Stages Of Phase Transformation Research Articles

Thermal and mechanical stability of retained austenite in high carbon steel: An in-situ investigation

High carbon multiphase steels have attracted a lot of interest in industrial applications due to their high resistance against abrasion. The structure of these steels comprises of mixed martensitic structure, plates and laths, and retained austenite. The presence of retained austenite phase plays a very important role in properties of these steels. In this paper, two in-situ approaches have been used to study thermal and mechanical stability of retained austenite. By in-situ x-ray diffraction, the start and end of phase transformation temperatures of retained austenite were identified. For the sample with higher retained austenite percentage, phase transformation started at lower temperature because by increasing the percentage, required barrier energy for phase transformation will be reduced. Also, different stages of phase transformation were investigated using a nano-indentation method. Phase transformation from austenite to martensite was identified in the mechanical stress range of 60–70mN.

OPTIMIZATION OF MECHANICAL CHARACTERISTICS OF STEEL AND ALLOYS BY NANOSTRUCTURIZATION OF THEIR DEFECT STRUCTURE AT THE BORDER OF PHASE PRETRANSFORMATION WITH THE USE OF ACOUSTIC EMISSION METHOD

The influence of the duration of stay of the steels 30HGSA and R18 as well as titanium alloy VT 20 in the conditions of polymorphic pretransformation and transformation conditions on the mechanical properties is investigated. It is shown that the effect of phase pretransformation allows in wide range controlling the steel defect structure and mechanical properties. The “operation” at polymorphic transformation border promotes to regulate the defect structure density and to optimize mechanical properties of alloys. It is proposed to use the acoustic emission method for phase transformation stage detection and for the development of heat treatment technology with maximize use of the phase pretransformation effects.

Nucleation and growth of crystals at the intermediate stage of phase transformations in binary melts

An exact analytical solution of integro-differential equations, which describe the phase transitions in supercooled binary melts, is constructed with allowance for fluctuating rates of crystal growth. A complete solution of the corresponding Fokker–Planck and balance equations is found for arbitrary nucleation kinetics and growth rates of crystals. In limiting cases, the obtained solutions transform to earlier known solutions for a special form of diffusivity in the space of crystal sizes and purely kinetic and “diffusion-kinetic” growth rates of nuclei.

The Phase Transformation and Melting Behavior of Micro-Nano Iron

In this paper the melting behavior of micro-nanoiron particle in a medium diameter of 100nm, 200nm, 300nm and 500nm was studied using a DSC (Differential Scanning Calorimetry). The results show that micro-nanoiron particle experiences three stages of phase transformation from room temperature to 1400°C, α-Fe to β-Fe, β-Fe to γ-Fe and solid to liquid. Temperatures for the former two phase transformations hardly change with diameter of iron powder while for last transformation temperature increases when the diameter raising. The melting point of 500nm iron particle is 157°C lower than the bulk iron. Melting enthalpy is no longer a constant but a number decrease with diameter decreasing.

The effect of Ru on precipitation of topologically close packed phases in Re – containing Ni base superalloys: Quantitative FIB-SEM investigation and 3D image modeling

A new approach to clarify the ruthenium effect on the precipitation of topologically close packed (TCP) phases is described in the paper. It is based on focused ion beam – scanning electron microscopy (FIB – SEM) dual beam methodology as well as three-dimensional imaging. The high-temperature capabilities of nickel base superalloys can be improved by alloying with refractory elements. With excessive refractory element content or excessive exposure to high temperature, brittle TCP phases precipitate resulting in a drop of strength. The undesirable phase transformation can be suppressed by addition of ruthenium. Although the effect is well known, its real mechanism remains open. In the present paper, the volume fraction and particle density, as well as the exact three-dimensional morphology of TCP phases as measured by FIB-SEM will be presented. The effect of ruthenium content and time of exposure is studied quantitatively. The results show that increased Ru additions slow down all stages of phase transformation and also reduce the equilibrium TCP volume fraction. The Ru effect might be due to either reduced driving force for precipitation or reduced interfacial energy.

Optimizing the mechanical properties of steel and alloys by adjustment of the defect nanostructure

The influence of the residence time of 30XΓCA and P18 steel and also VT20 titanium alloy in conditions of polymorphous austenitic and bainitic pretransformation and transformation on their mechanical properties is investigated. The results show that phase transformations permit regulation of alloy structure and properties over a broad range (from properties typical of the quenched state to those of tempered metal). By working at the boundary of polymorphous transformation, the density of the defect structure may be equalized, and the mechanical properties of the alloys may be optimized. Acoustic emission may be used to determine the stage of phase transformation. On that basis, heat treatment that permits the most effective use of the phase transformation is developed.

Baric polymorphism: characteristic functions and critical indexes

Polymorphous phase transition induced by hydrostatic pressure has been considered as a percolating process. Host matrix of the initial solid phase with nuclei of a new phase arising therein due to phase transformation was treated as an inhomogeneous medium. Electrical conductivity of the medium was the control parameter for searching over the stage of phase transformation. Effective-medium approximation, based on the Bruggeman model, has been improved to fit with percolation theory postulates. Functional connections between volume fraction of a new polymorphous phase and a pressure value were calculated for three investigated cases: n-ZnO, n-GaAs, and n-InAs on the base of the proposed approach.

Textures and Rare Earth Elements Composition of Banded Iron Formations (BIF) at Njweng Prospect, Mbalam Iron Ore District, Southern Cameroon

The REE signature of banded iron formations (BIF) and martite-goethite mineralization of the Njweng ridge (South Cameroon) are used here to decipher the nature of the ocean during the period of BIF precipitation. The textures of typical BIF categories are also presented as the only sedimentary feature. Two types of BIF facies are present at Njweng: the oxide and silicate facies. These facies show two stages of phase transformations. The first is the transformation of the original magnetite mineral into martite by oxidation resulting in trellis texture; the second is a simultaneous transformation by hydration into goethite and dehydration into martite. The samples typically show LREE depleted patterns relative to HREE, a characteristic similar to that of modern seawater. Also the strong positive Eu anomalies from the samples indicate the involvement of hydrothermal plumes of volcanic origin such as at mid ocean ridges (MOR). Yttrium (Y) has a strong positive anomaly pointing to the transportation of REEs as aqueous complexes within these ancient seas. The BIF samples have no positive Ce anomalies, suggesting that the oceans at that time were more reducing with no Ce fractionation than in present day oceans. The martite-goethite mineralization rather shows a positive Ce anomaly that derived from the enrichment in REE during the weathering of REE-bearing minerals. These results are integrated into existing literature on REE in BIFs worldwide and provide new insights into these heretofore unstudied BIFs of the northern edge of the Congo Craton.

Magnetic Field-Induced Granular Pearlite at Early Stages of Phase Transformation

ffects of high magnetic fields (HMF) up to 19.81T on pearlite phase transformation are studied by examination of the microstructures of a Fe-0.47C-2.3Si-3.2Mn (wt %) alloy partially isothermally processed above the eutectoid temperature. The results show that granular pearlite (GP) can be obtained at earlier transformation stages. The evolution of the granular pearlite is always accompanied by the formation of lamellar pearlite. TEM analysis reveals the existence of sub-grain boundaries within GP colonies and indicates that the nucleation of ferrite matrix in GP belongs to multiple nucleation mechanism. Most of carbides at the early stage of pearlite formation are found to precipitate at the α/γ interface--the growing front of ferrite phases, and some of coarse carbides can further develop into thin lamellar cementite.

Interface Motion and Interface Mobility of the Partitioning Phase Transformations in Fe-Mn-C and Fe-C Alloys: A Cyclic Phase Transformation Approach

A cyclic phase transformation concept has been proposed to investigate the growthkinetics of the austenite (γ) to ferrite (α ) and vice versa in Fe-Mn-C and Fe-C alloys. In the caseof cyclic partial transformations in Fe-Mn-C alloys, two new and special stages are observed:a stagnant stage in which the degree of transformation does not vary while the temperaturechanges and an inverse phase transformation stage, during which the phase transformationproceeds in a direction contradictory to the temperature change. The local equilibrium (LE)and paraequilibrium (PE) are both applied to analyzing the new observations. The stagnantstage was found to be caused by the Mn partitioning, while the inverse phase transformationstage was due to equilibrium conditions not being reached at the transition temperatures.A mixed-mode model is applied to simulating the cyclic phase transformation in Fe-C alloy,and it is found that the cyclic phase transformation concept is a very promising method forinvestigating the interface mobility.

Inversion boundary induced grain growth in ZnO ceramics: from atomic-scale investigations to microstructural engineering

In semiconducting materials special boundaries play the key role in crystal growth. They introduce an abrupt structural and chemical anisotropy, which is readily reflected in an unusual microstructure evolution, whereas their local structure affects the physical properties of semiconducting materials. These effects, however, can be exploited to tailor the electronic and optical properties of the materials, as demonstrated in this review. The presented topic fits in the field of preparatory stage of phase transformations, manifested through evolution of chemically induced structural faults. In the noncentrosymmetric structure of ZnO, inversion boundaries (IBs) are the most common type of planar faults that can be triggered by the addition of specific spinel-forming dopants (Sb2O3, SnO2, TiO2). In addition to conventional HRTEM techniques several new methods were developed to resolve crystallography and atomic-scale chemistry of IBs. The absolute orientation of the polar c-axes on both sides of the IB was determined by a novel quantitative microdiffraction method, providing a reliable identification of crystal polarity in noncentrosymmetric crystals. To determine sub-monolayer quantities of dopants on the IB, we developed a special technique of analytical electron microscopy using concentric electron probe (CEP) in EDS or EELS mode, providing more accurate and precise results than any other available technique. Knowing the local crystal chemistry of IBs we were able to design experiments to identify their formation mechanism. IBs nucleate in the early stage of grain growth as a dopant-rich topotaxial 2D reaction product on Zn-terminated surfaces of ZnO grains. Soon after their nucleation, ZnO is epitaxially grown on the inherent 2D phase in an inverted orientation, which effectively starts to dictate anisotropic growth of the infected crystallite. In very short time the grains with IBs dominate the entire microstructure in ZnO ceramics via IB-induced exaggerated grain growth mechanism. This phenomenon was used to design physical properties of ZnO-based varistor ceramics, whereas the bottom-up approach demonstrated here provides the basic tool for microstructural engineering of functional materials in virtually any system that is prone to formation of special boundaries.

Investigation of the heterogeneous nucleation in a peritectic AlNi alloy

The goal of this research is a systematic investigation aiming at a quantitative understanding of the four different stages of peritectic solidification: 1) heterogeneous nucleation, 2) peritectic reaction, 3) peritectic transformation and 4) direct growth of the peritectic phase. Solidification experiments of the peritectic alloy Al-36wt.% Ni are carried out in differential scanning calorimetry (DSC), and the recently developed multi-phase-field techniques are employed to quantitatively model and simulate the peritectic phase transformation, by taking into account the precise thermodynamic description of the Al-Ni system. A previously developed phase-field model that had been successfully employed in the Al-Ni system is adapted here and calibrated by a novel method that simulates not only the final solidification microstructure but also DSC traces. The resulting kinetics of the phase-transformation processes are presented underlying the above four stages of phase transformation in detail.

Microstructural engineering of ZnO-based varistor ceramics

In ceramic materials, special boundaries play the key role in crystal growth. They introduce an abrupt structural and chemical anisotropy, which is readily reflected in an unusual microstructure evolution, whereas their local structure affects the physical properties of polycrystalline materials. These effects, however, can be exploited to tailor the electronic and optical properties of the materials, as demonstrated in this review. The presented topic is related to a preparatory stage of phase transformations, manifested through the evolution of chemically induced structural faults. In non-centrosymmetric structure of ZnO, inversion boundaries (IBs) are the most common type of planar faults that is triggered by the addition of the specific IB-forming dopants (Sb2O3, SnO2, TiO2). In addition to conventional TEM techniques, new methods were developed to resolve crystallography and atomic-scale chemistry of IBs. The absolute orientation of the polar c-axes on both sides of an IB was determined by micro-diffraction, providing the most reliable identification of crystal polarity in non-centrosymmetric crystals. To determine sub-monolayer quantities of dopants on the IB, we developed a special technique of analytical electron microscopy using concentric electron probe (CEP) in EDS or EELS mode, providing more accurate and precise results than any other technique. Knowing the local crystal chemistry of IBs, we were able to design experiments to identify their formation mechanism. IBs nucleate in the early stage of grain growth as a dopant-rich topotaxial 2D reaction product on Zn-terminated surfaces of ZnO grains. Soon after their nucleation, ZnO is epitaxially grown on the inherent 2D phase in an inverted orientation, which effectively starts to dictate anisotropic growth of the infected crystallite. In very short time, the grains with IBs dominate the entire microstructure via IB-induced exaggerated grain growth mechanism. This phenomenon was used to design physical properties of ZnO-based varistor ceramics, whereas the bottom-up approach demonstrated here provides the basic tool for microstructural engineering of functional materials in virtually any system that is prone to the formation of special boundaries.

Application of cyclic partial phase transformations for identifying kinetic transitions during solid-state phase transformations: Experiments and modeling

A series of cyclic partial phase transformation experiments has been performed to investigate the growth kinetics of the austenite to ferrite phase transformation, and vice versa, in Fe–Mn–C alloys. Unlike the usual phase transformation experiments (100% parent phase → 100% new phase), in the case of cyclic partial transformations two special stages are observed: a stagnant stage in which the degree of transformation does not vary while the temperature changes, and an inverse phase transformation stage, during which the phase transformation proceeds in a direction contradictory to the temperature change. The experimental results have been analyzed using paraequilibrium and local equilibrium diffusional growth models. Only the local equilibrium model was shown to predict the new features of the cyclic phase transformation kinetics. The stagnant stage was found to be due to Mn partitioning, while the inverse phase transformation is caused by non-equilibrium conditions when switching from cooling to heating and vice versa.

<i>In Situ</i> Observation of Solidification Behavior in a Q235 Steel by Using Electric Pulse Modification

In the unmodified and EP-modified melt of Q235 steel, the solidification behavior including the nucleation and growth of crystals has been observedin situby use of a confocal scanning laser microscope combined with an infrared image furnace, and meanwhile, described the characteristic of phase transformation. The result shows that the every stage of phase transformation is advanced; the phase transformation time is shortened, and temperature is reduced during heating proceeds. On the other hand, it can be observed that the nucleation density of solid phase increases and has an even distribution among the liquid phase.

Polytypic transformations of the HfCr2 Laves phase – Part I: Structural evolution as a function of temperature, time and composition

HfCr2-Laves phase alloys of different composition have been produced by arc-melting followed by homogenization by annealing at temperatures at which the C14-high temperature modification is stable. Polytypic phase transformations have been induced by subsequent annealing at different, lower temperatures for various times. The resulting (micro-) structure of the Laves phase has been characterized by X-ray powder diffraction (XRPD) and high-resolution transmission electron microscopy (HRTEM). At temperatures between 1300 °C and 1225 °C, a microstructure forms, which can be regarded as composed of randomly alternating C36 and C15 lamellae. At lower annealing temperatures (1050 °C-1150 °C), the C36 stacking sequences do not occur. Instead a lamellar C14/C15 microstructure develops, representing an incomplete C14 → C15 transformation. A phase map for the Laves-phase part of the Hf-Cr system has been constructed. The XRPD patterns are not a superposition of the diffraction patterns corresponding to the different occurring Laves-phase polytypes; instead a common, average diffraction pattern can be discerned which can be ascribed to coherent diffraction of the polytypes constituting the lamellar microstructure. This coherent diffraction by the irregular layer-stacking sequence induces broadening of the reflections corresponding to the specific polytypes, whereas the “fundamental” reflections, occurring for all polytypes, remain unbroadened. From the diffraction patterns two scalar parameters can be obtained, which describe the stage of phase transformation: one based on the intensity of a selected polytype reflection and the other one based on the average unit cell parameter ratio, c/a, of the lamellar microstructure.

Relationship between the Bandgap and Electrochemical Behavior on TiO<sub>2</sub> Nanoparticles Prepared Sonochemically

Nanocristalline TiO2 obtained by a facile and environment-friendly sonochemical method was subjected to thermal treatment in the temperature range of 400-900 °C in order to produce variable anatase-rutile phases ratio. The relationship between the optical bandgap and the electrochemical behavior was studied. All the stages of phase transformation of the as-prepared sample such as: nucleation, growth and coarsening were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that phase transformation mechanism stems from the redistribution of energy in the system and a critical particle size. On the other hand, the samples were characterized by UV-vis spectroscopy for the bandgap studies. The optical band gap of as-prepared sample increases to 3.31 eV with respect to 3.20 eV for bulk-anatase. This expansion could be attributed to quantum size effect. The i-E characteristics of samples with variable anatase-rutile ratio were obtained using cyclic voltammetry technique in a 0.5 M H2SO4 solution at room temperature. The foremost charge magnitude was obtained when anatase had a critical size of 17 nm. Analyzing both particle size for anatase and rutile, we observed that when rutile is the dominating phase and its size difference larger in 35% than anatase, the current reaches its minimum values. Based on electrochemical results, the optimal particle size and content phases control are important in order to obtain an increase in the electrochemical performance in the Hydrogen Evolution Reaction (HER) zone

Ultrafine ferrite formation through isothermal static phase transformation

A novel thermomechanical route has been developed to produce an ultrafine-grained structure through warm deformation of metastable austenite followed by isothermal static transformation to ferrite. The thermomechanical parameters influenced the evolution of ferrite phase transformation (i.e. nucleation and growth), resulting in different levels of ferrite grain refinement. For the first time grains as fine as 200 nm were produced through a traditional diffusional transformation of austenite to ferrite. The use of a model Ni–30Fe austenitic alloy showed that the strain has a strong effect on the nature of the intragranular defects developed throughout the microstructure. At a low strain, microbands were the dominant intragranular features. There was a transition strain above which a complex cell/subgrain substructure with high misorientation angle appeared in the vicinity of prior austenite grain boundaries. This region was extended with strain and promoted significant ferrite nucleation sites at an early stage of phase transformation, resulting in the local formation of nanosized ferrite grains near the prior austenite grain boundaries with ultrafine grains towards the interior of the original austenite grain.

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.

Optical applications of glass-ceramics

Glass-ceramics are usually formed through passing several stages of phase transformations (nucleation, crystallization of metastable and stable crystalline phases). The properties of glass-ceramics depend on at what stage the process was stopped. The phase assemblage of glass-ceramics is controlled by the composition of the initial glass and heat-treatment conditions. Raman and optical spectroscopy, in combination with X-ray diffraction and small angle X-ray scattering techniques are used to study the behavior of the nucleation and crystallization of lithium, magnesium and zinc aluminosilicate glasses. Specific heat-treatments has given the opportunity to achieve a desired phase assemblage ensuring unique properties of glass-ceramics by interrupting the process at a certain stage. Examples of such glass-ceramic developments include thermal shock resistant color filters, passive Q-switchers for eye-safe Er:glass lasers, as well as transparent materials based on rare-earth titanates with pyrochlore structure with luminescence in the range of 1.5 μm. The approach to developing the dual active–passive media for the eye-safe spectral region is demonstrated for the first time.