Thermodynamics Of Transformation Research Articles

Kinetics and Thermodynamics of the Phase Transformation in the Nanocrystalline Substance—Gas Phase System

This study presents a model of the reaction of a nanocrystalline substance within the gas phase, where diffusion of gas reactants in the volume of the nanocrystallites is a rate-limiting step. According to the model calculations carried out, the rate of diffusion across the phase boundary located on the nanocrystallite surface limits the rate of the process. It was stated that in chemical processes with a phase transformation, the critical concentration of absorbate depends on two factors: the specific active surface area of the adsorbent and the difference in chemical potentials between the gas phase and the equilibrium potential at which the phase transformation occurs. When the actual adsorbate potential in the gas phase is much greater than the equilibrium potential of the nanocrystallite with the largest specific active surface, nanocrystallites undergo phase changes in the order according to their specific active surfaces from the largest to the smallest. In a process where the actual adsorbate potential is close to an equilibrium one, nanocrystallites undergo phase transformation in the order of their specific active surface from the smallest to the largest.

Thermodynamic Prediction of the Impact of Carbon on Bainite Formation, Including the BCT Transformation

There exists strong experimental evidence that bainitic ferrite is formed as a supersaturated solid solution of carbon within a tetragonally-distorted body-centered iron structure (BCT), where carbon preferentially occupies the octahedral site. Despite this, the BCT structure has not yet been accounted for in the computational analysis of the thermodynamics of the bainite transformation. In the present work, we propose to calculate the onset of the bainite transformation based on the T0′ concept, including the effect of Zener-ordering. This mechanism stabilizes the BCT structure, makes it energetically more favorable than BCC and leads to a significantly higher solubility of C compared Fe-BCC. The computational predictions are made based on a recent reassessment of low-T Gibbs energies and finally compared to experiments. The important role of C is emphasized, with the Fe-C system used as a showcase.

Tailoring microstructure evolution and austenite stability of TRIP steels by Rare-Earth micro-alloying

The microstructure evolution and austenite stability of Rare Earth (RE)-alloyed Transformation Induced Plasticity (TRIP) steels subjected to intercritical annealing and isothermal bainitic transformation have been investigated for superior strength-ductility performance. Structure morphology, austenitizing temperature and phase proportion during thermomechanical process were carefully compared, confirming that minor RE additions indeed refined the microstructure, and affected the thermodynamics of austenite and bainite transformation. Austenitizing ranges of TRIP steels were lifted with increasing RE content then stabilized when exceeding 360 ppm; the intercritical austenite reduced but dissolved more CMn atoms, hence the bainite transformation in RE-alloyed TRIP steels was significantly inhibited, which elevated the retained austenite quantity and thermal stability simultaneously. Therein, more austenite nucleated from the structure fragments with RE-refined lamellar morphologies and strictly grew into acicular counterparts or aggregated into clusters. The aggregated austenite, composed of several subgrains with [011¯] coaxial relations or Σ3 60°〈111〉 misorientation but coherent interfaces, were engulfed or being swallowed up by coalescing ferrites in sluggish growth, guaranteeing the higher mechanical stability than intergranular austenite. In this study, the product of strength and elongation of TRIP steels after optimized RE-alloying was increased by 38% as compared to that without RE additions. The micro-alloying effect of RE elements in TRIP steels highlights the innovations of new-generation automotive steels.

Effect of Magnetic State of Austenite on Thermodynamics and Kinetics of Martensitic Transformation in Steel

Effect of Magnetic State of Austenite on Thermodynamics and Kinetics of Martensitic Transformation in Steel

Expansion of bond dissociation prediction with machine learning to medicinally and environmentally relevant chemical space

Bond dissociation energetics underpin the thermodynamics of chemical transformations where bonds are broken or formed and can also be used to predict reaction rates and selectivities.

Fine-Tuned Reactivity of N-Containing Naphthol Analogues.

6-Hydroxyquinoline and 3-hydroxyisoquinoline as N-containing naphthol analogues were tested in modified Mannich reactions (mMr’s). In the case of 6-hydroxyquinoline, the outcomes of the attempted Mannich reactions were strongly influenced by the amine components. Aminoalkylation of this substrate with reagents 1-naphthaldehyde and N-benzylmethylamine led to the isolation of a diol regarded as a stabilised water adduct of an ortho-quinone methide (o-QM), of which formation can be ascribed to the presence of a hydroxide ion in a relatively higher concentration generated by the bulky and basic amine component with decreased nucleophilicity. The classical Mannich base was isolated as a single product when the amine component was replaced for morpholine, featuring nucleophilicity rather than basic character under the applied reaction conditions. Starting from the isomer substrate 3-hydroxyisoquinoline, independently on the nucleophile (methanol or morpholine) besides the formation of the classical Mannich base, the nucleophilic attack at position one of the heterocyclic substrate was also observed. The DFT analysis of the acceptor molecular orbitals of the potential electrophilic components and the thermodynamics of the assumed-possible transformations demonstrated that this regioselective addition is a feasible process on the investigated heterocyclic skeleton. DFT modelling studies also suggest that besides the steric bulk, the orbital-controlled electronic properties of the aryl group, originating from the aldehyde components, have a strong influence on the ratios and the NMR-monitored interconversions of the C-1-substituted 3-hydroxyisoquinolines and the classical Mannich bases formed in multistep reaction sequences. On the basis of the DFT analysis of the thermodynamics of alternative pathways, a reaction mechanism was proposed for the rationalization of these characteristic substrate-controlled interconversions.

Microscopic mechanism of nanoscale shear bands in an energetic molecular crystal (α-RDX): A first-order structural phase transition

Nanoscale shear bands formed in many energetic molecular crystals upon shock compression [including 1,3,5-trinitro-$s$-triazine (RDX)] are considered as a defect-free mechanism for formation and growth of hot spots which control detonation initiation. Using classical molecular dynamics, we predict the formation of similar nanoscale shear bands in the \ensuremath{\alpha}-RDX crystal subjected to quasistatic isothermal uniaxial compression indicating a common mechanism of shear strain localization under both shock and quasistatic conditions. In the framework of the Ginzburg-Landau phenomenology coupled with the coarse-grained (CG) Helmholtz free energy of the crystal from first principles, we explore the thermodynamics of stress-induced lattice transformations under quasistatic uniaxial load. We show that the shear banding exhibits a critical behavior associated with a first-order structural phase transition with bands of localized twinning strain as transient microstructure. Analysis of the CG Helmholtz free energy suggests that the stress-induced core softening of the effective intermolecular interaction is a fundamental mechanism for a structural phase transition leading to the nanoscale shear bands.

Surface transformation thermodynamics of alkaline earth carbonates using first-principles calculations

Alkaline Earth (AE) carbonates are ubiquitous minerals with important roles in both industry and the environment. For example, CaCO3 helps regulate biogeochemical cycles and the amount of CO2 present in the atmosphere through cycles of dissolution and re-precipitation. Knowing the atomistic response of carbonates to changes in their chemical environment, such as by surface termination, composition, or pH, will allow for better controls of both naturally occurring processes and industrial applications. Here first-principles methods were used to investigate the initial stages of carbonate dissolution by comparing surface exchanges and ion release processes from the aragonite mineral phase. The focus was on computing the thermodynamics of CaCO3, SrCO3, and BaCO3 surface transformations and to link key features in surface structure to changes in the thermodynamics of ion release. The overall cycle of surface cation exchange, subsurface atom release, and subsequent surface healing was predicted to occur for a wide range of pH values. Surface bicarbonate reorganization, measured as a change in vertical height, dictated in part the thermodynamics of subsurface release, and it was shown that subsurface anion release was the most energetically costly part of the thermodynamic analyses. Moreover, these cycles resulted in thermodynamically stable defect structures that i) may be a better starting point for creating more accurate models of CO2 release and sequestration in aqueous environments, ii) could be used to guide future spectroscopic analyses, and iii) lead to improvements in technologies as diverse as nanomedicine, sensing, and water treatment.

Cryogenically martensitic transformation and its effects on tempering behaviors of bearing steel

Herein, the cryogenically martensitic transformation and its effects on the tempering behaviors of bearing steel was investigated. An in-situ magnetic method was used for obtaining the thermodynamics of the martensitic transformation at sub-zero Celsius temperature from in respects of athermal and isothermal conditions. Microstructure evolution was also characterized systematically to investigate the mechanisms of the martensitic transformation. The results showed that the athermally martensitic transformation of samples experienced three different stages, where the martensite formed slowly at 300–230 K, rapidly at 230–120 K, and slowly again at 120–77 K during the continuous cooling process. With the increase of cooling rate, both the start and finish temperatures of the transformation rise, while the maximum transformed rate decreases. The content of retained austenite transformed isothermally is much smaller as compared to the athermal condition. The amount of isothermally formed martensite does not show linear relationship with the temperature, where an optimal isothermal temperature for GCr15 steel is 193 K in this work. XRD measurement result showed that the content of retained austenite decreased with the decrease of treating temperature in practical DCT, where the content was reduced up to 4.2% after DCT at the lowest temperature combined with tempering. DCT enhanced the precipitation of θ-carbides during tempering and altered the precipitation behavior by advancing the precipitated temperature of θ-carbides from 340 °C to 315 °C. The tempered hardness of samples was improved by DCT, which can be attributed to the enhanced carbides precipitation during tempering and lower content of retained austenite after DCT. The improvement was also affected by tempering and austenitizing temperatures. It was concluded that the performance improvement induced by DCT was most evident if DCT was conducted between higher temperature quenching and lower temperature tempering. DCT also affected the decomposition of retained austenite during tempering by decreasing the activation energy of decomposition by up to 38.13 KJ/mol, which indicated a lower thermal stability of retained austenite after DCT. Microstructure observation indicated that the substructure of the new formed martensite was dominated by ultra-fine twins, which derived from the accommodated strain induced by martensitic transformation during DCT. The transformation strain induced by DCT also causes more dislocations in the microstructure, which provides more nucleation sites for the precipitation of θ-carbides during tempering.

Basic Promotors Impact Thermodynamics and Catalyst Speciation in Homogeneous Carbonyl Hydrogenation.

Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. In particular, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of well-established and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally, we confirm that by varying the base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of the reaction medium, on its own capable of improving the catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation.

Origin of the isothermal B2 → B19′ transformation in NiTi-based shape memory alloys

The aim of this paper was to study the isothermal forward martensitic transformation in binary, ternary, and quaternary NiTi-based shape memory alloys to find the origin of this phenomenon. The results obtained showed that the isothermal B2 → B19′ transformation was observed only in the NiTi-based alloys with substitutional defects. The critical concentration of copper was found that completely suppressed the martensitic transformation both on cooling or isothermal holding in Ti-Hf-Ni-Cu alloys. The thermodynamics of the martensitic transformation that occurred in the presence or absence of the substitutional defects was discussed, and the reason for the influence of the substitutional defects on the martensitic transformation was clarified. It was verified that the formation of the martensite phase on isothermal holding was caused by the local fluctuation of the substitutional defect concentration which led to fulfilment of the thermodynamic condition for the martensitic transformation. It was shown that the maximum volume fraction of the isothermal martensite was limited by the maximum volume of martensite that corresponded to the thermal elastic equilibrium and appeared on cooling of the alloy without substitutional defects. The results allowed to conclude that the martensitic transformation remained to be thermal elastic in the NiTi-based alloys with a non-stoichiometric composition however, the martensite nucleation was controlled by the thermally activated migration of substitutional defects.

Investigation of austenite decomposition behavior and relationship to mechanical properties in continuously cooled medium-Mn steel

The austenite decomposition behavior in medium-Mn steel was investigated here by combining thermo-mechanical controlled processing (TMCP) and continuous cooling processes. Microstructures were characterized by means of optical microscopy, electron probe micro analyzer, scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy. Furthermore, the effect of rolling reduction and cooling path on microstructural evolution and mechanical properties were studied. The results indicated that the austenite region was expanded by manganese, and the ferrite, pearlite, and bainite were not nucleated before martensite transformation even when the cooling rate was decreased to 0.07 °C/min. The approach to tailor yield ratio without compromising tensile strength depends on the control of martensite lath morphology and internal dislocation density. The resistance of lattice to shear transformation increases with the increasing strain hardening of prior austenite, which leads to decrease martensite-start (Ms) temperature. The martensite formed at lower transformation temperatures exhibited relatively high dislocation density and refined laths, resulting in high work-hardening capacity during the early stage of plastic deformation. The crack propagation can be effectively deviated or even terminated by high angle grain boundaries (HAGBs) and the nonparallel structures of martensite packets. These studies on the kinetics and thermodynamics of martensitic transformation confirmed the viability of processing continuously cooled medium-Mn steel in the hot rolling production line.

Use the Force: Review of High-Rate Actuation of Shape Memory Alloys

Typical shape memory alloy actuators provide a unique combination of large stresses and strains that result in work-per-volume larger by more than two orders of magnitude than all other actuation methods that are based on active materials. High-rate actuation of shape memory alloys can provide improved energy efficiency, and shorter response and total actuation times, along with large travel-per-wire-length, with respect to slow-rate SMA applications. In this article, we review the different aspects of high-rate actuation of shape memory alloy wires in the high-driving-force regime. We briefly survey previous experimental results about the kinetics and thermodynamics of the phase transformation in view of its practical implications. New experimental results, regarding energy efficiency, total actuation time, repeatability, and fatigue, are presented and discussed. The paper provides general design guidelines for obtaining high actuator performance, as well as guidelines for selecting the source of the electric pulse and its parameters. Finally, we construct and solve detailed simulations of actuator response that can serve as accurate design tools.

Thermochemical Investigation of Oxyanion Coordination in a Zirconium-Based Metal-Organic Framework.

Porous materials possess high internal surface areas and void fractions that make them valuable in several applications, including gas storage, heterogeneous catalysis, and water purification. Despite the plentiful effort allocated to porous materials research annually, few methods exist to directly monitor and characterize chemical events occurring within a pore's confines. The crystalline nature of zeolites, covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) permit structural characterization by X-ray diffraction; yet, quantifying the thermodynamics of chemical processes and transformations remains tedious and error ridden. Herein, we employ isothermal titration calorimetry (ITC) to determine the full thermodynamic profile of oxyanion adsorption in a zirconium-based MOF, NU-1000. To further validate this method, which we recently introduced to the field, we replicated ITC experiments as bulk adsorption measurements to demonstrate the correlation between the extracted stoichiometric parameter from ITC thermograms and the MOF uptake capacity. Moreover, based on the calculated association constants, we accurately predicted which analytes might be able to displace others. For example, dihydrogen phosphate can displace selenate and sulfate because of its higher association constant (ΔGphosphate = -5.41 kcal/mol; ΔGselenate = -4.98 kcal/mol; ΔGsulfate = -4.77 kcal/mol). We monitored the exchange processes by titrating oxyanion-functionalized MOF samples with a more strongly binding analyte.

Investigation of Grain Boundary Influence on the Thermodynamic Phase Stability of Nanocrystalline Iron by Using the Molecular Dynamics Simulation Method

The thermodynamics of allotropic transformation in nanocrystalline Fe is studied. For this purpose, the grain boundary structure is initially examined using a novel method based on molecular dynamics (MD) simulation, the results of which are exploited to determine grain boundary thickness and excess volume for different grain sizes. The dilated crystal model and the quasi-harmonic Debye approximation method are subsequently employed to calculate the values of Gibbs free energy for grain boundaries of different allotropic phases. Using the quantitative parameters obtained from the simulations, the relevant Gibbs free energy curves are plotted in terms of grain size and analyzed for different temperatures. This is followed by deriving stability curves for the γ-Fe and α-Fe phases in terms of grain size for different temperatures. Ultimately, the parameters obtained from MD simulation studies and the stability diagrams thus obtained are used to explore the changes in the γ→α allotropic transformation temperature of iron nanocrystal for different grain sizes. It will be that allotropic transformation temperature declines from 900 to 300 K with rising grain size from 4 to 33 nm.

Surface Transformations of Lead Oxides and Carbonates Using First-Principles and Thermodynamics Calculations.

Lead (Pb)-containing solids find widespread commercial use in batteries, piezoelectrics, and as starting materials for synthesis. Here, we combine density functional theory (DFT) and thermodynamics in a DFT + solvent ion model to compare the surface reactivity of Pb oxides and carbonates, specifically litharge, massicot, and cerussite, in contact with water. The information provided by this model is used to delineate structure-property relationships for surfaces that are able to release Pb as Pb2+. We find that Pb2+ release is dependent on pH and chemical bonding environment and go on to correlate changes in the surface bonding to key features of the electronic structure through a projected density of states analysis. Collectively, our analyses link the atomistic structure to i) specific electronic states and ii) the thermodynamics of surface transformations, and the results presented here can be used to guide synthetic efforts of Pb2+-containing materials in aqueous media or be used to better understand the initial steps in solid decomposition.

Thermodynamics of Phase Transformation and Microstructure Evolution of 18Cr2Ni4WA Carburized Steel during High‐Temperature Tempering

The microstructural evolution and phase transformation mechanism in 18Cr2Ni4WA carburized steel during high‐temperature tempering is investigated using a thermodynamic model. The experimental results demonstrate that the retained austenite in the matrix decreases from the surface to the core, and martensite transform from acicular to lath‐type. Furthermore, the hardness distribution of the carburized layer increase initially, and then decrease. Based on thermodynamic calculations, the driving forces for the decomposition of martensite decomposition and nucleation of cementite in martensite are larger than those for the decomposition of austenite during tempering. Furthermore, cementite preferentially precipitate in martensite. The driving force for the overall phase transformation of the decomposition of the retained austenite is negative, and the driving force for the nucleation of cementite in the low‐carbon retained austenite is positive, indicating that cementite can nucleate only in high‐carbon retained austenite. The low‐carbon retained austenite transform into secondary martensite on cooling after the tempering process while the driving force gradually increase. The decomposition of the initial quenched martensite during the tempering process leads to a decrease in the overall hardness of the carburized layer. The secondary martensite obtained from the retained austenite contribute to the increase in the surface hardness.

Emerging investigator series: first-principles and thermodynamics comparison of compositionally-tuned delafossites: cation release from the (001) surface of complex metal oxides

For a set of compositionally tuned complex metal oxides we predict the thermodynamics of aqueous surface transformations that lead to metal release.

Effect of the precipitates on the thermodynamics of the martensitic transformations in [formula omitted]-rich [formula omitted]-[formula omitted]-[formula omitted] thin films

The wide-spread multiple transformation in Ti-rich Ni-Ti shape memory alloys produced by Ti2Ni nano-precipitates was investigated. For this purpose, a series of amorphous Ti-Ni-Co thin films with the same composition were subjected to different annealing treatments. The characteristic temperatures and heat of the transformations were measured by four-point probe resistivity and differential scanning calorimetry. A critical review of the thermodynamics of thermally-induced thermoelastic martensitic transformations is presented. The values of the frictional work and the stored elastic energy associated with the thermoelastic transformations were assessed from the transformation temperatures, and related with the transformation heats measured in the samples annealed at different temperatures. It was observed that the stored elastic energy decreases as the annealing temperature rises due to a change in the distribution of the Ti2Ni precipitates, in concordance with an increase of the transformation heat measured by the calorimeter. Nevertheless, the values of the frictional work were independent of the annealing temperature. The influence of the dissipation of the stored elastic energy on the transformation temperatures and thermal hysteresis is discussed. Finally, a better understanding of the wide-spread multiple transformation, as an effect of the local stresses introduced by Ti2Ni nano-precipitates in the matrix, is proposed.

Thermodynamics and Kinetics of γ-Al2O3 and AlOOH Transformations under Hydrothermal Conditions

We have studied the kinetics of boehmite nanopowder formation during hydrothermal treatment of γ-Al2O3 nanopowder in a 1.5% HCl solution at 200, 170, and 150°C. The results demonstrate that the temperature-dependent reaction rate constant follows the Arrhenius equation. The Ea of the process has been determined to be 84 kJ/mol. The thermodynamics of γ-Al2O3 nanopowder conversion into boehmite during hydrothermal treatment at 150°C has been studied by differential scanning calorimetry. The heat of vaporization of water from a two-phase nanosystem (γ-Al2O3 + forming boehmite) has been determined to be 8, 16, and 22 kJ/mol H2O, which points to an active role of water with a low heat of vaporization in the initial stages of the hydrothermal treatment of the γ-Al2O3 nanopowder. The heat effect of the АlООН → γ-Al2O3 conversion in the nanopowders is lower than the reference value by 7 kJ/mol AlOOH, which is attributable to the small particle size and low structural perfection of the synthesized boehmite (AlOOH).