Study Of Phase Transformations Research Articles

The effect of chemical composition and annealing condition on the microstructure and tensile properties of a resource-saving duplex stainless steel

Edge cracking phenomenon occurs in ‘Mn + N′ duplex stainless steels because of inferior thermoplasticity that is mainly induced by the non-uniform deformation between ferrite and austenite phase with different strength during hot rolling, limiting its application. To address this problem, a Fe–19Cr-0.6Al–1Ni–12Mn duplex stainless steel with an optimized alloy composition was trial-prepared, where Ni is substituted by Mn and Cr is partially substituted by Al. This enabled a high content ferrite to be present during hot rolling to reduce the adverse effect of austenite on thermoplasticity. Thus, the edge cracking of hot rolled plates was solved. Furthermore, the formation of austenite was governed by annealing after cold rolling. In view of this, it is significant to study phase transformation and precipitation in duplex stainless steel during annealing. A large number of experimental results indicated that there were two types of transformation during the annealing process: the transformation of ferrite to austenite and recovery and recrystallization of ferrite. After annealing at lower temperature, the austenite was characterized by the banded structure at the grain boundaries of ferrite. Besides, the formation of austenite restricted recrystallization of ferrite. With the increase of annealing temperature, the growth of austenite grain would be enhanced, and the banded austenite evolved into bamboo morphology or island morphology. When the annealing temperature was greater than 900 °C, recrystallization of ferrite was promoted, and transformation of ferrite to austenite was inhibited. However, the precipitation of σ phase increased with the decrease of annealing temperature below 800 °C, reducing plasticity and pitting resistance for the investigated steel.

Experimental and molecular dynamics studies of phase transformations during cryogenic thermal cycling in complex TiNi-based crystalline/amorphous alloys

In TiNi-based crystalline/amorphous alloys, superelasticity in crystalline phase coordinating the dislocation sinking in amorphous phase lead to a high ductility and outstanding anti-fatigue properties. We performed cryogenic thermal cycling, between 77 K and 303 K, on the complex TiNi-based alloys consisting of a major B2 austenite phase, an interdendritic amorphous phase, and a minor B19′ martensite phase in the as-cast state. The critical martensitic phase transformation stress (σm) increased with the number of thermal cycles, reaching a maximum at 10 cycles. The initial B19’ martensite which is confined in the amorphous phase transformed to B2 austenite due to thermal induced stable transformation. A lamellar structure of alternating amorphous and crystalline layers dominantly grew into the amorphous matrix as a consequence of the thermal fatigue during the cryogenic thermal cycling. Initial cell for the molecular dynamic simulations was carefully prepared to contain three different phases. Cyclic compressive loading and cryogenic thermal cycling simulations were consistent with the experimental results.

First-principles computation of magnesium doped CaZrO3 perovskite: A study of phase transformation, bandgap engineering and optical response for optoelectronic applications

In the present work the structural, electronic and optical properties of pure CaZrO3 have been tuned by the magnesium (Mg) doping concentrations (1.41%, 2.82% and 4.23%) by first-principles computation, based on the density functional theory (DFT), implemented within the CASTEP code with Perdew-Burke-Ernzerhof-Generalized-Gradient-Approximations (PBE-GGA) exchange correlation functional and USP (ultra-soft pseudo-potential). From structural results optimized lattice parameters and unit cell volume are obtained, well in match with literature, and with 4.23% doping concentration of Mg structural phase transformation, cubic to pseudo-cubic tetragonal, along with a noticeable influence on electronic band gap squeezing, with the appearance of new k-symmetry points at Brillouin zone, is observed. Electronic band structure shows that with, 1.41% and 2.82% doping concentration of Mg, the indirect band gap of host changes to direct one while with 4.23% doping concentration it remains same and value of host band gap is reduced from 3.279eV to 2.189eV. The phase transformation and reduction of band gap is explained with partial and total density of sates and have great influence on the optical properties. Analysis of optical properties with Mg doping reveals that the absorption edges of the doped CaZrO3 show the red shift (1.9eV–0.65eV) whereas the static refractive index almost remain same. Electron energy loss spectra is observed to be consistent with absorption spectra. The doping of Mg concentration changes positively in electronic and optical properties and it would be very potential candidate for optoelectronic applications.

Observations of grain-boundary phase transformations in an elemental metal.

While the theory of grain boundary (GB) structure has a long history1 and the proposition that grain boundaries can undergo phase transformations was made already 50 years ago2,3, the search for these GB transitions has been in vain. The underlying assumption was that multiple stable and metastable states exist for different grain boundary orientations4,5,6. The terminology complexion was recently proposed to distinguish between those interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexions and transitions between them have been characterized mostly in binary or multicomponent systems 8,9,10,11,12,13,14,15,16,17,18,19. Novel simulation schemes have provided a new perspective on the phase behavior of interfaces and showed that grain boundary transitions can occur in a multitude of material systems20,21,22,23,24. However, their direct experimental observation and transformation kinetics in an elemental metal remained elusive.Here, we show atomic scale grain boundary phase coexistence and transformations at symmetric and asymmetric [11-1] tilt grain boundaries in elemental copper (Cu). We found the coexistence of two different grain boundary structures at Σ19b grain boundaries by atomic resolution imaging. Evolutionary grain boundary structure search and clustering analysis21,25,26 finds the same structures and predicts the properties of these GB phases. Furthermore, finite temperature molecular dynamics simulations explore their coexistence and transformation kinetics. Our results demonstrate how grain boundary phases can be kinetically trapped enabling atomic scale room temperature observations.Our work paves the way for atomic scale in situ studies of grain boundary phase transformations in metallic grain boundaries. In the past, only indirect measurements have indicated the existence of interfacial transitions9,15,27,28,29. Now, their atomic scale role on the influence of abnormal grain growth, non-Arrhenius type diffusion or liquid metal embrittlement can be explored.

Investigations of Crystalline Phases in Glasses under Various Annealing Conditions

Studies of the temperature dependence of the electrical properties of glasses show that the high-temperature annealing in glasses observed irreversible processes. These processes lead to changes in electrical conductivity, dielectric permittivity, and hence the electrical capacitance, dielectric loss tangent, and other parameters. Obviously, this is due to structural changes in the glass as a result of high-temperature annealing. In this regard, this paper presents studies of structural and phase transformations in glasses used for the production of microchannel plates in the process of high-temperature annealing in vacuum and in the air atmosphere at different times. The studies were conducted by x-ray phase and X-ray diffraction analysis, as well as X-ray fluorescence elemental analysis.

Studies of structural-phase changes in a modified cement stone method by derivatographic analysis

The use of a complex nanodispersed additive consisting of a superplasticizer and nanoparticles (sol of nanosilica and carbon nanomaterial) have a significant effect on the hydration, hardening, and curing of building composites, predetermining their durability.The results of studies of phase transformations occurring in a cement stone modified with this additive are presented. The studies were carried out using the method of thermal analysis, which allows to obtain quantitative information about the change in the mass of the sample as a result of its heating and at the same time register this change. Thermal analysis has a number of advantages over other research methods, the flexibility of setting up an experiment, the rapid removal of information, the ability to automate data processing, and a small amount of substance. A comprehensive study of the properties of a material when it is heated allows a detailed and deeper study of the nature of the processes occurring in it. The study of physical and chemical processes was carried out on a combined thermogravimetric analysis device and differential scanning calorimetry METTLER TOLEDO, which is designed to measure the thermodynamic characteristics (heat and temperature of phase transitions and physico-chemical reactions), as well as detect changes in the mass of materials in the temperature range from 25 to 1600 degrees C. The article discusses the intervals of weight loss of samples containing an additive of superplasticizer and a complex nanodispersed additive in age those 1 and 28 days. To find out what effect a superplasticizer has on a cement sample, an additional sample No. 1, consisting only of cement and water, was examined at 28 days of age.

Dilatometric Study of Phase Transformations in 5 Mn Steel Subjected to Different Heat Treatments.

The work presents results of phase transformation kinetics of hot-rolled 5% Mn steel subjected to different heat treatments. Three different schedules were introduced: isothermal holding in a bainite region, coiling simulation and intercritical annealing. The evolution of microstructure components was investigated using dilatometric and metallographic analyses. According to obtained results, the medium-Mn steel exhibits high resistance for γ/α transformation during the bainite transformation and coiling simulation (upon cooling from the austenite region). During 5 h isothermal holding, no bainite and/or ferrite formation was detected. This results in the formation of martensite upon cooling to room temperature. Differently, when the steel was subjected to the intercritical annealing at 720 and 700 °C (upon heating from room temperature), a final microstructure consisted of ferrite, martensite and retained austenite. At 700 °C, no fresh martensite formation was detected upon cooling to room temperature. This means that the austenite was enriched in carbon during the intercritical annealing step enough to keep its thermal stability.

In Situ Neutron Diffraction Study of Phase Transformation of High Mn Steel with Different Carbon Content

In situ neutron diffraction was employed to examine the phase transformation behavior of high-Mn steels with different carbon contents (0.1, 0.3, and 0.5 wt.%C). With increasing carbon contents from 0.1 C to 0.5 C, the austenite phase fraction among the constituent phases increased from ~66% to ~98%, and stacking fault energy (SFE) increased from ~0.65 to ~16.5 mJ/m2. The 0.1 C and 0.3 C steels underwent phase transformation from γ-austenite to ε-martensite or α’-martensite during tensile deformation. On the other hand, the 0.5 C steel underwent phase transformation only from γ-austenite to ε-martensite. The 0.3 C steel exhibited a low yield strength, a high strain hardening rate, and the smallest elongation. The high strain hardening of the 0.3 C alloy was due to a rapid phase transformation rate from γ-austenite to ε-martensite. The austenite of 0.5 C steel was strengthened by mechanical twinning during loading process, and the twinning-induced plasticity (TWIP) effect resulted in a large ductility. The 0.5 wt.% carbon addition stabilized the austenite phase by delaying the onset of the ε-martensite phase transformation.

The Study of Structure Formation Evolution at High Pressure with a Shear in a Corrosion-Resistant Austenitic-Ferritic Steel 03Kh14N10K5M2YuT with Different Initial States

The article presents results of studies of phase transformations and structure formation that form the necessary complex of physical and mechanical properties of austenitic-ferritic steel with a structural ratio of 50:50. The structure formation at high pressure with a shear of steel 03Kh14N10K5M2YuT with different initial states is studied. After the action of intense plastic deformations, a submicrocrystalline structure arises. This causes interest in these deformations.

Thermal flows influence on the change of temperature stresses in surface and inner layers of refractories

The work is devoted to the study of the heat fluxes effect on refractory materials and the temperature stresses with a temperature gradient. These parameters were determined using experimental setup allows to investigate the micro- and -macro level structural processes of a material. Macro- and microstructural studies have shown that a heat flow intense and appeared thermal stresses can contributes to the formation of structural defects and the rupture of intercrystalline bonds. Thermal stresses and the changeable loads occurring during this process can form complex structures in the material in the form of disconnected blocks separated by pores. Electron microscopic studies of structural and phase transformations under the exposure of heat pulse fluxes showed a layer-by-layer change in the internal structure with the formation of microcracks at temperatures from 500 °C to 1500 °C and heating rates in the range from 5 °C/min to 45 °C/min. It is identified that the change of temperature stresses depends on the phase changes during thermal effect on the refractory material.

Мандельштам-бриллюэновское рассеяние света в растворах бычьего сывороточного альбумина при различных концентрациях в окрестности тепловой денатурации

Results of Brillouin light scattering studies of phase transformations in bovine serum albumin (BSA) in the temperature range 300 – 380 K in the protein solution with concentration 50 and 5 mg/ml are presented. In the analysis of the results, the sequence of phase transformations in a concentrated BSA solution (100 mg/ml) was used as a reference. It was shown that sequence of BSA phase transformations modified with a decrease in concentration. For instance, in the vicinity of denaturation, the region with fibril-like aggregate disappears, and the properties of the gel-like high temperature phase change.

Phase transformation during continuous heating in a β-quenched Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe alloy

As a new near β titanium alloy designed by NIN (the Northwest Institute for Non-ferrous Metal Research), the phase transformations of the β-quenched Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe alloy during continuous heating have been investigated by means of in situ dilatometer test coupled with X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The β-quenched Ti-5321 alloy with an initial structure consisting of metastable β phase and dense ωath, which was subjected to continuous heating treatment and underwent the following transformations: β + ωath→ β + ωiso→ β + α+ ωiso→ β + α. The ωisophase precipitated at 300 ℃ with a ~3 nm ellipsoidal morphology. When continuously heated to 390 ℃, a coexistence of both ωiso and α phase was observed in the β matrix, the platelet-like α phase closely nucleating at the ω/β interface with ~1 nm in width and ~8 nm in length. The α phase subsequently developed into lamellar structure when heated to higher temperatures, with a width of ~20 nm at 570 ℃ and ~85 nm at 690 ℃ respectively. Meanwhile, these α laths were uniformly distributed and composed of two distinct orientations within the microstructure. Finally, it can be found that the α morphology is directly associated with the formation and decomposition of the metastable ωisophase which may lead to the homogeneous and fine distribution of α precipitates. Based on the study of phase transformations occurring during the continuous heating process presented in this paper, an efficient guidance to engineer the microstructures and mechanical properties of this new near β titanium alloy was offered.

Гидроксид титана как прекурсор для получения функциональных материалов

It was found that finely ground powders can polymorphically transform into other nonequilibrium crystalline phases in the study of phase transformations of X-ray amorphous titanium hydroxide (TH) in the process of mechanical activation in the presence of additives in the form of zinc compounds. Judging by the increase in the intensity of the peaks in the diffraction patterns, it can be stated that amorphous phase crystallizes in the form of anatase and brookite as a result of high-energy exposure. It is shown that the lower the degree of hydration of titanium hydroxide, the efficiency of the mechanical effect of the additive on phase transformations is higher. The following trend for the effect of the introduced modifier is observed — ZnSO4·7 H2O > > Zn(NO3)2·6 H2O > ZnO. This dependence is due to the combination of physical and chemical transformations of the material located in the field of intense mechanical influence, confirmed by data on the calculation of crystallite size and microstrains. The conversion of excess mechanical energy into thermal energy initiates chemical processes with the formation of solid titanium-zinc solutions, which accelerate the restructuring of the crystalline structure upon calcination of the modified titanium hydroxide according to the scheme: anatase-brookite-rutile. The obtained results have the prospects of practical application in the production of the so-called “rutile” nuclei used in the industrial production of titanium dioxide for various purposes.

Unsupervised learning of dislocation motion

The unsupervised learning technique, locally linear embedding (LLE), is applied to the analysis of X-ray diffraction data measured in-situ during the uniaxial plastic deformation of an additively manufactured nickel-based superalloy. With the aid of a physics-based material model, we find that the lower-dimensional coordinates determined using LLE appear to be physically significant and reflect the evolution of the defect densities that dictate strength and plastic flow behavior in the alloy. The implications of the findings for future constitutive model development are discussed, with a focus on wider applicability to microstructure evolution and phase transformation studies during in-situ materials processing.

Study of phase transformations in Co/CoCo2O4 nanowires

The paper presents the results of a systematic study of the influence of thermal annealing on changes in the structural properties and phase composition of metallic nanostructures based on cobalt, as well as the possibilities of using an anode material for lithium ion batteries. It was established that as a result of thermal annealing, a four-stage phase transition of nanowires (Co-C/Co-H) → (Co-C) → (Co-C/CoCo2O4) → (CoCo2O4) is observed, accompanied by a rearrangement of the crystal structure, coarsening of the crystallites and the formation of oxide growths on the surface of nanowires. It has been established that with an increase in temperature up to 700 °C, a partial destruction of the oxide layer occurs, which confirms the results of a decrease in the degree of crystallinity and an increase in stresses and strains in the structure. In the course of life tests, the prospect of using nanostructures as the basis for anode materials of lithium-ion batteries has been established. For the initial sample, the number of cycles is about 300 cycles. For oxide nanostructures, an increase in the amount of the resource lifetime of the anode material is observed and in the case of annealed at 500 °C, the resource lifetime exceeds 500 cycles. That corresponds to the average lifetime of the anode materials based on silicon or carbon nanostructures. The dependences of degradation rate of nanostructures in highly concentrated solutions of sulfuric acid were obtained. That will later make it possible to predict the amorphization rate and oxidation of nanowires.

Unsupervised Learning of Dislocation Motion

The unsupervised learning technique, locally linear embedding (LLE), is applied to the analysis of X-ray diffraction data measured in-situ during the uniaxial plastic deformation of an additively manufactured nickel-based superalloy. With the aid of a physics-based material model, we find that the lower-dimensional coordinates determined using LLE appear to be physically significant and reflect the evolution of the defect densities that dictate strength and plastic flow behavior in the alloy. The implications of the findings for future constitutive model development are discussed, with a focus on wider applicability to microstructure evolution and phase transformation studies during in-situ materials processing.

Comparative study of phase transformation of Al2O3 nanoparticles prepared by chemical precipitation and sol-gel auto combustion methods

Al2O3 nanoparticles were prepared by Chemical Precipitation (CP) and Sol-Gel Auto Combustion (SGAC) methods and a comparative study on structural behaviour of the nanoparticles was carried out. Nanoparticles prepared by CP gradually transformed to the thermodynamically stable α-phase of alumina at an annealing temperature of 1100 °C whereas pure α-alumina directly obtained for the nanoparticles prepared by sol gel method during the combustion stage. Surface morphology showed the agglomeration of particles after calcining at a higher temperature. Nanoparticles prepared by SGAC method at low temperature showed the same thermal behaviour as those annealed at 1100 °C after being prepared by CP method.

Waste-activated sludge disruption by dry ice: bench scale study and evaluation of heat phase transformations

The freezing process consists of dissipating heat from the product until the final temperature is lower than the temperature of crystallisation of that product. Freezing can be used for numerous applications, including for disruption of waste-activated sludge (WAS). The aim of this study was to calculate the estimated amount of heat conveyed between the solidified carbon dioxide and the WAS, in the following ratios: 0.25:1; 0.5:1; 0.75:1 and 1:1. In heat of phase transformations, dry ice sublimation, water solidification, the amount of heat transferred by other substances and heat transferred from the sludge (dry sludge) were taken into account during the process of WAS freezing. Heat changes on the surface of WAS were registered using a thermovision camera. The effectiveness of WAS disintegration was confirmed by several biochemical parameters such as soluble chemical oxygen demand (increase over 14 times), degree of disintegration (48%), proteins (increase over 5 times), carbohydrates (increase almost 7 times), RNA (increase by 2.23 mg L−1), ammonia nitrogen (increase over 23 times), phosphates (increase almost 27 times) and turbidity (increased over 7 times). It was found that dry ice pretreatment of WAS can be an intriguing alternative for the conventional methods used.

Phase transformations and compatibility in helical structures

We systematically study phase transformations from one helical structure to another. Motivated in part by recent work that relates the presence of compatible interfaces with properties such as the hysteresis and reversibility of a phase transformation (Chluba et al., 2015; Ni et al., 2016; Zarnetta et al., 2010; Song et al., 2013), we give necessary and sufficient conditions on the structural parameters of two helical phases such that they are compatible. We show that, locally, four types of compatible interface are possible: vertical, horizontal, helical and elliptical. We discuss the mobility of these interfaces and give examples of systems of interfaces that are mobile and could be used to fully transform a helical structure from one phase to another.These results provide a basis for the tuning of helical structural parameters so as to achieve compatibility of phases. In the case of transformations in crystals, this kind of tuning has led to materials with exceptionally low hysteresis and dramatically improved resistance to transformational fatigue. Compatible helical transformations with low hysteresis and fatigue resistance would exhibit an unusual shape memory effect involving both twist and extension, and may have potential applications as new artificial muscles and actuators.

Optimization of Induction Quenching Processes for HSS Roll Based on MMPT Model

To improve the comprehensive performance of high speed steel (HSS) cold rolls, the induction hardening processes were analyzed by numerical simulation and experimental research. Firstly, a modified martensitic phase transformation (MMPT) model of the tested steel under stress constraints was established. Then, the MMPT model was fed into DEFORM to simulate the induction quenching processes of working rolls based on an orthogonal test design and the optimal dual frequency of the induction quenching process was obtained. The results indicate that the depth of the roll’s hardened layer increases by 32.5% and the axial residual tensile stress also becomes acceptable under the optimized process. This study provides guidance for studying phase transformation laws under stress constraints and the optimization of complex processes in an efficient manner.