Isothermal Transition Research Articles

Relationship of microstructure on the mechanical and splitting fracture properties of a high carbon forging steel for connecting rod

AbstractThe influence of austenitizing temperature and isothermal transition temperature on the mechanical properties and splitting characteristics of C70S6 were investigated. The maximum ultimate tensile strength and yield strength were 1032 and 625 MPa, respectively, when the lamellar spacing of sorbite was 0.195 μm. The greatest elongation was 17% when the size of the pearlite colony was 13 μm, which suggested an approach for enhancing the properties of pearlite steel at a low cost. The splitting property performed excellent when the proeutectoid ferrite content is 0.3%, the prior austenite grain is 38.9 μm, and the yield ratio is 0.57 currently. The connection between the process performance and the microstructure parameters was established, allowing process improvement to be guided accurately.

Observation of an isothermal glass transition in metallic glasses.

Glass transition, commonly manifested upon cooling a liquid, is continuous and cooling rate dependent. For decades, the thermodynamic basis in liquid-glass transition has been at the center of debate. Here, long-time isothermal annealing was conducted via molecular dynamics simulations for metallic glasses to explore the connection of physical aging in supercooled liquid and glassy states. An anomalous two-step aging is observed in various metallic glasses, exhibiting features of supercooled liquid dynamics in the first step and glassy dynamics in the second step, respectively. Furthermore, the transition potential energy is independent of initial states, proving that it is intrinsic for a metallic glass at a given temperature. We propose that the observed dynamic transition from supercooled liquid dynamics to glassy dynamics could be glass transition manifested isothermally. On this basis, glass transition is no longer cooling rate dependent, but is shown as a clear phase boundary in the temperature-energy phase diagram. Hence, a modified out-of-equilibrium phase diagram is proposed, providing new insights into the nature of glass transition.

Investigation of the isothermal curing and glass transition behaviors of epoxy–imidazole systems

The isothermal curing behavior of an epoxy–imidazole system was investigated through thermal, mechanical, and compositional analyses. Precise control over temperature and time afforded the monitoring of the progression of the curing reaction and enthalpy relaxation via fast scanning calorimetry. The real-time tracking of the glass transition temperature (Tg) during isothermal curing revealed a temperature-dependent relationship between Tg and thermally estimated conversion, distinct from epoxy–amine systems. Unlike previous research suggesting plasticization through cyclic-compound formation in epoxy–imidazole systems, such compounds were not detected in the early curing stages in this study. Furthermore, an endothermic reaction from imidazole regeneration was suggested to overlap with the main exothermic reaction of polymerization. Therefore, assessing thermally estimated conversion in the intermediate curing stage through differential scanning calorimetry was deemed practically impossible for epoxy–imidazole systems. The Tg values of resins with different thermal histories were equal after complete curing under high-temperature/long-duration conditions.

Isothermal martensitic transformation in Ti-Ni-Cu-Co shape memory alloy: Insight from a thermally activated kinetic model

Recently the isothermal martensitic transformation in shape memory alloys (SMAs) has been reported in many literatures, and several models have been proposed to interpret the isothermal and athermal kinetics. However, the underlying mechanisms remain inadequately understood. In this work, the isothermal transformation from B2 to B19 is confirmed in Ti-Ni-Cu-Co melt-spun ribbons at the temperature range between Ms and Mf. It reaches a saturation point at every isothermal temperature Tiso, and the saturation points correspond to the f−T curve at the cooling rate of 0.5 K/min. The experimental results indicate that the isothermal accumulation of martensite is a relaxation process from the transient state to the thermoelastic balance one. A thermally activated kinetic model is developed in this study to characterize the isothermal and athermal kinetics. The model is able to estimate the evolution of martensite volume fraction under any temperature path T(t) and it agrees with the experimental results well. According to the model, the effects of elastic energy, nucleation density, and activation energy on the kinetics are investigated. Among those, a small nucleation density ni as well as a large activation energy Q will result in a significant isothermal transition. In this work, the slighter isothermal effects originate from the higher value of ni. As for the non-stoichiometric SMAs, the higher value of Q is responsible for the accumulation of martensite at the isothermal process. Accordingly, the present work provides a novel view from a kinetic model to understand the isothermal martensitic transformation in SMAs.

Critical storage conditions of pequi pulp microparticles and kinetics of degradation of nutritional properties

AbstractThe understanding of water sorption behavior and the determination of glass transition temperature (Tg) is essential in stability studies of dried foods, to determine their best storage or processing condition. Water sorption isotherm and glass transition temperature of spray‐dried pequi pulp microparticles were evaluated to obtain storage critical condition of spray dried pequi pulp, which was 25 °C and 62% of relative humidity (RH). Water sorption isotherm at 25 °C was determined by static gravimetric method, using saturated salt solutions, while Tg by differential scanning calorimetry. Total carotenoids, antioxidant capacity, lipid oxidation, and color parameters were evaluated throughout 120 days at 25 and 35 °C and RH of 23 and 43% to determine the powder kinetic degradation. All quality parameters presented decay during storage. Total carotenoids and antioxidant capacity by ferric reduction antioxidant power method exhibited a second‐order kinetic, while antioxidant capacity by DPPH assay and lipid oxidation a first and zero‐order kinetics, respectively. Both temperature and RH negatively affected nutritional properties. The storage condition that better preserved the desirable characteristics of spray‐dried pequi pulp microparticles was 25 °C and RH of 23%.Practical ApplicationsOne of the greatest challenges for the food industry is to extend food stability against environmental factors. Especially for those food components very unstable such as pigments, antioxidant components, flavor, and so forth. The sample we used for this study was pequi fruit which has high potential in the food industry because of its sensory and nutritional qualities due to the phytochemicals present in the fruit, such as carotenoids and vitamins A and C. However, pequi is highly perishable and generally susceptible to degradation reactions. We successfully applied microencapsulation for pequi pulp protection and showed the environmental conditions that will preserve its functional properties for longer periods. Our results are relevant to extending the bioactive properties of food components.

Dimer-parity-dependent odd-even effects in photoinduced transitions to cholesteric and twist grain boundary smectic-C^{*} mesophases: Experiments and simulations.

We describe investigations on the influence of the flexible spacer parity and length of the guest photoactive liquid-crystalline dimers in guest-host mixtures exhibiting photoinduced transitions involving isotropic (I), cholesteric (N^{*}), and twist grain boundary smectic-C^{*} (TGBC^{*}) phases. Despite a small concentration (3 wt. %) of the guest molecules, the transition temperatures and their photodriven shift (δT) show a strong odd-even parity (of the dimer) dependent effect, with the even-parity systems having a larger value than their odd-parity counterparts; δT is larger for the N^{*}-TGBC^{*} transition than for the I-N^{*} one. The photocalorimetric measurements corroborate these features in addition to showing that, in comparison with the absence-of-ultraviolet (UV) case, the transition enthalpy (ΔH) of the I-N^{*} transition in the UV-on case is diminished by 33 and 12% for the mixtures with even- and odd-parity dimers, respectively. The duration for relaxation from the isothermal photodriven transition also exhibits general features of an odd-even influence. Molecular dynamics simulations demonstrate the presence of significant conformational heterogeneity and associated shift in the conformational space on photostimulation of the guest molecules. The change in the effective shape and nematic order parameter is more pronounced in the even-parity system.

Cyclic heat engines with nonisentropic adiabats and generalization to steady-state devices including thermoelectric converters.

For heat engines (including refrigerators) the separation of total entropy production in reversible parts ΔS and irreversible contributions has proved to be very useful. The ΔS are entropies for ideal lossless processes at the hot- and cold side and are important system parameters. For Carnot-like heat engines performing finite-time cycles, the concern was raised in a preceding paper that the ΔS are not always independent from irreversibilities, if initial and final working fluid temperatures T_{f}(t) differ in the isothermal transitions. It turns out that the ΔS are unchanged and independent, if T_{f} (t) evolution is optimized for entropy minimization and apparent inconsistencies are cleared up. If nonisentropic transitions in the adiabatic cycle branches are taken into account, the difference of cold- and hot-side entropy reversibilities is equal to the entropy production in the adiabats. Maximization of cooling power is studied for various irreversible entropy models. The concepts are extended to noncyclic steady-state engines. Power maximization and efficiency calculations are performed exactly analytically. This serves as prerequisite for the hitherto unsolved problem of an accurate definition of reversible and irreversible entropy parts in thermoelectric (TE) converters in the case of inhomogeneous three-dimensional material distributions. It is revealed that for nonconstant Seebeck coefficients, additional terms to the Joule heat arise that destroy positive generator performance in the limit of heat conductance k→0, in contrast to the traditional constant material properties model. Thus, the concept of improving TE materials by reducing k is in question and an adapted figure of merit Z is presented to deal with the situation.

Long-Term Isothermal Phase Transformation in Lead Zirconate.

Lead zirconate PbZrO3 has been the subject of research interest for several dozen years. Recently, even its antiferroelectric properties have started to be questioned, and many researchers still deal with the so-called intermediate phase below Curie temperature (TC), whose existence is not fully understood. It turns out that PbZrO3 doped with Nb exhibits below TC phases with complex domain structures. One of them undergoes self-organization taking place at a constant temperature, and transforms, after several minutes, into a lower phase. This isothermal transition was investigated through dielectric, pyroelectric current and Raman scattering measurements. Discontinuities accompanied it in the permittivity and pyroelectric current. The obtained Raman spectra proved that those discontinuities are strictly linked with the isothermal transition between two intermediate phases. The ordering process in lead sublattice stimulated by thermal fluctuations is discussed as a driving force for this peculiar phenomenon.

Simultaneously improved the strength dramatically and eliminated HAZ softening of DP980 steel pulsed-arc welding joints by PWHT

A pathway of post-weld heat treatment (PWHT) to eliminate heat affected zone (HAZ) softening and improve the strength of the welded joints of DP980 steel was developed. It is found that the high temperature isothermal transition at 750–950 °C followed by water-quenching significantly influences the strength and fracture location by changing the microstructures of the welded joint. The block martensite is distributed discretely in the island ferrite after PWHT, which causes ferrite to deform continuously in space. At the same time, the microstructure of joints is more uniform after PWHT. As a result, the highest strength (1146 MPa) can be obtained by PWHT at 950 °C, which is about 1.35 times that of the as-welded joint.

Investigation about basic heat transfer law in the micro pulse tube

Pulse tube is one of the main parts of the pulse tube cryocooler. Ideally, the pulse tube works under adiabatic environment without heat transfer loss. But in real case, entropy is generated in the pulse tube itself from heat exchanged with the walls, turbulent mixing of the hot and cold gas segments, circulation of the gas within the tube, and end effect losses from the adiabatic to isothermal transition in the cycle. All these processes reduce the enthalpy flow in the pulse tube from the ideal acoustic power. However, no 1D model can sufficiently simulate these effects in the past. Also, most of the 1D models about the pulse tube part with a heat transfer coefficient is far from accurate, especially for micro pulse tube in which the heat transfer is more obvious. This work revealed that the basic heat transfer changes instantaneously and shows different tendencies in different time of one cycle, and no suitable law or explanation could fare well the phenomenon eventually. The phase angle changes from negative to positive that results in the relatively heat flux along the pulse tube becomes smaller, that could help the pulse tube to make a good performance.

Transition kinetics of mixed lipid:photosurfactant assemblies studied by time-resolved small angle X-ray scattering

HypothesisPhotoswitchable surfactants are used in the design of many light-responsive colloids and/or self-assemblies. Photo-isomerization enables to control molecular equilibrium, and triggers transient reorganizations with possibly out-of-equilibrium intermediate states that have been overlooked. Here, we address this question by an in depth structural investigation of intermediate lipid-surfactant assemblies that occur during fast isothermal photo-triggered transition in lipid:surfactant mixtures. ExperimentsThe structural parameters of mixed assemblies of azobenzene-containing cationic surfactant (AzoTMA) and dioleoylphosphatidylcholine (DOPC) lipids were studied by light scattering and time-resolved small angle X-ray scattering. Structural and compositional information about the assemblies and unimers in bulk were determined at the photostationary states, as well as at intermediate kinetic states formed during UV or blue light illumination. FindingsDOPC:AzoTMA systems form mixed assemblies representative of phospholipid:cationic surfactant mixtures, that evolve from spheroid, to rod-like micelles, and vesicles with increasing DOPC fraction. Transient assemblies detected during the photo-triggered kinetics are similar to the ones found in stationary states. But changes of AzoTMA unimers in bulk can be considerably faster than mass reorganizations of the mixed assemblies, suggesting that out-of-equilibrium conditions are transiently reached. Mass reorganization of the surfactant-enriched assemblies is much faster than in the lipid enriched ones, providing insight into the role of lipids in a slow reorganization of the assemblies.

Photochromic spiropyran-based liquid crystals

We present a new type of light responsive self-assembling structures based on a spiropyran photoswitchable unit. The spiropyran central part was modified by functional groups, which enabled the connection of two structurally different elongating parts. Two series of compounds were prepared with reversed orientation of the ester as a linkage group in the molecular core. The properties of the target materials were studied by differential scanning calorimetry and polarized light optical microscopy, which confirmed the presence of a nematic phase for most of the materials. In case of a trifluromethyl-substituted compound, a sequence of nematic – smectic A – smectic C phase was detected and confirmed by X-ray diffraction analysis. The photochromic properties of compounds were studied in solution as well as in mesophases. In the condensed state, UV light-induced switching from the spiropyran form to the merocyanine dye led to isothermal transition from the liquid crystalline phase to the isotropic liquid.

Phase-field investigation on the peritectic transition in Fe-C system

We adopt a thermodynamically consistent multi-phase, multi-component phase-field model to investigate the morphological evolution of peritectic transition in carbon steel though 2-D and 3-D simulations. By using phase-field method, we rationalize the peritectic solidification in both 2-D and 3-D simulations under different liquid supersaturations as well as on the δ particle with distinct microstructures. Through the comparison between 2-D and 3-D simulation results, we clarify the reason for the different growth rate of γ phase in two and three dimensions. In 3-D simulation, we observe the unequal growth rate of γ phase in radial and axis directions. In addition, a novel measurement method is proposed to determine the dynamic contact angle. We anticipate that the simulation results can be applied to interpret the isothermal peritectic transition with a liquid supersaturation in alloys.

Microencapsulation of okara protein hydrolysate by spray drying: physicochemical and nutritive properties, sorption isotherm, and glass transition temperature

The objective of this work was to evaluate the physicochemical and nutritive properties, sorption isotherm, and glass transition temperature (Tg) of okara protein hydrolysate microencapsulated with maltodextrin or modified starch by spray drying. Freeze-dried hydrolysate without a wall material was obtained as a non-encapsulated sample. The type of wall material had no significant influence on the powder properties. There were differences between encapsulated and non-encapsulated samples, mainly in the sugar profile, mean particle size (∼13 and 208 µm, respectively), and anhydrous powder Tg (∼100 and 55 °C, respectively). Microencapsulation improved the critical relative humidity of storage from 25% to ∼55%.

Irreversible entropy production in low- and high-dissipation heat engines and the problem of the Curzon-Ahlborn efficiency.

Heat engines performing finite time Carnot cycles are described by positive irreversible entropy functions added to the ideal reversible entropy part. The model applies for macroscopic and microscopic (quantum mechanical) engines. The mathematical and physical conditions for the solution of the power maximization problem are discussed. For entropy models which have no reversible limit, the usual "linear response regime" is not mathematically feasible; i.e., the efficiency at maximum power cannot be expanded in powers of the Carnot efficiency. Instead, a physically less intuitive expansion in powers of the ratio of heat-reservoir temperatures holds under conditions that will be inferred. Exact solutions for generalized entropy models are presented, and results are compared. For entropy generation in endoreversible models, it is proved for all heat transfer laws with general temperature-dependent heat resistances, that minimum entropy production is achieved when the temperature of the working substance remains constant in the isothermal processes. For isothermal transition time t, entropy production then is of the form a/[tf(t)±c] and not just equal to a/t for the low-dissipation limit. The cold side endoreversible entropy as a function of transition times inevitably experiences singularities. For Newtonian heat transfer with temperature-independent heat conductances, the Curzon-Ahlborn efficiency is exactly confirmed, which-only in this unique case-shows "universality" in the sense of independence from dissipation ratios of the hot and cold sides with coinciding lower and upper efficiency bounds for opposite dissipation ratios. Extended exact solutions for inclusion of adiabatic transition times are presented.

Phase-Field Investigation on the Peritectic Transition in Fe-C System

We adopt a thermodynamically consistent multi-phase, multi-component phase-field model to investigate the morphological evolution of peritectic transition in carbon steel though 2-D and 3-D simulations. By using phase-field method, we rationalize the peritectic solidification in both 2-D and 3-D simulations under different liquid supersaturations as well as on the δ particle with distinct microstructures. Through the comparison between 2-D and 3-D simulation results, we clarify the reason for the different growth rate of γ phase in two and three dimensions. In 3-D simulation, we observe the unequal growth rate of γ phase in radial and axis directions. In addition, a novel measurement method is proposed to determine the dynamic contact angle. We anticipate that the simulation results can be applied to interpret the isothermal peritectic transition with a liquid supersaturation in alloys.

Assessment of Anticaking Agent on Caking Behavior of Jujube Amorphous Powder via Glass Transition and State Diagram

Jujube (Ziziphus jujuba Mill.) powder is hygroscopic and highly prone to caking during storage which could significantly reduce product quality. Anticaking agents are usually added to maintain flowability of powders and prevent caking. Caking behaviors of jujube powder and blends with anticaking agents (calcium stearate, magnesium stearate, and silicon dioxide at 1, 2, 3% w/w) as affected by temperature (25 °C, 35 °C, 45 °C), mixing speed (22, 73, 96 rpm), and mixing time (1, 5, 10, 15, 20 min at 25 °C) of powders exposed at 75% RH for 5 h were investigated by FT4 powder rheometer. Energy of crust that formed during caking was determined. Results showed that anticaking agents resulted in significant effect on the moisture sorption and crust strength formed during caking of jujube powders. Calcium stearate (2% w/w) proved to be the most effective to improve the flowability and delay onset of powder caking with significant reduction in the crust strength of powder cake from 170 to 2 mJ/g when powders exposed to 75% RH for 5 h at 25 °C. Mixing speed at 73 rpm and mixing time of 5~20 min could present better flowability of jujube powders and delay the onset of caking. Lower (22 rpm) and higher (96 rpm) mixing speeds and higher temperatures (45 °C) could speed up caking rate in powder blends. State diagrams established based on moisture sorption isotherm and glass transition curves showed that jujube powders with and without calcium stearate addition were stable at 25 °C when water content was lower than 0.095 and 0.088 g water/g sample, respectively.

Catalysis by Imaging: From Meso- to Nano-scale

In-situ imaging of catalytic reactions has provided insights into reaction front propagation, pattern formation and other spatio-temporal effects for decades. Most recently, analysis of the local image intensity opened a way towards evaluation of local reaction kinetics. Herein, our recent studies of catalytic CO oxidation on Pt(hkl) and Rh(hkl) via the kinetics by imaging approach, both on the meso- and nano-scale, are reviewed. Polycrystalline Pt and Rh foils and nanotips were used as µm- and nm-sized surface structure libraries as model systems for reactions in the 10–5–10–6 mbar pressure range. Isobaric light-off and isothermal kinetic transitions were visualized in-situ at µm-resolution by photoemission electron microscopy (PEEM), and at nm-resolution by field emission microscopy (FEM) and field ion microscopy (FIM). The local reaction kinetics of individual Pt(hkl) and Rh(hkl) domains and nanofacets of Pt and Rh nanotips were deduced from the local image intensity analysis. This revealed the structure-sensitivity of CO oxidation, both in the light-off and in the kinetic bistability: for different low-index Pt surfaces, differences of up to 60 K in the critical light-off temperatures and remarkable differences in the bistability ranges of differently oriented stepped Rh surfaces were observed. To prove the spatial coherence of light-off on nanotips, proper orthogonal decomposition (POD) as a spatial correlation analysis was applied to the FIM video-data. The influence of particular configurations of steps and kinks on kinetic transitions were analysed by using the average nearest neighbour number as a common descriptor. Perspectives of nanosized surface structure libraries for future model studies are discussed.

High-Pressure Behavior of Nickel Sulfate Monohydrate: Isothermal Compressibility, Structural Polymorphism, and Transition Pathway.

Single crystals of synthetic nickel sulfate monohydrate, α-NiSO4·H2O (space-group symmetry C2/c at ambient conditions), were subject to high-pressure behavior investigations in a diamond-anvil cell up to 10.8 GPa. By means of subtle spectral changes in Raman spectra recorded at 298 K on isothermal compression, two discontinuities were identified at 2.47(1) and 6.5(5) GPa. Both transitions turn out to be apparently second order in character, as deduced from the continuous evolution of unit-cell volumes determined from single-crystal X-ray diffraction. The first structural transition from α- to β-NiSO4·H2O is an obvious ferroelastic C2/c–P1̅ transition. It is purely displacive from a structural point of view, accompanied by symmetry changes in the hydrogen-bonding scheme. The second β- to γ-NiSO4·H2O transition, further splitting the O2 (hydrogen bridge acceptor) position and violating the P1̅ space-group symmetry, is also evident from the splitting of individual bands in the Raman spectra. It can be attributed to symmetry reduction through local violation of local centrosymmetry. Lattice elasticities were obtained by fitting second-order Birch–Murnaghan equations of state to the p-V data points yielding the following zero-pressure bulk moduli values: K0 = 63.4 ± 1.0 GPa for α-NiSO4·H2O, K0 = 61.3 ± 1.9 GPa for β-NiSO4·H2O, and K0 = 68.8 ± 2.5 GPa for γ-NiSO4·H2O.

Influence of isothermal structural transition on the magnetic properties of Cr doped Bi0.86Nd0.14FeO3 multiferroics

In this work, we have studied the crystal structure, microstructure, magnetic properties, and the effect of isothermal structural transition on the magnetic properties of Bi0.86Nd0.14Fe1-xCrxO3 (0.02 ≤ x ≤ 0.1) ceramic compounds. The analysis of X-ray diffraction patterns reveals a mixture of the R3c rhombohedral and PbZrO3-type orthorhombic phases. With increasing Cr concentration, the PbZrO3-type phase percentage is gradually increased from 33% for x = 0.02–63% for x = 0.1 at the expense of the R3c phase. The magnetic properties of compounds are mainly attributed to the destruction of the long-range incommensurate cycloidal spin structure in the orthorhombic symmetry. The isothermal structural transition (IST) is observed at room temperature when samples are stored in laboratory conditions. A remarkable change of magnetic properties is observed, which is believed originated from the IST and spin frustration at the phase boundary.