Nanocrystalline Mixture Research Articles

Molecular dynamics study on the effect of cementite structure on the mechanical properties of pearlite

The effect of cementite structure on the mechanical properties of pearlite were investigated using molecular dynamics simulations. Three types of cementite structures were considered: single crystal, nanocrystalline, and a mixture of nanocrystalline and amorphous structures. The study found that regardless of the cementite structure, the ferrite phase in the pearlite exhibited plastic deformation first due to the activation of its internal {112} 〈111〉 slip system during tensile loading, resulting in macroscopic yielding of the pearlite. However, the difficulty of plastic deformation in the ferrite phase varied with different cementite structures. Compared to the other two types of cementite structures, the ferrite phase in the pearlite containing single crystal cementite exhibited the most difficult plastic deformation, leading to the highest peak stress. As the strain increased, the plastic deformation in the ferrite transferred through the ferrite-cementite interface to the cementite. After the plastic deformation transferred to the cementite, the different cementite structures exhibited distinct mechanical responses: the single crystal cementite structure experienced cleavage fracture, the nanocrystalline cementite structure underwent plastic deformation induced by grain boundary slip, while the nanocrystalline-amorphous mixture cementite structure showed shear deformation in the amorphous cementite and plastic deformation induced by grain boundary slip in the nanocrystalline cementite. Due to the influence of cementite mechanical response mechanism, the flow stress of the pearlite was minimal when the cementite in the pearlite were in single crystal structure. The study also revealed that when the cementite was in nanocrystalline structure, the flow stress of pearlite decreases with the grain size decreasing from 4.25 nm to 2.68 nm. When the cementite is a mixture of nanocrystalline and amorphous structure, the flow stress of pearlite decreases as the thickness of amorphous layer increases from 1 nm to 2 nm.

Mechanical and magnetic properties of phase separated Cu50Fe50 (at%) alloys synthesized by mechanical alloying and spark plasma sintering

A bulk Cu50Fe50 (at%) alloy was prepared by mechanical alloying (MA) of elemental Cu and Fe powders for 20 h, followed by the spark plasma sintering (SPS). All the powder and bulk samples were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical and magnetic properties of the bulk samples were also evaluated. From 1 h to about 10 h of milling, the formation of nanocrystalline Cu-rich face-centered cubic solid solution (FCC γ phase) and Fe-rich body-centered cubic solid solution (BCC α phase) was observed. Above 10 h of milling, the γ and α phases merged to form a single-phase face-centered cubic (γS) solid solution till 20 h. Subsequently, the 20 h MA powder was consolidated using spark plasma sintering (SPS) at three different temperatures: 519 ℃ (0.5 Tm), 677 ℃(0.6 Tm), and 835 ℃ (0.7 Tm), where Tm is the average melting point of the alloy (1310 ℃). In all three samples, the metastable γS phase decomposed into the nanocrystalline Cu-rich γ and Fe-rich α phases during SPS. Notably, the 0.7 Tm sample exhibited a better balance between strength and ductility by demonstrating a compressive yield strength of 886 MPa and an ultimate compressive strength of 1166 MPa with a ductility of 15%. The high strength and ductility are attributed to a bimodal grain size distribution, i.e., a few microns thick and continuous Cu-rich γ grain boundary phase and an intragranular nanocrystalline mixture of γ and α phases. In addition to its excellent mechanical properties, the 0.7 Tm sample displayed favorable soft ferromagnetic characteristics exhibiting a saturation magnetization (Ms) of 66 emu/g and a coercivity (Hc) of 16 Oe.

Mechanical alloying and phase transition of immiscible Al/Zn system during high-pressure torsion

Two half-disk samples of pure Al and pure Zn were mechanically alloyed via high-pressure torsion (HPT) processing, followed by post-deformation annealing (PDA). The microstructure evolution of the Al–Zn alloy was studied by scanning electron microscopy, transmission electron microscopy and molecular dynamics (MD) simulations. The results indicated that the HPT-processed Al/Zn assembly was presented as a mixture of nanocrystalline and amorphous phases. The deformation-induced special orientation of (0001)Al//(111)Zn facilitated the interatomic diffusion, and the dislocation density reached 2.17×1017 m−2 under the pinning effect of high solid solubility. Nanocrystalline, high diffusion degree, and high local dislocation density may primarily accounted for the crystalline-to-amorphous transformation in Al–Zn alloy. Moreover, the results indicated a bimodal grain size distribution of 150–250 nm and 500–900 nm, and Zn atoms were enriched at the grain boundaries, upon subsequent PDA. Under the effect of this special heterogeneous microstructure, the prepared alloy exhibited an excellent plasticity with 160% of tensile elongation.

Structure and performance of iridium electrocatalysts for hydrogen evolution reaction obtained by electrodeposition and double glow plasma: A comparative study

Iridium (Ir), with low overpotential and excellent stability, is a promising catalyst for the hydrogen evolution reaction (HER) in alkaline electrolyte. Structure and HER performance of Ir electrocatalysts fabricated by electrodeposition (ED) and double glow plasma (DGP) were investigated. ED Ir nano electrocatalyst with rough surface was not completely covered and consisted of a mixture of nanocrystalline and amorphous. Homogeneous and dense DGP Ir electrocatalyst with micro-size was composed of many coarse particles. After ablation, the Ir grains coarsened, and many holes were formed. Boasting a large electroactive surface area and high electrical conductivity, ED Ir electrocatalysts exhibited a superior electrocatalytic activity in 1.0 M KOH solution, requiring only 0.044 V overpotential to obtain a current density of 10 mA·cm−2, with a low Tafel slope of 34 mV·dec-1 and a high exchange current density of 0.669 mA·cm−2, compared with DGP Ir electrocatalysts with and without high-temperature treatment. The electrocatalytic activities of the electrocatalysts were in order of ED Ir > DGP Ir > ablation Ir electrocatalysts.

Hydrogen Gas Sensing Properties of Mixed Copper-Titanium Oxide Thin Films.

Hydrogen is an efficient source of clean and environmentally friendly energy. However, because it is explosive at concentrations higher than 4%, safety issues are a great concern. As its applications are extended, the need for the production of reliable monitoring systems is urgent. In this work, mixed copper-titanium oxide ((CuTi)Ox) thin films with various copper concentrations (0-100 at.%), deposited by magnetron sputtering and annealed at 473 K, were investigated as a prospective hydrogen gas sensing material. Scanning electron microscopy was applied to determine the morphology of the thin films. Their structure and chemical composition were investigated by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The prepared films were nanocrystalline mixtures of metallic copper, cuprous oxide, and titanium anatase in the bulk, whereas at the surface only cupric oxide was found. In comparison to the literature, the (CuTi)Ox thin films already showed a sensor response to hydrogen at a relatively low operating temperature of 473 K without using any extra catalyst. The best sensor response and sensitivity to hydrogen gas were found in the mixed copper-titanium oxides containing similar atomic concentrations of both metals, i.e., 41/59 and 56/44 of Cu/Ti. Most probably, this effect is related to their similar morphology and to the simultaneous presence of Cu and Cu2O crystals in these mixed oxide films. In particular, the studies of surface oxidation state revealed that it was the same for all annealed films and consisted only of CuO. However, in view of their crystalline structure, they consisted of Cu and Cu2O nanocrystals in the thin film volume.

On the possibility of fabricating fully austenitic sub-micron grained AISI 304 stainless steel via equal channel angular pressing

The paper concerns possibility of increasing yield and ultimate stress of austenitic stainless steels by equal channel angular pressing (ECAP). To this end, an AISI 304 stainless steel subjected to eight passes of ECAP in a 105◦ die at 350◦ C. The evolution of microstructure as a function of induced plastic strain (number of passes) was studied in different length scales using light and scanning transmission electron microscope (STEM). An insight into fine details of dislocation structure was possible via X-ray diffraction profile analysis. Changes in the structure of processed samples were correlated with their mechanical properties as determined by tension tests and microhardness measurements. Light microscopy revealed a shear banded microstructure. STEM observations demonstrated co-existence of three types of grain structure: (a) short elongated grains/subgrains of 71 nm average width formed within the shear band regions, (b) a mixture of coarse (200–350 nm in size) and nanocrystalline (15– 70 nm) equiaxed grains developed outside the shear bands, and (c) nano-twin lamellae of 3–20 nm width inside nanocrystalline grains. XRD peak profile analysis using modified Warren–Averbach approach yielded dislocation density as ρ = 4.71 × 1015 m−2 after eight passes. ECAP-induced changes in the microstructure led to a remarkable increase in yield strength of the alloy (YS=1498 MPa, about 3.5 times the initial amount). At the same time, a moderate ductility of εf = 12% was maintained, primarily due to a large post-necking elongation. This behavior was explained by an increase in the strain rate sensitivity of ECAP-ed material in combination with the contribution from transformation induced plasticity, TRIP effect, caused by formation of ∼ 22 vol% martensite during room temperature straining.

Fabrication of Ti–N and Ti–Al–N Coatings on Mg–Li Alloy by Surface Mechanical Nanoalloying Treatment under Nitrogen Atmosphere at Room Temperature

Fabrication of Ti–N and Ti–Al–N Coatings on Mg–Li Alloy by Surface Mechanical Nanoalloying Treatment under Nitrogen Atmosphere at Room Temperature

Superior anticorrosion performance of crystal-amorphous FeMnCoCrNi high-entropy alloy

In order to explore metal material with better corrosion resistance, this work presented a new crystal-amorphous FeMnCoCrNi high entropy alloy (HEA) mode as the mixture of nanocrystalline and amorphous shell fabricated by magnetron sputtering technique. Due to the wide and remarkable Cr-enrichment of amorphous shell, which provides higher driving force and diffusion rate for Cr atom, the crystal-amorphous FeMnCoCrNi HEA exhibits strong passivation behavior because of the formed passive layer about 40 nm thick. The corrosion current density of the crystal-amorphous FeMnCoCrNi HEA is about 0.147 nA•cm−2 that effectively inhibit pitting corrosion, indicating the superior anticorrosion performance. Then, the crystal-amorphous FeMnCoCrNi HEA presented herein provides a new mode for developing advanced materials with outstanding anticorrosion ability.

(Invited) Metal Oxide Based Micromechanical Sensors for Sensitive Gamma Rays Detection

In this talk, I will present our ongoing efforts on developing sensitive gamma rays dosimeter using two sensing techniques namely, metal oxide-coated microcantilever and Quartz tuning forks. The primary objective of these studies is to exploit the appealing characteristics (i.e. low in cost, sensitive and rapid) of the abovementioned techniques in developing gamma radiation dosimeters. In this first study, we have investigated the potential of exploiting TiO2 thin films as a sensing layer on Silicon microcantilevers for detection of gamma-ray radiations. Silicon microcantilever and wafers are both used for this purpose. All samples were exposed to gamma rays produced by 60Co with different doses ranging between 0 kGy and 40 kGy. The optical properties of silicon-coated by TiO2 at 200°C ALD grown and 100 nm thickness were studied before and after irradiation using X-ray diffraction (XRD) spectrum, scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE) and Atomic force microscopy (AFM). The optical studies by X-ray diffraction showed that change of our film from the nanocrystalline mixture of rutile and anatase phases to anatase by irradiation, this change appeared as (Δf) in AFM whereas the response of sensors to the doses between 0KGy and 20KGy was very sensitive and linear. The results of the spectroscopic ellipsometry measurements showed the same linearity change in the thickness, roughness and optical constants with irradiation doses were fitted using the Cauchy model. In the second study, gold coated quartz tuning forks were exposed to gamma radiations and we found that QTFs underwent a clear change in response to increasing gamma dosage. Our results show that both techniques are excellent candidates for dosimetry applications.

Data Storage in a Nanocrystalline Mixture Using Room Temperature Frequency-Selective and Multilevel Spectral Hole-Burning

Data Storage in a Nanocrystalline Mixture Using Room Temperature Frequency-Selective and Multilevel Spectral Hole-Burning

Influence of Alumina Particles on Tribology of Autocatalytic Ni-P Coatings at High Temperature

The present work considers the effects of incorporation of hard Al2O3 particles on the structure, microhardness, and tribological behavior of electroless Ni-P coatings at room temperature and elevated temperature. Ni-P (9% P) coating shows a typical amorphous structure that changes to a mixture of nanocrystalline and amorphous structure due to the addition of alumina particles. The incorporation of Al2O3 particles is found to enhance the overall hardness and wear resistance of the Ni-P coating. Exposure to high temperature during tribological tests acts as brief heat treatment, initiating microstructural changes in the coating which further increases the hardness of the deposit. The scanning electron micrograph of the worn surface of the coating reveals both abrasive and adhesive wear phenomena governing the wear mechanism at elevated temperature. The development of the oxide layer is another important characteristic of the coatings examined under high temperatures (around 500°C).

Solid Phase Interaction of the Recondensed Finely Dispersed Mixture (VC0.40O0.53–C) with Titanium Hydride

The processing of industrial waste and highly hazardous substances using extreme techniques of formation, in addition to else, of nanocrystalline materials, which include plasmachemical synthesis according to the plasma recondensation scheme in low-temperature nitrogen plasma, is an urgent issue of chemical technology. In the work, an attempt was made to plasma recondensation of a mechanical mixture consisting of crystalline hydrates of the VnOn·xH2O type and gas soot in an 1 : 1 ratio. The solid phase interaction of the VC0.40O0.53–Cfree nanocrystalline mixture with TiH2 hydride in various proportions in the course of high-temperature vacuum sintering was studied. Based on the data of X-ray diffraction, scanning electron microscopy, and EDX analysis, the main patterns of solid phase interaction in the system (VC0.40O0.53–Cfree)–TiH2 were described, and schemes of the processes were proposed occurring during sintering.

Incorporation mechanism of tungsten in W-Fe-Cr-V-bearing rutile

Abstract Rutile is a common mineral in many types of ore deposits and can carry chemical or isotopic information about the ore formation. For closer understanding of this information, the mechanisms of incorporation of minor elements should be known. In this work, we have investigated natural rutile crystals with elevated concentrations of WO3 (up to 17.7 wt%), Cr2O3,tot (7.5), V2O3,tot (4.1), FeOtot (7.3), and other metals. X-ray absorption spectroscopy (XAS) of rutile at the Fe K, Cr K, V K, and W L1 and L3 edges shows that all cations are coordinated octahedrally. The average oxidation state of V is +3.8, and that of Cr is near +4. Shell-by-shell fitting of the W L3 EXAFS data shows that W resides in the rutile structure. Raman spectroscopy excludes the possibility of hydrogen as a charge-compensating species. High-resolution TEM and electron diffraction confirm this conclusion as the entire inspected area consists of rutile single crystal with variable amounts of metals other than Ti. Our results show that rutile or its precursors can be efficient vehicles for tungsten in sedimentary rocks, leading to their enrichment in W and possibly later fertility with respect to igneous ore deposits. Leucoxene, a nanocrystalline mixture of Ti and Fe oxides, is an especially suitable candidate for such a vehicle.

Understanding the effect of bimodal microstructure on the strength–ductility synergy of Al–CNT nanocomposites

The research work presents a novel methodology to fabricate Al–CNT nanocomposites possessing excellent amalgamation of strength and ductility. A unique combination of processing techniques comprising of ball milling and spark plasma sintering was utilized for consolidating the samples. High-energy ball milling of as-received Al powder and subsequent addition of CNT into the milled Al powder led to a substantial enhancement in the mechanical properties of the resultant composites. Al compact comprising of as-received microcrystalline powder displayed a tensile strength of 105 MPa, which increased to 161 MPa for Al compact with ball-milled nanocrystalline powder. The respective values of ductility reduced from 41.9 to 6.6%. Adding 0.5 wt% CNT in the later compact led to a further upsurge in the tensile strength to 217 MPa; however, the nanocomposite exhibited a further reduction in the ductility to 4.7%. To address this problem, bimodal microstructure was introduced in Al–CNT nanocomposites by using a mixture of nanocrystalline and microcrystalline Al powders (10, 20, 30, 40 and 50 wt%) as the matrix. Al–CNT nanocomposite containing 30 wt% microcrystalline Al powder exhibited a remarkably improved strength–ductility synergy showing 149% higher yield strength and 78% higher tensile strength than the Al compact with microcrystalline grained matrix and 130% higher elongation than the Al–CNT nanocomposite with nanocrystalline grained matrix. Furthermore, to assess the effectiveness of bimodality and CNT reinforcement, experimentally determined value of yield strength of Al30M70N–CNT was assessed with respect to the theoretically calculated value. It was found that the experimentally obtained yield strength of the nanocomposite was about 94% closer to the predicted value endorsing the effectiveness of the designed fabrication route.

Evaluating the effect of grain size distribution on thermal conductivity of thermoelectric materials

The influence of grain size (d) on the thermal conductivity (k) of thermoelectric (TE) materials has been well established through experimental studies. However, the effect of grain size distribution, described by S n , on k has not been reported before. Since thermal conductivity is a key contributor to the figure of merit (ZT) for thermoelectric materials, studying the effect of grain size distribution, an important microstructural descriptor, on k is necessary. In the current study we are evaluating the effect of S n on the k of thermoelectric materials by using data reported in literature on bismuth telluride (Bi2Te3) and lead telluride (PbTe). We first check for correlations between k and d. In literature, mathematical correlations between lattice thermal conductivity (k l ) and d have already been reported but the same is missing for electronic thermal conductivity (k e ) and d. By analysing literature data for bismuth telluride and lead telluride at 300 K, we identified a linear correlation between k e and d, wherein an increase in d leads to an increase in k e . This dependence of k e on d was combined with the dependence of k l on d to establish the overall dependence of k on d. Subsequently, the grain size distribution effect was imposed by using a log normal distribution. The analysis revealed that for a given grain size, an increase in S n leads to lowering of the thermal conductivity of the material. The analysis was also extended to bimodal grain size distributions wherein the microstructure was designed in a way to contain a mixture of both nanocrystalline and microcrystalline grains.

Integrated Thin Film Battery Design for Flexible Lithium Ion Storage: Optimizing the Compatibility of the Current Collector‐Free Electrodes

AbstractThe design of flexible full‐cell configuration relies on the light‐weight arrangement, structural robustness, compositional compatibility, and shape conformability of the coherent components. In this article, a general and scalable spin‐coating approach is developed to integrate the flexible, current‐collector‐free cathode and anode in the thin film batteries with the layer‐stacked configuration. The design of the ternary composite anode involves the reduced graphene oxide encapsulation of the MoS2/N‐doped carbon microrods composite (rGO‐MoS2/NC) via the facile hydrothermal‐calcination process. In the similar structure design of the cathode, the ball‐milled nanocrystalline mixture of LiMn2O4 and LiVPO4F is wrapped within the GO interfacial protection layer. Furthermore, the slurries of electroactive materials with the predetermined areal capacity ratio of negative to positive electrodes (N/P ratio) are cast onto both sides of the nano‐SiO2 modified polyethylene (SiO2‐MPE) separator via the facile spin‐coating process. The prototype full‐cell configuration delivers a high reversible capacity, satisfactory capacity retention at the high rate, as well as good cyclability under different mechanical loadings. This integrated thin film battery model showcases its potential use in the flexible electronic devices.

Investigation of the plasma electrolytic oxidation mechanism of titanium

Abstract A critical parameter in Plasma Electrolytic Oxidation (PEO) is the applied current composed of electronic current caused by sparking and ionic current caused by diffusion of electrolyte ions into the oxide. A wide spectrum of current densities was applied on pure titanium to investigate the PEO mechanism. The growth process and oxide characteristics were investigated by studying the ionic/electronic current contributions during the PEO stages. The contribution of electronic current was found to dominate at low current densities (30 and 40 mA/cm2). The large number of plasma discharges at this condition increases the porosity and coating surface roughness. These discharges provide enough energy to raise the temperature facilitating formation of both stable rutile and metastable anatase. By increasing the current density, the incorporation of ionic current increases resulting in the formation of dense anatase coatings. The highest growth rates achieved at a balance between ionic and electronic charges. HRTEM showed that all coatings are composed of a top amorphous layer produced from electrolyte quenching and an inner layer consisting of a mixture of nanocrystalline and amorphous regions and pores. The interlayer forming at the substrate/coating interface was found to play a key role in the growth mechanism during PEO processing.

Fracture Strength and Surface Magnetic Flux Uniformity in an Isotropic Polymer-Bonded Ring-Shaped Nd-Fe-B Magnet

Ring-shaped (RS) polymer-bonded magnets were produced from a mixture of an isotropic nanocrystalline Nd-Fe-B powder with a nominal composition of 59.8 wt.% Fe, 29 wt.% Nd, 5 wt.% Zr, 4.8 wt.% Co and 1.4 wt.% B and a variety of epoxy resins as binders, using a compaction-molding technique. The morphology and average particle size of the powders are determined by an SEM. The magnetic properties of the magnets were measured using a permeameter. Magnetic flux on the surface of the magnets was determined by a Gauss meter. The mechanical properties of the RS specimens were determined using a tensile fracture strength (FS) test. The effects of polymer type and amount, hardener amount, applied pressing pressure and curing temperature and time on FS were investigated. The experimental results showed that at optimal conditions of 8 wt.% solid epoxy, 3.5 wt.% solid hardener, a pressing pressure of 900 MPa and a curing time of 8 h at 170 °C, a maximum tensile strength of ~ 36 MPa was achieved. Surface magnetic flux uniformity in optimal mechanical conditions was also found to be optimum at around ± 40 G, which is one of the best flux uniformities in bonded magnets.

S-incorporated TiO2 coatings grown by plasma electrolytic oxidation for reduction of Cr(VI)-EDTA with sunlight.

The plasma electrolytic oxidation (PEO) technique was used to prepare photocatalytic S-TiO2 coatings on Ti sheets; the incorporation of the S ions was possible from the electrolyte for modifying the structural and optics characteristics of the material. In this work, substrates of Ti (ASME SB-265 of 20 × 20 × 1mm) were used in a PEO process in 10min, using constant voltage pulses of 340V with frequency of 1kHz and duty cycles of 10% and of 30%. Solutions with H2SO4 (0.1M) and CH4N2S (52 and 79mM) were used as electrolytes. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy (EDS) were utilized to analyze the surface morphology, crystalline phase, and chemical composition of the samples. According to the results, the catalyst coatings had microporous structure and contained anatase-rutile TiO2 nanocrystalline mixture, until 73.2% rutile and 26.8% anatase in the samples grown with 30% duty cycle and the lowest concentration of CH4N2S. From the EDS measurements, the incorporation of sulfur ions to the coatings was 0.08wt%. 99.5% reduction efficiency of Cr(VI)-EDTA with sunlight was observed after 2h; it was determined by diphenyl carbazide spectrophotometric method. These coatings have potential for effective sunlight heterogeneous photoreduction of this toxic, cumulative, and non-biodegradable heavy metal that contaminates the soil and water and is a serious risk to sustainability, ecosystems, and human health.

Microstructure evolution and deformation mechanism of NiTiFe shape memory alloy based on plane strain compression and subsequent annealing

Microstructure evolution and deformation mechanism of NiTiFe shape memory alloy (SMA) are investigated during plane strain compression and subsequent annealing at 400 °C. In the case of various deformation degrees, an inhomogeneous plastic deformation occurs in the NiTiFe SMA undergoing plane strain compression. With increasing deformation extent, a mixture of nanocrystalline and amorphous phases can be generated. Annealing significantly influences the microstructures of NiTiFe SMA subjected to plane strain compression. Consequently, in the annealed NiTiFe SMA, the distributions of grain size, high angle grain boundary, subgrain boundary and geometrically necessary dislocation (GND) density are captured using electron backscatter diffraction (EBSD). The maximum Schmid factor of all the slip systems of each grain in NiTiFe SMA is obtained on the basis of <111>/{110}, <100>/{110} and <100>/{010} slip systems. It can be concluded that prior to mechanically-induced martensite phase transformation, dislocation slip is responsible for plane strain compression of NiTiFe SMA, where <111>/{110} slip system plays a dominant role.