Initial Phase Transformation Research Articles

A Novel Design of a Molten Salt Bath Structure and Its Quenching Effect on Wire Transformation from Austenite to Sorbite

The technology for obtaining sorbite by isothermal quenching of high-temperature molten salt has been used by more and more factories to produce wires with high tensile strength. In this paper, the controlling cap and bottom pipeline of the original salt bath are redesigned. The mathematical model previously proposed is used to simulate the redesigned salt bath model, and the flow field is analyzed in detail. The redesigned and original controlling cap are compared in detail by applying third-generation vortex identification technology. Then, by using the inverse heat transfer method, the heat transfer coefficient (HTC) during the boiling heat transfer stage of the wire rod in molten salt is calculated by taking advantage of quenching experimental data, on the basis of which the original model is corrected. Finally, a new salt bath design is proposed, which divides the salt bath into two parts. The first salt bath at 515 °C is used to cool the austenitized wire and complete the initial phase transformation. The second salt bath at 560 °C is used to prevent the transformation from retained austenite to bainite, and to induce its transformation from retained austenite to sorbite.

Uncertainty and sensitivity analysis of boundary conditions during water droplet phase change regimes

This paper presents the heat and mass transfer model of a liquid droplet in the consistently changing condensation, transit and equilibrium evaporation regimes, and iterative scheme of numerical simulation of the instantaneous temperature of a semi-transparent droplet in the case of complex radiative-convective heating. The whole cycle of the phase transition regimes of medium and large water droplets flowing in the flue gas was modeled, taking into account the application of water spraying into the flue gas of a biofuel furnace. Simulation results expressed as thermal, energy and phase transformation parameters of water droplets are analyzed in the universal Fourier time scale. The factors defining the change of heat transfer regime of a water droplet are termed as the suppression of droplet slipping in flue gas flow at initial phase transformation stage, and the decrease of radiation absorption at the final stage. Particular attention is paid to the evaluation of the influence of parameters defining the interaction of complex transfer processes on the droplet's phase transformations performing the sensitivity analysis of the heat and mass transfer parameters for the “hypothetical” droplet case (the diameter and the slipping velocity in to gas did not change). It is substantiated, that the main parameters defining the sensitivity of water droplets equilibrium evaporation temperature are flue gas humidity, droplets dispersity, flue gas temperature and droplets slipping velocity.The performed statistical uncertainty and sensitivity analysis showed that the uncertainty of heat and mass transfer parameters describing the phase transformations of a water droplet until equilibrium evaporation regime is affected mostly by droplet dispersity, slipping velocity, flue gas temperature and pressure. The size of droplets and their slip velocity in the flue gas flow were identified as very important parameters for the interaction of complex heat and mass transfer processes of a sprayed water. The main uncertainties were defined at the transition from condensing to transit evaporation regime, and it was defined that with a confidence interval of at least 95 % and accepting a tolerance limit with a probability of at least 95 %, the transit evaporation regime starts at the range Fo ≈ (0.02–0.1). The droplet surface temperature, i.e. parameter determining the droplet phase transformations and its dynamics, could be assessed with a mean error of ±10 %, accepting 95 % probability with 95 % confidence level.

Resistance Spot Welding Behavior of Automotive Steels

In present-day automobiles, new high-strength steel grades are being extensively utilized, which have different alloy concentration, microstructure and properties, and results in different spot welding behavior. In the present work, the effect of weld current was studied on four widely used automotive grades, such as IF 270, TRIP 690, DP 780 and TRIP 980, in a laboratory welding machine. The welding current was varied between 5 and 15 kA. Weld quality was assessed based on tensile shear, mode of failure, weld nugget diameter, metallographic observations and hardness tests. Experimental results showed that weld strength increased with an increase in base metal tensile strength but with a drop at the point of expulsion. Variation in hardness along the cross section was found to be higher in high-strength steels. The initial microstructure and phase transformation during cooling affected the final properties. Critical currents for peak strength, expulsion, sticking and failure mode transition were determined for each grade for industrial applications. This study helped in determining the criterion-based optimized welding parameters for specific applications of these automotive-grade steels.

The study of the structural characteristics and catalytic activity of Co/CoCo2O4 nanowires

The aim of this work is to study the prospects of using Co/CoCo2O4 nanowires as catalysts for the reduction reactions of para-nitroaniline to para-phenyldiamine, as well as CO conversion. Interest in these structures is due to their physicochemical properties, spinel structure, leading to an increase in catalytic activity, high stability to external influences and degradation over a long period of operation, etc. The initial nanowires were obtained by electrochemical deposition using polymer template matrices. In turn, the method of electrochemical synthesis makes it possible to control with high accuracy not only the geometry of nanostructures, with the possibility of producing nanostructures with a large aspect ratio of 1:100–1:1000, but also to scale this technology for producing nanostructures on a semi-industrial scale. The use of thermal annealing to initiate phase transformations contributes to the complete rearrangement of the nanowire structure along its entire length, with the formation of a stable phase of the CoCo2O4 spinel type. During the study of the catalytic activity of Со/CoCo2O4 nanowires it was found that the use of oxide nanowires makes it possible to carry out catalytic PNA–PPD reduction reactions, while the initial nanostructures do not exhibit catalytic activity for these compounds. As a result of the study of the catalytic activity of CO gas oxidation, it was found that the presence of Со3+ cations in the structure of oxide nanowires leads to intensification of CO adsorption processes with the formation of intermediate СО2* compounds and an increase in the rate of CO conversion.

Molecular dynamics study on the Li diffusion mechanism and delithiation process of Li2MnO3

Li2MnO3 is critical component in the well-studied Li-excess cathode materials xLi2MnO3·(1- x)LiMO2 for achieving high lithium storage capacity. In this article, the diffusion of Li ions in Li2MnO3 is studied using molecular dynamics (MD) simulation with well-behaved empirical force fields obtained by fitting against the crystal structure from experiment and phonons calculated using Density Function Theory (DFT). We have found two possible tetrahedral hopping channels, 0-TM and 1-TM(Mn4+) channel, which are differentiated by the face sharing octahedral cations. Simulation results show that the 0-TM channel is active for Li hopping, while 1-TM(Mn4+) channel is inactive. During the delithiation process, the Li ions in the transition metal (TM) layer are firstly removed, then those in the Li layer. However, the Li ions will be trapped in the tetrahedral 0-TM channels as long as the four face sharing octahedral sites are cleared. Up to x = 1.0 for Li2-xMnO3, almost all the Li ions are located at the tetrahedral sites, forming a regular array along a axis. The de-intercalation of tetrahedral Li ions requires a high voltage (>5.2 V vs. Li/Li+), limiting the practical capacities measured in lab. The diffusion of Mn ions into the Li layer is observed in a deeper delithiated structure (x = 1.2 for Li2-xMnO3), indicating an initial phase transformation to a spinel-like structure. However, the Mn ions are mainly trapped in the tetrahedral sites in the Li layer, instead of the octahedral sites in spinel-like structure. A few of Mn ions diffusing into the octahedral sites in Li layers have no face sharing tetrahedral Li ions, revealing a further Li de-intercalation is imperative for the complete phase transformation. During the delithation process, the oxygen sublattice shows a strong stability. In the deeply dilithiated structure Li0.8MnO3, the local distortion of the oxygen sublattice is observed, which may indicate the oxygen release at this high delithation stage. Our model is not stable for x ≥ 1.4 in Li2-xMnO3. Other charge compensation mechanism should be considered in this high delithiation stage, eg. oxygen release.

Enhanced electrochemical performance of Li1.2Ni0.13Co0.13Mn0.54O2 by surface modification with the fast lithium-ion conductor Li-La-Ti-O

The drawbacks of Li-rich layered oxides, such as large initial irreversible capacity, phase transformation and electrochemical performance degradation, are closely related to the surface states of the Li-rich layered oxide. To overcome these problems, a new surface modification method to the Li-rich layered oxide is proposed, in which the fast lithium-ion conductor (Li-La-Ti-O composite, LLTO) is coated on the surface of Li1.2Ni0.13Co0.13Mn0.54O2 (LRO) particles. SEM, EDS and HRTEM analyses reveal that an entire and homogeneous coating layer is formed. Electrochemical performances display that the LLTO coated samples, especially the LRO-LLTO5 material, exhibit better cycle stability, higher rate capabilities and less voltage decay. To investigate the origins of enhanced electrochemical performances for LLTO coated samples, the electrode processes at various cycles and the structural stability of the samples are studied by EIS and DSC. The protective effect of LLTO coating layer to the bulk crystallographic structure of cathode material is revealed by HRTEM images. Our study shows that the reduced interface impedances and definitely protective effect resulting from the complete and homogeneous LLTO coating layer are responsible for the enhanced electrochemical performances of the LLTO coated samples.

Insights into the role of grain refinement on high-temperature initial oxidation phase transformation and oxides evolution in high aluminium Fe-Mn-Al-C duplex lightweight steel

Influence of grain size on internal oxidation behaviour at 1273K in dry air of Fe-20Mn-8Al-0.45C (wt.%) has been investigated. The measured weight gains are in parabolic relation to oxidation time and parabolic rate constant degrades with the reduction of grain size. Grain size affects compositions of oxides in the initial oxidation process and diffusion abilities of cations. Fine-grained alloy becomes less permeable to oxygen than coarse-grained alloy. Grain refinement accelerates phase transformation and promotes the rapid formation of pure Al2O3 layer, contributing to the slightly improved oxidation resistance. A brief schematic model is proposed to explain oxides evolution.

Initial precipitation and coarsening of the γ phase in inverse Ni–Al alloys

Coalescence coarsening and Ostwald ripening as well as the kinetics of γ precipitation in the γ′ matrix of inverse Ni–Al alloys are investigated by phase field simulations. The ordering of the γ′ matrix during the initial phase transformation of inverse Ni–Al alloys is faster than clustering, and disordering of the γ precipitates is faster than dissolution. As the Al content increases from 20.2 to 20.8 at.% Al, the initial precipitation and growth of γ precipitates decelerates, and the volume fractions of γ precipitates decreases with higher Al content. The rate constants of steady-state coarsening decrease with decreasing volume fraction of γ precipitates, the particle size distributions (PSDs) are broader than the PSDs predicted by Lifshitz–Slyozov–Wagner theory at steady-state coarsening, and the PSD becomes broad with increasing volume fraction of γ precipitates. The simulated morphology and kinetics variation of the γ precipitates are similar to previous experimental results.

Analysis of the Electric and Thermal Effects on Mechanical Behavior of SS304 Subjected to Electrically Assisted Forming Process

Significant improvements in deformation resistance and ductility of metals are observed in the electrically assisted forming (EAF) process. Both electroplastic effect (EPE) induced by electric current and thermal effect associated with Joule heating have been proposed to explain the phenomenon. However, there are still arguments in the contribution of the EPE in EAF process. In this paper, both electrically assisted tension tests (EAT) and thermally assisted tension tests (TAT) were conducted on SS304 specimens at the same temperature. The existence of EPE is investigated, and the contribution of EPE is also distinguished with thermal effect numerically by considering the initial yield stress, dislocation hardening, and martensite phase transformation. It is shown when the temperature is around 34 °C, the electric current of 50 A/mm2 in EAT induces additional stress reduction of 16% in the short-range internal stress (effective stress) involved in the initial yield stress and volume reduction of 45.2% in martensite formation compared with results in TAT. However, the effect is not obvious for the cases of 100 A/mm2 and 150 A/mm2 when the temperature is above 100 °C. By comparing the storage coefficient and recovery coefficient of dislocation in EAT and TAT, it indicates that electric current has no additional activation effect on dislocation movement of SS304.

Joining of Zirconia Reinforced Metal-Matrix Composites

Metal-matrix composite materials, based on a metastable austenitic stainless steel reinforced with a magnesia partially stabilised zirconia have been prepared by a ceramics-derived extrusion technology. Using this powder metallurgical method enables the shaping of lightweight cellular structures as well as bulk specimens with a variety of steel/ceramic ratios at room temperature. However, the extrusion of composite structures is limited by the uniform cross section throughout its entire length. Joining of these metal-matrix composite preforms after sintering by conventional welding techniques is a challenging task. The presence of ceramic fractions may lead to several complications and the subsequent heat exposure during joining may initiate phase transformations in both metastable components resulting in a deterioration of the mechanical properties of the composite material. An adapted ceramics-derived joining technology allows the combination of varying TRIP-steel/zirconia composite materials. The main features are the machining and joining of the parts in their dry green state at room temperature before their thermal treatment. Thus, the material’s consolidation and the formation of the joint take place simultaneously. The ability of joining different parts offers the possibility to create structures for complex applications and testing conditions. The key to advanced properties of the joining zone are the base materials, the surface treatment of the parts, and the paste used for joining. The joining process of different base materials, the mechanical properties, and the microstructure of sinter-joint samples are presented.

Acceleration of the initial phase transformation of mineralization by phosvitin

Phosvitin has a similar structure and similar properties to the phosphorylated proteins that play an important role in biomineralization, suggesting that phosvitin may have similar regulation properties. This study investigated the effect of phosvitin on regulating the phase transformation of the mineral calcium phosphate in a biomimetic mineralization solution; the characterization techniques used were Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and fluorescence spectroscopy. The results clearly demonstrated that phosvitin significantly promotes the initiation of phase transformation, accelerated the transformation process and shortened the transformation time from 6 to 0.5h. Phosvitin was involved in the phase transformation and incorporated into or strongly absorbed on the mineral, as evidenced by the protein peaks observed in the FTIR spectra and XRD patterns. The effects of the substrate-addition sequence on the phase transformation demonstrated that the phosvitin-Ca2+ interaction played a key role in the regulation of mineralization. Compared with those for BSA, the results revealed that the role of phosvitin in mineralization is closely associated with its high level of phosphorylation. This study provides useful information about using phosvitin as a potential candidate for biomaterials.

001]-oriented nonepitaxial growth in L1-ordered FePt thin film by SiO2 addition and rapid thermal annealing

A highly L10-ordered FePt film with a crystalline [001] orientation was prepared by SiO2 addition and rapid thermal postannealing on an oxidized Si substrate. In order to clarify the mechanism of L10 ordering and the crystalline orientation of a nonepitaxial FePt film, we investigated the effect of the preparation conditions on the film structure. The crystalline quality was strongly dependent on the additive amount of SiO2; a high chemical order without the distribution of the chemical order and a nearly perfect [001] orientation were obtained at a SiO2 composition of 10 vol %. The nucleation of L10 grains with a high chemical order and a [001] orientation was observed from an initial phase transformation stage at a low annealing temperature. The as-deposited FePt–SiO2 film consisted of a nanocrystalline structure in which SiO2 segregated at the grain boundaries; SiO2 diffused with the grain growth during thermal annealing. Because the in-plane tensile stress among the grains was a dominant origin for the [001] texture formation, two-dimensional grain growth with the annihilation of grain boundaries played a crucial role in the [001] ordering in nonepitaxial FePt films. It was also crucial to maintain a high stress during annealing, and the [001] ordering was enhanced by rapid heating.

Design and application of a high-temperature microfurnace for anin situX-ray diffraction study of phase transformation

Thermal treatment of mineral ores such as ilmenite can initiate phase transformations that could affect their activation or deactivation, subsequently influencing their ability to dissolve in a leaching agent. Most laboratory-based X-ray diffraction (XRD) studies were carried out ex situ in which realistic diffraction patterns could not be obtained simultaneously with occurring reactions and were time-consuming. The availability of synchrotron-radiation-based XRD not only allows in situ analysis, but significantly shortens the data recording time. The present study details the design of a robust high-temperature microfurnace which allows thermal processing of mineral ore samples and the simultaneous collection of high-resolution synchrotron XRD data. In addition, the application of the manufactured microfurnace for in situ study of phase transformations of ilmenite ore under reducing conditions is demonstrated.

Relativistic simulations of the phase-transition-induced collapse of neutron stars

An increase in the central density of a neutron star may trigger a phase transition from hadronic matter to deconfined quark matter in the core, causing it to collapse to a more compact hybrid-star configuration. We present a study of this, building on previous work by Lin et al. (2006). We follow them in considering a supersonic phase transition and using a simplified equation of state, but our calculations are general relativistic (using 2D simulations in the conformally flat approximation) as compared with their 3D Newtonian treatment. We also improved the treatment of the initial phase transformation, avoiding the introduction of artificial convection. As before, we find that the emitted gravitational-wave spectrum is dominated by the fundamental quasi-radial and quadrupolar pulsation modes but the strain amplitudes are much smaller than suggested previously, which is disappointing for the detection prospects. However, we see significantly smaller damping and observe a nonlinear mode resonance which substantially enhances the emission in some cases. We explain the damping mechanisms operating, giving a different view from the previous work. Finally, we discuss the detectability of the gravitational waves, showing that the signal-to-noise ratio for current or second generation interferometers could be high enough to detect such events in our Galaxy, although third generation detectors would be needed to observe them out to the Virgo cluster, which would be necessary for having a reasonable event rate.

Liquefaction, Cyclic Mobility, and Failure of Silt

It is known that the mechanical properties of low-plasticity silt are similar to those of sand, and yet silts are frequently used as coastal reclamation materials in many cities and industrial areas and will thus be susceptible to liquefaction. Samples of a low-plasticity silt have been tested under monotonic and cyclic loading under isotropic and anisotropic stress conditions to characterize liquefaction, cyclic failure, and to develop an empirical model describing its cyclic strength. A sedimentation technique produced samples that had the highest susceptibility to liquefaction. Contractive behavior of monotonically loaded samples was triggered when the stress path reached an initial phase transformation (IPT) in both compression and extension tests. The samples became dilative after reaching a phase transformation (PT) point. The cyclic shear behavior of the silt samples prepared using the sedimentation method and consolidated at various initial sustained deviator stress ratios was examined in terms of two different failure criteria: a double amplitude axial strain εa,DA=5% for reversal conditions; or axial plastic strain εa,P=5% for nonreversal. For isotropically consolidated samples the initial phase transformation determined from undrained monotonic extension tests was the boundary between stable and contractive behavior. For anisotropically consolidated samples failure was defined by a bounding surface formed by undrained monotonic compression tests. An empirical model was developed relating the number of cycles to failure under conditions of both liquefaction and cyclic mobility to the initial anisotropic sustained deviator stress and cyclic deviator stress ratio.

Thermal and Mineral Properties of Al-, Cr-, Mn-, Ni- and Ti-Substituted Goethite

AbstractMineralogical and thermal characteristics of synthetic Al-, Cr-, Mn-, Ni- and Ti-bearing goethites, synthesized via alkaline hydrolysis of metal-ferrihydrite gels, were investigated by powder X-ray diffraction and differential thermal analysis. Shifts in unit-cell dimensions were consistent with size of substituent metal ions and confirmed the incorporation of Al3+, Cr3+, Mn3+, Ni2+ and Ti4+ in the goethite structure. A weight loss of 6.2 wt.% for goethite containing 12.2 mol.% Ti, being significantly less than for stoichiometric goethite, is consistent with the replacement of Fe by Ti in the goethite structure coupled with the substitution of O2− ions for OH− (i.e. proton loss). These data provide the first confirmation of the direct replacement of Fe by Ti within goethite. Formation of multiple dehydroxylation endotherms for goethite containing 4.5 mol.% Al, 15.3 mol.% Mn and 12.2 mol.% Ti was not attributed to the decomposition of surface OH groups or related simply to the crystallinity of precursor goethite (‘high-a’ vs. ‘low-a’) as defined by the magnitude of a. Instead, endotherm doublet formation was associated with weight loss due to the dehydroxylation of goethite remaining after initial phase transformation to protohematite and to the evolution of OH− associated with the rapid increase in crystallite size of protohematite directed primarily along the a direction. Development of the first endotherm is due to initial dehydroxylation and transformation to protohematite. With continued heating of well ordered goethite or goethite containing moderate to high levels of substituent cations, domain growth along the a direction is delayed or inhibited to a critical point that provides enough thermal energy to enable goethite transformation to proceed to completion and for proto-hematite domain growth to occur. This results in the formation of a second endotherm. For less well ordered goethite and/or goethite containing only low levels of foreign metal cations, protohematite domain growth is not inhibited and proceeds continuously with heating to give only a single endotherm.

Initial phase transformation diagram determination for the CD3MN cast duplex stainless steel

This work concerns the development of a time–temperature-transformation (TTT) diagram for cast CD3MN, a duplex stainless steel. Samples were solution heat treated and quenched to temperatures ranging from 700 to 900 °C and held for times from 1 to 1000 min. Initial sigma phase precipitation was noted as low as 700 °C, with the nose of the transformation curve at approximately 800 °C and 100 min.

O2 Structure Li2 / 3 [ Ni1 / 3Mn2 / 3 ] O 2: A New Layered Cathode Material for Rechargeable Lithium Batteries. I. Electrochemical Properties

Layered Li‐Mn oxide‐based materials with the unconventional O2 structure were prepared by ion exchanging Na for Li in the corresponding P2 structure sodium bronzes, . Powder X‐ray diffraction (XRD) was used to confirm the purity and structure of the phase. The Bragg peaks from can be indexed based on a hexagonal unit cell with dimensions a = 2.8609 Å and c = 10.061 Å, and the diffraction pattern can be approximately matched assuming that the atoms are in 2a and 2b positions of space group . However, the initial structure is more complex, because some observed diffraction peaks (i.e., 111) are forbidden in this space group. In situ XRD and electrochemical testing were used to investigate the behavior of O2‐type as a cathode for rechargeable lithium batteries. It exhibits a large reversible capacity of approximately 180 mAh/g divided into two plateaus centered near 2.8 and 3.9 V. In situ XRD proves that after an initial phase transformation, during which all forbidden diffraction peaks vanish, the extraction and insertion of lithium occurs without any first order phase transitions. The material remains layered and does not transform to the spinel structure during cycling. It shows good capacity retention vs. cycle number at both 30 and 55°C. Differential scanning calorimetry measurements on charged electrodes show exotherms that are about one order of magnitude less intense than those from charged electrodes. © 2000 The Electrochemical Society. All rights reserved.

Fundamental Formation Laws of Phase Composition, Structure and Properties of Aluminium Materials Manufactured by Mechanical Alloying

The present investigation objective is to develop the technological process for strengthened aluminium alloys formation of different density based on mechanical alloying. Standard aluminium and magnesium powders were used as initial materials. Lithium was introduced in free state and binded in hydroxide LiOH. Granulated composition with mean particle size 0.2 ... 0.3 mm is the product of mechanical alloying. Thermal treatment of granulated compositions and hot compaction of semiproducts initiate phase transformations which contribute to thermodynamic equilibrium state of the alloys. Alloy base is characterized by micro- or small crystalline structure and presents a solid solution of magnesium and lithium in aluminium strengthened by nanocrystalline inclusions of aluminium oxides, magnesium and aluminium carbides which results in high strength of said alloys at low and high temperatures.

Application of the weight function and boundary collocation method to the calculation of initial phase transformation zones

Transformation-toughened ceramics exhibit stress-induced phase transformation with the tetragonal phase changing to the monoclinic phase. These transformations will affect the stress field in the surroundings of crack tips. A special procedure—based on the fracture mechanics weight function—is used to compute the stress field at the onset of stable crack propagation. Calculations are performed for the special case of small-scale transformation zones as well as for zones around finite cracks in a finite body. It can be shown that the real stress field deviates from that computed on the base of the applied stress intensity factor. An interesting result which can be shown is that the crack opens due to the phase transformation zone, although the stress intensity factor is zero.