The Time-temperature-transformation Research Articles

In Situ Observation and Investigation of Mold Flux Crystallization by Using Double Hot Thermocouple Technology

The crystallization processes of mold fluxes for casting low-carbon (LC) and medium-carbon (MC) steels were investigated by using double hot thermocouple technology (DHTT) in this article. The results showed that the glass phase was first formed at the cold side thermocouple (CH-2), when the LC mold flux (mold flux for casting low-carbon steel) was exposed to the temperature gradient of 1773 K (1500 °C) to 1073 K (800 °C); then, the fine crystals were precipitated at the liquid/glass interface and grew toward glass and later on to liquid phase. However, the crystals were directly formed at CH-2 when MC flux (mold flux for casting medium-carbon steel) was under the same thermal gradient. The growth rate of MC flux crystals was much faster than that of LC ones. Scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS) analyses suggested that the crystals formed in LC mold flux were mainly dendritic cuspidine Ca4Si2O7F2, and the crystals formed from the liquid phase were larger than those from the glass. For MC mold flux, the earlier precipitated crystals were large dendritic Ca4Si2O7F2, whereas the later ones were composed of equiaxed Ca2Al2SiO7 crystals. The results of DHTT measurements were consistent with the time-temperature-transformation (TTT) diagrams and X-ray diffraction (XRD) analysis.

Evaluation of Glass-Forming Ability for Metallic Melts by Phase Field Approach

Evaluation of glass-forming ability (GFA) is important in the development of amorphous alloys. Based on phase field theory, the kinetic model of liquid-to-solid phase transition is build, and the time-temperature-transformation (TTT) diagram is plotted according to the phase field simulations of isothermal phase transformation kinetics for a model system. Furthermore, the critical cooling rate for glass formation is calculated on the basis of the TTT curve and is taken as the intrinsic criteria of reflecting the GFA for metallic melts.

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.

Analysis of crystallization kinetics of undercooled Fe-B hypereutectic alloy using DSC technique

Crystallization kinetics of undercooled Fe-B hypereutectic alloy was researched using differential scanning calorimetry (DSC) technique. With the combination of melt-fluxing and high pure argon gas to purify and protect the melts, high undercooling up to 410 K was obtained successfully. The comparison of time dependent transformation fraction at different undercoolings has shown that the crystallization rate increased firstly with increasing Δ T until 354 K and then, the crystallization process was retarded with the further increase of Δ T. Applying quantitative thermodynamic calculation, the evolution of the nucleation and growth rates as a function of temperature was described. On this basis, the temperature–time-transformation (TTT) curve was thus constructed, which gives a reasonable explanation of the observed experimental results.

Temperature-time transformation diagram for Pb(Zr,Ti)O3 thin films

In this paper, we describe an analytical model to define the temperature-time- transformation (TTT) diagram of sol-gel deposited Pb(Zr,Ti)O3 thin films on platinized silicon substrates. Texture evolution in film occurred as the pyrolysis and thermal annealing conditions were varied. We demonstrate that the developed model can quantitatively predict the outcome of thermal treatment conditions in terms of texture evolution. Multinomial and multivariate regression techniques were utilized to create the predictor models for TTT data. Further, it was found that multinomial regression can provide better fit as compared to standard regression and multivariate regression. We have generalized this approach so that it can be applied to other thin film deposition techniques and bulk ceramics.

Crystallization Behavior in Fluoride-Free Mold Fluxes Containing TiO 2/ZrO 2

The time-temperature-transformation (TTT) diagrams of fluoride-free mold fluxes containing TiO 2/ZrO 2 were constructed through the confocal scanning laser microscope (CSLM) technique. It was found that the crystallization temperature of F-free mold fluxes containing TiO 2 or ZrO 2 increases, while incubation time decreases with increasing basicity. The crystallization tendency increases with the zirconia addition in slag melt, as it may be thought that the addition solubility is limited in molten slag and the solid particles act as heterogeneous nucleation sites. Three types of crystal morphologies were observed, corresponding to different crystallization mechanisms. CaSiO 3 and Ca 3Si 2O 7 precipitate in the slag with low basicity, and Ca 2SiO 4 was formed with increasing basicity. The addition of TiO 2 promotes the precipitation of CaTiO 3. The logarithm of crystal growth rate increased with increasing isothermal temperature, suggesting that the crystal/melt interface reaction is the controlling step in the present experiment.

Crystallization behavior of F-free mold fluxes

The time-temperature-transformation (TTT) diagrams of F-free mold fluxes was constructed using single hot thermocouple technique (SHTT) and confocal scanning laser microscopy (CSLM) to study the crystallization behavior of F-free mold fluxes. The tendency of crystallization is found to increase whereas the incubation time decreases with increasing basicity. Zirconia addition enhances the crystallization tendency due to its limited solubility in the slag melt and the solid particles acting as nucleation sites. Pseudo-wollastonite is found to precipitate in the slag with low basicity (CS-1 and CS-2), kilchoanite and larnite are formed with further increasing basicity (CS-3), and larnite is finally formed as the basicity beyond unit (CS-4). The crystal morphology changes with varying compositions and isothermal temperatures. The measured growth rate is found to be linear with time under isothermal conditions and decreases with increasing isothermal temperature.

Development of Tensile Ductility of Zirconium-Based Bulk Metallic Glass at Room Temperature by Thermoplastic Deformation

Bulk metallic glass (BMG) is a kind of alloy with amorphous structure and it has better thermal stability because of its atomic composition. In particular, zirconium-based BMGs are expected to apply for structural materials, because they exhibit excellent mechanical properties, a high corrosion resistance, etc. BMGs exhibit a brittle fracture behavior caused by the rapid propagation of a shear band under tensile load at room temperature (RT). On the other hand, BMGs show the high formability due to very large plastic deformation under high temperature condition. And sometimes BMGs are crystallized by thermoplastic deformation even though its thermal condition is never beyond the time-temperature-transformation (TTT) curve of the material. When crystalline particles make the propagation of the shear band stop and deflect, such an effect can bring about an apparent inelastic strain under the tensile load at RT. In this study, the development of the ductile property with the inelastic strain of Zr55Cu30Al10Ni5 BMG at RT was attempted by applying crystallization with thermoplastic deformation. The experimental results suggested that the thermoplastic deformation with strain rate of 2.0 × 10-4s-1 or 3.0 × 10-3s-1 can bring about the inelastic strain to Zr55Cu30Al10Ni5 BMG at RT under tensile loading by precipitating the crystalline particles in the material.

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.

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.

Influence of isothermal aging on σ precipitation in super duplex stainless steel

The time-temperature-transformation (TTT) curve of the 00Cr25Ni7Mo4N duplex stainless steel was obtained with a Formastor-digital thermal dilatometer, and the influence of isothermal aging on σ precipitation was studied by metallographic observation, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results show that the decomposition of ferrite phase is accompanied by the formation of σ phase at 750–1000°C, especially in the range of 800–900°C. The longer the aging time, the higher the amount of σ precipitation. The area fraction of various phases remains at a certain value upon the completion of ferrite deformation. The temperature of 850°C is the most sensitive transaction temperature, the incubation time for the formation of σ precipitation is less than 1 min, and aging for 20 min leads to the complete transformation of ferrite. The σ phase is formed preferentially at the α/α/γ junction, and then grows along the α/α boundary in the matrix.

Age-hardening behaviour of two metastable beta-titanium alloys

Two novel β-titanium alloys, of compositions Ti–10V–2Ni and Ti–10V–6Cu (wt.%) and with distinct potential for athermal ω-phase formation, have been selected and the sensitivity of the age-hardening responses to the cooling rate post solution treatment investigated. Upon rapid water quenching, the solution-treated Ti–10V–2Ni alloy contained a significantly higher volume fraction of ω-phase than the Ti–10V–6Cu alloy. It was observed that the Ti–10V–6Cu alloy is remarkably insensitive to cooling rates in the range of water quenching to air cooling, in terms of both microstructural evolution and age-hardening behaviour, when aged isothermally at 350 °C and 500 °C. The microstructure is thermally stable and the hardness remains approximately constant for extended aging (48 h) at both temperatures, implying that it may have potential as a deep-hardenable alloy. In comparison, the aging response of the Ti–10V–2Ni alloy was marginally influenced by cooling rate when aged at 500 °C, the effect being ascribed to autocatalytic formation of α-phase in the course of slow air cooling. The results are discussed with respect to the β-phase stability and the time-temperature-transformation (TTT) characteristics of metastable beta-titanium alloys.

Glass formation and crystallization of a simple monatomic liquid

A simple monatomic system in two dimensions with a double-well interaction potential is investigated in a wide range of temperatures by molecular-dynamics simulation. The system is melted and equilibrated well above the melting temperature, and then it is quenched to a temperature 88% below the melting temperature at several cooling rates to produce an amorphous state. Various thermodynamic quantities are measured as functions of temperature while the system is heated at a constant rate. The glass transition is observed with a sudden increase in the energy and the glass transition temperature is shown to be an increasing function of the cooling rate in the preparation process of the amorphous state. In a relatively high-temperature region, the system gradually transforms into crystals, and the time-temperature-transformation curve shows a typical nose shape. It is found that the transformation time to a crystalline state is the shortest at a temperature 14-15 % below the melting temperature and that at sufficiently low temperatures the system does not transform into a crystalline state within an observation time in our simulation. This indicates that a long-lived glassy state is realized.

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.

Isothermal Martensitic Transformation in Sensitized SUS304 Austenitic Stainless Steel at Cryogenic Temperature

We have investigated martensitic transformation behavior in a sensitized SUS304 austenitic stainless steel to know the stability of the austenitic phase at cryogenic temperatures. We found that the sensitized specimen exhibits an isothermal martensitic transformation when the specimen is held in the temperature range between 60 and 260K. The time-temperature-transformation (TTT) diagram corresponding to the formation of 0.5 vol% of α'-martensite shows a double-C curve with two noses located at about 100 and 200 K. An in-situ optical microscope observation has revealed that the double C-curve is due to two different transformation sequences. That is, the upper part of the C-curve is attributed to the direct γ (fcc) → α' (bcc) martensitic transformation and the lower part of the C-curve is due to the successive γ (fcc) → e' (hcp) → α' (bcc) martensitic transformation. The direct γ → α' transformation occurs in the vicinity of grain boundaries, while the successive γ → e' → α' transformation occurs near the center of each grain. The reason for appearing two types of isothermal transformation sequence in the sensitized SUS304 stainless steel will be due to the difference in concentration by sensitization heat-treatment.

Consolidation of [(Fe<SUB>0.5</SUB>Co<SUB>0.5</SUB>)<SUB>0.75</SUB>Si<SUB>0.05</SUB>B<SUB>0.2</SUB>]<SUB>96</SUB>Nb<SUB>4</SUB> Metallic Glassy Powder by SPS Method

Metallic glasses have been reported to exhibit excellent properties, such as high strength, high corrosion resistance, high wear resistance, which result from their amorphous structure. Because of a drastic reduction in their viscosities in supercooled liquid region, metallic glasses have an excellent workability through viscous deformation for the production of various industrial parts. Recently, much attention has been paid to Fe-based metallic glasses because of their rich resources in addition to their excellent mechanical and magnetic properties. However, due to their poor glass-forming ability, the size of the Fe-based bulk metallic glass by conventional casting techniques is limited. In the present investigation, nearly fully densified disk-shaped compacts of [(Fe0:5Co0:5)0:75Si0:05B0:2]96Nb4 metallic glass were produced from a gas-atomized amorphous powder by spark plasma sintering (SPS). The processing temperature and the time that assure the supercooled liquid state of the compacts were determined from the Time-Temperature-Transformation (TTT) diagram that was constructed by isothermal differential scanning calorimetry. The mechanical properties of the consolidated samples were evaluated by compression test. Comparatively low values of the fracture stress, Young’s modulus and yielding stress of SPSed sample compared to the casted samples were observed, which is discussed on the basis of the integrity of the interparticle bonding. [doi:10.2320/matertrans.M2009093]

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

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

The estimation of temperature and hardness distribution in heat-treated complex-shaped components

The quality or obtainable characteristics of any heat-treated component depend on the extent of the temperature distribution over the entire surface and cross-section of the component exposed during the different stages of heat treatment. It becomes more critical in the case of complex-shaped components, such as gears, for the shape plays a major role in controlling the distribution of temperature. This work presents a temperature distribution analysis using the Finite Element Method (FEM) and mathematical modelling approaches, carried out on gears during quenching in different quenchants. The temperature distribution data thus obtained from both analyses are used to draw cooling curves. The phase transformation kinetics is studied by superimposing the cooling curves on the Time-Temperature-Transformation (TTT) diagram of the material. The hardness achievable on the gears is obtained by estimating the martensite content using the Koistinen-Marburger model and the Avrami model.

Phase transformation in Cu–Cr–Zr–Mg alloy

After solid solution treatment, the effects of aging processes on the microstructure and electrical conductivity of Cu–Cr–Zr–Mg lead frame alloy were investigated in order to determine the phase transformation and the time–temperature-transformation (TTT) behavior. The results show that aged at 470 °C the fine precipitation of an ordered CrCu 2(Zr,Mg) phase is found in copper matrix as well as fine Cr and Cu 4Zr, aged at 550 °C the phase CrCu 2(Zr,Mg) is decomposed into Cu 4Zr and Cr phase, aged at 600 °C the precipitates is bcc Cr only. The phase transformation kinetics equation was deduced from the Avrami empirical formula based on the linear relationship between the electrical conductivity and the volume fraction of the phase transformation.

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