The Time-temperature-transformation Research Articles

The comparative study of crystallization transformation in Zr-Cu-Ag metallic glasses

The crystallization transformation in Zr-Cu-Ag metallic glasses have been comparatively studied. The crystallization enthalpy ΔHx lies in the range of −56.18 ∼ −46.90 J/g for Zr50Cu38Ag12 metallic glass and −55.69 ∼ −44.70 J/g for Zr50Cu36Ag14 metallic glass in their respective isothermal temperatures. The average crystallized volume fraction fmax corresponding to the maximum value of transformation rate [df/d(tIso − τ)]max is about equal to 0.52 and 0.55 for Zr50Cu38Ag12 and Zr50Cu36Ag14 metallic glasses. It is found that at the same isothermal temperature, [df/d(tIso − τ)]max becomes smaller, the time–temperature-transformation (TTT) diagram shifts to the right and time tmax required to reach [df/d(tIso − τ)]max becomes longer when Cu is replaced with Ag, which indicates that the thermal stability will be improved as Ag content rises in these metallic glasses. In addition, the type of growth is interface-controlled growth for Zr50Cu38Ag12 and Zr50Cu36Ag14 metallic glasses.

Study on Quench Sensitivity during Isothermal Treatment of 7A65 Aluminum Alloy.

The quenching sensitivity of 7A65 aluminum alloy was investigated using interrupted quenching experiments. The time-temperature transformation (TTT) and time-temperature performance (TTP) curves of the alloy were determined. The results indicate that the nose temperature is about 320 °C and the quenching sensitivity temperature range is from 240 °C to 360 °C. During the isothermal treatment, the supersaturated solid solution resolves to the equilibrium phase of η (MgZn2), and the precipitation rate is the largest at about 320 °C. Through transmission electron microscope (TEM) observation and X-ray diffraction (XRD) tests, it was found that with the extension of the isothermal holding time, the originally dispersed η' phase gradually decreases until disappear, and the number of η phase increases and gradually grows up at the grain boundary or around the Al3Zr particles. The rod-like η phase at the grain boundary is distributed from discontinuous distribution to chain-like continuous distribution, and the precipitation free zone (PFZ) is gradually generated and widened as the holding time is extended. At the nose temperature, the driving force of nucleation is high, and the diffusion rate is fast, which promotes the precipitation and growth of η phases. The coarse η phase weakens the mechanical properties. According to the results, it is recommended to increase the cooling rate at the sensitivity temperature range to reduce the precipitation of the η phase and decrease the quenching cooling rate from solution temperature to 360 °C to reduce residual stresses in the components.

Evolution of austenite lattice parameter during isothermal transformation in a 0.4 C low alloyed steel

The evolution of internal stress during displacive transformation is a topic of continuous debate. Neutron diffraction was used to study the isothermal bainite transformation in a 0.4 C low alloyed steel from 773 to 623 K to provide a clearer basis for discussion regarding the change in the austenite lattice parameter. According to diffraction profile analysis, fresh bainite possesses a body-centered tetragonal structure, and its c/a ratio decreases rapidly over time. The austenite lattice parameter increases or decreases depending on whether the transformation temperature is above or below the nose of the Time-Temperature-Transformation (TTT) curve. This isothermal transformation behavior can be divided into two categories: above and below the nose of TTT curve, which correspond to the upper and lower bainites, respectively. The internal stresses caused by the transformation strains are relaxed by dislocation motion and vacancy formation. The yielded dislocations and vacancies not only affect the broadening of both austenite and bainitic ferrite diffraction peaks but also the lattice parameter. The first-principles calculations demonstrate that the austenite lattice parameter decreases as the vacancy density increases, which may account for the experimental observation in lower bainite.

Influence of oscillatory shear on nucleation in metallic glasses: A molecular dynamics study

The process of crystal nucleation can be accelerated or retarded by ultrasonic vibration, which is particularly attractive for the addictive manufacture and thermoplastic forming of metallic glasses, however, the effect and mechanism of oscillatory loading on the nucleation process are still elusive. Here, by using molecular dynamics simulation, the changes in the time–temperature-transformation (TTT) curve under oscillatory external loading are systematically investigated in two typical binary alloys. A glass forming ability dependent response to the external loading is found, and the shortest incubation time is insensitive to the external loading, while the corresponding temperature can be significantly shifted. Within the framework of classical nucleation theory, a fitting formula is proposed to describe the simulation data quantitatively. In contrast to stationary shear, the elastic stress, rather than the strain rate, is the key parameter to control the evolution of TTT curve under oscillatory loading. Furthermore, the model shows that oscillatory loading can decouple the mobility and nucleation in the deeply supercooled liquid, hence the deformation ability can be enhanced while the nucleation is suppressed, which is particularly helpful for the forming and manufacturing of metallic glasses.

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.

Crystallization behaviors and kinetic of argon oxygen decarburization slag via various SiO2 content: Conditions for obtaining amorphous state

The crystallization behaviors and morphology of industrial AOD slag with various SiO2 levels were studied. The time-temperature-transformation (TTT) and continuous-cooling- transformation (CCT) diagrams of argon oxygen decarburization (AOD) slag were obtained using confocal laser scanning microscopy (CLSM). The results show that the main crystalline phases evolved from dicalcium silicate, merwinite, and bredigite to an amorphous state when the dosage of SiO2 increased to 15 wt% in the quenching slag. Crystallization behaviors of AOD slag follow the Johnson-Mehl-Avrami (JMA) crystallization kinetic model. The Avrami exponent and the activation energy increase to 2.382 and 83.68 kJ/mol, respectively in the 15 wt% SiO2 samples. Thus, the crystal size shrinks, and the crystallization capacity of slag decreases. The critical cooling rate reaches the minimum value of 2052.5±34 °C/min in the 15 wt% SiO2 samples. This research showed that AOD slag modified by SiO2 can enhance the potential of preparing supplementary cementitious materials for added-value applications.

Effect of Na2O addition on crystallization behavior and properties of 25 wt%Al2O3–SiO2–CaO non-metallic inclusion-type oxides

The 25 wt%Al2O3–SiO2–CaO inclusion with high basicity has the characteristics of high melting point and strong crystallization tendency. The effect of Na2O addition on the crystallization behavior and properties of 25 wt%Al2O3–SiO2–CaO non-metallic inclusion-type oxides was systematically investigated in the current study. The time-temperature-transformation (TTT) diagrams and continuous-cooling-transformation (CCT) diagrams were constructed introducing the single hot thermocouple technology (SHTT). The crystallization temperature decreased from 1640.15 K to 1514.15 K with the Na2O content increased from 0 to 6.92 wt%, then rose to 1564.15 K when the content of Na2O was equal to 11.81 wt%. Above 1473.15 K, the incubation time was first prolonged and then shortened with the increase of Na2O content. The crystallization tendency was first inhibited and then promoted with increasing the Na2O addition. The XRD result showed that the precipitated crystals of the oxide system transformed from Ca2Al(AlSiO7) to NaCaAlSi2O7 when the content of Na2O exceeded 6.92 wt%. The FESEM micrograph showed that the morphology of crystals from all samples demonstrated spherical or rod-like. The calculated melting point decreased from 1840.81 to 1680.29 K then rose to 1755.56 K with the increase of Na2O content. The crystallization tendency or ability of the oxide system could be predicted by the initial crystallization potential, the viscosity at the melting point, the rate of change of viscosity with temperature as well as the activation energy of crystallization.

Effect of basicity on the crystallization behavior of 25 wt.%Al2O3-SiO2-CaO non-metallic inclusion-type oxides

As a typical inclusion system in Si-Mn killed steels, the plasticity or deformability of inclusions during rolling process is significantly influenced by the crystallization behavior. The effect of basicity on the crystallization behavior of 25 wt.%Al2O3-SiO2-CaO non-metallic inclusion-type oxides was systematically investigated in the current study. A series of 25 wt.%Al2O3-SiO2-CaO system slags with different basicity were prepared to represent inclusions with corresponding compositions. The time-temperature-transformation (TTT) diagrams and continuous-cooling-transformation (CCT) diagrams were constructed introducing the single hot thermocouple technology (SHTT). The CCT and TTT tests result indicated basicity tended to promote the crystallization ability of slags, which was proved by the changes of crystallization temperature, critical cooling rate and crystallization incubation time. The maximum crystallization temperature and critical cooling rate could reach 1675.15 K (1402 °C) and 18 K/s when basicity increased to 1.55. The increase of basicity could also shorten the crystallization incubation time. The XRD results showed the precipitated crystal was gehlenite (2CaO•Al2O3•SiO2). The initial crystallization potential was proposed to reflect the crystallization ability of slags, which was calculated by FactSage, ranging from 0.317 to 1.052 with increasing basicity. Thermodynamic analysis indicated larger initial crystallization potential and lower viscosity around the melting point would be more favorable for the crystallization of melt. Under kinetic consideration, the activation energy of crystallization was calculated, decreasing from 196.511 kJ/mol to -187.649 kJ/mol when basicity increased from 1.25 to 1.55. Overall, the initial crystallization potential and crystallization activation energy could semi-quantitatively characterize the crystallization ability of slags.

Niobium nanoparticle-enabled grain refinement of a crack-free high strength Al-Zn-Mg-Cu alloy manufactured by selective laser melting

Grain refinement is an effective way to avoid hot cracking of non-weldable high strength aluminum alloys during additive manufacturing. In the present work, primary Al3Nb with the tetragonal D022 structure was discovered to be an extremely effective inoculant for Al-Zn-Mg-Cu alloys manufactured by selective laser melting (SLM). The columnar crystals were transformed into fine equiaxed crystals with an average grain size of ~1.9 µm through adding 1.5 wt% Nb nanoparticles, eliminating the defects such as cracks and porosity. The high number density of primary Al3Nb and the coherent interface with high lattice matching between the primary Al3Nb and the Al matrix are the main reasons for grain refinement. The Time-Temperature-Transformation (TTT) curves of the precipitation kinetics curves of primary Al3X (XNb, Zr, Ti, Sc) show that primary Al3Nb has the advantage in grain refinement compared with Al3Sc and Al3Ti. Due to grain refinement and defect elimination, the 1.5 wt% Nb-modified Al-Zn-Mg-Cu alloy after T6 heat treatment exhibits excellent mechanical properties, with a tensile strength of ~ 505 ± 12 MPa, and an elongation after fracture of ~ 12.3 ± 1.3%.

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.

Detailed investigation of quench sensitivity of 2050 Al-Cu-Li alloy by interrupted quenching method and novel end quenching method

The quench sensitivity of 2050 Al-Cu-Li alloy was determined by the time-temperature-transformation (TTT) curves and time-temperature-property (TTP) curves based on conductivity and hardness. Additionally, a novel end quenching test was used to investigate the quench sensitivity of cold-rolled sheets with a thickness of 2 mm which cannot be tested by conventional methods. The microstructure evolution during interrupted quenching (IQ) treatment and end quenching test was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation and coarsening of Cu-rich phases during quenching are the main factors affecting the quench sensitivity of the alloy. Meanwhile, these Cu-rich phases lead to the uneven distribution of precipitates (T1 and θ′), which results in the decreased tensile properties. The influence of precipitate free zones (PFZs) on elongation during end quenching test was clarified and the high quench sensitive temperature zone was found to be 260 ~440 °C. This work can help to optimize the quenching process and improve the mechanical properties of the 2050 Al-Cu-Li alloy.

Thermal processing map and thermoplastic forming map of Zr-based bulk metallic glass in the supercooled liquid region

In order to make the bulk metallic glass (BMG) better used in micro electro mechanical system (MEMS), the rheological behavior of (Zr0.6336Cu0.1452Ni0.1012Al0.12)99.4Y0.6 BMG in the supercooled liquid phase region (SCLR) was studied, and the thermal processing map and the thermoplastic forming map were constructed in this paper. The Time-Temperature-Transformation (TTT) curve was established according to the crystallization incubation time of (Zr0.6336Cu0.1452Ni0.1012Al0.12)99.4Y0.6 BMG at different temperatures in the SCLR, which provided a theoretical basis for the time and temperature selection of thermal forming. Furthermore, the results show that the flow stress and viscosity coefficient of the BMG in the SCLR are strongly dependent on temperature and strain rate. The flow stress tends to decrease with the increase of temperature and the decrease of strain rate. When the temperature is 703 K with the strain rate of 5 × 10−4 s−1, the flow stress value is only 5 MPa. Meanwhile, the viscosity coefficient of the alloy remain approximately constant at 703 K, showing Newtonian fluid characteristic. It can be concluded that the alloy has excellent thermal processing property and thermoplastic forming (TPF) ability at temperature of 698–703 K, and thermoplastic forming ability is less affected by the strain rate within this temperature range.

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.

Effect of Nb minor addition on the crystallization kinetics of Zr-Cu-Al-Ni metallic glass

The crystallization kinetics of Zr55Cu30Al10Ni5 and (Zr55Cu30Al10Ni5)98Nb2 melt-spun metallic glasses in non-isothermal and isothermal conditions were studied by differential scanning calorimetry. According to the non-isothermal and isothermal conditions, the crystallization activation energy for the Nb-containing sample is lower than the Nb-free sample, suggesting that crystallization transformation becomes easier by Nb microalloying. The time-temperature-transformation (TTT) diagrams demonstrate that the Nb minor addition delays the onset of crystallization by shifting the incubation time to longer times, but then accelerates the crystallization progress by shortening the endset time of crystallization, yielding a narrower crystallization transformation region. The Avrami exponents for both non-isothermal and isothermal conditions have larger values in Nb-containing sample, revealing increased nucleation rates in this glassy alloy. In the isothermal condition, the Avrami exponent values in both alloys are either above or below 2.5, indicating that the governing crystallization mechanism is diffusion-controlled three-dimensional growth with either decreasing or increasing nucleation rate.

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 of BiFeO3 phase synthesized by citrate–nitrate route and a synergetic effect for its stabilization

BiFeO3 has been synthesized from the nitrate salts of bismuth and iron at different temperatures at 500, 600 and 700 °C. The time–temperature-transformation (TTT) has been analysed at the annealing time duration of 1, 2, 3, 4, 5, 6 and 7 h. Thermo-gravimetric study of the starting solid has been analysed from room temperature to 800 °C and the reaction kinetics have been investigated from 400 to 700 °C using Flynn-Wall-Ozawa model which has also been correlated with the differential thermal analysis (DTA). Gibbs free energies of the formation of BiFeO3, Bi2Fe4O9 and Bi25FeO39 have been calculated to investigate the reason for the maximum percentage of pure BiFeO3 phase formation at 600 °C. Based on the calculation, though later two compounds are thermodynamically destabilizing secondary phases of BiFeO3, its synergetic stabilization has been supported by the diffusion kinetics and their activation energies.

The role of ternary alloying elements in eutectoid transformation of U–10Mo alloy part II. In and ex-situ neutron diffraction-based assessment of eutectoid phase transformation kinetics in U-9.8Mo-0.2X alloy (X = Cr, Ni or Co)

Exploring the effects of minor ternary alloying additions, typically impurity elements, on the phase stability of U–10Mo is important for preventing undesirable phase decomposition during processing or during service. This work examines the influence small ternary additions of Cr, Ni, and Co. Both in-situ and ex-situ neutron diffraction measurements made during and after high temperature (450–525 °C) exposures were used to better define the influence of these elements on the time-temperature-transformation (TTT) behavior of U–10Mo, providing information which is complementary to electron microscopy investigations of the same alloy systems performed in the first part of this work. Minor additions of Ni and Co decrease the γ-phase stability at all temperatures investigated. Signatures of U6X (X = Ni or Co) compounds were shown to be present in amounts of up to 6 wt% in the heat treated alloys, suggesting that the initial precipitation of this phase may catalyze further γ-phase decomposition. On the other hand, the Cr containing alloys were observed to have nearly the same, and in some cases slower, phase transformation kinetics when compared to the binary U–10Mo control samples. The results of the study have enabled preliminary estimates of the TTT curves for the ternary alloys.

Heterogeneous melting kinetics in polycrystalline aluminum.

The heterogeneous melting kinetics of polycrystalline aluminum is investigated by a theoretical model which represents the overall melting rate as a functional of the Weibull grain-size-distribution. It is found that the melting process is strongly affected by the mean-grain-diameter, but is insensitive to the shape parameter of the Weibull distribution. The temperature-time-transformation (TTT) diagrams are calculated to probe dependence of the characteristic timescale of melting on the overheating temperature and the mean-grain-diameter. The model predicts that the heterogeneous melting time of polycrystalline aluminum exponentially depends on temperature in high temperature range and the exponent constant is an intrinsic material constant independent of the mean-grain-diameter. Comparisons between TTT diagrams of heterogeneous melting and homogenous melting are also provided.

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.