Isochronal Transformation Research Articles

Shear strain rate sensitivity and crystallisation kinetics investigation in melt spun Cu64Zr36 binary metallic glass

Abstract Shear strain rate effect on crystallisation behaviour and characteristic temperatures of the three well-known glass formers in Cu–Zr binary amorphous system, namely Cu50Zr50, Cu56Zr44 and Cu64Zr36, were investigated. The crystallisation behaviour of Cu64Zr36 was uniquely found to be heavily dependent on shear strain rate. Crystallisation kinetics were studied through Isochronal transformation and isothermal transformation. The activation energy of crystallisation of each case was compared and contrasted. Johnson-Mehl-Avrami (JMA) analyses were also employed to study its kinetics behaviour. Finally, high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) combined with laser-assisted local electrode atom probe (LEAP) investigation revealed no detectable clustering or phase separation.

A Non-Isokinetic Approach for Modeling Solid-State Transformations: Application to Crystallization of a Fe-B Amorphous Alloy.

Solid-state phase transformations like crystallization of amorphous alloys can be described by an analytical model incorporating a nucleation index a. However, this model cannot be used to examine isochronal transformations with abrupt changing of enthalpy differences performed with differential scanning calorimetry. Based on the model, a non-isokinetic approach is proposed and applied to analyze the isochronal crystallization kinetics of Fe85B15 amorphous alloy. The approach enabled us to obtain the kinetic parameters and activation energies for nucleation and growth.

Investigating the Kinetic Parameters of SiO2-Al2O3-CaO-CaF2-K2O Oxyfluoride Glass

Oxyfluoride glass-ceramics containing CaF2 nanocrystals are kind of attractive materials for the optical applications due to their low phonon energy and high transparency. Moreover, the crystallization control and consequently, the kinetic properties are important for oxyfluoride glasses. Therefore, in the present research, the crystallization kinetics of isochronal transformation of the 37.26SiO2-28.11Al2O3-7.73CaO-26.89CaF2-4.5 K2O (wt%) glass have been determined upon the basis of maximum transformation rate using Differential Thermal Analysis (DTA) technique. Hence, it is concluded that the crystallization of the mentioned glass is a process controlled by Avrami nucleation, three-dimensional diffusion-controlled growth, and anisotropic growth impingement mode. The effective activation energy Qp =181 kJ.mol-1, growth exponent n=2.272, nucleation activation energy QN= 123, and growth activation energy QG=211 have been determined.

Conversion of isothermal and isochronal crystallization in a supercooled liquid through additivity rule

In the present work, the compatibility of additivity rule with the crystallization behavior of an Au50Cu25.5Ag7.5Si17 glass forming alloy on the conditions of both isothermal and isochronal transformations is investigated. Taking advantages of the ultrafast heating and cooling rates of the Flash DSC technique, we verified the additivity rule in a solidification process for the first time. It is proved to be effective in predicting the beginning and the end of isochronal crystallizations during continuous heating and cooling from the kinetic analysis of corresponding experimental isothermal transformation. The predicted isochronal curves are fairly in accordance with the experimental results, especially for the incubation and crystallization processes during rapid solidification of the glass forming liquid. The compatibility of additivity rule enables a conversion between isothermal and isochronal transformations for the crystallization of supercooled liquid even during solidification from the melt.

Martensite–austenite transformation kinetics of high Cr ferritic heat-resistant steel

Abstract The isochronal transformation behavior from martensite (α′) to austenite (γ) of modified high Cr ferritic heat-resistant steel was analyzed using the kinetic information extracted from differential thermal analysis results. The thus obtained α′→γ transformation kinetic process has been described by a JMAK-like model including three overlapping processes: site saturation nucleation, diffusion-controlled growth, and impingement correction for random distribution of nuclei. It is found that both the diffusion of alloying components and the inhibition of carbide precipitates strongly affect the α′→γ transformation kinetic processes. The α′→γ transformation (at relatively low heating rates) is of diffusion-controlled nature, and the obstruction of carbides during the migration of the α′/γ interface leads to a decrease in the pre-exponential factor of diffusion coefficient.

Thermal stability and phase transformations of martensitic Ti–Nb alloys

Aiming at understanding the governing microstructural phenomena during heat treatments of Ni-free Ti-based shape memory materials for biomedical applications, a series of Ti–Nb alloys with Nb concentrations up to 29 wt% was produced by cold-crucible casting, followed by homogenization treatment and water quenching. Despite the large amount of literature available concerning the thermal stability and ageing behavior of Ti–Nb alloys, only few studies were performed dealing with the isochronal transformation behavior of initially martensitic Ti–Nb alloys. In this work, the formation of martensites (α′ and α″) and their stability under different thermal processing conditions were investigated by a combination of x-ray diffraction, differential scanning calorimetry, dilatometry and electron microscopy. The effect of Nb additions on the structural competition in correlation with stable and metastable phase diagrams was also studied. Alloys with 24 wt% Nb or less undergo a transformation sequence on heating from room temperature to 1155 K. In alloys containing >24 wt% Nb α″ martensitically reverts back to β0, which is highly unstable against chemical demixing by formation of isothermal ωiso. During slow cooling from the single phase β domain α precipitates and only very limited amounts of α″ martensite form.

Research on splitting phenomenon of isochronal martensitic transformation in T91 ferritic steel

T91 steel is a representative type of ferritic heat-resistant steel currently used in power plant components, and is a potential candidate for structural steel in nuclear reactors. The isochronal martensitic transformation behaviors during continuous cooling after austenitization in T91 ferritic steel were systematically investigated by high-resolution dilatometry and microstructure observation. The splitting phenomenon of martensitic transformation is accompanied with the precipitation of needle-like M3C particles, which is suppressed by rapid cooling after austenitization. The appearance of this splitting is ascribed to the concentration gradient caused by the consumption of alloy element in process of the formation of M3C. This concentration gradient results in the appearance of wide martensitic laths ahead of the generation of normally narrow laths. These two types of martensitic laths possess different M s (martensitic start transformation) temperatures, which are attributed to the splitting transformation phenomenon.

In situ investigation of the martensitic transformation in Fe–12wt.%Ni–0.6wt.%C steel at subzero temperatures

Abstract Martensitic transformation in a Fe–12 wt.%Ni–0.6 wt.%C alloy was investigated with magnetometry and in situ (synchrotron) X-ray diffraction at sub-zero Celsius temperature. In situ X-ray diffraction at the HZB-BESSY II synchrotron facility was applied to quantitatively determine the fractions of retained austenite and martensite formed during: (i) isochronal transformation, while cooling at a rate of 3 K/min from room temperature to 138 K, and (ii) isothermal transformation, while holding at 138 K. X-ray diffraction analysis was also used for a qualitative evaluation of the stress evolution in austenite during the transformation. Magnetometry was applied to monitor the overall kinetics for the same isochronal and isothermal transformations. The results are discussed in the light of the current theories on martensitic transformations.

Recent Developments for Isochronal Transformation Kinetics Model and their Applications

Analytical model for isochronal phase transformation kinetics attracts much attention for its advantages and importance. Adopting an effective treatment for the so-called temperature integral, an analytical model for isochronal transformation kinetics has been developed by considering various nucleation and growth modes. After applying to the analysis of the austenite-ferrite phase transformation recorded by the high-resolution dilatometric measurements, it is recognized that the developed analytical phase transformation model could be adopted effectively to determine the nucleation and/or growth mechanism prevailing during the isochronal phase transformation processes.

Evaluation of the maximum transformation rate for analyzing solid-state phase transformation kinetics

An evaluation of the maximum in the transformation rate of a solid-state transformation is given for isothermal and isochronal transformations, on the basis of a general, flexible, analytical transformation model incorporating different combinations of nucleation, growth and impingement modes. For isothermal phase transformations, the position of the peak maximum is determined by the growth exponent and the impingement mode whereas, for isochronal phase transformations, only the impingement mode influences the position of the peak maximum. A straightforward method has been developed for determining the mode of impingement and the separate activation energies for nucleation and growth. The results obtained by application of the proposed transformation-rate maximum analysis to model systems are in good agreement with the values assigned to the kinetic parameters of the model systems. The proposed recipes have also been applied successfully to experimental data on the crystallization of an initially amorphous Mg–Cu–Y alloy.

An integral fitting method for analyzing the isochronal transformation kinetics: Application to the crystallization of a Ti-based amorphous alloy

An integral fitting method has been developed to determine the phase transformation mechanism and to extract the kinetic parameters during the crystallization of amorphous alloys. The proper kinetic function of the phase transformation was firstly deduced. Theoretical differential scanning calorimetry curves were then calculated. All the kinetic parameters can be extracted by fitting the calculated differential scanning calorimetry curves to experimental data. We applied the integral fitting method to analyze the isochronal crystallization of the Ti 50Cu 42Ni 8 amorphous alloy. Results indicate that a transformation process considering impingement is more suitable to describe the crystallization kinetics of this alloy than using the traditional Johnson–Mehl–Avrami model. Mean values of the obtained kinetic parameters show strong heating rate dependence.

Consideration of the growth mode in isochronal austenite-ferrite transformation of ultra-low-carbon Fe–C alloy

The three cooling rates of 10, 100, 200 K/min dilatometry experiments are used to investigate the kinetics of the isochronal austenite (γ) to ferrite (α) transformation of Fe–0.0036wt.%C alloy. “Normal transformation” and “abnormal transformation” have both been observed for transformations at different cooling rates. In accordance with the thermodynamic characteristics of the γ→α transformation investigated here and previous kinetic considerations, a JMAK-like approach for the kinetics of isochronal phase transformations was developed that incorporates three overlapping processes: site saturation nucleation, alternate growth modes (from interface-controlled to diffusion-controlled to interface-controlled growth), as well as impingement for random distribution nuclei. The JMAK-like approach has been employed to fit the experimental results, and the fitting results show that for the γ→α transformation of the Fe–C alloy at all applied cooling rates, the growth mode evolves in the corresponding order: from interface-controlled to diffusion-controlled growth; from interface-controlled to diffusion-controlled to interface-controlled growth; and interface-controlled growth.

Approaches for isochronal transformation kinetics model and their application to the crystallization of amorphous alloys

An analytical model for isochronal phase transformation kinetics has attracted much attention for its advantages and importance. However, simple but exact analytical formulae of the isochronal transformation are unavailable because of the so-called temperature integral. Here an improved analytical model for isochronal transformation kinetics has been established by considering various nucleation and growth modes on using an improved treatment for the temperature integral. The powerful model-analysis method was also adopted to investigate the validity and effectiveness of the general and the improved isochronal transformation analytical model by applying them to two crystallization processes of amorphous alloys: a wide temperature interval one and a narrow one. The results showed that the improved model can provide a much better description for the narrow one while the improved and general models perform equally well in the wide one.

THE WFC3 GALACTIC BULGE TREASURY PROGRAM: A FIRST LOOK AT RESOLVED STELLAR POPULATION TOOLS

[Abridged] When WFC3 is installed on HST, the community will have powerful new tools for investigating resolved stellar populations. The WFC3 Galactic Bulge Treasury program will obtain deep imaging on 4 low-extinction fields. These non-proprietary data will enable a variety of science investigations not possible with previous data sets. To aid in planning for the use of these data and for future proposals, we provide an introduction to the program, its photometric system, and the associated calibration effort. The observing strategy is based upon a new 5-band photometric system spanning the UV, optical, and near-infrared. With these broad bands, one can construct reddening-free indices of Teff and [Fe/H]. Besides the 4 bulge fields, the program will target 6 fields in well-studied star clusters, spanning a wide range of [Fe/H]. The cluster data serve to calibrate the indices, provide population templates, and correct the transformation of isochrones into the WFC3 photometric system. The bulge data will shed light on the bulge formation history, and will also serve as population templates for other studies. One of the fields includes 12 candidate hosts of extrasolar planets. CMDs are the most popular tool for analyzing resolved stellar populations. However, due to degeneracies among Teff, [Fe/H], and reddening in traditional CMDs, it can be difficult to draw robust conclusions from the data. The 5-band system used for the bulge Treasury observations will provide indices that are roughly orthogonal in Teff and [Fe/H], and we argue that model fitting in an index-index diagram will make better use of the information than fitting separate CMDs. We provide simulations to show the expected data quality and the potential for differentiating between different star-formation histories.

Determination of nucleation and growth modes from evaluation of transformed fraction in solid-state transformation

On the basis of an analytical transformation model, the transformed fraction has been evaluated for isothermal and isochronal solid-state transformations. From the evolution of Avrami exponent n with annealing temperature T and the evolution of effective activation energy Q with transformed fraction f, the prevailing modes of nucleation and growth in the isothermal transformation can be determined. As for the isochronal transformation, however, a conversion of the experimentally obtained continuous-heating-transformation (CHT) diagram to the temperature–time-transformation (TTT) diagram should also be performed. Because the modes of nucleation and growth remain unchanged upon CHT–TTT conversion, the prevailing modes of nucleation and growth in the isochronal transformation can be determined. Using numerical calculations, the above evaluation has been tested. Finally, application in the crystallization kinetics of amorphous Pd–Ni–P–Cu and Mg–Cu–Y as measured by differential scanning calorimetry was described.

Examination of Kissinger's equation for solid-state transformation

Applying numerical calculations for isochronal transformations on heating assuming site saturation or continuous nucleation, the applicability of Kissinger's equation was examined, in combination with an analytical transformation model. Only if in the temperature integral the starting temperature for the transformation T is sufficiently larger than T 0, which indicates the temperature of the initial state, an exact value for effective activation energy Q can be deduced using Kissinger's equation. In the meantime, a recipe, derived from the transformation rate, is proved to be applicable to isochronal transformations, independent of T 0.

Improved analytical model for isochronal transformation kinetics

Analytical model for isochronal phase transformation kinetics attracts much attention for its advantages and importance. However, the simple but exact analytical formula of the isochronal transformation is unavailable because of the so-called temperature integral, and the asymptotic expansions have to be adopted to obtain approximate results. Here a generally used asymptotic expansion was proved divergent, and a reasonable approximation was proposed to obtain a more precise description as compared numerically to the previous one. Based on the proposed approximation, an analytical model for isochronal transformation kinetics was developed, which was proved more effective than the previous analytical model when the transformation occurs in a narrow temperature range and exhibited an identical form to the previous model when in a wide temperature interval.

Stellar Cluster Fiducial Sequences with the Advanced Camera for Surveys

We present color-magnitude diagrams of five Galactic globular clusters and one Galactic open cluster spanning a wide range of metallicity (-2.1 [Fe/H] +0.3), as observed in the F606W (broad V) and F814W (I) bands with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. These clusters are part of two large ACS programs measuring the star formation history in the Andromeda halo, tidal stream, and outer disk. In these programs, the clusters serve as empirical isochrones and as calibrators for the transformation of theoretical isochrones to the ACS bandpasses. To make these data more accessible to the community, for each cluster we provide a ridgeline tracing the stars on the main sequence, subgiant branch, and red giant branch, plus the locus of stars on the horizontal branch. In addition, we provide the transformation of the Victoria-Regina isochrones to the ACS bandpasses.

Ageing Kinetics of Al.-4.7 % Cu Alloy. Dilatometric and DTA Studies

Studies of transformation kinetics during ageing of Al-4.7 % Cu were performed with use of dilatometric and DTA methods. The dilatometric method was used for studies of isothermal ageing, DTA for studies of ageing during isochronal heating. In order to determine isothermal transformation kinetics the JMA (Johnson – Mehl – Avrami) equation was applied, for isochronal heating transformation kinetics Kissinger method was used. Ageing of Al- Cu alloys consist of precipitation processes and dissolution processes that overlap each other. These processes, during isothermal ageing were distinguished with help of the time exponent n of JMA equation that is very sensitive to change of the transformation mechanism. During isochronal ageing, precipitation processes produce heat, dissolution processes consume heat. That helps to distinguish them. Results obtained during isothermal studies were in agreement with isochronal heating studies. Activation energies for precipitation processes 324-530 K temperature range varied from about 50 kJ/mol to about 100 kJ/mole and complied with the values obtained before. For precipitation processes at 650- 735 K temperature range, activation energies varied from 226 to 300 kJ/mole. The results obtained were discussed with literature data.

An analytical model for isothermal and isochronal transformation kinetics

An analytical model for the kinetics of phase transformations has been discussed that combines three overlapping processes: nucleation, growth, and impingement. Two kinds of nucleation have been considered in particular: a mixture of site saturation and continuous nucleation and Avrami nucleation. In combination with either interface-controlled growth or volume diffusion controlled growth, and incorporating the effect of impingement of the growing particles, a general analytical description of the transformation kinetics has been given for both isothermally and isochronally conducted transformations. The corresponding kinetic parameters are time and temperature dependent. In specific, limiting cases, the model reduces to the so-called Johnson-Mehl-Avrami description of transformation kinetics. The analytical model has been verified by exact results obtained from numerical calculations. The influences of the different nucleation and growth modes on the time and temperature dependencies of the transformation rate and the kinetic parameters have been demonstrated.