic structure of the two systems are coincident with dense atomic packing scheme and the difference between major Voronoi polyhedron types (0, 3, 6, 0 for La65Ni35 and 0, 2, 8, 1, 0, 2, 8, 0 for La65Al35) in local structures is controlled by their ratio of solute to solvent atomic size. The high five-fold local symmetry structure gradually increases in both systems with the decrease of temperature, which validates its pivotal part in hindering crystallization. The electronic structure is studied with the partial density of states. It is found that the significant bond-shortening between La and Ni is due to the strong hybridization between Ni-3d and La-5d electrons and this result may play a key role in understanding composition related structure and properties in MGs.

U-based metallic glasses with superior glass forming ability

By using Al as the third and B as the fourth but minor alloying elements for the U66.7Co33.3 basic metallic glass, a series of U-Co-Al(-B) alloys were designed. The quaternary U-Co-Al-B alloys exhibit significantly improved glass-forming ability (GFA) than previously reported U-based metallic glasses. Low fragility (∼24) is found for these new U-based metallic glasses. The improvement in GFA would result from denser atomic packing in the undercooled liquids due to the presence of small B atoms. Some U-Co-Al(-B) glasses showed corrosion resistance comparable to that of U64Co34Al2 glass, known for premium anti-corrosive performance among the unveiled U-based glasses.

Investigation of the Structure and Magnetic Properties of Bulk Amorphous FeCoYB Alloy

The paper presents the results of structural and microstructural studies for the bulk Fe65Co10Y5B20 and Fe63Co10Y7B20 alloys. All the rods obtained by the injection casting method were fully amorphous. It was found on the basis of analysis of distribution of hyperfine field induction that the samples of Fe65Co10Y5B20 alloy are characterised with greater atomic packing density. Addition of Y to the bulk amorphous Fe65Co10Y5B20 alloy leads to the decrease of the average induction of hyperfine field value. In a strong magnetic field (i.e. greater than 0.4HC), during the magnetization process of the alloys, where irreversible processes take place, the core losses associated with magnetization and de-magnetization were investigated.

Structural signature in Au-based amorphous alloys

Here we elucidated that the anomalous shoulder at the left side of first main peak in structural factor is resulted from packing of AuAu bonds in the Au55Cu25Si20 amorphous alloy (AA), while the first main peak is largely from contributions of AuAu, AuCu and AuSi bonds. Local atomic packing of this glass is mainly composed of 9, 10 and 11-coordinated polyhedra rather than icosahedral-like clusters, in which the small sized clusters prefer avoiding each other while large ones prefer connecting themselves. This anomalous atomic packing structure limits its glass forming ability (GFA) and results in the anomalous shoulder of the first peak in structural factor. With the replacement of 5 at.% Cu atoms by Ag atoms, the spatial connectivity and the atomic packing density in Au55Cu20Ag5Si20 AA are largely increased, which homogenize atomic structures at least up to the second nearest neighbors and frustrate the geometry of competing crystalline-like nuclei, consequently, enhancing the GFA, as observed in experiments. This work illustrates how the atomic packing in an Au-based glass is altered by element substitution and sheds new insights to understanding of high GFA from the atomic scale and thus developing novel AAs with larger critical sizes.

Effect of component substitution on the atomic dynamics in glass-forming binary metallic melts

We investigate the substitution of early transition metals (Zr, Hf, and Nb) in Ni-based binary glass-forming metallic melts and the impact on structural and dynamical properties by using a combination of neutron scattering, electrostatic levitation (ESL), and isotopic substitution. The self-diffusion coefficients measured by quasielastic neutron scattering (QENS) identify a sluggish diffusion as well as an increased activation energy by almost a factor of 2 for Hf35Ni65 compared to Zr36Ni64. This finding can be explained by the locally higher packing density of Hf atoms in Hf35Ni65 compared to Zr atoms in Zr36Ni64, which has been derived from interatomic distances by analyzing the measured partial structure factors. Furthermore, QENS measurements of liquid Hf35Ni65 prepared with 60Ni, which has a vanishing incoherent scattering cross section, have demonstrated that self-diffusion of Hf is slowed down compared to the concentration weighted self-diffusion of Hf and Ni. This implies a dynamical decoupling between larger Hf and smaller Ni atoms, which can be related to a saturation effect of unequal atomic nearest-neighbor pairs, that was observed recently for Ni-rich compositions in Zr-Ni metallic melts. In order to establish a structure-dynamics relation, measured partial structure factors have been used as an input for mode-coupling theory (MCT) of the glass transition to calculate self-diffusion coefficients for the different atomic components. Remarkably,MCT can reproduce the increased activation energy for Hf35Ni65 as well as the dynamical decoupling between Hf and Ni atoms.

Atomistic Simulation of ZrNi Metallic Glasses Under Torsion Test

ZrNi metallic glass alloy nanowires (NWs) under torsion are studied using molecular dynamics simulations based on the many-body embedded-atom potential. The effect of cooling rate on the deformation mechanism and mechanical properties of ZrNi NWs is evaluated in terms of shear strain, torque, potential energy and radial distribution function. Simulation results show that for slower cooling rates, the NWs have larger packing density, whereas for faster cooling rates, the packing density of atoms decreases. The amount of deformation increases with increasing torsional angle before it reaches a critical torsional angle ([Formula: see text]. The torque required for deformation and the [Formula: see text] value increase with decreasing cooling rate, indicating a larger mechanical strength. Localized shear bands concentrate at regions with high shear strains, leading to the formation of torsional buckling.

Effect of crystal orientation on corrosion behavior of directionally solidified Mg-4 wt% Zn alloy

Microstructure and crystallographic orientation of directionally solidified Mg-4wt% Zn alloy were characterized by X-ray computed tomography (XCT) and electron backscatter diffraction (EBSD) in this study. Results reveal that Mg-4wt% Zn alloy with dendritic microstructure exhibits typical {0002} basal texture along growth direction. Based on this, the effect of grain orientation on corrosion behavior of directionally solidified Mg-4wt% Zn alloy in 0.9wt% NaCl solution was investigated. Result shows that {0002} oriented planes have better corrosion resistance than {112¯0} and {101¯0} ones, which is attributed to a synergistic effect of surface energy, atomic packing density and the stability of oxidation film.

Crystallization pathways and some properties of lithium disilicate oxynitride glasses

Abstract Lithium silicates have been used as model glasses for scientific and technological studies of glass-ceramics because they easily crystallize in the interiors, even without the addition of any nucleating agent. On the other hand, partial replacement of oxygens with nitrogens affects most properties of oxide glasses, but its effect on the crystallization kinetics of the glasses has been poorly documented. In this work, we report, for the first time, on the crystallization kinetics of nitrited lithium silicate glasses. The oxynitride glasses were prepared by partial substitution of oxygen by nitrogen, up to 6 at% N/(N+O), by melt-quenching the liquid under N 2 atmosphere inside a glove box. As expected, the density, microhardness, and Young's modulus of the glasses improved with increasing nitrogen content. Higher values of glass transition and crystallization peak temperatures were also obtained with an increase in the nitrogen content. Rietveld refinement analysis after adequate thermal treatment revealed that addition of nitrogen led to increasingly higher contents of lithium metasilicate at the expense of the (expected) lithium disilicate crystal phase. Crystallization kinetic parameters such as the activation energy and Avrami index were calculated using Ozawa's equations. These two parameters also increased with increasing nitrogen content, whereas the crystal growth rates decreased with increasing nitrogen content. The above-described changes in the properties of the oxynitride glasses are straightforwardly explained by the increase in the connectivity of the glass network, which results in enhancement of the atomic packing density owing to partial substitution of two-coordinated oxygens by three-coordinated nitrogens.