The Time-temperature-transformation Diagram Research Articles

Material extrusion thermal model mapped across polyetheretherketone isothermal and continuous cooling transformation charts

This study provides an insight into the polyetheretherketone (PEEK) crystallinity progression throughout the material extrusion (MEX) additive manufacturing process as a function of time and temperature, comparing it with the isothermal and continuous cooling transformation charts created over a wide range of isothermal crystallisation temperatures and constant cooling rates. The isothermal and non-isothermal crystallisation kinetics were explored using Differential Scanning Calorimetry (DSC) and Fast Scanning Calorimetry (FSC). The half-time, onset and ending of crystallisation were obtained for isothermal crystallisation temperatures between 150 °C and 330 °C, while the crystallisation under constant cooling was obtained using rates between − 0.5 K s −1 and − 45 K s −1 . The results were used to draw the Continuous Cooling Transformation (CCT) and the Time-Temperature Transformation (TTT) diagrams and calculate the Avrami numbers using the parallel Avrami model. These results were then compared to the degree of crystallinity as a function of time and temperature for the MEX process. To evaluate the crystallisation within the MEX process a 1D transient transfer heat model was used to obtain the printing thermal profile, which was replicated using the FSC technique. The results showed that for the MEX printing process, the crystallisation usually is a product of a combination of rapid cooling and heating processes followed by periods of greater thermal stability which, depending on the nature of the process, can approach a quasi-isothermal crystallisation process. By superimposing the process thermal profile on the TTT and CCT diagrams and comparing the crystallinity values measured from each point in the thermal profile, it was possible to understand the crystallinity evolution and the remelting of the build surface promoted by the subsequent printed layers. • Temperature fluctuations in the MEX process are a combination of ultra-fast dynamic cooling steps with heating and remelting. • The FSC and standard DSC allowed the replication of the modelled thermal profile. • The crystallinity varies as a function of local temperature and the fluctuations caused by the multi-layer printing process. • Higher chamber temperature increased the crystalline phase remelting, resulting in greater mobility of polymeric chains. • Shorter return times improved overall crystallinity levels due to more frequently heat input in the building surface.

Isothermal crystallization kinetics of an industrial-grade Zr-based bulk metallic glass

Bulk metallic glasses (BMGs), due to their amorphous structure, exhibit remarkable mechanical properties, and there is an increasing interest in their commercialization. For the industrial fabrication of BMG, knowledge about the isothermal crystallization kinetics of industrial-grade BMG is required. Previous investigations on isothermal crystallization kinetics are mainly based on high-purity samples with very good glass forming ability and/or mainly limited to the low temperature regime. In the present study, a systematic investigation on the isothermal crystallization kinetics of an industrial-grade Zr-based BMG (Zr59.3Cu28.8Al10.4Nb1.5 at.%, trade name: AMZ4) has been performed using conventional and flash differential scanning calorimetry.We report the time-temperature-transformation (TTT) diagrams of the AMZ4 with two different oxygen levels. The diagrams cover the temperature range from glass transition temperature up to liquidus temperature, that have the typical “C-shaped” noses. Faster crystallization of the higher oxygen level AMZ4 was observed, and the underlying mechanisms were investigated. The universal isothermal Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was employed to model the isothermal crystallization kinetics. Satisfactory match was achieved between the experimental facts and the JMAK model, and the interfacial energies between the crystalline phase and liquid were determined as ∼0.04 J/m2 for the industrial-grade AMZ4. The crystallization fraction dependence of Avrami index and activation energy is studied and found to be neglectable in the JMAK modeling. The critical casting thicknesses were estimated based on the TTT diagrams.

Phase transitions in Fe-(23−24)Ga alloys: Experimental results and modeling

A significant difference in properties and structure of Fe-Ga alloys in their equilibrium and metastable states limits their application. This necessitates an immediate understanding of the importance of cooling rate in the formation of alloy metastable and equilibrium structures. In this paper, we experimentally studied the structure of the Fe-(23−24) Ga alloys after both (i) cooling with different cooling rates and (ii) isothermal annealing between 300 and 575 °C up to 1800 h to report the alloy structure and present the Time-Temperature-Transformation (TTT) diagrams for the metastable to equilibrium transition in these alloys. The critical cooling rates were defined with respect to the formation of the equilibrium state. A theoretical model along with corresponding Monte Carlo simulations for modeling phase transitions and construction of TTT diagram for the same alloys were developed: simulated phase diagrams were analyzed with respect to our the experimental findings.

Isothermal Crystallization Monitoring and Time-Temperature-Transformation of Amorphous GDC-0276: Differential Scanning Calorimetric and Rheological Measurements.

Cold crystallization of amorphous pharmaceuticals is an important aspect in the search to stabilize amorphous or glassy compounds used as amorphous pharmaceutical ingredients (APIs). In the present work, we report results for the isothermal crystallization of the compound GDC-0276 based on differential scanning calorimetric and rheometric measurements. The kinetics of isothermal crystallization from the induction time to the completion of crystallization can be described by the classic Johnson-Mehl-Avrami (JMA) equation. The time-temperature-transformation (TTT) diagrams were constructed for two time points-that of induction and that of completion of crystallization. The results show that the rheological measurement for GDC-0276 has a better overall sensitivity in detection of the early stage nucleation and, consequently, detects the onset of crystallization sooner than does the differential scanning calorimetry. Rheological measurements were also used to obtain the temperature dependence of the viscosity of GDC-0276 and the relevant parameters were used in a modified form of the JMA model to describe the temperature dependence of the crystal induction and completion times, that is, the TTT diagram for the material. In the modification, we assumed that the kinetics followed the viscosity to the 0.75 power as suggested by the recent work of Huang et al. (Huang, C., et al., J. Chem. Phys.2018,149, 054503). The relationship and the possible impact on crystallization kinetics of the break-down of the Stokes-Einstein relation in glass-forming liquids are discussed. From the crystallization kinetics modeling, the solid-liquid interfacial surface tension σSL was obtained for GDC-0276 and was compared with that obtained from the melting point depression measurements of the material confined in nanoporous glasses. The differences between the values from the two methods are discussed.

Time-Temperature-Transformation (TTT) diagram of a carbon fiber epoxy prepreg

The Time-Temperature-Transformation (TTT) diagram of a commercially available carbon fiber epoxy prepreg system has been developed by determining the gelation and vitrification behavior using small amplitude oscillatory shear and differential scanning calorimetry (DSC), respectively. Rheological measurements were made on the neat matrix system through separation of the fiber from the matrix using acetone washing of the un-cured carbon fiber epoxy prepreg. Dynamic DSC experiments were performed on the as-received carbon fiber epoxy prepreg system, and their cure kinetics modeled using two phenomenological models. Development of the TTT diagram for the carbon fiber epoxy prepreg will aid in the development of the optimal molding conditions required to improve manufacturing cycle time.

Time-temperature-transformation (TTT) diagram of a dual-curable off-stoichiometric epoxy-amine system with latent reactivity

The time-temperature-transformation (TTT) diagram of the curing an off-stoichiometric amine-epoxy formulation with intermediate latent reactivity has been elaborated. The first curing stage is an epoxy-amine polycondensation taking place at low temperatures, while the second curing stage is an anionic homopolymerization of the excess epoxy groups, taking place at high temperatures and catalyzed by a latent base. The curing kinetics of the first and second curing stages have been analyzed separately by integral isoconversional procedures, and the kinetics data has been used to plot the isoconversion lines in the TTT diagram. The relationship between the glass transition temperature (Tg) and conversion has been determined experimentally and theoretically and used to determine the vitrification line in the TTT diagram. The results reflect the clearly different kinetic behaviour of both curing stages and a significant intermediate storage stability, which makes these curing systems highly attractive for multiple stage processing. The shape of the vitrification line also reflects the occurrence of different curing processes with different structure build-up. Safe processing and storage conditions can be easily defined on the basis of the TTT diagram. This is the first time the curing behaviour of dual-curing formulations is illustrated in such a clear and concise way thanks to the TTT diagram.

Characterization of Oxide Scales Formed on Plain Carbon Steels in Dry and Wet Atmospheres and Their Eutectoid Transformation from FeO in Inert Atmosphere

Thermal gravimetric analysis (TGA) was used to investigate plain carbon steel exposed to dry air and wet air containing 4 vol% water vapor at 750 °C to simulate the formation of tertiary scale during hot-strip rolling. After the exposure to oxidation condition, the specimens were isothermally held at temperature in the range of 300–500 °C in inert argon (Ar) atmosphere for 10–240 min, and the isothermal phase transformation behavior of FeO was investigated. The morphology, element distribution, and phase composition of oxide scales after isothermal transformation were characterized by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction, respectively. The experimental results indicate that wet air accelerated the growth rate of the oxide scale. Moreover, the time–temperature-transformation (TTT) diagrams of oxide scales grown in dry and wet air conditions were constructed to elucidate the isothermal phase transformation behavior of the FeO layer. The rate of isothermal phase transformation in the scale formed in wet air was significantly delayed compared to that in dry air, which caused the C-type TTT diagram to shift to the right. Two mechanisms elucidating the effects of initial oxidation atmosphere on the subsequent isothermal phase transformation behavior of FeO during inert atmosphere annealing were proposed.

Investigation of quench sensitivity of high strength 2219 aluminum alloy by TTP and TTT diagrams

The time-temperature-transformation (TTT) curve and time-temperature-property (TTP) curve of 2219 aluminum alloy were determined by an interrupted quench technique. The results show that the quench sensitivity temperature range of 2219 aluminum alloy is from 300 °C to 480 °C and the nose temperature is about 440 °C. The transmission electron microscopy (TEM) observation indicated that, during isothermal holding at 440 °C, a number of coarse equilibrium rod-shaped θ particles precipitated and grew fast, resulting in loss of solutes and decrease of subsequent ageing hardening effect. In the quench sensitive range, primary precipitating reasons of rod-shaped θ particles are high diffusion rate of solute atoms and large phase transformation driving force. Therefore, for large 2219 aluminum alloy forgings, the quenching cooling rate should be increased as high as possible in the quenching sensitive range (300–480 °C) to obtain high mechanical properties, and decreased properly from the solution temperature to 480°Cin order to reduce the residual stress.

Effect of slag composition on the crystallization of synthetic CaO–SiO2–Al2O3–MgO slags: Part I—Crystallization behavior

This study investigates the effect of composition on the crystallization behavior of synthetic blast furnace slags. The time–temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams for various slags are obtained using a modified single hot-thermocouple technique (SHTT). The results show that by an increase in basicity, the crystallization tendency increases, shifting the TTT and CCT diagrams to shorter times. This would indicate higher cooling rate requirements for more basic slags in order to prevent their crystallization. Both crystallization temperature and slag basicity affected the size and growth of the crystals. For slags with NBO/T≥1.6, crystallization kinetics is faster, the crystalline phases are opaque, and the size of crystals is highly dependent on the temperature. On the other hand, for slags of low basicity, the crystalline phases are translucent and nucleation is limited, leading to growth of few large crystals.

TTT diagram of martensitic transformation under magnetic field in a Ni45Co5Mn36.5In13.5 (at.%) alloy

Abstract We have investigated the time–temperature-transformation ( TTT ) diagram of a magnetic shape memory alloy Ni 45 Co 5 Mn 36.5 In 13.5 under magnetic field. We found that the TTT diagram under 2 T magnetic field shows a clear C -curve with its nose located approximately at 150 K. The incubation time increases with increasing magnetic field. Such a magnetic field dependence of the TTT diagram can be interpreted by considering magnetic field dependence of potential barrier for the martensitic transformation.

A Kinetic Study of the Effect of Basicity on the Mold Fluxes Crystallization

The effect of basicity on the mold fluxes crystallization was investigated in this article. The time-temperature-transformation (TTT) diagrams and continuous-cooling-transformation (CCT) diagrams of mold fluxes with different basicity were constructed by using single, hot thermocouple technology (SHTT). The results showed that with the increase of basicity, the incubation time of isothermal crystallization became shorter, the crystallization temperature was getting higher, and the critical cooling rate of continuous cooling crystallization became faster. The X-ray diffraction analysis suggested that calcium silicate (CaO·SiO2) was precipitated at the upper part of the TTT diagram and cuspidine (Ca4Si2O7F2) was formed at the lower part, when the basicity of mold fluxes was within 1.0 to 1.2. However, when basicity was 0.8, only the cuspidine phase was formed. A kinetic study of isothermal crystallization process indicated that the increase of the basicity tended to enhance the mold flux crystallization, and the crystallization activation energy became smaller. The crystallization mechanism of cupsidine was changing from one-dimensional growth to three-dimensional growth with a constant number of nuclei, when the basicity of mold fluxes varied from 0.8 to 1.2.

Time-temperature-transformation measurements of FePt thin films in the millisecond regime using pulse laser processing

A section of the time-temperature-transformation (TTT) curve has been measured in the millisecond regime to describe the A1 to L10 transformation of 10 nm FePt thin films. Short time annealing was accomplished using a pulsed neodymium-doped yttrium aluminum garnet laser operating at a wavelength of 1064 nm. The temperature-time profile of the films was measured using an optical pyrometer and a platinum thin film resistor and it was numerically modeled. Effective thermal pulse widths were determined from the time dependence of the atomic diffusion coefficient calculated from the measured temperature profile. The measured TTT diagram involving average order parameter is consistent with theoretical predictions of TTT diagrams involving ordered volume fraction and shows that partial ordering can be obtained with a single effective thermal pulse as short as 1.1 ms.

Simulating Time Temperature Transformation Diagram of Steel Using Artificial Neural Network

Design and development of steel is essentially governed by the Time-Temperature-Transformation (TTT) diagram. The diagram predicts the phase evolution during isothermal transformation schedules for a given chemistry. Selection of chemistry for obtaining a desired microstructure in steel under isothermal schedule needs determination of the TTT diagrams either by extensive experimental exercise or by rigorous thermodynamic calculations. Artificial neural network (ANN) technique has recently been employed as a versatile tool to predict the CCT diagrams of steels. The present work aims to identify a favorable composition capable of yielding an ultrafine bainitic microstructure by isothermal holding of austenite at low homologous temperature. To achieve this, TTT diagrams of varying compositions have been predicted a priori to reduce the required experimental trials. The exercise has led to the development of bainitic microstructure of nanoscale dimension in steel having 0.7C-2.0Mn-1.5Si-0.3Mo-1.5Cr (wt%). Experimental trial with the predicted composition of bainitic steel resulted into attractive combination of strength and ductility.

Effect of lattice-mismatch-induced strains on coupled diffusive and displacive phase transformations

Materials which can undergo slow diffusive transformations as well as fast displacive transformations are studied using the phase-field method. The model captures the essential features of the time-temperature-transformation (TTT) diagrams, continuous cooling transformation (CCT) diagrams, and microstructure formation of these alloys. In some materials systems there can exist an intrinsic volume change associated with these transformations. We show that these coherency strains can stabilize mixed microstructures (such as retained austenite-martensite and pearlite-martensite mixtures) by an interplay between diffusive and displacive mechanisms, which can alter TTT and CCT diagrams. Depending on the conditions there can be competitive or cooperative nucleation of the two kinds of phases. The model also shows that small differences in volume changes can have noticeable effects on the early stages of martensite formation and on the resulting microstructures. -- Long version of cond-mat/0605577 -- Keywords: Ginzburg-Landau, martensite, pearlite, spinodal decomposition, shape memory, microstructures, TTT diagram, CCT diagram, elastic compatibility

Resistivity Study of Eutectoid Decomposition Kinetics of α-Fe<SUB>2</SUB>Si<SUB>5</SUB> Alloy

The kinetics of the eutectoid decomposition a → β + Si in a sintered α-Fe 2 Si 5 alloy under isothermal conditions has been studied by using electrical resistivity technique. X-ray diffraction was applied to determine therelation of resistivity and transformed volume fraction by quantitative analysis. The time-temperature-transformation (TTT) diagram for the reaction was obtained in the temperature range of 873-1148 K. The TTT diagram shows a typical C shape and gets nose at 1073 K. A general expression of Johnson-Mehl-Avrami (JMA) equation was proposed by introducing a constant, which is associated with spatial distribution of nucleation. The mechanism of the transformation was discussed in the theoretical frame of the modified JMA theory. The Avrami exponent was found to change with temperature, n = 3.8 above 1073 K and n = 3.0 in the region of 873-1073 K. The results indicated that an interface controlled three-dimensional growth is responsible for the β formation in the eutectoid decomposition under the conditions of decreasing nucleation rate above 1073 K but zero nucleation rate (site saturation of nucleation) in the region of 873-1073 K. The activation energy associated with the eutectoid decomposition, obtained in the lower temperature range, was 132.8 kJ/mol.

Time-Temperature-Transformation Diagram within the Bainitic Temperature Range in a Medium Carbon Steel

The time-temperature-transformation (TTT) diagram within the medium temperature range of medium carbon steel has been determined. A single type of C-curve is found within the bainite temperature range for the studied steel. Distinct reaction C-curves were not observed for both types of microstructure, upper bainite and lower bainite in the TTT diagram. Experimental results on the kinetics of the isothermal formation of bainite at different temperature have demonstrated that both type of microstructure, upper and lower bainite, possesses similar overall transformation kinetics. Some applications of phase transformation theory towards the formation of bainitic microstructures are discussed, with particular emphasis on the bainite start temperature, transition temperature from upper to lower bainite, martensite start temperature and the thickness of bainitic plates. Bainite can be regarded as a non-lamellar mixture of ferrite and carbides, but within this broad description, it is possible to identify two classical morphologies, traditionally named upper and lower bainite. Both upper and lower bainite consist of plates of ferrite, known as sub-units, separated by cementite. The plates of ferrite grow in aggregates called sheaves of bainite. Within each sheaf, the plates of ferrite are parallel and share a common crystallographic orientation. The essential difference between upper and lower bainite is with respect to the carbide precipitation. In upper bainite, the bainitic ferrite is free of precipitation; carbides grow from the regions of carbon enriched austenite, which are trapped between the sub-units of ferrite. By contrast lower bainitic ferrite contains a fine dispersion of plate-like carbides within the bainitic ferrite plates. Both upper and lower bainite are formed by the propaga- tion of displacive ferrite sub-units followed by the partition of the excess carbon into the residual austenite. Cementite can then precipitate from the enriched austenite between the ferrite plates. The growth of the bainitic sub-units is accompanied by a change in the shape of the transformed region, which can be described as an invariant-plane strain with a large shear component. The parent austenite cannot accommodate the large shape deformation elastically and relaxes by plastic deformation in the region adjacent to the bainite. This plastic deformation stifles the growth of bainite sub-units before they hit any obstacle, and the transformation proceeds with the formation of a new sub-unit. 1) The mechanism of transformation continues until the carbon concentration of the residual austenite reach the To curve, at which ferrite and austenite with identical chemical compo-

Time-temperature-transformation analysis of an alkyd-amino resin system

The curing behaviour of a thermosetting acrylic-amino-formaldehyde resin matrix has been investigated by rheological, thermogravimetric, and thermomechanical analysis. The time-temperature-transformation (TTT) diagram of the solventless resin matrix has been constructed. The gelation curve could be fitted with a power-function of Tgel=118.14·tgel-01238. With the help of this function the conditions for gelation can be calculated, and storability of the matrix can be predicted. The onset temperature of thermooxydative decomposition is 220°C, and the maximum loss of mass belonging to this temperature was found to be 16.3%, corresponding to the ultimate conversion of curing. From iso-Tcure diagrams determined by isothermal thermogravimetric analysis, the iso-curing time (iso-tcure) and iso-mass loss curves of the TTT diagram have been constructed. The iso-Tcure diagrams determined by TMA measurements, served the construction of the iso-Tcure and iso-glass transition temperature (iso-Tg) diagrams. The Tg of the fully cured system (Tg∞) was found to be 30°C. The iso-Tg line of 30°C represents the ultimate conversion of the cured system, up to which no degradation takes place. This curve runs below the ultimate conversion determined by thermogravimetric measurements, meaning that above 160°C decomposition takes place simultaneously with curing reactions as supported by the increasing tangent of the straight lines fitted to the last section of thermogravimetric iso-Tcure diagrams. The Tg of the matrix before the cure (Tg0) is less than 0°C. This means that the system is in its rubbery physical state during the curing process.

Vitrification and determination of the crystallization time scales of the bulk-metallic-glass-forming liquid Zr58.5Nb2.8Cu15.6Ni12.8Al10.3

The crystallization kinetics of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 were studied in an electrostatic levitation (ESL) apparatus. The measured critical cooling rate is 1.75 K/s. Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the first bulk-metallic-glass-forming liquid that does not contain beryllium to be vitrified by purely radiative cooling in the ESL. Furthermore, the sluggish crystallization kinetics enable the determination of the time-temperature-transformation (TTT) diagram between the liquidus and the glass transition temperatures. The shortest time to reach crystallization in an isothermal experiment; i.e., the nose of the TTT diagram is 32 s. The nose of the TTT diagram is at 900 K and positioned about 200 K below the liquidus temperature.

Crystallization and Glass Forming Ability of Supercooled Pd-Cu-Ni-P Melt

The thermal stability, GFA and crystallization behavior of a highly purified Pd 40 Cu 30 Ni 10 P 20 supercooled liquid was examined. Under continuous cooling, the critical cooling rate for glass formation for the highly purified alloy is the same as that (0.100K/s) for the fluxed ordinary alloy, though the magnitude of supercooling is enhanced by about 80K upon by the purification treatment. The enhancement is presumed to result from the elimination or decrease of the quenched-in nuclei. The Time-Temperature-Transformation (TTT) diagram was constructed experimentally under isothermal annealing of the supercooled melt. The nose point in the TTT diagram is located at 683 K and 80 s. By utilizing the high thermal stability of the supercooled liquid, in-situ TEM observation was successfully carried out. In the isothermal annealing at 683 K, crystalline particle with a diameter of about 15 nm abruptly precipitated from the molten particle with a diameter of 40 nm and no significant grain growth was observed during further annealing. The incubation time was measured to be 3180s. This value is much longer than that of the sample obtained by the HV/HT-DSC measurement. The difference between the two incubation times indicates that the nucleation is the event dominated by statistical probability.

α‐Melt‐mediated crystallization of 1‐palmitoyl‐2‐oleoyl‐3‐stearoyl‐sn‐glycerol

AbstractThe α‐melt‐mediated crystallization of 1‐palmitoyl‐2‐oleoyl‐3‐stearoyl‐sn‐glycerol (POS) has been investigated by differential scanning calorimetry (DSC), combined with polarized‐light microscopy. Starting from a completely liquid state, the melt was first cooled down and maintained at a temperature, T1, during a time, t1, where the α‐phase formed. Then it was heated to a temperature, T2, above the melting point of α for isothermal solidification into a solid phase, which was identified as δ. Based upon DSC solidification peaks, the time‐temperature‐transformation (TTT) diagram of POS was constructed for these solidification conditions and was compared with the TTT diagram of direct crystallization from the melt. The α‐melt‐mediated solidification showed accelerated kinetics of the δ‐phase. The effects of T1 and t1 were also studied: at short t1, crystallization was faster with a decreasing value of T1, whereas the opposite trend was observed for a longer plateau at T1. These tendencies were interpreted in terms of three competing phenomena: the density of δ‐nuclei that can form during the plateau at T1, α‐δ solid‐state transformation, and memory effects of molecule arrangements in the α‐remelted phase.