Formation Of Short-range Order Research Articles

Formation of short-range order in Al–Ge–Fe melts: influence of composition

ABSTRACTThe short-range order in Al–Ge–Fe melts has been studied by X-ray diffraction and reverse Monte Carlo simulations in wide concentration range. Influence of the replacement of one component by another while the content of third component is constant on the formation of a local structure of ternary melts has been discussed. It has been shown that at Ge content less than 30 аt. % Ge atoms are uniformly distributed in the volume of the Al–Ge–Fe melts and atomic clusters with structure similar to the liquid germanium are formed at content more than 30 аt. % Ge. The addition of the third component (Ge or Al) to the binary Al–Fe or Fe–Ge melts, correspondingly, results in competition between Al and Ge atoms in formation of the local structure around Fe atoms. The obtained concentration dependences of the nearest neighbour distances point out that the ternary interactions take place in the Al–Fe–Ge melts.

Effects of carbon and nitrogen on austenite stability and tensile deformation behavior of 15Cr-15Mn-4Ni based austenitic stainless steels

The austenite stability and deformation behavior of 15Cr-15Mn-4Ni based austenitic stainless steels alloyed with C or N are studied. Stability of austenite against martensitic transformation was enhanced with increasing C or N content, while N was a more effective element at an equivalent concentration. With the addition of interstitial elements, both yield and tensile strengths increased. The more pronounced effect of N than C on the yield strength was likely to be due to the formation of short range ordering (SRO) and its higher binding energy with dislocations. The strain hardening behavior was described by classifying the deformation regime into two parts. At low strain, short-range obstacle was a main governing factor, while strain-induced martensitic transformation and dislocation interaction controlled the strain hardening at high strain. The difference between C and N on strain hardening at low strain arose from the existence of SRO which induced the formation of planar slip in N containing alloys. On the other hand, the difference in strain hardening behavior at high strain was attributed to the difficulty of SRO recovery in N containing alloys and a larger increase of SFE by N addition in comparison with C.

Influence of Minor Additions on Icosahedral Short-Range Order (ISRO) and Its Consequences on Nucleation and Solidification Growth Kinetics in fcc Alloys

The present contribution reviews the recent progress related to the influence of icosahedral short-range order (ISRO) and icosahedral quasicrystal (iQC) formation on the solidification of fcc alloys through minor solute element additions. From intensive crystallographic analysis of multi-twinned region in as-cast Al-based and Au-based fcc alloys, it has been shown recently that a so-called iQC-mediated nucleation mechanism occurs when a few hundred ppm of Cr and Ir, respectively, are added to the melt (Kurtuldu et al. in Acta Mater 61(19):7098–7108, 2013; Acta Mater 70:240–248, 2014). Similarly, it appears that the growth directions of dendrites in Al–Zn/Cr are also influenced by ISRO in the liquid, thus showing an attachment kinetics effect (Kurtuldu in Influence of trace elements on the nucleation and solidification morphologies of fcc alloys and relationship with icosahedral quasicrystal formation, 2014). Similar observations have been made recently in pink gold alloys with Ir additions, when the solidification speed is increased (Zollinger et al. in Acta Mater, 2018).

Dust particles of finite dimensions in complex plasmas: thermodynamics and dust-acoustic wave dispersion

Grounded on the premise that dust particles are charged hard balls, the analysis in Davletov et al. (Contrib. Plasma Phys., vol. 56, 2016, 308) provides an original pseudopotential model of intergrain interaction in complex (dusty) plasmas. This accurate model is engaged herein to consistently treat the finite-size effects from the process of dust particle charging to determination of the thermodynamic quantities and the dust-acoustic wave dispersion in the strongly coupled regime. The orbital motion limited approximation is adopted to evaluate an electric charge of dust grains immersed in a neutralizing background of the buffer plasma. To account for finite dimensions of dust particles, the radial distribution function is calculated within the reference hypernetted-chain (RHNC) approximation to demonstrate a well-pronounced short-range order formation at rather large values of the coupling parameter and the packing fraction. The evaluated excess pressure of the dust component is compared to the available theoretical approaches and the simulation data and is then used to predict the dust-acoustic wave (DAW) dispersion in the strongly coupled regime under the assumption that the dust particles charge varies in the course of propagation. In contrast to many previous investigations, it is demonstrated for the first time ever that for DAWs the charge variation of dust particles should necessarily be taken into account while evaluating the dust isothermal compressibility.

Cation distribution and magnetic properties of Zn-substituted CoCr2O4 nanoparticles

CoCr2O4 is a normal spinel where Co occupies the tetrahedral (A) site and Cr occupies the octahedral (B) site; it is important to examine the cation distribution and magnetic properties by substituting a non-magnetic ion like Zn. In this context, we have synthesized pure phase ZnxCo1-xCr2O4 (x = 0.05, 0.1) of crystallite size 10 nm through conventional co-precipitation technique. Fourier transform of Co, Zn, and Cr K-edge spectra obtained from extended X-ray absorption fine structure demonstrates that while Co and Zn prefer the A site, Cr strongly occupies the B site. The paramagnetic to long range ferrimagnetic transition, TC, decreases from 97 K in CoCr2O4 (bulk) to 87.4 K at x = 0.1 with an intermediate TC of 90 K at x = 0.05. The decrease in TC is ascribed to decrease in A-B exchange interaction confirming the preferential occupation of Zn2+ ions towards the A site. The spin-spiral transition, TS, decreases from 27 K in bulk (CoCr2O4) to 24 K at x = 0.1 followed by a spin lock-in transition, TL, observed at 10 K which remains unchanged with increase in Zn concentration. The diffuse neutron scattering in both compositions shows the evidence of long range spiral ordering in contrast to the simultaneous formation of long and short range order in single crystals of CoCr2O4. The decrease in maximum magnetization from 9 to 8 emu/g and an increase in coercivity from 3.2 to 5.2 kOe at 2 K with an increasing Zn concentration from 0.05 to 0.1 have been explained by considering the Yafet-Kittel model.

Interdiffusion in lithium–lead melts

Interdiffusion in lithium-lead melts containing 10.0, 15.0, 18.1, 20.0, 25.1, 30.2, 38.2, 40.1, 43.1, 46.7, 50.2, 60.2, and 70.0 at. % Pb was investigated using gamma-ray attenuation technique in the temperature range from 720 to 1030 K. It has been found that the concentration dependence of the interdiffusion coefficient has a maximum in the vicinity of 20 at. % Pb. This phenomenon is shown to relate to a tendency of formation of short-range order in liquid alloys of lithium−lead.

Thermal ageing and short-range ordering of Alloy 690 between 350 and 550 °C

Thermal ageing of Alloy 690 triggers an intergranular (IG) carbide precipitation and is known to promote an ordering reaction causing lattice contraction. It may affect the long-term primary water stress corrosion cracking (PWSCC) resistance of pressurized water reactor (PWR) components. Four conditions of Alloy 690 (solution annealed, cold-rolled and/or heat-treated) were aged between 350 and 550 °C for 10 000 h and characterized. Although no direct observation of ordering was made, variations in hardness and lattice parameter were attributed to the formation of short-range ordering (SRO) in all conditions with a peak level at 420 °C, consistent with the literature. Prior heat treatment induced ordering before thermal ageing. At higher temperatures, stress relaxation, recrystallization and α-Cr precipitation were observed in the cold-worked samples, while a disordering reaction was inferred in all samples based on a decrease in hardness. IG precipitation of M23C6 carbides increased with increasing ageing temperature in all conditions, as well as diffusion-induced grain boundary migration (DIGM).

Ab initio modeling of decomposition in iron based alloys

This paper reviews recent progress in the field of ab initio based simulations of structure and properties of Fe-based alloys. We focus on thermodynamics of these alloys, their decomposition kinetics, and microstructure formation taking into account disorder of magnetic moments with temperature. We review modern theoretical tools which allow a consistent description of the electronic structure and energetics of random alloys with local magnetic moments that become totally or partially disordered when temperature increases. This approach gives a basis for an accurate finite-temperature description of alloys by calculating all the relevant contributions to the Gibbs energy from first-principles, including a configurational part as well as terms due to electronic, vibrational, and magnetic excitations. Applications of these theoretical approaches to the calculations of thermodynamics parameters at elevated temperatures (solution energies and effective interatomic interactions) are discussed including atomistic modeling of decomposition/clustering in Fe-based alloys. It provides a solid basis for understanding experimental data and for developing new steels for modern applications. The precipitation in Fe–Cu based alloys, the decomposition in Fe–Cr, and the short-range order formation in iron alloys with s–p elements are considered as examples.

Influence of icosahedral short range order on diffusion in liquids: A study on Al-Zn-Cr alloys

It has been shown recently that minute additions of Cr in Al-Zn alloys refine the solidified grain structure and induce multiply-twinned grains which can only be explained if the icosahedral symmetry is considered (Kurtuldu et al., 2013). It has been suggested that Cr addition to Al-Zn alloys promotes the formation of icosahedral short-range order (ISRO) in the liquid, leading to the formation of icosahedral quasicrystals (iQC) acting then as a template for the nucleation of the fcc Al-phase. If ISRO exists in liquid Al-Zn-Cr alloys, this should influence diffusion of solute elements in the liquid. The present study focuses on the effect of Cr addition on Zn diffusion in liquid Al-Zn-Cr alloys. The solute diffusion coefficients of Zn and Cr in the liquid were deduced from the solute profiles in the quenched liquid ahead of a planar solid-liquid interface. By comparison with the same measurement in Al-Zn alloys, it is shown that Cr indeed slows down the diffusion (and mobility) of Zn atoms, an effect which reinforces the hypothesis of ISRO in Al-Zn liquid induced by Cr.

Locally Structural Evolution of Silicon Anode in Lithium-Ion Battery Investigated By in-Situ Synchrotron X-Ray Diffraction and Absorption Spectroscop

With the possible exhaust of fossil oil, effective utility of energy becomes an important issue, clean energy and rechargeable energy are thus highly developed. In which, rechargeable lithium-ion batteries attract much attention because of its extensive use such as portable electronic devices, powering hybrid electric vehicles (HEV) and zero-emission vehicles (ZEV). While, capacity and charging rate are needed to be highly improved for the utility in practice. Advancing battery with silicon anode thus becomes increasingly emphasized in the recent years because of the ultra-high theoretical capacity. But the dramatic volume expansion of silicon anode hinders its real utility. Lots of studies focus on this phenomenon and try to overcome this issue with specific design of nano-structure and so on. But one can’t get constant rule to stabilize silicon anode because the real mechanism of volume expansion has not been explored in detail. We thus intend to explore the structural evolution of silicon anode in detail by in-situ charging/discharging synchrotron x-ray techniques, to dig out the mechanism of volume expansion in silicon anode, in this study. Initially, hollow silicon spheres are synthesized by the reduction of SiO2 surface and selective corrosion. After the measurements of CV, capacity and life time, in-situ charging/discharging synchrotron x-ray diffraction and absorption spectral analyses are used to probe structural variation in silicon anode. At the first cycle of lithiation, silicon structure gradually transforms from crystalline to amorphous and finally changes to Li15Si4 alloy phase. The created defects in silicon anode contribute to the structural disintegration and reconstruction as indicated by the x-ray line profile analysis. The change in local structure can also be evidenced by x-ray absorption near edge spectroscopy (XANES). Lithium ions cause Si-Si bond-breaking and result in the formation of identical short range ordering of Li15Si4. With the macro and micro analyses of structures, volume expansion of silicon anode with phase transformation has be expounded in detail. Results in this study could be the fundamental to design silicon anode with stable structure, facilitating the utility of Li-ion battery with ultra-high capacity in practice.

Formation of chemical short range order and its influences on the dynamic/mechanical heterogeneity in amorphous Zr–Cu–Ag alloys: A molecular dynamics study

The chemical short range order of metallic glasses is expected to be correlated with their mechanical properties. In this article, classic molecular dynamics simulations of amorphous Zr45Cu45Ag10 alloys were carried out to reveal such links in metallic glasses. Our calculations of Warren–Cowley parameter indicate the growth of chemical short range order during supercooling process, which also depends on the effective cooling rates. The chemical short range ordering is related to the energetic stability of the system. Based on the chemical preference or avoidance for different bonds, the model is separated into Cu-rich regions and Ag-rich regions. Simulated structural relaxation and shear loading process were performed to study how chemical bonds affect the distribution of dynamic and mechanical heterogeneity in our systems. The Cu-rich regions exhibit slower dynamics and higher shear resistance, whereas Ag-rich regions have faster dynamics and prefer to be plastically deformed.

Characterization of citric acid induced transformation of short-range-order minerals in Alfisol, Inceptisol and Vertisol of India

To quantify the citrate induced transformation of short range order (SRO) minerals, a controlled column experiment was conducted with three soils representing three soil orders. The soils were saturated with six concentrations of citric acid (CA) viz. 0, 5, 10, 20, 40, 100 mg L−1 in 0.01 M CaCl2 solution, with an addition of fresh CA at 15, 30, 45, 60 days. After 60 days whole soil was collected and separated into clay and clay-humus complex of colloidal and non-colloidal fractions and analyzed for SRO. Maximum amorphous ferri-alumino silicate (AFAS) formation was observed in Alfisol treated with 10 mg L−1 citric acid for both the size fractions. The citric acid concentration of 20 mg L−1 resulted in maximum formation of oxalate extractable allophane plus imogolite and total oxides in Inceptisol colloidal clay. Crystalline iron content in colloidal clay was decreased with increasing concentration of citric acid. Maximum ferrihydrite and crystalline iron were observed in Alfisol in both the clay fractions. It may be concluded that SRO minerals formed or dissolved depending upon the concentration of CA and the most favourable concentration for maximum SROs formation was in the range of 20–40 mg L−1.

Strain-rate sensitivity, activation volume and mobile dislocations exhaustion rate in nanocrystalline Cu–11.1 at%Al alloy with low stacking fault energy

The strain rate sensitivity, activation volume and exhaustion of mobile dislocation density in nanocrystalline (NC) Cu–11.1at%Al alloy with low stacking fault energy were studied by strain rate jump tests and repeated stress relaxations at room temperature. It was found that in comparison with NC Cu with similar grain size, the strain rate sensitivity of NC Cu–Al alloys is much small, which might due to the low SFE and the tendency of chemical short range order formation during plastic deformation. The small physical activation volume (~20b3) was determined, which could be ascribed to the dislocation processes mediated by grain boundaries, twin boundaries. Meanwhile, a lower exhaustion rate of mobile dislocation density was found, possibly due to the low stacking fault energy enhancing dislocation storage capability, the high densities of nanotwins preserving mobile dislocations and a lower dislocation velocity in concentrated alloys.

Short-to-medium range order of Al–Mg metallic glasses studied by molecular dynamics simulations

Molecular dynamics simulations of Al–Mg metallic glasses with a wide composition range have been conducted. We made use of a variety of analytical methods to study their amorphous structure. Pair distribution functions were constructed to determine the interatomic distances, and we found good agreement with reported simulation and experimental results. Coordination number analyses revealed that Mg atoms are more likely to serve as neighbour atoms in Al-centred icosahedral clusters in the middle concentration range or to form larger polyhedra at the Al-rich end. The presence of the 155-type pairs demonstrated that icosahedral ordering is predominant in the Al–Mg alloy. Results of Voronoi tessellation showed that Al-centred icosahedra are abundant in most compositions of the Al–Mg alloy and will lead to the formation of short-range order. The icosahedral clusters are highly shared with one another forming the medium-range order and there are less than 3% isolated icosahedra in most of the compositions of the Al–Mg alloy. Finally, the sharing schemes of icosahedra are represented by the splitting of the second peak of each pair distribution function.

Structural basis for supercooled liquid fragility established by synchrotron-radiation method and computer simulation

Metallic melts above the liquidus temperature exhibit nearly Arrhenius-type temperature dependence of viscosity. On cooling below the equilibrium liquidus temperature metallic melts exhibit a non-Arrhenius temperature dependence of viscosity characterized by liquid fragility phenomenon which origin is still not well understood. Structural changes and vitrification of the Pd42.5Cu30Ni7.5P20 liquid alloy on cooling from above the equilibrium liquidus temperature are studied by synchrotron radiation X-ray diffraction and compared with the results of first-principles calculations. Subsequent analysis of the atomic and electronic structure of the alloy in liquid and glassy states reveals formation of chemical short-range order in the temperature range corresponding to such a non-Arrhenius behavior. The first-principles calculations were applied to confirm the experimental findings.

Effect of Magnetism on Short-Range Order Formation in Fe-Si and Fe-Al Alloys

Short-range order formation in dilute Fe-Si and Fe-Al alloys has been investigated by statistical Monte Carlo simulations with effective interactions deduced from first principles calculations for different magnetic structures of bcc Fe. We find that the variation of the magnetic order from ferromagnetic to paramagnetic leads to significant changes in effective cluster interactions and, as follow, in short-range order parameters of alloys. It is shown in agreement with experiment the B2 type short-range order is formed above the Curie temperature, TC, while the D03 type short-range order is preferred below TC.

Effect of cooling rate on magnetoelasticity and short-range order in Fe–Ga alloys

For Fe–Ga single crystals, faster cooling rates achieved by metallic quench and ice-water quench extended the tetragonal magnetostriction constant, (3/2) λ 100 , to higher Ga contents than reported previously. The maximum magnetostriction measured was 420 ppm at a composition of 21.4 at.% Ga. Using these faster cooling rates, the formation of short-range order (SRO) was obviously suppressed in A2 single phase, while this change in SRO did not significantly affect (3/2) λ 100 .

Raman spectroscopy of pseudomorphic Si 0.989C 0.011/Si superlattices

Raman spectroscopy is used here to study pseudomorphic Si 0.989C 0.011/Si superlattices grown by molecular beam epitaxy. The high crystalline quality of the samples was tested by a high resolution X-ray diffraction experiment. The lineshape of the LO Si–Si peak shows an asymmetry, which correlates with the increase of the alloy layer width. The Raman spectra show three additional peaks in the high energy side above the LO mode of Si. One of them is due to the local vibration of the C substitutional atoms, and the other two can be attributed to the formation of short range order with C atoms occupying second and third nearest-neighbor places. On the low energy side of the LO Si–Si mode, we have observed two other peaks associated with the relaxation of the Si atoms around the substitutional C. Although the X-ray experiments show clear evidence of superperiodicity, no indication of the superlattice formation could be observed in the parallel polarized Raman spectra, where the folded acoustic modes are allowed.

Recovery of electrical resistivity, short-range order formation and migration of defects in electron-irradiated Fe–4Cr alloy doped with carbon

Resistivity recovery (RR) data of Cr4 alloy doped with carbon (Cr4C) after irradiation with 5 MeV electrons are presented and compared with RR data of non-doped Cr4 alloy. Analysis of the defect- and short-range order-induced parts of RR has confirmed the proposed earlier scheme of the evolution of Frenkel pair defects on post-irradiation anneal in Cr4 and Cr9. Vacancies start free migration around 205–210 K; however, the related peak of stage III is invisible in conventional RR plots. Interstitials atoms (IAs) trapped in stage I at configurations of several Cr atoms start their long-range migration at 220 K. Migrating vacancies are captured by carbon atoms in Cr4C while IAs are not. Dissociation of vacancies from carbon atoms at 350 K gives rise to a decoration of carbon atoms with Cr atoms. Indications are seen that a release of vacancies captured by atoms of residual nitrogen takes place around 250–260 K.

Self-consistent chemical model of partially ionized plasmas

A simple renormalization theory of plasma particle interactions is proposed. It primarily stems from generic properties of equilibrium distribution functions and allows one to obtain the so-called generalized Poisson-Boltzmann equation for an effective interaction potential of two chosen particles in the presence of a third one. The same equation is then strictly derived from the Bogolyubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy for equilibrium distribution functions in the pair correlation approximation. This enables one to construct a self-consistent chemical model of partially ionized plasmas, correctly accounting for the close interrelation of charged and neutral components thereof. Minimization of the system free energy provides ionization equilibrium and, thus, permits one to study the plasma composition in a wide range of its parameters. Unlike standard chemical models, the proposed one allows one to study the system correlation functions and thereby to obtain an equation of state which agrees well with exact results of quantum-mechanical activity expansions. It is shown that the plasma and neutral components are strongly interrelated, which results in the short-range order formation in the corresponding subsystem. The mathematical form of the results obtained enables one to both firmly establish this fact and to determine a characteristic length of the structure formation. Since the cornerstone of the proposed self-consistent chemical model of partially ionized plasmas is an effective pairwise interaction potential, it immediately provides quite an efficient calculation scheme not only for thermodynamical functions but for transport coefficients as well.