Influence of Oxygen on the Kinetics of Omega and Alpha Phase Formation in Beta Ti–V

Metallurgical and Materials Transactions A - A detailed understanding of the kinetics of phase formation in $$beta $$ -stabilised titanium is of decisive importance for the applicability of these...     

The Redox Kinetics of the Oxidation for CO on EuFeO_3 Catalyst

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Morphology of Oxide Scale and Oxidation Kinetics of Low Carbon Steel

The oxidation kinetics and composition of oxide scales on low carbon steel(SPHC)were studied during isothermal oxidation.Thermogravimetric analyzer(TGA)was used to simulate isothermal oxidation process of SPHC for 240min under air condition,and the temperature range was from 500to 900 ℃.Scanning electron microscope(SEM)was used to observe cross-sectional scale morphology and analyze composition distribution of oxide scales.The morphology of oxide scale was classical three-layer structure.Fe2O3developed as whiskers at the outermost layer,and interlayer was perforated-plate Fe3O4 while innermost layer was pyramidal FeO.From the oxidation curves,the oxidation mass gain per unit area with time was of parabolic relation and oxidation rate slowed down.On the basis of experimental data,the isothermal oxidation kinetics model was derived and oxidation activation energy of SPHC steel was 127.416kJ/mol calculated from kinetics data.     

Kinetics of the σ-phase formation in Fe-Cr-Co hard magnetic alloys

Three magnetic hard alloys 22Kh15K, 25Kh15KYuBF and 30Kh5K of the Fe-Cr-Co system are used to study the kinetics of σ phase formation after quenching from 1300–1100°C and tempering in the temperature range 700–800°C by measuring HV ₃₀₀ hardness, X-ray diffraction, and metallographic analysis. The main σ-forming element in these alloys is shown to be cobalt. The rate of formation of σ-phase during tempering is maximal at 750°C.     

Oxidation Kinetics of an Al–6 wt % Li Alloy Modified by Cerium

The oxidation kinetics of an Al–6 wt % Li alloy modified by ≤0.5 wt % Ce is studied by thermogravimetry. The oxidation rate of the initial alloy is shown to increase with addition of cerium to the alloy or with temperature. Herewith, the apparent activation energy decreases from 35.2 kJ/mol for the initial alloy to 27.7 kJ/mol for the alloy with 0.5 wt % Ce. The oxidation of the alloys is found to occur according to a hyperbolic mechanism.     

Kinetics of sub-grain coalescence—A reconsideration of the theory

The theory for the kinetics of sub-grain coalescence originally developed by Li [J. appl. Phys.33, 2958 (1962)] has been modified; the original theory significantly underestimates the observed times for coalescence in aluminium at temperatures below 350°C. The first modification considers the climb mobility of dislocationloops rather than that of isolated edge dislocations. For mobility limited by bulk diffusion the result is similar to that given by Li, but mobility limited by pipe diffusion is possible for dislocation loops and is found to give much higher mobilities at temperatures less than 350°C. An additional modification is the relaxation of Li's assumption of a uniform dislocation array now known to be incorrect. A simple computer simulation of coalescence by independent climb of a set of dislocation loops, is shown to give non-uniform dislocation distributions very similar to those observed. In combination with the pipe diffusion model for climb mobility, the simulation matches the reported kinetics of coalescence. Finally, the model for coalescence developed here, involving interaction of different dislocation loops, is shown to account for the known heterogeneity of sites for coalescence in aluminium. It is predicted that coalescence requirestwo adjacent high-angle boundaries to act as dislocation sinks.     

Kinetics of Formation of Large-Dimension Rare Earth Inclusion in Steels

The elemental distribution within the large-dimension rare earth inclusion in steels was studied through analysis on plane distribution of elements by EPMA (Electron Probe Microanalyzer). The kinetics and mechanism of formation of RE inclusion were discussed, taking 5CrNiMo steel with the addition of RE and other microalloying elements as example. RE inclusions nucleate on oxides and sulphides with high melting point and grow up separately, then integrated by relatively high surface activity, which is a particular characteristic of RE elements, in liquid phase and during the course of solidification. Finally, the fine and dispersed RE inclusions merge and grow into a large-dimension one.     

Kinetics for Formation of TbFe2 in the Ca-Tb4O7-Fe System

The (TbDy)Fe2 rareearthscolossalmiagnetore sistacematerialwasstudiedsince 1980s .Itisanotherrareearthsfunctionalmaterialthatcametofrontafterrareearthspermanent magnetmaterialandthetrade namesisTerfenol D[1] .TheTerfenol DconsistsoftwokindsofmaterialsTbFe2 andDyFe2 ,whichhavemaxi mumpositiveandnegativemagnetocrystallineanisotropismcoefficient.NormallyusingDy ,Tb ,andhighpurityFeorDyFe2 andTbFe2 interalloyasrawma terials ,andtheywerefoundedinvacuumnon self consumearcfurnacetoproduce (TbD…     

Function of Oxygen in Mo-La2O3 Cathode

Different atomic ratio La/O film cathodes were prepared by pulsed laser deposition under various vacuum conditions. The emission properties were measured and their surface composition was investigated with in situ AES analyses. The function of oxygen in thermoionic emission of Mo-La2O3 cathode was investigated. Excess oxygen is negative to electron emission because lanthanum is easy to become stable La2O3, but proper proportion oxygen can weaken the evaporation of lanthanum and conduce to cathodes＇ stable work. The formation of oxygen vacancies enhances semiconductor property of La-O compound and improves greatly the performance of cathodes.     

Kinetics of the fcc → hcp Phase Transformation in Cu-Ge Solid Solutions Upon Isothermal Aging

The kinetics of the ζ-phase formation from a supersaturated α-Cu(Ge) solid solution (i.e., transformation from the fcc crystal structure to the hcp crystal structure) containing 10.8 at. pct Ge [at isothermal temperatures of 573 K, 613 K, and 653 K (300 °C, 340 °C, and 380 °C)] were studied by X-ray diffraction (XRD) for phase fraction determination. Both in situ and ex situ annealing experiments were performed. The transformation kinetics were modeled on the basis of a versatile modular model. The transformation kinetics complied with a site-saturation nucleation mode and strongly anisotropic interface-controlled growth mode in association with a corresponding impingement mode: diffusion of Ge (towards the stacking faults, SFs) does not control the transformation rate. Transmission electron microscopy (TEM) investigations showed that segregation of Ge at the stacking faults (SFs) takes place (relatively fast) prior to the structural transformation (fcc → hcp).     

Kinetics of the amorphous to icosahedral phase transformation in AlCuV alloys

Single phase icosahedral samples are obtained by annealing melt-spun Al₇₅Cu₁₅V₁₀ amorphous alloys. The kinetics of this amorphous to icosahedral phase transformation were measured isothermally and nonisotheramally by differential scanning calorimetry and from changes in the electrical resistivity. Transmission electron microscopy and energy dispersive X-ray analysis indicate that the transformation proceeds polymorphically by nucleation and growth, ruling out a “micro-quasicrystal” model of the glass in this system. A standard Johnson-Mehl-Avrami analysis of the isothermal, transformation data yields Avrami exponents in the range 2–2.5, which are inconsistent with a polymorphic transformation. These anomalous Avrami exponent arise from an inhomogeneous distribution of quenched-in nuclei. Fits are made to a kinetic model assuming a constant nucleation rate and growth on these quenched-in nuclei. An analysis of the nucleation rates obtained from these fits gives an estimate for the interfacial energy between the icosahedral phase and the glass of 0.002 J/m² ⩽ α ⩽ 0.015 J/m², demonstrating that the short range order must be similar on both sides of the interface.     

Kinetics of cellular dissolution in an AlZn alloy

Dissolution of the lamellar precipitate by cell boundary migration has been studied in an Al-18.9at.% Zn alloy in the temperature range 554–637 K. Microstructural observations have revealed that the process of dissolution in this alloy is cellular mode of transformation in the early stages. The boundary diffusivities were calculated by using the theory of Petermann and Hornbogen modified for cellular dissolution. The diffusivities calculated from the experimental data are four orders of magnitude higher than the volume diffusivities. From the Arrhenius plots, activation energies of 51.5 ± 2 and 47.5 ± kJ mol⁻¹ were obtained from the temperature dependence of diffusivity and mobility respectively. These values are about half the activation energy required for volume diffusion of Zn in Al and compare very well with the activation energy of 60 kJ mol⁻¹ for the cellular precipitation in this alloy system. There exists a range of temperature between 502 and 532 K where the two kinetic processes, cellular precipitation and dissolution, are equally probable. The forward migration of the grain boundary during cellular precipitation is acted upon by the back pull of dissolution and the migrating grain boundary remains motionless.     

Fluid–Solid Reaction Kinetics for Solids of Non-basic Geometries: Application of the Law of Additive Times in Combination with the Shape-Factor Method

Metallurgical and Materials Transactions B - Fluid–solid reactions often occur under the mixed rate control of chemical and mass transfer processes. The results of this work demonstrate that...