Controlled Phase Transformation Research Articles

Inclusions and Microstructure of Steel Weld Deposits with Nanosize Titanium Oxide Addition

Nanosize TiO2 particles were added directly into welding molten pool through electrode for the difficulty of accurate control of oxygen potential and production processing parameters. The characteristics of phase transformation and thermal behavior of inclusions for Fe‐C‐Mn‐Si‐Ti‐O system and Fe‐C‐Mn‐Si‐TiO2 system were analyzed. Results show that the added TiO2 particles are more helpful for the formation of Mn‐Ti‐O complex inclusion and can induce the decrease of phase transformation temperature of austenite to ferrite. Intragranular ferrite can be obtained under the condition of continuous cooling transformation with cooling rate of 293 K/s–373 K/s. The inclusions in steel welds are spherical in shape and mainly composed of TiO2, Ti3O5, Ti2O3, MnO, and SiO2. The mean size of inclusions is 0.67 μm. These complex inclusions can supply a large number of nucleating cores for their precipitation at higher temperature, which will disturb the growth of columnar crystal during solidification. Moreover, Mn‐containing titanium oxides will promote the transformation of austenite to intragranular ferrite for the formation of manganese depleted zones in steel welds around oxides. So it can be concluded that nanosize titanium oxide added directly in welding molten pool can be effectively used to control phase transformation and achieve fine and favorable microstructure.

A study on transformation of some transition metal oxides in molten steelmaking slag to magnetically susceptible compounds

Sustainable development of steelmaking requires solving a number of environmental problems. Economically feasible and environmentally friendly recycling of slag wastes is of special concern. Research of the team representing National Metallurgical Academy of Ukraine, Royal Institute of Technology, Carnegie Mellon University and URS Corp revealed a possibility of the controlled phase transformations in the liquid silicate melts followed by formation of the magnetically susceptible compounds. This approach enables selective recovery of metal values from slag. In this paper, the results obtained and further research directions are discussed. A possibility to exploit physical properties of the transition metals, typical for the metallurgical slags (such as Fe, Mn, V and others), and corresponding specific properties of their compounds, such as non-stoichiometry, mixed valency, pseudomorphosis, thermodynamic stability etc, in production of value-added materials from slag wastes is discussed. The results of the studies of thermodynamics and kinetics of oxidation in slags followed by phase transformation with binary, ternary and complex oxides under various physicochemical conditions are discussed in the view of their application for production of the materials with predefined physical properties. Peculiarities of precipitation in slags with various basicities are analysed and demonstrate capacity of the proposed approach in the production of the material with a given structure and size - for example, nano-sized crystals with structure of spinel. The approaches towards industrial realization of the developed method are also discussed.

Controllable Phase Transformation from ZnO to Zn-HDS in the Alcohol System

Controllable Phase Transformation from ZnO to Zn-HDS in the Alcohol System

Effect of Titanium Oxide on Phase Transformation for Low Alloy High Strength Steel Welds

Aimed at improving mechanical properties of high strength low alloy steel welds, oxides with high melting point and high stability were added into steel liquid. By thermodynamics calculation and thermal simulating technology, phase transformation characteristics of low alloy steel weld with TiO2addition were investigated. The results showed that TiO2added in welds led to the decrease of phase transformation temperature and the wideness of γ→α transformation temperature range. The starting and finishing transformation temperature of ferrite respectively dropped about 311K and 486K on the cooling condition of 373K/s. Moreover, the complex inclusions of TiOx-MnO-SiO2-MnS with spherical shape and 0.67μm mean size formed in weld metal. In addition, only acicular ferrite existed in the titanium oxide added welds. So it was concluded that titanium oxide can effectively be used to control phase transformation and then achieve fine and favorable microstructure.

Spiky Mesoporous Anatase Titania Beads: A Metastable Ammonium Titanate‐Mediated Synthesis

A complex titania nanostructure of monodisperse spiky mesoporous anatase beads composed of anatase nanocrystals with diameters of less than 15 nm in the core and much larger hollow-cone shaped spikes on the surface was fabricated using a facile solvothermal process in the presence of ammonia. This proceeded through a controllable phase transformation from an amorphous titania to a metastable amorphous titania/ammonium titanate core-shell structure then finally to anatase titania. The size of the spiky anatase nanostructures can be increased from approximately 55 × 100 nm to 160 × 410 nm (square edge × length) by increasing the ammonia concentration used in the solvothermal treatment step from 2.2 to 17.4 wt. %. Such hollow-cone shaped nanostructures, as revealed by HRTEM characterization, are single crystals elongated along the c axis of the tetragonal anatase titania. The resultant spiky titania beads have high surface areas of up to 112 m(2) g(-1) and pore diameters and pore volumes that vary depending on the ammonia concentration and solvothermal treatment time. The morphological evolution and crystallization process of the spiky titania beads was investigated using SEM and XRD techniques. A metastable amorphous titania/ammonium titanate core-shell structure evolved from the smooth amorphous precursor beads producing a "fluffy" titanate intermediate, on further heating the final spiky mesoporous titania beads were clearly observed. This titanate-phase-mediated approach allows control over the size of the nanocrystals in the core of the bead, as well as the anatase spikes on the surface, and thereby, tuning of the surface area and porosity of the resultant products. The spiky mesoporous titania beads have been used to prepare working electrodes for dye-sensitized solar cells achieving a solar to electric power conversion efficiency of 10.30 %, indicating their potential for application in the photovoltaic field. Such complex titania nanostructures would have a number of other possible applications, such as photocatalysis, lithium ion batteries, and catalysis.

Enhancement of the electrochemical capacitance of TiO2 nanotube arrays through controlled phase transformation of anatase to rutile

Here, we report the fabrication of self-organized titania (TiO(2)) nanotube array supercapacitor electrodes through controlled phase transformation of TiO(2), with aerial capacitances as high as 2.6 mF cm(-2), which far exceeds the values so far reported in the literature. The role of phase transformation in the electrochemical charge-discharge behaviour of nanocrystalline TiO(2) nanotubes is investigated and discussed in detail. The ease of synthesis and the exceptional electrochemical properties make these nanotube arrays an alternative candidate for use in energy storage devices.

Topological order in 1D Cluster state protected by symmetry

We demonstrate how to construct the Z2*Z2 global symmetry which protects the ground state degeneracy of cluster states for open boundary conditions. Such a degeneracy ultimately arises because the set of stabilizers do not span a complete set of integrals of motion of the cluster state Hamiltonian for open boundary conditions. By applying control phase transformations, our construction makes the stabilizers into the Pauli operators spanning the qubit Hilbert space from which the degeneracy comes.

Investigation of the final stages of solidification and eutectic phase formation in Re and Ru containing nickel-base superalloys

The microstructure resulting from the final stages of solidification—commonly referred to as eutectic islands—has been analysed in detail for three nickel-base superalloys containing Re and Ru. Focused ion beam 3-D reconstruction and EBSD-analysis were used to clarify the origin of different eutectic structure types. One common type of parent 3-D eutectic structure was identified. The solidification process of the final solidifying liquid has been further investigated by electron probe microanalysis mappings along with DICTRA simulations. Two models for diffusion controlled phase transformations are shown to present a fair description of the solidification sequence.

Galvanostatic Intermittent Titration Technique for Phase-Transformation Electrodes

A novel galvanostatic intermittent titration technique (GITT) and a novel potentiostatic intermittent titration technique (PITT) for phase-transformation electrodes were developed by integrating mixed control phase-transformation theory with traditional GITT and PITT methods. The contribution of the strain accommodation energy to the thermodynamic driving force for phase transformation was assessed. These novel GITT and PITT methods can be used to determine the true ion diffusion coefficient and the interface mobility of phase-transformation electrodes in the two-phase region. To demonstrate the utility of this method, the lithium ion diffusion coefficient and the interface mobility of two LiFePO4 samples with different particle sizes were obtained in the two-phase region. The lithium ion diffusion coefficient in the two-phase region as measured using phase-transformation GITT was on the order of 10−13 cm2/s in the β phase (Li1−yFePO4) and 10−12 cm2/s in the α phase (LixFeO4), which is similar to the diff...

Preparation of Calcite and Aragonite Complex Layer Materials Inspired from Biomineralization

Abalone shell is natural inorganic/organic hybrid material, which is biologically constructed by using two calcium carbonates, aragonite and calcite. Aragonite can provide an enhanced mechanical support for the shell, and the stable phase, calcite, acts as the outer layer. In the current study, large-area aragonite film is deposited onto a calcite film in the presence of magnesium ion to biomimetically construct an aragonite−calcite complex structure. A calcite film is fabricated on a silicon wafer by a controlled phase transformation of amorphous calcium carbonate film at 120 °C, which is used as a substrate to induce the deposition of the aragonite layer with the addition of magnesium ion. We demonstrated the importance of the transition process from calcite to aragonite during the formation of the aragonite−calcite complex layer. This study could be used to mimic a sharp transition process of calcite to aragonite in vitro without any organic macromolecules.

Effect of zirconium addition on the austenite grain coarsening behavior and mechanical properties of 900 MPa low carbon bainite steel

The ultra-fine bainitic microstructure of a 900 MPa low carbon bainitic Cu-Ni-Mo-B steel was obtained by a newly developed relaxation precipitation control (RPC) phase transformation processing. In a pan-cake like prior-austenite grain, the microstructure consisted of lath bainite, a little of abnormal granular bainite, and acicular ferrite. The effect of zirconium carbonitrides on the austenite grain coarsening behavior was studied by transmission electron microscopy (TEM). The results show that, the lath is narrower with increasing cooling rate. The ratio of all kinds of bainitic microstructure is proper with the intermediate cooling rate; and Zr-containing precipitates distribute uniformly, which restrains austenite grain growing in heat-affected welding zone.

Construction of coral-like complex: Controlled growth of vaterite submicron hairs on flat films and hemispheres

The complexes of vaterite with the overall macromorphology of some type of coral were biomimetically constructed in the present study. The films and hemispheres of vaterite were obtained by the phase transformation of amorphous calcium carbonate on silicon wafer and mica, respectively. Subsequently, the growth of vaterite submicron hairs was induced on the vaterite substrates in the presence of poly (acrylic acid). It was found that poly (acrylic acid) and vaterite substrates were the key factors in the formation of submicron hairs of vaterite, which could be explained by the solution-precursor-solid (SPS) mechanism. Our work demonstrated an achievement of crystal morphogenesis of calcium carbonate by the controlled phase transformation and crystal growth.

Functional Gradation of Low Alloy Steel by Differentially Controlled Phase Transformation

Functional Gradation of Low Alloy Steel by Differentially Controlled Phase Transformation

A Study on the Microstructure of Ultra Low Carbon Bainitic Steels by RPC Technique

The effect of tempering process on the microstructure of ultra low carbon bainitic (ULCB) steel produced by relaxation precipitation controlled phase transformation (RPC) has been investigated by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The results showed that the final microstructure mainly contained lath-like bainitic ferrite, granular bainite and martensite-austenite (MA) constituent in ULCB steels. On tempering at 650°C a slight increase was detected in the effective grain size as the strain-induced precipitates pinned up the dislocation walls and subgrains. After tempering at 700°C, bainitic ferrite laths started to coarsen and polygonal ferrite occurred. The effective grain size of ULCB steels in as-rolled condition was 1.5 μm at the tolerance of 10o~15o measured by EBSD technique.

Instabilities in adiabatic transformations of first-order phase transitions in a model with bistable units

In this paper we study adiabatic transformations in a model of first-order phase transformations made of superposition of bistable units. A differential equation with hysteresis operators is derived and how to compute numerically the T ( H ) trajectories is discussed. The numerical method is applied to an idealized phase transformation displaying an instability. We discuss the differences of the temperature change between the controlled magnetic field case and the controlled phase transformation case.

Phase Stability and Transformation in Titania Nanoparticles in Aqueous Solutions Dominated by Surface Energy

The surface free energy of small particles in an aqueous solution consists of the electrostatic energy of charged surfaces and the interfacial energy. For nanoparticles in an aqueous solution, the two terms can be modified by solution chemistry and be manipulated to control phase stability and transformation kinetics. Here we show that the phase stability of titania (TiO2) nanoparticles strongly depends on the solution pH. At small sizes, rutile is stabilized relative to anatase in very acidic solutions, whereas in very basic solutions anatase is stabilized relative to rutile and brookite. Rutile is the stable phase at large particle sizes regardless of pH. These results indicate that the activity of potential determining ions (protons or hydroxyl groups) is a factor that can determine the phase stability of nanoparticulate titania in aqueous solutions at pH values far from the point of zero charge of titania. The phase transformation proceeds via a dissolution−precipitation mechanism under hydrothermal conditions.

Modelamento termodinâmico e cinético por meio do método Calphad do processamento térmico e termoquímico de aços

Na década de 1970 Kaufman e Bernstein realizaram trabalho pioneiro sobre modelamento numérico da termodinâmica de sistemas multicomponentes e fundaram o grupo CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry), que tem como propósito promover a termodinâmica computacional e desenvolver programas computacionais para: (i) avaliar e validar dados experimentais (e teóricos) para incorporá-los às bases de dados auto-consistentes, (ii) representar as propriedades termodinâmicas de sistemas multicomponentes, (iii) modelar processos tecnológicos. Além de programas para modelamento termodinâmico, vários programas computacionais CALPHAD têm sido desenvolvidos, também, para calcular a cinética de transformações de fase controladas por difusão, os quais têm interfase com programas de calculo termodinâmico e com bases de dados de mobilidades atômicas. No presente trabalho relatam-se diferentes exemplos do uso do método CALPHAD para o modelamento matemático de diferentes processamentos térmicos e termoquímicos de aços, os quais correspondem a estudos de casos realizados no departamento de Engenharia Metalúrgica e de Materiais da Escola Politécnica da Universidade de São Paulo. Por meio de modelamento CALPHAD foi possível otimizar os parâmetros de processamento durante: a nitretação de aços de alta liga, o processamento térmico de aços TRIP, a produção de nitretos CrN e Cr2N a partir de pó de cromo, o tratamento térmico de solubilização de aços inoxidáveis e o tratamento térmico de decomposição de carbonetos em aços ferramenta.

Numerical treatment of diffusional phase transformation through fully implicit control volume method

Solid state, diffusion controlled phase transformation kinetics with a moving boundary has been quantified using a fully implicit, fixed grid, finite difference method based on the control volume approach. In a departure from the usual modeling techniques for phase change problems, the region undergoing phase change has also been considered as a control volume. A new equation for the interface flux balance has been obtained that minimises the mass balance error that normally plagues the numerical solution of moving boundary problems. The model has been validated with the calculated phase thickness based on binary equilibrium diagram and available experimental data in the literature for the Cu–Zn system and a good match has been obtained. The results obtained by the present formulation are compared with those obtained from the other models. In addition to the improved accuracy of the prediction because of elimination of the mass balance error, the proposed method has the usual advantages of a fully implicit scheme.

Computer Simulation of Phase Decomposition in Magnetic Materials Based on the Phase-Field Method

During the last decade, the phase-field method has emerged across many fields in materials science as a powerful tool to simulate and predict complex microstructure evolutions. Since phase-field methodology has an ability to model complex microstructure changes quantitatively, it will be possible to search for the most desirable microstructure by using this method as a design simulation, i.e. through computer trial-and-error testing. In order to establish this methodology, the flexible quantitative modeling for various types of complex microstructure changes using the phase-field method must first be needed. In this study, as the typical examples for the modeling of the complex microstructure changes using phase-field method, recent simulation results for the diffusion controlled phase transformations and microstructure developments in magnetic materials are demonstrated.

Improvement of 9Cr-ODS martensitic steel properties by controlling excess oxygen and titanium contents

The effects of different percentages of oxygen and titanium on the mechanical properties and microstructure of 9Cr-ODS steel were investigated. It was shown that the high-temperature strength was drastically improved by controlling the atomic ratio between excess oxygen and titanium ( x in TiO x ) around 1.0. When x is around 1.0, the elongated alpha-grains containing the ultra-fine and close oxide particle dispersion were generated in addition to equiaxed martensite grains. Furthermore, the number density of oxide particle in the martensite grains increased. The control of phase transformation and oxide particle dispersion by adjusting oxygen and titanium contents should be a key technology to achieve superior high-temperature strength.