Isothermal Temperature Range Research Articles

Phase transformation kinetics in a coarse-grain Ti17 alloy determined by laser ultrasonics and dilatometry

Different methods are used to determine the phase transformation titanium alloys. While ex-situ methods require a large number of samples and accurate quantification of the phases, in-situ methods can provide the kinetics of transformation continuously and with less samples. However, the results are based on the changes of physical properties, and the interpretation of the results in terms of phase transformation are not direct. This study compares two in-situ methods to measure the kinetics of phase transformation for a Ti17 alloy, namely: laser ultrasonics for metallurgy (LUMet) and dilatometry. While LUMet is coupled to a Gleeble® 3500 thermo-mechanical simulator with ohmic resistance heating, the dilatometer device uses induction heating. We performed two types of experiments by heating the specimens above the β-transus temperature (∼ 865 °C) and 1) and then fast cooled below the β-transus temperature and held at different temperatures, or 2) continuously cooled at different rates. The β→α + β phase transformation is inferred by the evolution of the longitudinal wave speed with the LUMet method and by the change in the length of the specimen by dilatometry. We obtained the information about the isothermal phase transformation using the normalized changes in length for the dilatometric data and in longitudinal ultrasonic velocity. In the investigated isothermal temperature range (580 – 700 °C), the β →α + β transformation is completed within one hour of holding with the fastest transformation rates at 580 °C. For the continuous cooling treatments, the small change in length measured by dilatometry makes it impractical to use the lever rule to accurately determine the relative transformation fraction. However, we could successfully apply the lever rule to the LUMet data. Furthermore, the derivatives of the change in length and ultrasonic velocity are used to estimate the transformation temperatures as a function of the cooling rates. Further, the in-situ measurements are supplemented with ex-situ characterization of the microconstituents after heat treatments using scanning electron microscopy.

Investigation of The Effect of Semi-Solid Temperatures and Holding Times at Isothermal Range On Microstructure of AZ31 Mg Alloy

Mg alloys have poor formability due to their hexagonal tightly packed (HCP) crystal structure, low active shear system at room temperature, and anisotropic behavior in polycrystalline structure. While this situation hindered the development and commercialization of Mg alloys to some extent in the past, today developments in machining, liquid forming and semi-solid forming technology have removed these obstacles. Semi-solid forming of alloys has been the subject of intense R&D since it was discovered in the 1970s. In the semi-solid forming method, it is aimed to obtain ideal microstructure of the alloy, including excellent fluidity, appropriate flow control, adjustable viscosity and controllable grain morphology, thanks to equiaxed spherical solid particles surrounded by liquid. In this study, it is aimed to achieve the spherical microstructure required for the semi-solid shaping of the extruded AZ31 Mg alloy by heating to semi-solid temperatures and isothermal holding in the semi-solid temperature range. To accomplish this objective, rapid heating in the induction coil and controlled holding in the isothermal temperature range were applied to the alloy. Heating the extruded AZ31 Mg alloy to a semi-solid temperature (565-630°C) and subsequently rapid cooling processes lead to many intermetallic Mg17Al12(γ) compounds with heterogeneous distribution both in the equiaxed primary Mg(α) grains and at the grain boundaries as well as a very little eutectic at the grain boundaries. As the semi-solid temperature value increased, the degree of spheroidization increased. However, it was found that the grain growth reached a maximum at one point and there was no significant change in the shape factor as a result of controlled holding time.

Effects of Isothermal Temperature on Dislocation Density in Bainite Transformation of 4140 Steel

To relate the bainitic microstructures to the mechanical properties of steel, the average dislocation density needs to be determined. Using X-ray diffraction and diffraction line broadening analysis, this research quantifies the average dislocation density in the four bainite phase matrices, (upper bainite, upper and lower bainite mixture, lower bainite, lower bainite and martensite mixture), which are transformed in a wide range of isothermal temperatures. The effects of isothermal temperatures on the average dislocation density are assessed for different thermal dynamic driving forces in terms of activation energy and cooling rate. It is found that as isothermal holding temperature is increased, the dislocation density in the bainite matrix decreases from 1.55 × 1017 to 8.33 × 1015 (m−2) due to the reduction in the plastic deformation in the austenite in the transformation. At the same time, the activation energy required decreases only after passing the martensite and lower bainite mixed phase. A new method for better estimating the average dislocation density in bainitic steel is also proposed.

Establish time-temperature-transformation diagram based on dilatometry results and microstructural evolutions in an AISI 1010 steel

A time-temperature-transformation (TTT) diagram has been constructed using dilation results. The phase transformations have been confirmed through microstructural changes of AISI 1010 steel. The strength of this steel has been improved for various applications using heat treatments. Experiments for a series of steel samples have been conducted between 310-730°C isothermal temperature range using a thermo-mechanical simulator Gleeble®3800. The austenitised microstructure of the steel has been transformed into a combination of ferrite, pearlite, widmanstätten ferrite, massive ferrite, upper and lower bainite, and martensite, etc., when held at different isothermal temperatures. Ferrite along with pearlite have been observed at higher isothermal temperatures while bainite with martensite have been observed at lower isothermal temperatures. Two C-curves have been observed in pearlite and bainite transformation regions.

Effect of isothermal temperature on bainite transformation, microstructure and mechanical properties of LSFed 300M steel

The influence of isothermal treatment on the microstructure and mechanical properties of laser solid formed (LSFed) 300 M steel was investigated within the isothermal temperature range of 280–440 ℃. The result showed that the microstructure of LSFed 300 M steel isothermal with treatment of 440 ℃ consisted of upper bainite and small amount of martensite. As the decreasing isothermal temperature, the amount of upper bainite gradually decreased, the content of the martensite and lower bainite gradually increased. In addition, the superledges have been formed in the lower bainite. Microstructure of LSFed 300 M steel was almost martensite when the isothermal temperature was 280 °C. The hardness in LSFed 300 M steel presented a significant change with the decrement of isothermal temperature. The hardness decreased positively with isothermal temperature when it is more than 400 ℃; while, it increased inversely with the decreasing the isothermal temperature when the temperature is lower than 400 °C. The tensile strength and yield strength changed little when the isothermal temperature decreased from 440 °C to 320 °C, while sharply increased when the isothermal temperature was 280 °C. The elongation of LSFed 300 M steel was the highest (27 %) when the isothermal temperature was 320 °C.

Bainite Transformation-Kinetics-Microstructure Characterization of Austempered 4140 Steel

Bainite transformation is a kinetic process that involves complex solid diffusion and phase structure evolution. This research systematically studies the bainite transformation of austempered 4140 steel in a wide range of isothermal temperatures, in which four bainite phases structures were generated: upper bainite; mixed upper bainite and lower bainite; lower bainite and mixed lower bainite and martensite. The kinetics of bainite transformation has been described with a linear trend using an Avrami n-value. It was found that the bainitic ferrite sheaves grow with widthwise preference. The sheaves are stable when half-grown and are variable in length, due to austenite size limit or soft/hard impingement, or autocatalytic nucleation, or these conditions combined. The full-grown upper/lower bainite sheaves were found to be 1.9 μm/1.2 μm in width under the conditions of this study. Each individual bainite sheave is lath-like instead of wedge-like. The upper bainite sheaves mostly appear as broad-short-coarse lath, while the lower bainite sheaves appear as narrow-long-fine lath. The overall bainite transformation activation energy ranges from 50–167 kJ/mol.

Effect of Carbon Content on Bainite Transformation Kinetics and Microstructure of 4140/4150 Steels

4140 steel and 4150 steel are different only in their carbon content. A comparison between the kinetics and microstructure of the bainite transformation of both steels has been carried out under a wide range of isothermal temperatures, in which four bainite phase matrices were generated. The four phases are upper bainite, mixed upper and lower bainite, lower bainite and mixed martensite and lower bainite (lower bainite transformed under Ms temperature). The bainite transformation kinetics-morphology characteristics of the four phases were studied and compared. Kinetics behavior patterns are shown with the developed TTT diagrams and kinetics plots- (bainite volume fraction vs isothermal holding time). The upper bainite and lower bainite transition temperatures are different and the bainite reaction time span is shorter when carbon content is higher. The n values in the Johnson-Mehl- Kolmogorov-Avrami (JMKA) equation have a wider deviation when carbon content is higher, indicating more bainite reaction variation. The activation energy required for the bainite phase transformations were compared and it was found that higher activation energy is needed when carbon content is higher. The subtle microstructure morphology differences are illustrated. This work will aid future development of 4140/4150 steels with bainitic microstructures having improved mechanical properties.

Comparison of Semisolid Microstructure Evolution of Wrought Nickel Based Superalloy GH4037 with Different Solid Fraction

The research on semisolid metal forming of Ni-based superalloys is limited because of the difficulties in preparing semisolid billets. Traditional methods for fabricating semisolid billets are SIMA and RAP. In order to decrease the processing procedures, a new method called semi-solid isothermal treatment of wrought superalloy (SSITWS) was used to fabricate semisolid billets in this paper. In order to get the optimal parameters to fabricate Ni-based GH4037 semisolid billets, the effects of isothermal temperature and soaking time on microstructure evolution were investigated. The isothermal temperature ranges from solidus to liquidus, and the soaking time ranges from 10min to 60min. The results show that near-globular grains can be obtained only in the semi-solid state with appropriate solid fractions. When the solid fraction was high, the shape of grains was irregular and the average size of grains was large and inhomogeneous. When the isothermal temperature reached 1350°C, spherical grains appeared, and many small intragranular liquid droplets existed. During the appropriate isothermal temperature range, from 1350°C to 1380°C, the shape of solid grains was globular and the size of solid grains was homogeneous. At the same time, the size of the grains and the roundness of the grains increased with the increasing of isothermal temperature.

High-temperature oxidation of graphite

Graphite is used in extreme environments, such as high-temperature gas-cooled reactors and anodes for high-temperature fuel cells, because of its outstanding irradiation performance and oxidation resistance. Its oxidation resistance at high temperature depends on the porosity, ash content, oxidizer concentration, burn-off degree and degree of graphitization. The rate of oxidation was determined for different graphite grades (GLM50, GLM and SPSS) at the isothermal temperature range from 773 to 1273 K in a controlled dry oxygen environment. The degree of graphitization was quantitatively measured using selected area electron diffraction patterns and Raman spectra and was shown to affect the oxidation resistance of the graphite grade. The lowest degree of graphitization (0·67) and a high average pore radius (15·4 nm) in GLM decrease the rate of oxidation due to limited active sites for oxidation. The pore radius and the high graphitic structure reduce the average activation energy of GLM50 (41·8 kJ/mol) compared to those of GLM (48·5 kJ/mol) and SPSS (58·5 kJ/mol) in the boundary-layer-controlled regime in the temperature range 1073–1273 K.

Enhanced Thermal Conductivity and Durability of a Paraffin Wax Nanocomposite Based on Carbon-Coated Aluminum Nanoparticles

Phase change materials (PCMs) based on latent heat energy storage techniques over a nearly isothermal temperature range have been regarded as a promising strategy to meet the highly efficient thermal management system for electric vehicles (EVs). Ordinarily, paraffin wax is applied as a most widespread PCM especially in medium- and low-temperature storage, owing to a large latent heat that is nontoxic and noncorrosive. However, paraffin wax is limited because of its poor thermal conductivity, resulting in a significant decrease of the heat transfer rates. Here, we present a novel paraffin wax nanocomposite filling with carbon-coated aluminum nanoparticles (Al–C/PW), in which carbon-coated aluminum nanoparticles (Al–C) can form a network, resulting in a remarkable increase of the thermal conductivity. The results show that the thermal conductivity is 0.189W/(m·K) with 4 wt % Al–C under 25 °C, which is about 206.5% compared to the pure paraffin, simultaneously keeping a good thermal durability. Our work pav...

High temperature hydrogenation of Ti–V alloys: The effect of cycling and carbon monoxide on the bulk and surface properties

In the present work, the high temperature (425–575 °C) hydrogen storage properties of Ti–V alloys have been studied, both at static conditions and in a flow of hydrogen gas. The selected isothermal temperature range is considered as an optimal condition for hydrogen sorption enhanced steam reforming. When hydrogenation and dehydrogenation were performed in pure hydrogen gas and in vacuum, a large reversible hydrogen capacity of 3.95 wt. % H was obtained, demonstrating completeness of the formation and decomposition of (Ti,V)-dihydrides. However, when cycling was performed in a flow of hydrogen gas, the reversible hydrogen capacity decreased to ∼2 wt. % H caused by the formation of stable lower (Ti,V)-hydrides. A further decrease in the reversible hydrogen storage capacity took place when pure hydrogen gas was replaced by a mixture of hydrogen and carbon monoxide CO. This decrease was caused by the formation of an oxygen rich layer on the surface of the alloy, which was partially blocking the hydrogen exchange between the surface and the bulk of the sample.

Preliminary study on co-gasification behavior of deoiled asphalt with coal and biomass

The co-gasification behavior of deoiled asphalt (DOA) with coal and biomass were investigated by a thermogravimetric analyzer (TGA). The gasification experiments were conducted under CO2 atmosphere within an isothermal temperature range from 900 to 1100°C. The physical properties of the samples were examined by X-ray diffraction (XRD), scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS), N2 and CO2 adsorption and inductively coupled plasma-atomic emission spectrometry (ICP-AES). Compared with coal or biomass, the low surface area, high graphitization degree and the low ash content are the main reasons for the low reactivity of DOA. The co-gasification of coal and DOA does not show synergetic effect, while the combination of biomass and DOA shows higher gasification reactivity than that of being calculated. The synergetic effect is mainly caused by the alkali metals. Further study shows the transfer of the potassium from the surface of biomass to DOA greatly increases the active sites of the DOA, which leads to obvious improvement of the co-gasification reactions. Meanwhile, the gasification experiments of adding coal and biomass ashes to DOA also support the above explanations.

Effects of Ca content on the microstructure of semisolid Mg–13Al alloy produced via isothermal heat treatment

Mg–13Al–xCa (x=0, 0.3, 0.6, 1.0 and 3.0wt.%) alloys were designed for semisolid forming, and the effects of Ca content on the microstructures of semisolid Mg–13Al alloys produced via isothermal heat treatment were investigated. The results show that with increasing quantity of Ca added, the average size of solid particles in semisolid slurry has a trend of first increase and then decrease. And the solid particles in Mg–13Al alloy without Ca addition are smaller and more spherical than those in the alloys with Ca addition. These changes in semisolid microstructure associate with the distribution of low melting point phases and the morphology of primary α-Mg phases at as-cast state. The melting temperature of Al2Ca phase in Mg–13Al–xCa alloys is lower than that of primary α-Mg phase. With the increasing of Ca content, Al2Ca phases will become an important part of the liquid matrix in semisolid slurry. Among the semisolid Mg–13Al–0.3Ca, Mg–13Al–0.6Ca, Mg–13Al–1Ca and Mg–13Al–3Ca alloys, the best rheological parameters are obtained for the 3.0wt.% Ca containing alloy. The optimum isothermal temperature range for Mg–13Al–3Ca alloy is between 515 and 550°C.

Crystallization Behavior of Rutile in the Synthesized Ti-bearing Blast Furnace Slag Using Single Hot Thermocouple Technique

The present paper constructed the time-temperature-transformation (TTT) diagram of the synthesized titanium-bearing blast furnace (Ti-BF) slag using single hot thermocouple technique (SHTT) in order to study the crystallization behavior of rutile. A combination of X-ray Diffraction (XRD), Electron Probe Micro Analysis (EPMA) and Scanning Electron Microscope (SEM) equipped with Energy-dispersive X-ray spectroscopy (EDX) were applied to determine the structure and the composition of the crystals in the synthesized Ti-BF slag. It was found that rutile with rod shape was formed in the wide range of isothermal temperatures from 1160°C to 1320°C, and CaMgSi2O6, CaAl2Si2O8 as well as CaTiSiO5 were precipitated with further decreasing isothermal temperature. At a fixed isothermal temperature, the diameter of rutile increased slightly, whereas the length of rutile increased linearly with holding time. When increasing the isothermal temperature, the diameter of rutile increased linearly, while the growth rate of the length of rutile initially increased and followed by a decrease with further increasing isothermal temperature. The growth rate of the length of rutile had a maximum value (7.74 μm/s) at 1260°C. The mechanism of crystal growth was also discussed, and the results indicated that rutile with one-dimensional growth was observed in the slag melt.

Cocrystallization mechanism of poly(3-alkyl thiophenes) with different alkyl chain length

The crystallization rates of poly(3-alkyl thiophene) (P3AT) cocrystals having different alkyl chain length (e.g. hexyl and octyl) of the components are measured using differential scanning calorimetry (DSC) technique. Two pairs of cocrystals with varying compositions of the components viz. poly(3-octyl thiophene) (P3OT(R), regioregularity 89mol%) and poly(3-hexyl thiophene) [P3HT(R), regioregularity 92mol% and P3HT-2 regioregularity 82mol%] are used. In both the systems the isothermal temperature range (TR) in the same time scale of crystallization is found to decrease with increasing alkyl chain length in the blends. The crystallization rate at the same Tc decreases with increasing alkyl chain length P3AT concentration and the Avrami exponent values of cocrystals are same with those of the component values. The low Avrami exponent values (0.23–1.16) in all the samples suggest the presence of rigid amorphous portion which can not diffuse out quickly from the crystal growth front (soft impingement). Analysis of crystallization rate using Laurintzen–Hoffman (L–H) growth rate theory indicates that there is regime-I to regime-II transition in all the samples. The product of lateral (σ) and end surface energy (σe) values are found to decrease with increasing the concentration of longer alkyl chain P3AT in the blend. Analysis of σ values according to a theory of Hoffman et al. [Hoffman JD, Miller RL, Marand H, Rotiman DR. Macromolecules 1992;25:2221. [14]] indicates that there is chain extension of the components in the melt of the blends, however, the entropy of cocrystallization has different sign to the two systems. Cocrystallization in P3HT(R)/P3OT(R) system is an entropy driven process but that in P3HT(2)/P3OT(R) system is entropy forbidden process. A possible explanation of cocrystallization in the later system has been attributed from small interaction between the components.

Dissolution of different graphite grades during sintering of PM steels

Abstract The dissolution behaviour of graphite during sintering of Fe–0.8%C and the resulting properties were studied using standard high quality natural graphite and an artificial graphite grade recently developed for ferrous powder metallurgy. Sintering was done in high purity N2 atmosphere for 60 min, the temperature being varied. It showed that dissolution of carbon takes place mainly in the isothermal temperature range 750–950°C, for the artificial graphite the maximum dissolution rate being observed at a temperature about 50° lower than for the natural graphite. This also affects the properties, especially the hardness, of the specimens sintered in this temperature range while at standard sintering temperatures of about 1120°C both graphite grades yield well comparable results. Therefore, selection of the graphite grade seems to be important especially for pre-sintering of specimens prior to further treatment, dissolution of the graphite increasing the hardness but also improving the inter-particle strength.

Cocrystallisation in blends of poly(vinylidene fluoride) samples: 4. Kinetics study

The overall crystallisation rates of poly(vinylidene fluoride) (PVF 2) samples [KF PVF 2; head to head defect = 3.5 mol%, KY PVF 2 head to head defect = 5.3 mol%] and their cocrystals are measured. At the same undercooling (Δ T) the crystallisation rate of cocrystals is faster than that of the pure component with identical head to head defect concentration. The isothermal temperature range and the Avrami exponent vary linearly with composition. Analysis of the crystallisation rate using Lauritzen—Hoffman growth rate theory indicates regime I–II transition of KF PVF 2 sample and regime II–Ill transition for KY PVF 2 and their cocrystals. The product of surface free energies (σσ e) is lower for the cocrystal than that of the pure component. Assuming the fold surface energy (σ e) as invariant for the above two PVF 2 samples, the lateral surface energy (σ) values are calculated and have been interpreted according to a recent theory of Hoffman et al. ( Macromolecules, 1992, 25, 2221) The results indicate a small extension of the PVF 2 chains in the melt of the blend. The entropy of cocrystallisation has been calculated from the above theory and has been found to be ∼0.6 cal deg −1 mol −1. A model for explaining the higher crystallisation rate of the cocrystal compared with that of the pure components at the same Δ T has been presented.

Investigation into the GC separation of enantiomers on a trifluoroacetylated cyclodextrin. I. Effect of analyte structure on stereoselectivity for alcohols

AbstractMore than 30 enantiomeric alcohols have been analyzed, without prior derivatization, by gas chromatography using a fused silica capillary column coated with octakis(3‐O‐trifluoro‐acetyl‐2,6‐di‐O‐n‐pentyl)‐γ‐cyclodextrin. Most were analyzed over a range of isothermal temperatures from 35 to 70°C. Enantiomeric separations were observed for most of the analytes, even at temperatures as low as 35°C.The stereoselectivity of the stationary phase was found to depend on the length of the longest carbon chain attached to the stereogenic centre in 2‐ and 3‐hydroxy alkanes, the relative positions of the methyl and hydroxyl substituents in methylsubstituted alcohols, and the effects of multiple bonds in the analyte molecule. Thermodynamic data calculated from the results suggest that the enantiomers of all the analytes are resolved by a similar process. Retention and thermodynamic data are presented and possible mechanisms discussed.

Kinetics of cellulose pyrolysis modelled by three consecutive first-order reactions

Isothermal thermogravimetry can be used to help elucidate complex reaction mechanisms, particularly if data are represented as ‘DMM plots’, i.e., as - d M′/d t vs. M′, where M′ is dimensionless mass and t is time. In a previous work it was shown that the simplest homogeneous reaction scheme consistent with the DMM curves of cellulose pyrolysis consists of three consecutive first-order reactions. In this work it is further shown that other mechanisms, such as nucleation and growth (Avrami-Erofeev equation) and diffusion, are also inconsistent with the experimental DMM curves. The experimental isothermal temperature range of the previous work is extended up to around 350°C to allow the calculation of the kinetic parameters of each of the three consecutive reactions. The stoichiometric parameters obtained by optimization for each of the reactions confirm former conclusions: the first reaction releases some volatile matter; the second reaction releases practically no volatiles, probably consisting of a rearrangement or activation of the solid; the third reaction releases most of the total volatile matter. Activation energies obtained vary between 139 and 209 kJ/mol.

Study of cure of epoxy resins by torsional braid analysis

The use of torsional braid analysis (TBA) for the dynamic mechanical characterisation of polymers in composites has been inhibited to some degree by difficulties associated with both the measurement and processing of data. Methods for overcoming difficulties associated with spurious motions of the TBA pendulum and for the rapid and optimum processing of raw output data are outlined. A description follows of the application of the modified TBA technique to the analysis of six nominally similar epoxy resins systems. It is shown, by the presentation of comparative data, that the TBA method gives highly reproducible results, with little scatter, in studies both of gelation and vitrification times over a wide range of isothermal temperatures, and of activation energies for the processes involved, and also in studies of dynamic mechanical behaviour over a wide temperature range.