The Time-temperature-transformation Research Articles

Study on quenching sensitivity of 7097 aluminum alloy

The time-temperature-property (TTP) and the time-temperature-transformation (TTT) curves of 7097 aluminum alloy are determined by interrupted quenching experiments in order to investigate the quench sensitivity. The quenching sensitive zone is 230 °C–370 °C, and the nose temperature and transformation time are about 320 °C and 1.2 s, respectively. The evolution of quench-induced precipitates during different isothermal temperatures treatment has been observed by the transmission electron microscope. The number and size of quench-induced precipitates are gradually increased with the extension of isothermal holding time. Moreover, the quenching sensitivity of 7097 aluminum alloy is lower than that of 7055 aluminum alloy and 7050 aluminum alloy, but higher than that of 7085 aluminum alloy, and the influence of alloy composition on the quenching sensitivity is investigated.

Heat treatment behavior of alternator shafts steels

The present paper analyzes properties such as the hardness, the tensile and toughness behaviour and the microstructure for multiple bars of C40 and C45 steel used for shafts in alternators. The materials have been manufactured by rolling processes, followed by normalization treatments or forging processes followed by normalization treatments. Heat treatments are performed on steels to produce different microstructural phases that affect the mechanical properties. Microstructural changes during austenite decomposition depend on the transformation temperature region and cooling rate. Over the years, many experimental studies have been performed to reveal the microstructural changes during isothermal holding or continuous cooling of austenite for various kinds of steels. The isothermal transformation (IT) diagram, which is also called the time-temperature-transformation (TTT) diagram, and the continuous cooling transformation (CCT) diagram have been developed to graphically characterize the decomposition transformation as a function of time and temperature. However, when these diagrams are based on experimental measurements, several limitations are present. Each diagram is limited because the temperatures at which specific transformations occur can vary due to several factors, such as: the chemical composition of steel, the solid solution condition in the austenite prior to cooling and transformation, the presence of precipitates in austenite, the prior austenite grain size, and the applied stresses during the transformation.

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

Abstract 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.

Effect of pre-existing nuclei on crystallization during laser welding of Zr-based metallic glass

Abstract In this study, experiments are carried out to demonstrate that in laser welding of a Zr-based (Zr52.5Ti5Al10Ni14Cu17.9) bulk metallic glass (BMG), pre-existing nuclei nucleus density has significant influence on its crystallization behavior. Based on the classical nucleation/growth theory, it is concluded that a small amount of pre-existing nuclei in a BMG can shift the time-temperature-transformation (TTT) curve from a well-known ‘C-shape’ to a ‘e-shape.’ This result provides fundamental understanding on why the shape of the TTT curve for a heating process is different from that for a cooling process for the same BMG. Two quality factors were defined as a measure of the effect of pre-existing nucleus density. By integrating the classical nucleation/growth theory with the heat transfer model, the evolution of crystalline phase during laser welding for a BMG with pre-existing nuclei was studied, and the modeling predictions compared favorably with the experimental results.

Oxygen diffusion in amorphous and partially crystalline gallium oxide.

Oxygen transport in amorphous (a-GaO1.5) and partially crystalline (a/c-GaO1.5) gallium oxide was studied by means of 18O/16O isotope exchange experiments and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Thin films of a-GaO1.5 were deposited by pulsed laser deposition (PLD) on alumina substrates at room temperature in an oxygen atmosphere. Oxygen tracer diffusion coefficients D* and oxygen surface exchange coefficients k* were determined as a function of temperature, 300 ≤ T/°C ≤ 370, and as a function of oxygen partial pressure, 2 ≤ p(O2)/mbar ≤ 500 at a temperature of T = 330 °C. The activation energy of oxygen tracer diffusion in amorphous gallium oxide was found to be EA = 0.8 eV. In addition, the time-temperature-transformation (TTT) diagram of crystallisation of amorphous gallium oxide was determined.

Effect of cooling rate and isothermal temperature on the phase transformation and microstructure evolution in SWRH82B high-carbon steel

To study the effect of isothermal temperature and cooling rate on the phase transformation and microstructure evolution in SWRH82B steels, a thermal dilatometer was used in this study to generate the static continuous cooling transformation (CCT) and isothermal-transformation thermal-expansion diagrams. In addition, scanning electron microscopy was used to analyze the effect of process parameters on microstructural transformation in the steel specimens. The results suggest that with a decrease in the isothermal temperature, the incubation time and duration of phase transformation gradually decreased; further, the content of non-lamellar pearlite in the microstructure gradually increased at room temperature. Compared with the time-temperature-transformation (TTT) diagram of SWRH82B steel subjected to conventional process conditions (rapid cooling before isothermal treatment), the incubation periods observed in this study were significantly shorter. When the loop-laying temperature was 850 °C, austempering temperature was 630 °C, and cooling rate was 5 °C s−1, the incubation time and phase-transformation duration of the experimental steel were 9.60 and 28.52 s, respectively. With an increase in the cooling rate, the incubation period gradually increased, while the phase-transformation duration initially increased and then decreased. The room-temperature microstructure, obtained by austempering at 630 °C for 300 s, was mainly composed of well-distributed lamellar pearlite. With an increase in the cooling rate, the pearlite interlaminar spacing gradually reduced. However, the room-temperature microstructure, obtained by austempering at 550 °C for 300 s contained a large proportion of non-lamellar structures and the distribution of pearlite became disorganized.

Crystallization kinetics of Cu47Zr47Al6 and (Cu47Zr47Al6)99Sn1 bulk metallic glasses

Abstract Crystallization kinetics of Cu47Zr47Al6 and (Cu47Zr47Al6)99Sn1 bulk metallic glasses in the non-isothermal and isothermal conditions were investigated by differential scanning calorimetry (DSC). In non-isothermal condition, the crystallization activation energy was determined by various methods, and the Sn-containing glassy alloy showed higher crystallization activation energy than that of the base alloy, that means the Sn addition delayed crystallization transformation. In the isothermal condition, the Sn-containing BMG shows longer incubation time and wider crystallization transformation peak than that of the Sn free sample alloy, at the same annealing temperature. The time-temperature-transformation (TTT) diagram signified that the Sn addition delayed the crystallization and consequently improved the GFA. In isothermal conditions, the Avrami exponents vary from 3.75 to 5.31 and 3.88 to 4.08 for Cu47Zr47Al6 and (Cu47Zr47Al6)99Sn1 BMGs, respectively. According to Avrami's model, these numbers indicate that the crystallization of both glassy alloys is governed by three-dimensional growth with an increasing nucleation rate, while Sn addition decreases the nucleation rate.

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

Abstract 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 ( T g ) 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.

Phase prediction in high entropy alloys – A kinetic approach

Abstract A simple and completely predictive model has been developed to predict whether a multicomponent equiatomic alloy will form a single phase BCC, FCC, HCP or a combination of two or more solid solution phases or intermetallic compounds (IM) or an amorphous phase. This approach is based on the viscosity of alloys as a function of temperature, utilising the viscosities of its constituting elements, and suitably incorporating the crystal structure information. Some other parameters affecting viscosity of an alloy like atomic size of constituting elements, packing density of the unit cell, etc., are suitably incorporated into the model. The temperature-time-transformation (TTT) diagrams were generated with the help of the viscosity data of five widely experimentally examined alloys, CoCrCuFeNi, CoCrFeMnNi, AlCoCrFeNi, AlCuMgMnZn and ZrTiCuNiBe. The chance of formation of preferable lower order alloys has also been considered. In this regard, all the possible binary to quinary alloys that can form from the constituting elements have been studied. The formation of the single phase BCC, FCC, or formation of multi phases, IMs or an amorphous phase in these alloys has been excellently predicted by the model. It has also been revealed that AlCuMgMnZn alloy prefers to form a number of IMs with a rare HCP phase, which matches excellently with the experimental evidence. The most important part of the present work is that it acts as an efficient guide about the processing route that should be used to form an intended phase in a particular alloy via the critical cooling rate Rc obtained through the predicted TTT diagrams. Further, two alloys (AlCoCrFeNi and CoCrFeMnNi) could not be vitrified even via melt spinning route as predicted by the model. Almost all the equiatomic alloys found so far ranging from ternary to octanary according to literature have been studied by the present model. The phase formation in most of the alloys has been predicted correctly by the model.

Kinetics of oxidation and decomposition of soybean biodiesel evaluated by the TTT superposition theory and the Freeman-Carroll method

Abstract In the present work, the oxidation process of a commercial soybean biodiesel was investigated under oxidative atmosphere in different conditions of temperature (Tox) and time (tox). Fourier Transform Infrared Spectroscopy (FTIR) and thermogravimetry analysis (TGA) were used to characterize the oxidation and decomposition processes of biodiesel, respectively. For both experimental techniques we have applied the Time-Temperature-Transformation (TTT) superposition theory in order to explain the oxidation and decomposition phenomena. The FTIR spectra showed that when the oxidation times and temperature was increased the intensity of some characteristic bands suffered a decreasing, proving the oxidation occurrence on soybean biodiesel. The thermal stability studies (TG/DTG curves) have been appointed out that the process for Tox 80 °C displayed multiple-stage thermal decomposition processes which are ascribed to the evaporation and/or pyrolysis of the methyl esters and their sub products. The kinetic studies realized by the TTT superposition theory were comparable with that obtained by the Freeman-Carroll, Flynn-Wall-Osawa and Vyazovkin methods. The evaluation of kinetic compensation effect (KCE) has been found usefully to predict the isokinetic parameters as lnkiso, which was shown to have a unique value independently of the oxidation conditions.

A new approach for determining the critical cooling rates of nucleation in glass‐forming liquids

AbstractCrystallization of a liquid below liquidus temperature is a complex process due to simultaneous nucleation and growth of crystals. Nucleation is the crucial initial step of the crystallization process, and affects the glass‐forming ability, especially when there is a large overlap between nucleation and crystal growth versus temperature curves. From the temperature‐time‐transformation (TTT) diagram, one can estimate the critical cooling rate, , of glass‐formation, however this is time‐consuming. In this paper, we establish a simple approach to determine the using calorimetric and viscometric data. Based on the classical nucleation theory, the correlation between the crystallization onset temperature and cooling rate is described by combining two temperature‐dependent functions. The new approach is applicable to a wide range of glass‐forming systems. This work also gives insight into heterogeneous nucleation and glass formation kinetics.

Prediction of the glass forming ability in a Fe-25%B binary amorphous alloy based on phase-field method

Abstract The critical cooling rate ( R c ) calculated on the basis of the time-temperature-transformation (TTT) curve was taken as the intrinsic criteria of reflecting the glass forming ability (GFA) for the amorphous alloy. The TTT curves were plotted based on the isothermal Wheeler-Boettinger-McFadden (WBM) phase field method (PFM). By analyzing the TTT curves, the influence of different activation energy parameters E and Q for the interface mobility M and nucleation rate P respectively on R c and GFA were investigated for Fe-25%B (mole fraction) binary amorphous alloy in present paper. The results showed that the activation energy E for the interface mobility M had litter influence on the nose temperature and time for the TTT curves. Under the different thermal activation energy Q 1 , Q 2 and Q 3 for the nucleation rate P , the temperatures and times corresponding to the nose tip were 500 K, 600 K, 775 K and 6.2 × 10 − 4 s, 4.5 × 10 − 4 s, 9.5 × 10 − 4 s, respectively, and the calculated values of R c were 1.57 × 10 6 K/s, 1.93 × 10 6 K/s, 7.35 × 10 5 K/s, respectively, which were in the range of 6.4 × 10 5 ~ 6.8 × 10 6 K/s from the experimental measurements. Under the influence of thermal activation energy Q , the nose tip temperature increased, which meant Q controlled the nose tip temperature, while the higher Q would not get the perfect C shape. It was found that the values of R c calculated based on the TTT curves by the PFM were in good agreement with the reported experimental values, which verified the feasibility of the glass prediction and the research of the intrinsic physical mechanism of the GFA by the PFM.

Phase Equilibrium and Austenite Decomposition in Advanced High-Strength Medium-Mn Bainitic Steels

The work addresses the phase equilibrium analysis and austenite decomposition of two Nb-microalloyed medium-Mn steels containing 3% and 5% Mn. The pseudobinary Fe-C diagrams of the steels were calculated using Thermo-Calc. Thermodynamic calculations of the volume fraction evolution of microstructural constituents vs. temperature were carried out. The study comprised the determination of the time-temperature-transformation (TTT) diagrams and continuous cooling transformation (CCT) diagrams of the investigated steels. The diagrams were used to determine continuous and isothermal cooling paths suitable for production of bainite-based steels. It was found that the various Mn content strongly influences the hardenability of the steels and hence the austenite decomposition during cooling. The knowledge of CCT diagrams and the analysis of experimental dilatometric curves enabled to produce bainite-austenite mixtures in the thermomechanical simulator. Light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to assess the effect of heat treatment on morphological details of produced multiphase microstructures.

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

Abstract 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.

Crystallization characteristics of a coal slag and influence of crystals on the sharp increase of viscosity

Abstract In entrained flow gasifiers, crystallization inside the liquid slag can cause an increase of the slag viscosity, affecting flow along the wall and resulting in operational challenges. In the study reported here, a real Chinese ash was investigated for its crystallization behavior as well as the viscosity. Differential Scanning Calorimetry (DSC) and the Single Hot Thermocouple Technique (SHTT) were combined to measure the crystallization temperature and observe the crystallization process, constructing the temperature-time transformation (TTT) diagram. The ash was heated and then quenched to determine the crystalline phases. Unlike the synthetic slag mixed with oxides, the compositions of real ash had some variations after the long time heat treatment, which may attribute to the reducing minerals in the ash. Under inert atmosphere, gehlenite and spinel were the main crystals in cooling process. While under reduced atmosphere, the amount of crystals decreased especially gehlenite. Spinel was widely distributed inside the slag with irregular shape forming a network which caused the sharp increase of viscosity. The temperature of critical viscosity ( T cv ) was around 1310 °C corresponding to the spinel proportion calculated about 20%.

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

Abstract 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.

Intrinsic solidification behaviour of basaltic to rhyolitic melts: A cooling rate experimental study

Abstract Dynamic cooling-induced solidification experiments were run using six silicate glasses along the basalt–rhyolite join (B100 = 100 wt.% of basalt, R100 = 100 wt.% of rhyolite), i.e. B100, B80R20, B60R40, B40R60, B20R80 and R100; the glasses directly quenched from 1300 °C after a dwell of 120 min (experiment E0) contain 50–400 ppm H2O, ≪ 1 area% μm-sized bubble, and Fe2 +/Fetot between 0.34 and 0.46. Experiments were performed in Pt capsules at room pressure and fO2 of air, between 1300 and 800 °C using three different cooling rates of 0.0167, 3 and 30 °C/min; these cooling rates were run two times: E1–E2 experiments at 0.0167 °C/min, S1–E3 at 3 °C/min, and E4–E5 at 30 °C/min. In experiments E1 to E5, samples were annealed for 120 min at 1300 °C, whereas in the experiment S1 the samples were firstly heated for 30 min at 1400 °C followed by a dwell time of 2400 min at 1300 °C before cooling. In none of the run-products preferential crystallization at the melt/gas interface was observed. B100, B80R20 and B60R40 run-products have a low tendency to preferentially crystallize on Pt walls, while B40R60, B20R80 and R100 are not affected by the presence of Pt substrata. All run-products show homogeneous textures, except for B60R40 and B40R60 at 0.0167 °C/min in the E1 experiment. The duplicates of B40R60 and B60R40 at 0.0167 °C/min and B100 at 30 °C/min show differences in crystal content (> 4 and

Quench sensitivity of 6351 aluminum alloy

Abstract The quench sensitivity of 6351 alloy was determined by the time–temperature-transformation (TTT) curves and time– temperature-property (TTP) curves by an interrupted quench technique with measurement of as-aged hardness and as-quenched electro-conductivity. The microstructure transformation during isothermal treatment was studied by the transmission electron microscopy (TEM) and Avrami equation. The results showed that the electro-conductivity of the 6351 alloy increased and the hardness decreased with prolonging the holding time at a certain isothermal temperature. The TEM observation indicated that the supersaturated solid solution decomposed and needles β″ precipitated at the initial stage of isothermal holding. With the prolongation of holding time at the nose temperature, rod β′ and plate β phases formed. The isothermal transformation rate at 360 °C was the fastest, and became slow at 280 °C and reached the slowest at 440 °C. The nose temperatures of the TTT and TTP curves were about 360 °C and the high quench sensitive temperature range was 230–430 °C. The quench factor analysis indicated that the cooling rate should be more than 15 °C/s in the quench sensitive areas in order to get optimal mechanical properties.

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.