Necklace Structure Research Articles

High-temperature compressive behavior and kinetics analysis of Al0.4MnCrCoFeNi high entropy alloy

In the present study, AlxMnCrCoFeNi alloys (x = 0, 0.4) were produced by vacuum melting and casting. For this aim, ingots were subjected to the homogenization, cold roll, and annealing. X-ray diffraction analysis and SEM images indicated that the crystal structure is FCC solid solution for MnCrCoFeNi. Moreover, in addition to FCC solid solution, the alloy with aluminum (x = 0.4) ordered B2 precipitates. Besides, hot compressive tests were conducted at different temperatures and strain rates of 0.01, 0.05 and 0.1/s. Investigating the stress-strain diagrams and work hardening rate, it was revealed that twins played a role in the deformation mechanism. In the constitutive equation, the values of the stress exponent and activation energy were measured and they were 6.86 and 434 kJ mol−1, respectively. The exponential equations of peak stress and peak strain, as well as steady-state stress, were obtained due to the Zener-Hollomon parameter. Regarding the linear relationship between the work hardening rate and the dislocation annihilation coefficients, the activation energy of deformation changes at 600 °C. Finally, another effect of the Al addition was shown to be increase in initiation temperature of necklace structure about 200 °C in hot compression test. In addition, B2 nanoprecipitate and nanotwins indicated that there was twin deformation mechanism in alloy containing Al.

On the possibility to reduce ECAP deformation temperature in magnesium: Deformation behaviour, dynamic recrystallization and mechanical properties

This work investigates the effect of temperature on the dynamic recrystallization (DRX) behaviour and its consequence on the microstructure, texture, and mechanical properties in pure magnesium during equal channel angular pressing (ECAP) by route-A up to eight passes ( ε E f f = 9.3 ) . The ECAP temperature was reduced in steps for gradual grain refinement. Thereby the 8th pass could be conducted successfully at 200 °C , 150 °C and 27 °C , respectively. Irrespective of the 8th pass ECAP temperature, ~0.30 microstructure fraction was found to be DRXed. Continuous dynamic recovery and recrystallization (CDRR) predominantly occurred, resulting in the formation of fine grains with the necklace structure for all the conditions. However, distinctive and abnormally coarse CDRR grains evolved irrespective of the ECAP temperature. CDRR lead to the formation of Σ 13 a boundaries with [0002]- 30 ° axis-angle pair. Discontinuous DRX was also observed intermittently in the ECAP microstructures. The average grain size after the 8th pass ECAP at 200 °C , 150 °C and 27 °C was ~ 2.6 μ m , ~ 1.4 μ m and ~ 0.75 μ m μ m , respectively. The yield strength increases and the ultimate compressive strength (UCS) as well as strain to failure, decreases with grain refinement. The ambient temperature compressive strain hardening behaviour was initially sigmoidal, attributing to the evolution of { 10 1 ‾ 2 } 1 ‾ 011 extension twins. Sigmoidal flow behaviour reduces gradually as the grain size decreases with ECAP. No sigmoidality indicate slip-dominated deformation after 8th pass ECAP at 150 ° C and 27 ° C . • Strategic reduction in ECAP deformation temperature in Mg is possible to obtain ultrafine grains. • ~0.30 microstructure-fraction recrystallized after effective-strain of 9.3 regardless of ECAP temperature. • Continuous dynamic recovery & recrystallization occurred, forming fine and coarse grains. • Discontinuous dynamic recrystallization increases as the ECAP temperature reduced. • Yield strength increase, ultimate strength and ductility decrease with grain refinement.

Hot Deformation Behavior and Processing Map of a Novel Dilute Extruded Mg-Zn-Gd-Y-Nd Alloy

Background: The hot working characteristic of magnesium alloys have received considerable attention in order to evaluate the constitutive behavior of these alloys under hot working conditions with a view to evaluate constitutive conditions to optimize hot workability and control the microstructure so as to obtain consistent mechanical properties. Methods: The hot deformation response of a new dilute Mg alloy was investigated by means of a series of hot compression tests in the temperature and strain rate range of 375-450°C and 0.001-1 s-1, respectively. The stress-strain behavior, microstructure evolution and processing parameters optimization were studied carefully. Micro-structural characterization studies conducted on a series of deformed samples using optical microscopy revealed that during hot deformation, the main restoration mechanism was dynamic recrystallization (DRX). Results: In the final microstructure of the material, grain boundaries were thoroughly covered by layers of fine DRXed grains. Moreover, a strong twinning induced necklace structure was the most significant characteristic at high strain rates which was accompanied by smaller grain size in the domain material. Based on the measured stress-strain data, constitutive model was conducted on two regimes of low and high temperatures. Moreover, the processing map of the studied material was obtained and interpreted using dynamic material model (DMM). Conclusion: The processing map was built and divided into a feasible domain at high temperatures in the whole range of strain rates and two separated instable domains in the temperature range of 375 to 435°C at high and low strain rates of 1 and 0.001 s-1.

Dissimilar friction stir welding of AISI 430 ferritic and AISI 304L austenitic stainless steels

Dissimilar joints of AISI 430 ferritic and AISI 304L austenitic stainless steels were produced by friction stir welding process. A sound and defect-free joint was obtained at 1 mm tool offset towards the ferritic sample located in the advancing side, and at rotational and welding speeds of 560 rpm and 50 mm/min, respectively. The XRD measurements revealed the presence of approximately equal volume fractions of ferrite (51%) and austenite (49%) phases in the stir zone (SZ). The formation of low-angle grain boundaries through the occurrence of dynamic recovery along with the presence of shear texture components in both constituent phases of ferrite and austenite in the SZ approved the occurrence of continuous dynamic recrystallization throughout the evolved microstructure. Moreover, microstructural observations showed the formation of necklace structure through the microstructure of ferrite in the SZ. Taylor map approved the strain localization in the ferrite phase. Micro-hardness measurement indicated that the hardness value is increased in the SZ. The result of tensile test showed that fracture occurred from less ductile ferritic base metal.

Tunable Pearl Necklace Structured TiO2 Spheres on Carbon Nanotubes with Enhanced Electromagnetic Wave Absorption Performance

Due to the low density and high conduction loss of carbonaceous materials, they have been considered as candidates for compositing electromagnetic wave absorbing materials. In this study, the well-designed CNT@TiO2 composites with pearl necklace structure were successfully synthesized via a facile hydrolyzation method. The absorption performances of CNT@TiO2 under different loading ratio were characterized and showed significantly different performance on the electromagnetic wave absorption. The reflection loss value reaches to –47.5 dB in the pearl necklace structured CNT@TiO2 composites obtained with a thickness of 1.6 mm at 11.2 GHz. An effective absorption bandwidth was found to reach to 4.2 GHz with a coating thickness of 1.4 mm. More, the absorption frequency range can be diverse by adjusting coating thickness. We deduce that promising electromagnetic wave absorbing property of the composites is related to the interfacial polarization induced by pearl necklace structure, improved impedance matching, conduction loss and scattering within the materials.

Hot working of SP-700 titanium alloy with lamellar α + β starting structure using a processing map

Abstract In this study, hot deformation behavior of SP-700 titanium alloy was investigated by hot compression tests in the temperature range of 700-9508C and at strain rates of 0.001, 0.1, and 1 s-1. Final mechanical properties of the alloy (hot compressed at different strain rates and temperatures) were investigated using a shear punch testing method at room temperature. The flow curves of the alloy indicated that the yield point phenomenon occurs in the temperature range of 800- 9508C and strain rates of 0.1 and 1 s-1. The microstructural analysis showed that dynamic globularization of the lamellar α phase starts at 7008C and completes at 8008C. The alpha phase was completely eliminated from b matrix due to deformation- induced transformation at 8508C. The microstructure of specimens compressed at 8508C and strain rates of 0.001 and 0.1 s-1showed the serration of beta grain boundaries, whereas partial dynamic recrystallization caused a necklace structure by increasing strain rate up to 1 s-1. The specimen deformed at 7008C and strain rate of 1 s-1was located in the instability region and localized shear bands formed due to the low thermal conductivity of the alloy. The processing map of the alloy exhibited a peak efficiency domain of 54% in the temperature range of 780-8108C and strain rates of 0.001- 0.008 s-1. The hot deformation activation energy of the alloy in the α/β region (305.5 kJ mol-1) was higher than that in the single-phase β region (165.2 kJ mol-1) due to the dynamic globularization of the lamellar a phase.

Structural investigations of poly(ethylene glycol)-dodecylbenzenesulfonic acid complexes in aqueous solutions

Abstract The structures arising in mixed colloidal solutions of anionic surfactant dodecylbenzenesulfonic acid (DBSA) and neutral poly(ethylene glycol) (PEG) polymer in water for a wide range of concentrations and different molecular masses of the polymer were studied by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The micellization behavior of the system near the critical micelle concentration was characterized by fluorescence spectroscopy and measurements of the surface tension of the solutions. The assumption of the coexistence of free micelles, surfactant-polymer complexes in the form of a necklace structure, and polymer coils was confirmed by DLS and SANS. The typical sizes of micelles and complexes were estimated. It was found that there is an impact of PEG molecular mass on the ability of multi-micelle complex formation. The obtained results are important in particular for the PEGylation of magnetic nanoparticles in sterically stabilized aqueous ferrofluids.

Hot working of SP-700 titanium alloy with lamellar α + β starting structure using a processing map

AbstractIn this study, hot deformation behavior of SP-700 titanium alloy was investigated by hot compression tests in the temperature range of 700-9508C and at strain rates of 0.001, 0.1, and 1 s-1. Final mechanical properties of the alloy (hot compressed at different strain rates and temperatures) were investigated using a shear punch testing method at room temperature. The flow curves of the alloy indicated that the yield point phenomenon occurs in the temperature range of 800- 9508C and strain rates of 0.1 and 1 s-1. The microstructural analysis showed that dynamic globularization of the lamellar α phase starts at 7008C and completes at 8008C. The alpha phase was completely eliminated from b matrix due to deformation- induced transformation at 8508C. The microstructure of specimens compressed at 8508C and strain rates of 0.001 and 0.1 s-1showed the serration of beta grain boundaries, whereas partial dynamic recrystallization caused a necklace structure by increasing strain rate up to 1 s-1. The specimen deformed at 7008C and strain rate of 1 s-1was located in the instability region and localized shear bands formed due to the low thermal conductivity of the alloy. The processing map of the alloy exhibited a peak efficiency domain of 54% in the temperature range of 780-8108C and strain rates of 0.001- 0.008 s-1. The hot deformation activation energy of the alloy in the α/β region (305.5 kJ mol-1) was higher than that in the single-phase β region (165.2 kJ mol-1) due to the dynamic globularization of the lamellar a phase.

Comparative Study of Hot Deformation Behavior and Microstructure Evolution of As-Cast and Extruded WE43 Magnesium Alloy

Under compressive testing at 400 °C and a strain rate range of 0.05–5 s−1, the hot deformation behavior and microstructure evolution of an as-cast (AC), as-extruded (with a bimodal grain structure (named as Ex-1) or a relatively uniform fine grain structure (Ex-2)) WE43 alloy have been investigated and compared. The results indicate that the AC sample exhibits the highest peak stress, while the Ex-2 sample has the lowest value. Within the AC material, fine grains were firstly formed along the pancake-like deformed grains (as a necklace structure). The necklace structure was also formed within the Ex-1 and Ex-2 materials at high strain rates of 0.5 and 5 s−1. However, a lamellar structure that the coarse elongated grains divided by parallel boundaries was formed within the Ex-1 material. A relatively more homogeneous fine grain structure is achieved after a true strain of 1.0 within the Ex-2 material at a low strain rate of 0.05 s−1. In addition, a discontinuous dynamic recrystallization mechanism by grain boundary bulging is found to occur. After a true strain of 1.2, a (0001) fiber texture, a typical rare earth (RE) texture, and a relatively random texture are formed within the AC, Ex-1, and Ex-2 WE43 alloy material, respectively.

Microstructure Evolution and Dynamic Precipitation Behavior of Mg–10Gd–4Y–1Sm–0.5Zr Alloy During Backward Hot Extrusion

The microstructure evolution and dynamic precipitation behavior of Mg–10Gd–4Y–1Sm–0.5Zr alloy during backward hot extrusion are studied. The results show that the equiaxed grains become polygonal grains and fine DRX grains are generated which form a “necklace structure” along the initial deformed grain boundaries. With further increase of deformation, DRX grains volume fraction significantly increases and finally almost fully DRX extruded microstructure is obtained in the completely hot-extruded sample. The extrusion deformation results in increase of dislocation density and formation of dislocation cells. The subgrain boundaries continue to absorb dislocations and finally change from the low angle grain boundaries to the high angle grain boundaries. Hot extrusion deformation results in formation of dynamic precipitated phase, β phase particle, i.e., Mg5(Gd,Y) phase, with diameter of 45–145 nm, which is incoherent relationship with the matrix. In process of increasing deformation, the shape of β phase changes from spindle shape to disc shape and the distribution of β phase transforms from strip distribution along grain boundary to discrete distribution.

Nematic Liquid-Crystal Necklace Structure Made by Microfluidics System.

We report a necklace structure made of liquid crystal dispersed in poly(vinyl alcohol) (PVA) aqueous solution, which is fabricated by a microfluidic device. In the necklace structure, liquid crystal droplets that are tens of micrometers in diameter are connected by microtethers, which are birefringent, are not penetrating the droplets, and can be elastically stretched by applying external force. The necklace structure was analyzed by fluorescent confocal microscopy, and the tethers were made of liquid crystal and PVA composite. The elastic constant of the tether was determined by using laser tweezers to stretch the tether. The Whispering Gallery Modes circulating inside individual droplets in the necklace structure were also observed.

Height Variation of Gaps in 150‐km Echoes and Whole Atmosphere Community Climate Model Electron Densities Suggest Link to Upper Hybrid Resonance

AbstractRadar echoes from the daytime lower region near the magnetic equator, so‐called 150‐km echoes, have been puzzling researchers for decades. Neither the mechanisms that generate the enhanced backscatter at very high frequencies (typically 30–50 MHz), the sharp lower cutoff height, the intricate layering with multiple echo layers separated by narrow gaps, nor the modulation of the echoes by short‐period gravity waves is well understood. Here we focus on the diurnal variation of the echo layers—specifically, certain wide gaps in the vertical structure—which apparently descend in the morning, reach their lowest altitude near local noon, and ascend in the afternoon, sometimes described as necklace structure based on the appearance of the layers in range‐time‐intensity diagrams. Analyzing high‐resolution data obtained with the Jicamarca radar between 2005 and 2017, spanning more than one solar cycle, we find that (a) wide gaps and narrow lines occur in vertically stacked, systematically repeating pattern; (b) the gap heights vary with season and solar cycle; and (c) the gap heights can be associated with specific contours of plasma frequencies or electron densities. The last two findings are supported by simultaneous observations of VIPIR ionosonde reflection heights and by comparison of gap heights with electron density contours obtained with the WACCM‐X 2.0 global model. Finally, the wide gaps appear to coincide with the double resonance condition, where the upper hybrid frequency equals integer multiples of the electron gyrofrequency. This may explain why field‐aligned plasma irregularities are suppressed and enhanced radar backscatter is not observed inside the gaps.

Unique high-temperature deformation dominated by grain boundary sliding in heterogeneous necklace structure formed by dynamic recrystallization in HfNbTaTiZr BCC refractory high entropy alloy

Microstructural evolution of dynamically recrystallized (DRX) grains and grain boundary sliding (GBS) in the heterogeneous necklace structure of HfNbTaTiZr refractory high entropy alloy (RHEA) was studied systematically during high temperature deformation. Uniaxial compression testing was carried out to different strains at 1000 °C and a strain rate 10−3 s−1. Significant bulging of grain boundaries led initially to the formation of DRX grains. The fraction of DRX grains increased with strain, and typical necklace structures of fine (d ≤ 1.5 µm) DRX grains formed at strain ε ≥ 0.3. The DRX grains showed very limited grain growth, and heterogeneous microstructures composed of coarse unrecrystallized regions surrounded by the characteristic DRX necklace structure were formed at larger strains. Interrupted testing with marker grids revealed that DRX grains deformed by a GBS mechanism. The DRX necklace regions connected mesoscopically and also displayed diamond network morphologies, with unique “Y-shaped”, “T-shaped” and “X-shaped” junctions. The formation of different types of junctions were rationalized on the basis of GBS accommodated by local dislocation slip in unrecrystallized regions. The unrecrystallized regions showed preferred <001>/<111> micro-texture, consistent with conventional dislocation slip in the BCC crystals. On the other hand, the newly formed DRX grains initially had similar orientations to those of the parent grains, but they displayed a random texture with increasing strain, as expected from GBS. The randomized texture of DRX grains and the stability of DRX grains size represented GBS in the DRX necklace regions.

Hot Deformation Behavior and Processing Map of a Fe-11Mn-10Al-0.9C Duplex Low-Density Steel Susceptible to κ-Carbides

The hot deformation behavior of a Fe-11Mn-10Al-0.9C duplex low-density steel has been investigated, based on a series of isothermal compression tests at temperatures of 800-1100 °C and strain rates of 0.001-10 s−1. At relatively lower temperatures, with austenite decomposing, continuous dynamic recrystallization of ferrite and the formation of inter-granular κ-carbide were responsible for significant softening. At high deformation temperature where austenite was a dominant phase, dynamic recrystallization (DRX) is the restoration mechanism. Processing maps were developed at different plastic strains employing dynamic materials model and further verified through microstructural characterization. The optimal hot deformation condition at a large strain (0.7) was identified as deformation temperature of 950-1100 °C and strain rate of 0.01-1.0 s−1. In this domain, the original coarse grains could be replaced by the fine and uniform recrystallized grains, indicating that the high efficiency was dissipated by DRX. Meanwhile, two instable regions resulted from the formation of inter-granular κ-carbides and necklace structure should be avoided during hot processing.

Deformation response and microstructural evolution of as-cast Mg alloys AM30 and AM50 during hot compression

AbstractThe present study deals with the hot deformation behaviour of as-cast Mg alloys AM30 and AM50. The alloys have been investigated for mechanical and microstructural evolution in the temperature range 423–623 K and strain rate range of 10−3–10 s−1. A safe processing domain is identified for as-cast AM30 and AM50 alloys from the strain rate sensitivity map. The deformed microstructure of both AM30 and AM50 alloy reveals the occurrence of dynamic recovery and recrystallization for temperature above 523 K. Although the two alloys show similar behaviour in the hot deformation regime, they deviate at 623 K and low strain rate, where AM30 is fully recrystallized while AM50 attains necklace structure after deformation. This is likely due to the difference in the rate of dynamic recovery in the two alloys owing to the difference in Al concentration. Continuous dynamic recrystallization was the recrystallization mechanism active in the processing regime as deduced from the EBSD investigation.

Microstructural Evolution of Inconel 718 During Pancake Forging

Pancake forging of Inconel 718 was performed using a hydraulic press at temperature of 1273 and 1323 K. The effect of forging temperature on the microstructure of the pancakes was studied using optical microscopy. Variation in the microstructure was observed at different locations of the pancakes with fully recrystallized grains, partially recrystallized grains and necklace structure. Strain, strain rate and temperature across the pancake were determined through FEM analysis, and Johnson–Mehl–Avrami–Kolmogorov (JMAK) type model was employed to estimate the recrystallized grain size and volume fraction. There was close match between the predicted values and actual measurements.

Transport of a model diffusion probe in polyelectrolyte-surfactant hydrogels

Interactions of biopolymeric electrolytes with oppositely charged surfactants can result in micelle-like nanocontainers capable to dissolve hydrophobic substances. These micelle-like nanostructures can solve problems with hydrophobic nature of pharmaceutically active substances and be potentially usable for medical applications. In this study, two types of hydrogels were prepared: 1) based on hyaluronan and cationic surfactant septonex (carbethoxypendecinium bromide) and 2) based on diethylaminoethyl dextran and sodium dodecylsulphate. Nile red as a dye diffusion probe was incorporated into hydrogel in their preparation and then its release into surfactant solution and physiological saline was monitored. The release of NR from hydrogels was characterized by the effective diffusion coefficients of NR and structural parameters of hydrogels. The diffusion of NR in hydrogels was dependent on the molecular weight of polyelectrolyte and the ratio between charges of polyelectrolyte and surfactant. In the case of lower ratios, a bigger part of NR was probably incorporated in “pearl necklace structure” which suppressed its mobility. The increase of concentration of surfactant was connected with higher content of NR in free micelles or micelle-like aggregates and the increase in the release of NR. The formation of bigger micelles based on NR and SDS (in comparison with micelles based on NR and septonex surfactant) resulted in their lower diffusivity.

Evaluation of Hot Deformation and Dynamic Recrystallization Behaviors of Advanced Reduced-Activated Alloy (ARAA)

The hot deformation behavior of advanced reduced-activation alloy (ARAA) was investigated using hot torsion tests. The flow stress decreased as deformation temperature increased and as strain rate decreased. The flow behavior demonstrated the typical dynamic recrystallization (DRX). Based on the constitutive analysis of peak stress, the activation energy for hot deformation was found to be 330.3 kJ mol−1. Peak stress was analyzed as a function of the Zener–Hollomon parameter, and calculated and experimental values were in good agreement. A DRX kinetic model for ARAA was derived with deformation conditions based on the Avrami-type model. It was confirmed that the volume fraction of dynamically recrystallized grains increased as deformation temperature increased and as strain rate decreased. The necklace structure and grain boundary bulging were observed in the deformed microstructure of ARAA. The suggested DRX mechanism for ARAA during hot working is discontinuous DRX.

Boulder Array Effects on Bedload Pulses and Depositional Patches

AbstractThe influence of boulders in high gradient gravel‐bed rivers can be significant. Yet few studies have isolated their role in regulating bedload. It is hypothesized that boulder relative submergence and the corresponding eddies developing around boulders affect the frequency of bedload pulsations and the location of sediment deposits. Results show the occurrence of two distinct timescales in bedload exiting a boulder array, which were identified via time series analysis. These are the small‐scale periodicity, PS, and the large‐scale periodicity, PL. PS was identified in the range of 4–6 min and is believed to result from congested bedload movement due to reduced conveyance area within the array. PL was identified in the range of 8–46 min. It is suggested that PL corresponds to large bedload releases around boulders, which the authors consider to be caused by feedbacks between boulder eddies and bedload deposits. It is also found that boulder submergence and Froude number, Fr, influence the location of predominant deposition. At High Relative Submergence, deposition occurs in the boulder wake regions. At Low Relative Submergence, deposition instead occurs upstream of boulders in locations that depend on Fr. For Fr &lt; 1, material deposits in the stoss of boulders due to the necklace structure of the horseshoe vortex. However, for Fr &gt; 1, material deposits at the flanks of boulders due to the influence of local wave crests around boulders. The trapping efficiency of boulders reduces the dimensionless mean bedload transport rate by three orders of magnitude compared to conditions without boulders.

Effect of Post-Weld Heat Treatment on the Microstructure and Mechanical Properties of Friction Stir Welds of Cu–Cr–Zr–Ti Alloy

In the present study, the effect of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of the friction stir welded Cu–Cr–Zr–Ti alloy was studied with the aid of optical and transmission electron microscopy, tensile testing, and microhardness measurement. In the as-welded condition, the stir zone was characterized with fine recrystallized grains and thermo-mechanically affected zone showed a necklace structure. The microstructure of heat-affected zone and base metal showed equiaxed grains with annealing twins. Welded samples were subjected to three different PWHT: (a) direct aging, (b) solution treatment, and (c) solution treatment and aging. PWHT showed the effect on the microstructure of the weld with changes in the grain size and formation of precipitates. The microhardness across the weld was highest for the direct aged samples, whereas solution treated samples showed minimum hardness. The yield strength and elongation in as-welded condition were 158 MPa and 18%, respectively. Direct aging of the welded samples resulted in an improvement in the yield strength (171 MPa) with a decrease in ductility (13%). Significant softening of weld was observed after solution treatment, and yield strength was lower than that of base metal. Solution treatment and aging yielded an increase in elongation with a minor decrease in the yield strength. The increase in yield strength of the weld after aging is attributed to the formation of fine chromium precipitates.