Papers by Keyword: Phase Transformation

Abstract: Starting from H2TiO3, potassium teteratitanate (K2Ti4O9) nanofibres with the length of several micrometers and the diameter of 100nm were directly synthesized by solid state reaction. The novel phase transformation and structure change behavior was investigated by the X-ray diffraction technique (XRD), scanning electron microscopy (SEM) in details. In series of hydrothermal reaction, the K2Ti4O9 could transfer to anatase TiO2 and rutile TiO2. Aggregated anatase TiO2 particles and rod-like rutile TiO2 were produced respectively in 1M HCl solutions at 1600C and 2200C. When the 0.01M HCl solution was considered as solvent, the mixture of floral anatase TiO2 and K2Ti4O9 were present together.

Abstract: Co doped TiO2 nanoparticles and thin films were prepared from Ti(OC4H9)4 by sol-gel method. The influence of Co-doping and its content on the crystallite structure and phase composition of TiO2 was systematically investigated by SEM and XRD, the precursor was analysed by TGA. The photocatalytic activity of nanocomposites was investigated in the photocatalytic degradation of methyl orange. The results indicated that the inhibitory effect of Co-doping on the phase transformation of TiO2 from anatase to rutile receded obviously when the calcination temperature was changed to 650°C from 550°C. Under the ultraviolet light radiation, the favourite Co-doping concentration was 1% and the obtained Co doped TiO2 thin films exhibited higher photocatalytic activity than nanoparticles and pure TiO2 in the photodegradation of methyl orange.

Abstract: The deformation behavior of commercial Zr alloys with 1% and 2,5%Nb under compression at temperatures of the (α+β)-region of Zr-Nb phase diagram is considered on the basis of experimental data obtained by X-ray texture study of deformed samples. Mechanisms, responsible for plastic deformation of alloys by different temperature-rate regimes were determined on the basis of resulting textures. Among these mechanisms there are crystallographic slip and mutual displacements of crystallites along interphase boundaries. The latter mechanism sharply intensifies by grain fragmentation down to nanostructuring under conditions of α«β phase transformations. Texture features of deformed samples testify about interaction of plastic deformation with phase transformations and indicate that due to this interaction compression by optimal regimes promotes the utmost refinement of structure elements.

Abstract: The PALS spectra of alkanes and their binary mixtures were measured as a function of temperature and pressure. It was found that in rigid phase positronium locates in the interlamellar gap, while in the rotator phase in the vicinity of nonplanar (kinked) conformers. An interesting feature is equality of o-Ps lifetimes in rotator and liquid phases in spite of different mechanisms of Ps formation. In binary mixture of nonadecane and tricosane an instability of phase transition from rigid to rotator in the range of tricosane admixture (3-10)% was observed. The pressure and temperature dependences of o-Ps lifetime and intensity are similar, 1 K is roughly equivalent to 4 MPa. The effect of “overpressing” the rotator phase was demonstrated in the mixture containing 10% of tricosane. Unstable rotator phase was observed up to pressure of 110 MPa at 300 K, while at this pressure the stable rotator phase needs the temperature over 310 K.

Abstract: Phases’ evolution during the solidification of a hypoeutectic 3.92% C-equivalence cast iron was modelled by considering the cooling history of the alloy from the melt, thus including both solidification and solid state transformations. Simple Fourier model was used to combine macroscopic heat flow and microscopic kinetics for phase evolution. Different cooling rates were obtained by casting cylinders and stepped plates. Measured number of primary austenitic nuclei, eutectic cells and volume fraction of phases during solidification (graphite, a-ferrite, pearlite and cementite), are correlated with the cooling rate. Growth rate constants for primary austenite, are found to be  = 8.7E-7, and n = 2.3. Growth rate constants for primary graphite (types A, B, and C), are found to be  =5.7E-7, and n = 2. The model matches with the experimental work where the error percent of modelling volume fractions of pearlite, graphite, ferrite and cementite ranges between 0.2 and 1.5%.

Abstract: Austempered Ductile Iron (ADI) and fully ferritic ductile iron are frequently used to produce safety parts. A new kind of ductile iron (DI), usually referred to as “dual phase ADI” is currently under development. The matrix of this new material is composed of ausferrite (regular ADI microstructure) and free (or allotriomorphic) ferrite. This combined microstructure is obtained by subjecting DI to heat treatments comprising incomplete austenitization stages at temperatures within the intercritical interval (three phase α-γ-Gr field of the Fe-C-Si stable diagram) followed by an austempering step in a salt bath in order to transform the austenite into ausferrite. This work describes some alternatives to control morphologies and amounts of phases present in the final microstructure. A revision of the results obtained on this matter by the authors as well as by other researchers is also provided. A ductile iron melt was produced in a metal casting laboratory. Samples were heat treated following different thermal cycles to obtain a wide range of duplex microstructures, containing different amounts and morphologies of microconstituents. The results account for the fact that the transformations occurring within the α-γ-Gr field are strongly affected by the chemical composition, previous matrix, and holding times. The role played by the microsegregation and availability of sites for heterogeneous nucleation is also worth noticing. Starting from ferritic matrices and holding the samples at constant temperature within the intercritical interval, austenite nucleation and growth take place mainly in the last to freeze zones. On the other hand, when starting from fully recrystallised austenitic matrices, allotriomorphic ferrite can precipitate during the isothermal holding within the intercritical interval at the austenite grains boundaries forming a continuous net. The kinetics of the γ  α transformation is notoriously slower than the α  γ one. The influence that part size exerts on the final microstructure is also analyzed.

Abstract: The different dilatometric curves of continuous cooling transformation with the different cooling rates were determined by means of Gleeble-2000 thermal simulation machine. The CCT curve of heavy rail steel was obtained by measuring the dilatometric curves and metallographic analysis. And the effects and mechanisms of chromium on the phases transformation of heavy rail steel were investigated. It is found that, the heavy rail steel only takes place pearlite and martensite transformation during continuous cooling. The CCT curve of heavy rail steel is moved right with increasing chromium content, which indicates that chromium can obviously improve the stability of super cooled austenite and delay the pearlite transformation. When the content of chromium is increased from 0.21% to 0.40%, the shortest incubation time of pearlite transformation is increased from 25s to 52s. When the cooling rate ≤ 5°C s^-1, with increasing chromium content, both starting and finishing temperatures of pearlite transformation are decreased at different extent, meanwhile the pearlite is refined. In addition, when the content of chromium is increased from 0.21% to 0.40%, the critical cooling rate of quenching is decreased from 15°C s^-1 to 7°C s^-1.

Abstract: A thermomechanically coupled constitutive model for finite strain elasto-plasticity is formulated and numerically implemented. The model gives a physically sound description of an initially austenitic material influenced by martensitic phase transformation. The heat dissipated by plastic slip deformation and by phase transformation is allowed to influence the material behavior and appears as a key influencing factor on the growth of the martensitic phase. The model is calibrated using a common stainless steel as prototype material, allowing numerical simulations of crack propagation to be performed. Alterations of the crack growth behavior are observed as different simulation scenarios are compared.

Abstract: A power actuator based on a great volume expansion on hydrogenation of hydrogen storage alloys (HSAs) has been developed. The actuator has a bimorph structure consisting of Pd-Ni alloy and Cu-plating to convert the volume change into bending motion. The techniques to control the bending and rotation motions of the actuator were investigated by adjusting alloy composition, shape and the amount of hydrogen absorbed in the HSA. It is found that Pd-Ni alloy actuators exhibit a cyclic bending motion on hydrogen absorption and desorption cycles and the bending behavior can be controlled by controlling the hydrogen pressure. When a ribbon shaped actuator was deformed into the “L” shape on the transverse section, a rotation motion was observed without modifying the basic bimorph structure of the actuator.

Abstract: A drilling and compression method was designed to produce samples with artifical inner crack. According to the morphology, two types of crack were defined. After heating and holding at different temperatures, the crack healing results were observed under SEM. The starting temperature of crack healing is between 900°C and 950°C. Pseudo-equilibrium phase diagram of 12%Cr was calculated by the Thermo-Calc software to investigate the influence of transformation on the crack healing. It is found that the diffusive transformation has a positive influence on the crack healing behavior. Through experimental results and analysis, we conclude that crack healing is a physical process in association with diffusion, nucleation and grain growth. Any factor which can accelerate diffusion, nucleation and grain growth rate is favorable for the crack healing.