Mean Interparticle Spacing Research Articles

Elucidation on the correlation between thermal stability of Al13Fe4 intermetallic phase and mechanical properties of the Al-Fe alloy fabricated via friction stir alloying

Al-Fe alloy containing nanosized Al13Fe4 phase was fabricated by the solid-state multiple pass friction stir alloying (FSA) technique and subjected to heat treatment at different time-temperature conditions. During annealing, the diffusion of Fe atoms leads to the necking and fragmentation of the coarse plate shape Al13Fe4 IMC phase resulting in its spheroidization. However, as the annealing time is extended, the spheroidized Al13Fe4 phase undergoes coarsening and grows predominantly along a specific plane. This growth process transforms the originally spherical shape into an elliptical or small plate-like morphology. The grain size was insignificantly affected by the annealing time and temperature owing to the grain boundary pinning by the nanometric-sized IMC phase. The ultimate tensile strength of the Al-Fe alloy (149 MPa) is improved significantly at the annealing conditions of 200 °C/4hrs (165 MPa), 200 °C/6hrs (167 MPa), and 300 °C/2hrs (163 MPa). The improved UTS is attributed to the increased isotropic work hardening owing to the reduced IMC particle size and mean interparticle spacing in the Al-Fe alloy. The lowest work hardening rate is observed in the specimen annealed at 200 °C for 2 hrs (largest particle size) which is attributed to the stored energy in the material due to severe plastic deformation (SPD) associated with the FSA. The correlation between the work hardening rate, IMC particle size, and mean interparticle spacing at various annealing conditions are established.

STEM and HRTEM study on matrix microstructure and oxide particles in 11Cr ferritic/martensitic ODS steel

Fe–Cr oxide dispersion strengthened (ODS) steel is one of the candidate structural materials for high burn-up fuel cladding of sodium-cooled fast reactor and fusion reactor blanket due to its superior high temperature strength and excellent resistances to corrosion, creep and irradiation. The matrix structure and oxides in 11Cr ferritic/martensitic (F/M) ODS steel (Fe–11Cr–0.13C–1.3W–0.4Ni–0.3Ti–0.35Y2O3) have been characterized by scanning transmission electron microscopy (STEM) and high resolution transmission electron microscopy (HRTEM). The matrix of 11Cr F/M ODS steel is composed of tempered martensite (major) and residual ferrite (minor). The dispersion morphology of oxides of residual ferrite is much better, relative to that of tempered martensite, because the oxides in the former have much smaller mean diameter and inter-particle spacing and, however, significantly higher number density and volume fraction. For tempered martensite and residual ferrite, phase identifications on 41 and 69 nanoparticles with peak number fraction and, consequently, most significant contributions to the macroscopic properties, were accomplished, respectively. The proportions of Y–Ti–O, Y–Cr–O, TiO2 and Y2O3 in tempered martensite are 58.5%, 12.2%, 19.5% and 9.8%, respectively. The proportions of Y–Ti–O, Y–Cr–O, TiO2 and Y2O3 in residual ferrite are 65.3%, 8.7%, 15.9% and 10.1%, respectively. There is no considerable difference in phases and their corresponding number fractions of nanoparticles between tempered martensite and residual ferrite. Coherency and crystallographic orientation relationship between the lattice of oxides and that of the residual ferrite matrix are ubiquitous and, moreover, such coherency and crystallographic correlation between the lattice of oxides and that of the tempered martensite matrix were only occasionally detected. The mechanisms of the formation and polymorphic transition of various kinds of oxides, the structure origins of the much better dispersion morphology of the nanoparticles of residual ferrite, relative to that of tempered martensite, and, moreover, the strengthening mechanisms in the 11Cr F/M ODS steel were discussed.

Characterization of creep properties and the microstructure of a service-exposed low alloy CrMoV steel steam pipe

The effect of service degradation on the microstructure and creep properties of a 0.5%Cr-0.5%Mo-0.3%V steel steam pipe after an accumulated service time of 77,900 h was investigated and analysed. For comparison purposes, additional experiments and analyses were carried out on the same pipe in the as-received (service unused) conditions. The results of the microstructural analysis and the short-term creep testing confirmed that the high creep resistance of the investigated steel is caused predominantly by the dispersion of fine vanadium carbides, providing an effective precipitation hardening of the steel matrix. However, the microstructure is not in an equilibrium state, and as the size of the vanadium carbides and the accompanying mean interparticle spacing increase during the creep, the detrimental effect of vanadium carbide coarsening leads to a significant weakening in the precipitation hardening and, consequently, a decrease in the creep resistance. No creep cavitation was observed in the service-exposed steel. The short-term creep testing of the service-exposed and unused states of the steel was carried out in the power-law (dislocation) creep regime. Analysis of the creep data did not reveal any change in the operating creep deformation mechanisms as a response to the structural processes occurring during the long-term service. Instead, changes in the kinetics of creep flow and fracture were found.

Microstructure and mechanical properties of an ODS ferritic steel with very low Cr content

An ultra-low Cr oxide dispersion strengthened (ODS) steel with the composition of Fe-5Cr-4.5Al-0.6Zr (hereinafter abbreviated as 5CrAlZr ODS steel) were proposed and manufactured via mechanical alloying (MA) and hot isostatic pressing (HIP), followed by hot forging treatment. TEM results indicate that a large amount of fine δ-Y4Zr3O12 particles were formed within the grains and along the grain boundary, additionally, Y6ZrO11 and Al2Y4O9 nano-particles with larger size were also detected. The mean particles diameter (dp), the mean inter-particle spacing (λ) and number density (nv) of the nano-oxide particles in the 5CrAlZr ODS steel are about 10 nm, 80 nm and 1.23 × 1022 /m3, respectively. The room temperature ultimate tensile strength (UTS) and the total elongation (TE) of the fabricated 5CrAlZr ODS steel are 875 MPa and 21%, respectively, which are higher than those of traditional 5Cr-2W-0.24V steels. The UTS and TE of 5CrAlZr ODS steel at 700°C are 234 MPa and 58%, respectively. The strengthening mechanisms underlying were also discussed.

Numerical convergence of simulations of galaxy formation: the abundance and internal structure of cold dark matter haloes

ABSTRACT We study the impact of numerical parameters on the properties of cold dark matter haloes formed in collisionless cosmological simulations. We quantify convergence in the median spherically averaged circular velocity profiles for haloes of widely varying particle number, as well as in the statistics of their structural scaling relations and mass functions. In agreement with prior work focused on single haloes, our results suggest that cosmological simulations yield robust halo properties for a wide range of gravitational softening parameters, ϵ, provided: (1) ϵ is not larger than a ‘convergence radius’, rconv, which is dictated by two-body relaxation and determined by particle number, and (2) a sufficient number of time-steps are taken to accurately resolve particle orbits with short dynamical times. Provided these conditions are met, median circular velocity profiles converge to within ≈10 per cent for radii beyond which the local two-body relaxation time-scale exceeds the Hubble time by a factor $\kappa \equiv t_{\rm relax}/t_{\rm H}\rm{\,\, \buildrel\gt \over \sim \,\,}0.177$, with better convergence attained for higher κ. We provide analytic estimates of rconv that build on previous attempts in two ways: first, by highlighting its explicit (but weak) softening-dependence and, second, by providing a simpler criterion in which rconv is determined entirely by the mean inter-particle spacing, l, for example better than 10 per cent convergence in circular velocity for $r\rm{\,\, \buildrel\gt \over \sim \,\,}0.05\, l$. We show how these analytic criteria can be used to assess convergence in structural scaling relations for dark matter haloes as a function of their mass or maximum circular speed.

Mode-coupling instability in a single-layer complex plasma crystal: Strong damping regime

Mode-coupling instability (MCI) in a single-layer complex plasma crystal was studied experimentally in the regime of strong neutral gas damping. To trigger MCI, the discharge power was reduced at constant gas pressure. Surprisingly, at the onset of MCI the mean interparticle spacing became larger and the normalized neutral gas damping rate marginally increased. A steady-state regime where MCI occurred but was suppressed by the neutral gas friction was observed. Suppressed MCI caused heating up of the plasma crystal but did not lead to its melting. A theoretical model is proposed which describes well our experimental observations.

Aerodynamic generation of electric fields in turbulence laden with charged inertial particles

Self-induced electricity, including lightning, is often observed in dusty atmospheres. However, the physical mechanisms leading to this phenomenon remain elusive as they are remarkably challenging to determine due to the high complexity of the multi-phase turbulent flows involved. Using a fast multi-pole method in direct numerical simulations of homogeneous turbulence laden with hundreds of millions of inertial particles, here we show that mesoscopic electric fields can be aerodynamically created in bi-disperse suspensions of oppositely charged particles. The generation mechanism is self-regulating and relies on turbulence preferentially concentrating particles of one sign in clouds while dispersing the others more uniformly. The resulting electric field varies over much larger length scales than both the mean inter-particle spacing and the size of the smallest eddies. Scaling analyses suggest that low ambient pressures, such as those prevailing in the atmosphere of Mars, increase the dynamical relevance of this aerodynamic mechanism for electrical breakdown.

Effects of Zr Addition on Strengthening Mechanisms of Al-Alloyed High-Cr ODS Steels.

Oxide dispersion strengthened (ODS) steels with different contents of zirconium (denoted as 16Cr ODS, 16Cr-0.3Zr ODS and 16Cr-0.6Zr ODS) were fabricated to investigate the effects of Zr on strengthening mechanism of Al-alloyed 16Cr ODS steel. Electron backscatter diffraction (EBSD) results show that the mean grain size of ODS steels could be decreased by Zr addition. Transmission electron microscope (TEM) results indicate that Zr addition could increase the number density but decrease the mean diameter and inter-particle spacing of oxide particles. Furthermore, it is also found that in addition to Y-Al-O nanoparticles, Y-Zr-O oxides with finer size were observed in 16Cr-0.3Zr ODS and 16Cr-0.6Zr ODS steels. These changes in microstructure significantly increase the yield strength (YS) and ultimate tensile strength (UTS) of ODS steels through mechanisms of grain boundary strengthening and dispersion strengthening.

Work-hardenable Mg-based bulk metallic glass matrix composites reinforced by ex-situ porous shape-memory-alloy particles

Porous NiTi shape-memory-alloy particle-reinforced Mg-based bulk metallic glass matrix composites were prepared by an ex-situ method. The composites show a larger plastic strain, higher fracture strength and obvious work-hardening behavior compared with their monolithic glassy counterparts. The ex-situ porous NiTi particles hinder the rapid propagation of shear bands, which generates multiplied shear bands, like other ductile metal particles, and undergo a stress-induced martensitic transformation, which is unique and contributes to work hardening. Local regions within one porous particle satisfy the optimal conditions of bulk metallic glass matrix composite structure, i.e., the average particle size and mean inter-particle spacing are close to the processing zone size, and thereby further improve the plasticity by localized plastic deformation.

Fractal properties of stellar systems and random forces

The nearest neighbor distribution (Chandrasekhar 1943) is generalized to fractal stellar systems.For such systems an asymptotic distribution of the magnitude of large random forces and a formula for the effective mean interparticle spacing are derived. It is shown that in the case of a power-law distribution of conditional density the derived asymptotic fully agrees with the results obtained in terms of a general approach. It is concluded that large random forces in a fractal stellar medium are due entirely to the nearest neighbors (clumps) located inside the sphere of the effective radius determined from the generalized Holtsmark distribution.

Friction stir processing of Al/SiC composites fabricated by powder metallurgy

Friction stir processing (FSP) was applied to modify the microstructure of sintered Al–SiC composites with particle concentrations ranging from 4 to 16vol%. Two SiC particle sizes (490N and 800 grades) were examined. Following FSP, the hardness of the 4 and 8vol% of 490N grade SiC composites increased from 130HV and 145HV to 171HV and 177HV respectively. The increase was accounted for by the severe deformation occurring during FSP which uniformly distributed the SiC particles. The composites containing 16vol% SiC could not be fully consolidated using FSP, and contained residual pores and lack of consolidation which originated from the as-received sintered microstructure. The hardness correlated well with the mean inter-particle spacing for the SiC particles in the case of composites containing 4 and 8vol% SiC.

Bcc crystal-fluid interfacial free energy in Yukawa systems

We determine the orientation-resolved interfacial free energy between a body-centered-cubic (bcc) crystal and the coexisting fluid for a many-particle system interacting via a Yukawa pair potential. For two different screening strengths, we compare results from molecular dynamics computer simulations, density functional theory, and a phase-field-crystal approach. Simulations predict an almost orientationally isotropic interfacial free energy of 0.12k(B)T/a(2) (with k(B)T denoting the thermal energy and a the mean interparticle spacing), which is independent of the screening strength. This value is in reasonable agreement with our Ramakrishnan-Yussouff density functional calculations, while a high-order fitted phase-field-crystal approach gives about 2-3 times higher interfacial free energies for the Yukawa system. Both field theory approaches also give a considerable anisotropy of the interfacial free energy. Our result implies that, in the Yukawa system, bcc crystal-fluid free energies are a factor of about 3 smaller than face-centered-cubic crystal-fluid free energies.

Determination of the Surface Corrugation Amplitude from Classical Atom Scattering

The energy landscape of an atomic or molecular projectile interacting with a surface is often described in terms of a corrugation function that gives the classical turning point as a function of position vector parallel to the surface. It is shown here that the relative height variation of the corrugation function for scattering of atoms under classical conditions can be determined by a measurement of the maximum intensity in energy-resolved scattering spectra as a function of surface temperature. This is demonstrated by developing a semiclassical quantum theory of atomic scattering from corrugated surfaces and then extending the theory to the classical limit of large incident energies and high surface temperatures. Comparisons of calculations with available data for Ar atom scattering determine the corrugation amplitude for a molten In surface to be about 29% of the mean interparticle spacing in the bulk liquid.

Estimating thermo-osmotic coefficients in clay-rocks: I. Theoretical insights

The conditions to obtain a macroscopic model for the thermo-osmotic coupling coefficient which is needed for practical calculations of fluid flow in clay-rocks subjected to temperature gradients are investigated in this paper. A theoretical expression for the thermo-osmotic coupling coefficient proportional to the hydraulic conductivity was obtained. The theoretical expression of the thermo-osmotic conductivity involves the excess Gibbs energy of the fluid between adjacent charged surfaces. The interaction energy was calculated using disjoining pressure data. Our calculations suggest a crucial role of the so-called hydration or structural energy of interaction to explain the thermo-osmotic process. The mean pore size, i.e. the mean interparticle spacing, the concentration of the equilibrium solution and the temperature are determinant variables in this process. Some exploratory comparisons between the model and the available data for pure clays are proposed.

Nanogranular Au films deposited on carbon covered Si substrates for enhanced opticalreflectivity and Raman scattering

Electroless deposition of gold has been carried out on Si(100) surfaces precoated with laserablated carbon layers of different thicknesses, and the resulting substrates have beencharacterized by a host of techniques. We first established the porous nature of the amorphouscarbon layer by Raman and profilometric measurements. The Au uptake from the platingsolution was optimal at a carbon layer thickness of 90 nm, where we observed nanogranules of∼60–70 nm, well separated from each other in the carbon matrix (mean interparticle spacing∼7 nm). We believe that the observed nanostructure is a result ofAu3+ electroless reduction on the Si surface through porous channels present in theamorphous carbon matrix. Importantly, this nanostructured substrate exhibited highreflectivity in the near IR region besides being effective as a substrate for surfaceenhanced Raman scattering (SERS) measurements with enhancement factors up to107.

Effect of Nitrogen Content on Microstructural Aspects and Creep Behavior in Extremely Low Carbon 9Cr Heat-resistant Steel

Effects of nitrogen content on microstructure and creep strength in extremely low carbon 9Cr ferritic steel were investigated, focusing on the dislocation structure and the distribution of precipitates. The creep strengthof the high nitrogen steels was lower than that of the base steel, even though the initial lath width and prior austenite grain size in the high nitrogen steels are finer and larger, respectively. MX nitrides as well as large Cr 2 N particles were distributed along lath, block, packet and prior austenite grain boundaries in the high nitrogen steels after tempering. The large particles occupy the MX nitride precipitation sites. The coarsening of the MX nitride during creep exposure in the high nitrogen steels was faster than that of the base steel. The presence of large Cr 2 N particles after tempering and the higher coarsening rate of the MX nitride in the high nitrogen steels cause an increase in mean inter-particle spacing on boundaries, leading to a decrease in creep strength. The Z phase formation was observed after creep exposure in the gauge portion of all the steels. The Z phase formation in the high nitrogen steels occurs at relatively short term in contrast to the base steel. The short term precipitation of the Z phase in high nitrogen steels can contribute to a decrease in creep strength since the Z phase forms at the expense of the MX nitride which is a main strengthening obstacle.

623 炭化物形態変化法による CrMoV タービンロータ鋼のクリープ損傷評価

CrMoV turbine rotor steel is strengthened by dispersed fine carbides, however carbides precipitated and coarsened at grain boundary during creep exposure. The carbide structure also changes from Fe_3C to M_<23>C_6 type carbide as coarsening. The carbide structure was electrochemically isolated by solving the matrix and analyzed with X-ray diffraction analysis. The X-ray peak intensity ratio of M_<23>C_6 to Fe_3C is tentatively defined as J-parameter. In order to evaluate the efficacy of J-parameter for creep damage evaluation, creep interruption test was conducted. The J-parameter increased with increasing creep and also correlated well to mean interparticle spacing. The change in J-parameter was affected by the chemical composition of the steel.

自由落下金属液滴の偏平・凝固に及ぼす基材温度の影響

A freely falling experiment, in which a metal droplet fell freely and impinged on a flat substrate, was conducted as a simulation of the thermal spray process. The effect of substrate temperature on the flattening and solidification of the droplet was mainly investigated in this study.The transition of the splat pattern was recognized in the experiment, that is, the splat morphology of Ni and Cu droplet on the room temperature substrate was splash-type, while that on the high temperature substrate was disk-type. The cross section microstructure of the splat on the room temperature substrate was composed of an isotropic coarse grain, while that on the high temperature substrate was quite fine columnar structure. As the mean interparticle spacing in the splat changed transitionally with the substrate temperature, the solidification rate in the splat on the high temperature substrate was higher than that on the room temperature substrate. The unique porous microstructure and flowing pattern were observed in the bottom surface of the splat on the room temperature substrate while the flat microstructure without pore was recognized in that on the high temperature substrate. The difference of the solidification rate between these two kinds of splats seems to be attributed to the interface microstructure between splat and substrate. From the results obtained in this study, it can be concluded that the quite rapid solidification occured at the interface between splat and substrate when the droplet impinged on the cold substrate surface.

Low-field hopping among randomly-distributed sites with uncorrelated energetic disorder

The low-field mobility μ of a small concentration of charge carriers hopping among a random distribution of transport sites is studied, as a function of the mean interparticle spacing ρ and the temperature T, for model systems having different site-energy distribution functions. For a uniform density of states our calculations show that the mobility obeys empirical scaling laws similar to those found in the theory of variable-range hopping. For a binary distribution of site energies we observe a crossover as a function of site density between trap-limited conduction and trap-mediated conduction. For a Gaussian density of states our results confirm the quadratic inverse temperature dependence of lnμ found in Monte Carlo studies, although quantitative characterization of this dependence is found to depend sensitively on the degree of spatial disorder in ways that could impact the extraction of microscopic parameters from experimental data.

Mechanical properties of fine-crystalline two-phase materials

Results of experimental studies of composition-structure-mechanical properties achieved for various two-phase materials (e.g., Cn-Fe, Ti-TiC, Al-Be, TiAl-Ti and ZrO2-TiB2) produced by varpor deposition using electron-beam evaporation are presented. Maximum values of yield and tensile strengths were found, together with maxima in plasticity and fracture toughness, for condensates exhibiting particular microstructural features. Laminated deposits also exhibited structurally sensitive yield and elevated-temperature creep behavior for interlamellar spacings between 3 and 30 μm. The experimental values of strength and ductility were interpreted on the basis of volume fraction of the secondary phase, mean grain size of the matrix and secondary particles, and the mean interparticle spacing. High rate deposition of these alloys is proposed as a cost-effective, non-polluting, precise process for large-scale production of structural materials.