Temperature Transformation Diagram Research Articles

Application of time–temperature–transformation and time–pressure–transformation diagrams for cooling- and decompression-induced crystallization to estimate the critical cooling rate required to form glassy obsidian

The formation of glass from silicic magmas during eruption is an enigmatic process. To understand the formation of obsidian, we used time–temperature–transformation (TTT) and time–pressure–transformation (TPT) diagrams to estimate the critical cooling and decompression rates needed to form glassy obsidian. The TTT diagram is a contour map of the crystallized volume fraction as a function of the crystallization temperature and time. A contour line for a given crystal fraction in a time versus temperature diagram has a cone shape, with the “nose” of the curve corresponding to the minimum time and corresponding temperature required to generate a specific crystal fraction. We also present a TPT diagram, based on the TTT diagram, and liquidus curve for decompression-induced crystallization. We applied the TTT diagram to the Tokachi–Ishizawa obsidian from Shirataki, Hokkaido, Japan, and used it to estimate the critical cooling rate that would have been required to form its glassy texture by cooling-induced crystallization during eruption. For decompression-induced crystallization, we used the TPT diagram to estimate the critical decompression rate needed to form glassy obsidian. Based on our calculation results, the critical cooling and decompression rates are highly dependent on the interfacial energy between crystals and melt. We compared the estimated critical cooling and decompression rates with those estimated from microlite number density and a one-dimensional solution of the heat conduction equation. Our calculation results reveal that glassy obsidian can be formed during an eruption, particularly under conditions of high interfacial energy.

Modeling of Alloying Effect on Isothermal Transformation: A Case Study for Pearlitic Steel

Machine learning (ML) has been rapidly revolutionizing many fields and plays a vital role in materials science. To understand the microstructure of steels and even obtain optimized processing parameters, accurate predictions on the time–temperature–transformation (TTT) diagram are in urgent need. Herein, artificial neural network (ANN), support vector machine (SVM), and extreme learning machine (ELM) models are developed and compared for quantitatively predicting typical features, such as the nose tip temperature and incubation period, of TTT diagrams for pearlitic steels. As an important evaluating indicators of prediction accuracy, the correlation coefficient (R2), the root mean square error (RMSE), and the mean absolute percent error (MAPE) are used to evaluate the proposed models. The results show that the proposed SVM model exhibits higher prediction accuracy compared with the ANN and ELM and demonstrates the advantage of the machine learning approach in predictions of TTT diagrams. Moreover, a series of experiments demonstrate that the proposed SVM predictor performs favorably, with satisfactory prediction accuracy. The microstructures of the isothermal transformation products of the prepared pearlitic steel are investigated using dilatation and microscopy technique.

Blocky Alpha Transformation in Zircaloy-4 Weldments

Blocky alpha can be systematically generated during heat treatment of certain as-welded Zircaloy-4 microstructures. The volume fraction depends upon the exposure time and temperature of the postweld heat treatment. It is necessary to understand the transformation kinetics of the fast-cooled as-welded structure to determine the microstructures present after extended periods of high-temperature operation. An experimental test campaign was devised to observe the evolution of blocky alpha and to understand its transformation kinetics. A number of postweld heat treatments were performed at between 610°C and 760°C to form microstructures with various volume fractions of blocky alpha. Following microstructural analysis, the data were subsequently used to construct a time–temperature transformation (TTT) diagram, capable of predicting blocky alpha formation. The TTT diagram shows that blocky alpha is not expected to form below 600°C in any realistic timeframe and is therefore not an issue at typical civil pressurized water reactor operational conditions.

Prediction and Measurement of the Kinetics of Isothermal Crystallization and Devitrification of Mold Powder Slags

The rate and extent of formation of crystalline phases in the slag film that develops between the mold and the strand in steel continuous casting determine heat transfer and lubrication between them and therefore play crucial roles in shell integrity. In this work, a commercial mold powder for casting medium carbon (MC) thin steel slabs is selected for carrying out detailed comparisons between predicted and experimental results of the rates of isothermal crystallization and devitrification of liquid and glassy slag, respectively. Predictions are obtained with a kinetic model that combines the induction period (of Simon and Kolman) and the Kissinger methods to evaluate time–temperature–transformation (TTT) diagrams for the devitrification and crystallization reactions associated with the peaks appearing in non-isothermal thermo-analysis traces. To obtain experimental validation of the diagram, a high-temperature confocal laser scanning microscope (HT-CLSM) is used to rapidly heat or cool a sample, ~ 140 mg, of glassy powder or liquid slag, respectively, to selected isothermal conditions, where it is held for a prescribed length of time before quenching. Identification and quantification of the phases present in the quenched samples are done by a quantitative X-ray powder diffraction analysis (QXRPDA) method that uses a PONKCS (partial or not known crystal structure) procedure based on Le Bail and Rietveld strategies for refinement of diffraction patterns. The many experimental results indicate that the predicted TTT diagrams for devitrification and crystallization of the mold slag give a good estimation of the rate of the transformations that take place at different temperatures and reveal the differences in the reactions that occur along both paths.

Neutron and hard X-ray diffraction studies of the isothermal transformation kinetics in the research reactor fuel candidate U-8 wt%Mo.

Exposing uranium-molybdenum alloys (UMo) retained in the γ phase to elevated temperatures leads to transformation reactions during which the γ-UMo phase decomposes into the thermal equilibrium phases, i.e. U2Mo and α-U. Since α-U is not suitable for a nuclear fuel exposed to high burn-up, it is necessary to retain the γ-UMo phase during the production process of the fuel elements for modern high-performance research reactors. The present work deals with the isothermal transformation kinetics in U-8 wt%Mo alloys for temperatures between 673 and 798 K and annealing durations of up to 48 h. Annealed samples were examined at room temperature using either X-ray or neutron diffraction to determine the phase composition after thermal treatment, and in situ annealing studies disclosed the onset of phase decomposition. While for temperatures of 698 and 673 K the start of decomposition is delayed, for higher temperatures the first signs of transformation are already observable within 3 h of annealing. The typical C-shaped curves in a time-temperature-transformation (TTT) diagram for both the start and the end of phase decomposition could be determined in the observed temperature regime. Therefore, a revised TTT diagram for U-8 wt%Mo between 673 and 798 K and annealing durations of up to 48 h is proposed.

Vulcanization of EPDM rubber compounds with and without blowing agents: Identification of reaction events and TTT-diagram using DSC data

Vulcanization of industrial-like ethylene propylene diene termonomer rubber compound is studied using a differential scanning calorimetry (DSC). The analysis starts with DSC information to obtain the total transformation heat, followed by an isothermal-dynamic temperature ramp that captures diffusion-controlled reaction kinetics. The vulcanization is modeled by an auto-catalytic Kamal–Sourour model, complemented with a Kissinger model for the prediction of one energy of activation, DiBenedetto's equation for the glass transition temperature, and adjusted reaction constants to include diffusion mechanisms. Two rubber formulations, with and without blowing agents, containing crosslinking agents, primary and secondary accelerators, activators, promoters, and processing aids are studied. The identification and separation of multiple reaction events, occurring during crosslinking of the compound without a blowing agent, is done through a 2k design of experiments. Time–temperature–transformation (TTT) diagrams are calculated, integrating the kinetic model, thereby delineating processability windows, providing avenues for optimization, design, and online processing control. According to the kinetics and the TTT diagrams, the blowing agent induces several differences to the vulcanization reaction: decreases reaction temperatures while increasing reaction heats. It eliminates the exothermic peak before vulcanization and decreases the fully cured resin's glass transition temperature. Therefore, the presence of the blowing agent drives a shift in the vitrification line, resulting in a reduced operational window. POLYM. ENG. SCI., 55:2073–2088, 2015. © 2014 Society of Plastics Engineers

Method for time–temperature–transformation diagrams using DSC data: Linseed aliphatic epoxy resin

ABSTRACTA novel method to generate time–temperature–transformation (TTT) diagrams from Differential Scanning Calorimetry (DSC) data is presented. The methodology starts with dynamical DSC information to obtain the total transformation heat, followed by an isothermal‐dynamic temperature ramp that allows the inclusion of diffusion‐controlled reaction kinetic. The cure kinetics is modeled using an auto‐catalytic Kamal–Sourour model, complemented with a Kissinger model that allows the direct prediction of one energy of activation, DiBenedetto's equation for the glass transition temperature as a function of the cure degree and adjusted reaction constants to include diffusion mechanisms. The methodology uses a nonlinear least‐squares regression method following J.P. Hernández‐Ortiz and T.A. Osswald's methodology (J. Polym. Eng. 2004, 25, 23). A typical linseed epoxy resin (EP) presents two different kinetics control mechanisms, thereby providing a good model to validate the proposed experimental and theoretical method. TTT diagrams for EPs at two different accelerator concentrations are calculated from direct integration of the kinetic model. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40566.

Chemorheological behavior of β-SiAlON aqueous suspensions in gelcasting process

The chemorheological behavior of an aqueous suspension of monofunctional and difunctional monomers containing β-SiAlON was investigated for gelcasting process. The effect of various parameters on gel and processing times was also studied. The time–temperature–transformation (TTT) diagram was used to correlate the pertinent processing parameters (time and temperature) with the gelation and processing times during in situ polymerization. The significant difference noticed between TTT diagrams of gel system containing β-SiAlON and a pure gel system was a sharp shift of castable region to lower temperatures. This shift indicates that at the same monomers and initiator concentrations, the gelation can occur at lower temperature in the presence of SiAlON. These results clearly imply that the ingredient has accelerating effect on the polymerization reaction of acrylamide/methylene bisacrylamide. Reduction of polymerization activation energy from 72 kJ/mol for the monomer solution to 61 kJ/mol for a suspension containing SiAlON (35 vol%) confirmed this result. The impurities and surface groups of SiAlON are responsible for the accelerating effect of SiAlON. Furthermore, all suspensions showed shear thinning behavior, which were enhanced by β-SiAlON rate of concentration increase. POLYM. ENG. SCI., 53:2123–2128, 2013. © 2013 Society of Plastics Engineers

Solvent Effects on Cure 1-Benzyl Alcohol on Epoxy Cure

The effect of benzyl alcohol and phenol on the cure of epoxy resins is reported. The epoxy resin, a blend of the diglycidyl ethers of bisphenol A and bisphenol F, was cured with 4,4′-methylenebis(cyclohexylamine) diamine. Measurements of the cure and vitrification times were obtained using a vibrating plate curometer. Mechanical properties were assessed using dynamic mechanical thermal analysis (DMTA). Time temperature transformation (TTT) diagrams were constructed. Benzyl alcohol lowers the viscosity, aids cure and plasticizes the final product. Phenol and triethylenetetramine [TETA] significantly enhance the rate of cure and have a catalytic effect.

Computation of isothermal transformation diagrams of 42CrMo4 steel from dilatometer measurements with continuous cooling

AbstractTo generally predict the mechanical properties of steels following heat treatment, the isothermal time–temperature–transformation (TTT) diagrams must be known. The isothermal kinetics of the phase transformation is influenced by the austenitisation conditions and the deformation processes. Owing to the requirements of the process chain 'integrated heat treatment following hot-forging', the steels are austenitised at a comparatively high temperature and deformed before quenching. The TTT diagrams found in the literature only treat limited austenitisation temperatures and do not generally consider different deformation levels. However, the measurements of the corresponding TTT diagrams are both costly and time consuming. In addition to this, the isothermal transformations of low alloyed steels, which transform very quickly, cannot be measured using those dilatometers which are readily available.1 In this paper, the TTT diagrams of the 42CrMo4 steel austenitised at 1200°C and deformed at different le...

Time–Temperature–Transformation (TTT) Diagrams for Crystallization of Metal Oxide Thin Films

AbstractTime–temperature–transformation (TTT) diagrams are proposed for the crystallization of amorphous metal oxide thin films and their specific characteristics are discussed in comparison to glass‐based materials, such as glass‐ceramics and metallic glasses. The films crystallize from amorphous to full crystallinity in the solid state. As an example the crystallization kinetics for a single‐phase metal oxide, ceria, and its gadolinia solid solutions are reported made by the precipitation thin‐film method spray pyrolysis. The crystallization of an amorphous metal oxide thin film generally follows the Lijschitz–Sletow–Wagner (LSW) Ostwald ripening theory: Below the percolation threshold of 20 vol% single grains crystallize in the amorphous phase and low crystallization rates are measured. In this state no impact of solute on crystallization is measurable. Once the grains form primary clusters above the threshold the solute slows down crystallization (and grain growth) thus shifting the TTT curves of the doped ceria films to longer times and higher temperatures in comparison to undoped ceria. Current views on crystallization of metal oxide thin films, the impact of solute dragging, and primary TTT diagrams are discussed. Finally, examples on how to use these TTT diagrams for better thermokinetic engineering of metal oxide thin films for MEMS are given, for example, for micro‐Solid Oxide Fuel Cells and resistive sensors. In these examples the electrical properties depend on the degree of crystallinity and, thereby, on the TTT conditions.

Time–temperature–transformation cure diagrams of phenol–formaldehyde and lignin–phenol–formaldehyde novolac resins

AbstractIn this study, the time–temperature– transformation (TTT) cure diagrams of the curing processes of several novolac resins were determined. Each diagram corresponded to a mixture of commercial phenol–formaldehyde novolac, lignin–phenol–formaldehyde novolac, and methylolated lignin–phenol–formaldehyde novolac resins with hexamethylenetetramine as a curing agent. Thermomechanical analysis and differential scanning calorimetry techniques were applied to study the resin gelation and the kinetics of the curing process to obtain the isoconversional curves. The temperature at which the material gelled and vitrified [the glass‐transition temperature at the gel point (gelTg)], the glass‐transition temperature of the uncured material (without crosslinking; Tg0), and the glass‐transition temperature with full crosslinking were also obtained. On the basis of the measured of conversion degree at gelation, the approximate glass‐transition temperature/conversion relationship, and the thermokinetic results of the curing process of the resins, TTT cure diagrams of the novolac samples were constructed. The TTT diagrams showed that the lignin–novolac and methylolated lignin–novolac resins presented lower Tg0 and gelTg values than the commercial resin. The TTT diagram is a suitable tool for understanding novolac resin behavior during the isothermal curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Glass forming ability in Cu–Zr binary alloy: Effect of nucleation mode

In the present work, thermodynamic and kinetic calculations were combined to study the glass forming ability of Cu–Zr binary system. Applying thermodynamic association model, the primarily crystallized phase and its corresponding driving force can be determined firstly. Then, the whole Time–Temperature–Transformation (TTT) diagrams basing on homogeneous and heterogeneous nucleation modes have been constructed quantitatively. As for the former case, the critical cooling rate is underestimated by about three orders of magnitude in comparison with the experimental observation. As for the latter case adopting suitable values of catalytic factor f(θ) and potential nuclei N 1, the predictions agree well with the experimental results, and moreover, the nose temperature of TTT diagram is upper-shifted. This has been discussed using a competition of the atomic mobility and the reduced nucleation barrier, as well as its further effect on the nucleation and growth process.

New thermosets obtained by cationic copolymerization of DGEBA with γ-caprolactone with improvement in the shrinkage. II. Time–temperature–transformation (TTT) cure diagram

Mixtures of diglycidylether of bisphenol A (DGEBA) with different proportions of γ-caprolactone (γ-CL) were cured with ytterbium triflate as initiator. The curing was studied with differential scanning calorimetry (DSC) and thermo mechanical analysis (TMA). The results are presented in the form of a time–temperature–transformation diagram. The kinetic analysis was performed by means of the isoconversional integral procedure and the kinetic model was also determined using the Coats–Redfern method. Gelation was determined by means of combined experiences of DSC and TMA. The relationship between the glass transition temperature (Tg) and the degree of conversion α was determined by DSC. Using the isoconversional lines and the Tg-α relationship, the vitrificacion curve was obtained. The methodology developed makes it possible to obtain the TTT diagram using only no-isothermal experiments with equivalent results to those using classical isothermal procedures. The addition of γ-CL accelerates the curing and reduces the shrinkage after gelation and consequently the internal stresses in the material. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Characterization of Epoxy Prepreg Curing Process

ABSTRACT The isothermal and dynamic curing process of epoxy composite was studied using differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and other techniques. The variation of degree of curing with time and temperature was analyzed. The degree of curing was limited at any particular temperature because of the diffusion control in isothermal curing process. Half-life and maximum cure time were discussed in the analysis of the isothermal curing process. The modeling result from isothermal curing process indicated that degree of curing calculated with diffusion control agreed well with experimental data. The degree of curing calculated by two methods for dynamic curing process had a deviation with experimental data in either earlier or later cure stages. The relationship between T g and the degree of curing was described by two models. Both models agreed well with the experimental T g . The isothermal curing diagrams of time–temperature–transformation (TTT) and conversion–temperature–transformation (CTT) were constructed. Each region in TTT and CTT diagrams corresponded to the phase state of the curing process, so that the curing mechanism was clearly reflected in diagrams. The thermal stability analysis indicated the epoxy resin system was very thermally stable under temperature of 300°C.

Isothermal transformation of low-carbon microalloyed steels

The microstructures of isothermal transformation products and time–temperature–transformation (TTT) diagrams of two low-carbon microalloyed steels have been investigated by the means of dilatation and microscopy. The results show there are three independent “C-shape” curves in the TTT diagrams of low-carbon microalloyed steels, namely, the polygonal ferrite/pearlite transformation “C-shape” curve, the massive ferrite transformation “C-shape” curve and the bainitic transformation “C-shape” curve, respectively. The effects of the carbon and Mo content on massive ferritic transformation are discussed.

Time-Temperature-Transformation Diagrams for Borosilicate Glasses and Preparation of Chemically Durable Porous Glasses

To clarify the effects of glass composition and heat-treatment conditions on phase separation and crystallization, time–temperature–transformation (TTT) diagrams for the system (65 −x)SiO2·25B2O3·(10 −y)Na2O·yCaO·xZrO2, which is used for fabricating alkali-durable porous glasses, were determined. The microstructures and properties, such as resistance to alkaline attack as a function of ZrO2 content, of the porous glasses fabricated on the basis of the developed TTT diagrams were examined. CaO and ZrO2 additions were shown to affect the locations of the noses in the TTT curves for phase separation and crystallization. The substituting of CaO for Na2O was effective for retaining ZrO2 in the skeleton of the porous glasses. The alkali resistance of ZrO2-containing porous glasses was 8–10 times superior to that of the porous glass without ZrO2. The results demonstrate that TTT diagrams can be used as guides for heat-treatment scheduling in processing or predicting of glass-forming ability and the onset of phase separation and crystallization in glass-forming systems.

Crystallisation kinetics of Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG alloy during heating

The crystallisation behaviour of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass (BMG) alloy during heating from the amorphous state has been investigated. Upon heating, the nucleation process was not consistent with classical nucleation theory. This problem can be overcome by substituting the nucleation density with the number density of nuclei, N, for growth of the crystalline phase, which is estimated from the length scale of the microstructure. The growth of crystalline phase is the governing process for crystallisation of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy upon heating. The growth process is more important than the nucleation process for analysing the crystallisation mechanisms of the amorphous state. Non-isothermal heating DSC data have been analysed with a modified crystallisation kinetics model based on the non-isothermal transformation theory. Using this model, the onset crystallisation temperatures at various heating rates have been calculated. The calculated critical heating rate to avoid crystallisation is 190 K s−1, which matches well with the experimental value. The onset crystallisation times for isothermal annealing conditions have also been calculated and compared with the isothermal DSC data. Time temperature transformation (TTT) and continuous heating transformation (CHT) diagrams have been constructed quantitatively. The crystallisation kinetics of BMG alloys can be more reliably analysed with our modified crystallisation kinetics model than with several nucleation models.

Theoretical evaluation and the effect of thermal degradation on the Time–Temperature–Transformation (TTT) diagram of bisphenol A epoxies

AbstractA brief discussion about the Time–Temperature–Transformation (TTT) diagram is presented. Using diamino diphenylsulfone‐cured diglycidyl bisphenol A as a representative example, its TTT diagram is completed by including the thermal degradation. The theoretical diagram as obtained from the kinetics of curing and thermal degradation is compared with experimental data. The agreement is good, slight deviations are observed only in the time to vitrify above 150°C and the maximum available glass temperature, which is due to side reactions and onset of thermal degradation during curing. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3894–3896, 2003

Effects of time, temperature, and steel composition on growth and dissolution of inclusions in liquid steels

The current knowledge of stability and growth/dissolution kinetics of inclusions in liquid steels is synthesised to understand their growth and dissolution behaviour. In particular, it is shown that the effects of time and temperature on the diffusion controlled growth and dissolution behaviour of inclusions can be represented by a set of time–temperature–transformation (TTT) diagrams. The TTT diagrams for the growth of oxide, nitride, and sulphide inclusions show the characteristic C shape, with each plot having an optimal temperature where the highest growth rate is achieved. The inclusion dissolution rate depends strongly on the initial inclusion radius and temperature. Apart from the stability of various inclusions in a given composition of steel, the diagrams provide important kinetic information for diffusion controlled growth and dissolution of oxide, nitride, and sulphide inclusions.