Incubation Time For Crystallization Research Articles

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.

Crystallization characteristics and structural analysis of CaO–SiO2-based mold fluxes with different La2O3 contents

To elucidate deterioration mechanism of La2O3 on the crystallization performance of conventional mold fluxes, crystallization characteristics and structure of CaO–SiO2-based mold fluxes with various contents of La2O3 were studied. Results showed that adding 0 wt% to 15 wt% La2O3 increased initial crystallization temperature and critical cooling rate of slags. Crystalline phases changed from Ca4Si2F2O7 to CaLa2(SiO4)2 and Ca4Si2F2O7 with the increase in content of La2O3. Furthermore, crystallization incubation time was shortened gradually. Combined with structural analysis, the main reasons for enhanced crystallization characteristics of mold fluxes with La2O3 were following two factors. First, high-polymerized Q3 units were simplified by O2− released from La2O3 to form low-polymerized units Q2, Q1, and Q0. As a result, NBO/T of slags increased and the degree of polymerization decreased, resulting in the reduction of mass transfer resistance of ions and facilitating crystal nucleation. Second, with an increase in La2O3 content, La3+ exhibited strong attraction to simpler Q0 ([SiO4]4-) units due to its high field strength and large electric charge, which was conducive to the precipitation of rare earth phase. In this study, La3+ together with Ca2+ played charge balance effect and combined with Q0 units to form CaLa2(SiO4)2 phase at high temperature.

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.

Synthesis of Bulk Zr48Cu36Al8Ag8 Metallic Glass by Hot Pressing of Amorphous Powders

The critical cooling rate necessary for glass formation via melt solidification poses inherent constraints on sample size using conventional casting techniques. This drawback can be overcome by pressure-assisted sintering of metallic glass powders at temperatures above the glass transition, where the material shows viscous-flow behavior. Partial crystallization during sintering usually exacerbates the inherent brittleness of metallic glasses and thus needs to be avoided. In order to achieve high density of the bulk specimens while avoiding (or minimizing) crystallization, the optimal combination between low viscosity and long incubation time for crystallization must be identified. Here, by carefully selecting the time–temperature window for powder consolidation, we synthesized highly dense Zr48Cu36Ag8Al8 bulk metallic glass (BMG) with mechanical properties comparable with its cast counterpart. The larger ZrCu-based BMG specimens fabricated in this work could then be post-processed by flash-annealing, offering the possibility to fabricate monolithic metallic glasses and glass–matrix composites with enhanced room-temperature plastic deformation.

Effect of rare earth oxide on the crystallization behavior of CaO-Al2O3-based mold flux for rare earth heat-resistant steel continuous casting

New CaO-Al2O3-based mold flux for rare earth steel continuous casting was devised to restrain the slag-steel interface reaction. The effect of Ce2O3 on the crystallization behavior were investigated. The continuous cooling transformation tests show that the addition of Ce2O3 not only reduces the critical cooling rate of the mold flux but also reduces the initial crystallization temperature. Besides, the distribution of the initial crystallization temperature became more stable. The temperature time transformation tests show that the crystallization incubation time is slightly increased, the addition of Ce2O3 inhibit the isothermal crystallization process to a certain extent. The crystalline phases changed from LiAlO2+CaO to LiAlO2+CaCeAlO4. The precipitation of CaO was restrained by adding Ce2O3. With the combination of Ca2+, Ce3+, and [AlO69−]-octahedron units, CaCeAlO4 formed and precipitated in the crystallization process. Besides inhibiting the crystallization of CaO-Al2O3-based mold flux, adding rare earth oxide also can reduce the viscosity and the breaking temperature.

Crystallization behavior and melt structure of typical CaO–SiO2 and CaO–Al2O3-Based mold fluxes

Crystallization behavior and melt structure of two typical mold fluxes A (CaO–SiO2-based) and B (CaO–Al2O3-based) for casting high-aluminum steel were investigated using double hot thermocouple technology (DHTT), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results suggest that the crystallization temperature of Flux B is higher, and its crystallization incubation time is shorter compared with Flux A. The precipitated phase in Flux A is CaSiO3, whereas BaAl2O4 and Ca2Al2SiO7 form in Flux B. The structure analyses suggest that the degree of polymerization of Flux A is larger than that of Flux B. In addition, the major structural units of Flux A are Si–O–Si, Q0Si, Q1Si, Q2Si and Q3Si, but those of Flux B are mainly aluminate (Al–O–Al, Al–O-), aluminosilicate (Al–O–Si) and silicate units (Q0Si, Q1Si, Q2Si and Q3Si). These different melt structures are the main reasons why the precipitated phases in these two mold fluxes are different, and the crystallization ability of Flux A is weaker than Flux B.

Crystallization behavior of blast-furnace slag by single hot thermocouple technique

The crystallization behavior of blast-furnace slag under isothermal and continuous-cooling conditions was studied using the single hot thermocouple technique. The crystallization phases were obtained using FactSage software and X-ray diffractometry. The crystallization kinetic parameters were calculated by combining these results with the Johnson–Mehl–Avrami model. Under isothermal conditions, the shortest crystallization incubation time was 24 and 18 s when the temperatures were 1300 and 1150 °C, and the corresponding critical cooling rates were 4.5 and 14.3 °C/s, respectively. At 1270 °C, the slag was difficult to crystallize and the fiber-forming rate improved. When the continuous-cooling rate was 6.5 °C/s, the slag solidified into a glassy state. The main crystallization phases, gehlenite, akermanite, anorthite, and melangite, were most easily precipitated. The growth factors of melangite and anorthite were approximately 1.63 and 1.68, respectively, which indicated that the crystals nucleated on the surface and grew in two dimensions.

Effect of the Al2O3/SiO2 mass ratio on the crystallization behavior of CaO-SiO2-MgO-Al2O3 slags using confocal laser scanning microscopy

The crystallization behavior of a CaO-SiO2-MgO-Al2O3 slag system with varying Al2O3/SiO2 mass ratios from 0.03 to 1.10 has been investigated using a confocal laser scanning microscopy (CLSM). The resulting continuous cooling transformation (CCT) and time-temperature-transformation (TTT) curves showed that the initial crystallization temperature increased and the incubation time for crystallization slightly decreased with increasing Al2O3/SiO2 ratio. The crystal growth rate first increased and then decreased with decreasing isothermal temperature. X-ray diffraction (XRD) analysis suggested that Ca2MgSi2O7 or Ca3MgSi2O8 precipitated as the primary phase at lower Al2O3/SiO2 ratios, while the Ca2Al2SiO7 phase was preferred at higher Al2O3/SiO2 ratios. The observed crystalline phases correlated well with the expected thermodynamic predictions from FactSage. In addition, structural analysis using 27Al magic angle spinning nuclear magnetic resonance (27Al MAS-NMR) microscopy of the as-quenched slags indicated the presence of a higher ratio of tetrahedral [AlO4]5-structural units with increasing Al2O3/SiO2 ratio, which enhanced the polymerization of tetrahedral [AlO4]5- and [SiO4]4- structural units to form Ca2Al2SiO7.

Crystallization, high temperature deformation behavior and solid-to-solid formability of a Ti-based bulk metallic glass within supercooled liquid region

Empirical information of crystallization was first deduced using differential scanning calorimeter for a monolithic Ti44.4Zr6.6(Cu0.8Ni0.15Sn0.05)49 bulk metallic glass with 2 mm in diameter, characterized by a high activation energy and short incubation time for crystallization. Through the understanding of the compressive deformation modes as a function of temperature and strain rate within supercooled liquid region, processing map was constructed based on dynamic materials model. By overlapping time-temperature transformation curve onto processing map, macroscopic formability was found to have a good correspondence with the region of iso-efficiency contours having power dissipation efficiency above 0.4 without in-situ crystallization.

Microstructure Characterization and Kinetics of Crystallization Behavior of Tubular Spray Formed Fe43.2Co28.8B19.2Si4.8Nb4 Bulk Metallic Glass*

Abstract In this study the Fe43.2Co28.8B19.2Si4.8Nb4 (at.%) alloy was processed by spray forming in an industrial facility with the aim of investigating the formation of amorphous phases. The thicker layers (5–15 mm) presented partial/fully crystalline microstructure with onset of crystallization of the amorphous phase at T x = 595 °C. On the other hand, thinner layers (below 2.5 mm) presented fully amorphous structure, and glassy behaviour with glass transition (T g ) at 555 °C. The thermal stability and the crystallization kinetics of the alloy were studied using the isothermal DSC curves, measured at different heating rates and temperatures. Despite its good glass-forming ability (GFA) and high stability against crystallizations, its incubation time for crystallization is almost zero. In order to rule-out the complete crystallization mechanism, in situ temperature resolved studies using TEM were done. In addition, the amorphous part of the deposit was heat treated at various temperatures/times and the results were compared with the in situ crystallization behavior observed by TEM.

The role of yttrium and oxygen on the crystallization behavior of a Cu–Zr–Al metallic glass

The crystallization behavior of Cu–Zr–Al alloys with different contents of oxygen and yttrium was studied. Three compositions were chosen based on the good glass former Cu–45Zr–8Al with varying Y contents (0, 0.3 and 2%) added to getter the oxygen from the glass. Melt spun ribbons were prepared and their crystallization behavior was analyzed by differential scanning calorimetry (DSC).Heat treatments were performed at selected temperatures based on the DSC data and the phases formed were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results revealed that oxygen has a strong influence on the crystallization behavior, it changes the mode and the phases formed and results in the formation of a metastable crystalline phase prior to the formation of the stable phases.The Y was found to have a more subtle effect: it lowers the incubation time for crystallization and promotes heterogeneous nucleation of Cu10Zr7 and, therefore, adding Y to these alloys has this important drawback that is reported for the first time here. The mechanisms by which each of these elements acts are suggested. Finally the formation of defects in the Cu10Zr7 phase is reported.

Stress relaxation behavior of Fe–Co–Si–B–Nb metallic glassy alloys in their supercooled-liquid state

Iron-based metallic glasses have attracted much attention for use as microscale components in many devices because they provide unique processability with nanoscale precision. Fe-based metallic glasses, however, do not have enough resistance against crystallization, and therefore their incubation time for crystallization is too short. To prevent crystallization, the processing temperature cannot be set too high within the supercooled-liquid region, and high processing speeds are required. Taking this into consideration, the viscoelastic properties were observed during the viscous-flow microprocessing of a single metallic glassy particle of [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic glass to identify its relaxation behavior of on the micrometer scale during a compressive test at 833K. The relaxation behavior is found to be well described by the Kohlrausch–Williams–Watt model. The dependence of the ratio of the viscosity to the modulus of rigidity on the compressive speed during deformation was also investigated by constructing the master curve describing the behavior of the elasticity and viscosity fractions versus the apparent relaxation time. At a compressive strain rate of 4.5×10−2s−1, the Fe-based metallic supercooled liquid behaved as a viscoelastic fluid, although at a compressive strain rate of 4.5×10−3s−1, its behavior was almost viscous. From the master curve, it can be concluded that the effect of the compressive speed on the ratio of the viscosity to the modulus of rigidity can clearly be observed, and the viscous-flow behavior of a [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic supercooled liquid can be predicted at any processing speed.

Effect of Na2O and B2O3 on the Crystallization Behavior of Low Fluorine Mold Fluxes for Casting Medium Carbon Steels

An investigation has been conducted to study the effect of Na2O and B2O3 on the crystallization behavior of low fluorine (F) mold powders for casting medium carbon (MC) steels in this article. The results of this study indicated that B2O3 tends to lower the crystallization temperature and increase crystallization incubation time of the low F powders; however, Na2O plays an opposite role compared with that of B2O3. The crystalline phase of Ca11Si4B2O22 was formed in Sample D2 [F = 3 pct, Na2O = 10 pct, B2O3 = 8 pct (in wt pct)], which exhibited the most similar crystallization behavior to that of cuspidine, such that Sample D2 showed closest crystallization kinetics to that of a conventional high-F mold slag for casting MC steels. The precipitated crystalline phases for all the samples have been analyzed and discussed in the article.

Effects of B2O3and TiO2on Crystallization Behavior of Slags in Al2O3-CaO-MgO-Na2O-SiO2System

Nowadays, there are increasing demands for developing mould fluxes without fluoride due to environmental concerns. Slag bearing titanium oxide and/or boron oxide proved to be promising substitute for traditional mould fluxes with fluoride. In the present work, Crystallization behaviors of some slags in Al2O3–CaO–MgO–Na2O–SiO2–B2O3–TiO2 system were investigated using single hot thermocouple technique (SHTT), differential thermal analysis (DTA), and X-ray diffraction (XRD) technique. It was found that liquidus temperature of slag decreases with increases of B2O3 content. The XRD analysis on the crystallized samples shows that crystallization products are perovskite, gehlenite, and wollastonite. Time–temperature–transformation diagrams (TTT) for various slags have been constructed using SHTT technique. It was found that B2O3 can significantly increase the incubation time for crystallization of slags. Incubation time for crystallization of slags at nose temperature decreases with increase of titanium oxide content from 5% to 10%. The morphology of crystals was also investigated using SHTT technique. At higher temperature, dendrites and columnar crystal tend to form and grow with time progress, while at lower temperature, small crystals appear and the amount of crystal increase with time increasing.

Influence of nanofiller dimensionality on the crystallization behavior of HDPE/carbon nanocomposites

The isothermal and nonisothermal crystallization behavior of high density polyethylene (HDPE) containing various zero, one, and two dimensional (0-D, 1-D, and 2-D) carbon nanofillers were investigated by means of differential scanning calorimetry. For a given temperature, the isothermal crystallization incubation time of HDPE became longer with the addition of lower dimensional carbon nanofillers, and the isothermal crystallization rate got slower. The values of Avrami and Tobin exponents indicated that the isothermal crystallization of HDPE followed two-dimensional crystal growth in the presence of 2-D and 1-D carbon nanofillers, while exhibited three-dimensional heterogeneous crystal growth in the presence of 0-D carbon nanofillers. Contrary to the isothermal study, the nonisothermal crystallization of HDPE was accelerated in the presence of lower dimensional nanofillers. The nonisothermal crystallization data were finally analyzed using Ozawa and Mo methods. It was observed that only Mo approach could successfully describe the nonisothermal crystallization process of HDPE and HDPE/carbon nanocomposites. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Crystallization Behaviors of Slags through a Heat Flux Simulator

This paper investigates how the basicity and alumina content in synthetic slags influence the crystallization behavior that takes place in a heat flux simulator for mold slags. The purpose is to elucidate the variation in crystallization behavior for model slags that are expected to be glassy, partly crystalline and fully crystalline in order to correlate the changes in heat flux to the dynamic solid evolution that occurs in the simulator. Three levels of alumina content (3, 15 and 25 wt%) were chosen to investigate the heat transfer behavior through slag film which have different tendency of reaction with molten steel during continuous casting of high aluminum containing Transformation Induced Plasticity (TRIP) steels. A Confocal Scanning Laser Microscopy (CSLM) was used to develop TTT diagrams for the slags. XRD and SEM were also used to analyze the micro-structures of the crystalline phases.The measured heat fluxes through the mold slags tested were found to increase, as the crystallinity of the slag film decreased with decreasing basicity and alumina content. It was found that the crystallization temperature increased, while the incubation time for crystallization decreases with increasing basicity and alumina content. The increase in alumina content induced the precipitation of CaF2 during cooling process and hence a change in the crystalline phase from Ca4Si2O7F2 to Ca2Al2SiO7.

Altering the nucleation of thermally annealed hydrogenated amorphous silicon with laser processing

We demonstrate the use of laser processing to affect the nucleation of crystallites in thermally annealed hydrogenated amorphous silicon (a-Si:H) thin films. The influence of film H content and subcrystallization threshold laser fluence are investigated by x-ray diffraction measurements during in situ thermal annealing at 600 °C. All laser-treated films show a reduced incubation time for crystallization compared to as-grown films, with the largest differences exhibited for samples with higher film H and higher laser fluences. These results are consistent with multivacancy annihilation by laser processing, based upon a recently developed model for a nucleation center in a-Si:H.

Thermal stability and crystallization kinetics of Mg–Cu–Y–B quaternary alloys

The thermal stability and crystallization kinetics of the Mg 65Cu 25Y 10 and Mg 65Cu 25− x Y 10B x (numbers indicate at.%, x = 1–10) amorphous alloys are examined and compared in terms of non-isothermal and isothermal differential scanning calorimetry measurements. It is demonstrated that, with the minor addition of 1–5 at.% B, the incubation time for crystallization is prolonged and the activation energy is increased, suggesting an improvement of the thermal stability. Nevertheless, the overall nucleation and growth characteristics of the parent and B-additive alloys are similar. The minor B element appears to impose resistance to the crystallization of the major Mg 2Cu phase, possibly by blocking the path of Mg and Cu diffusion. With further addition of B upto 10 at.%, secondary crystallization phases such as MgB 4 or YB would be induced and thus lower the thermal stability.

Oxidation induced phase instability of Cu 54Zr 22Ti 18Ni 6 bulk metallic glass

Effects of isothermal oxidation on the chemical instability which triggers crystallization of Cu 54Zr 22Ti 18Ni 6 (numbers indicate at.%) bulk metallic glass were investigated. Comparing the isothermal heat flux of Cu 54Zr 22Ti 18Ni 6 bulk metallic glass within the supercooled liquid region in differential scanning calorimetry, it could be observed that the isothermal oxidation did decrease the incubation time for crystallization. What is further, this amorphous phase instability was enhanced with the decrease of particle size. Through this study, chemical instability due to preferential oxidation of alloying element and resulting deviation of chemical composition in intact metallic region needs to be considered in phase evolution of bulk metallic glass during material processing.

Simultaneous and different nucleation modes in undercooled Pd–Cu–Ni–P melts

Abstract Both Pd 40 Cu 30 Ni 10 P 20 alloys with a slightly off-eutectic composition and Pd 42.5 Cu 30 Ni 7.5 P 20 alloys with a eutectic composition exhibit high glass-forming ability and a low critical cooling rate for glass formation of 0.100 and 0.067 K/s, respectively. Due to the difference in the critical cooling rate, an incubation time for crystallization at 683 K for the eutectic alloy is 2.5 times longer than that for the off-eutectic alloy. In order to clarify the reason for the difference, nucleation and crystal grain growth behaviors of the undercooled off-eutectic and eutectic melts were investigated. Nucleation frequency was evaluated as 3.00×10 10 nuclei/m 3 s for the off-eutectic alloy and 4.77×10 9 nuclei/m 3 s for the eutectic alloy. In addition, it is found that the nucleation behavior changes from internal homogeneously dispersed mode to surface one. On the contrary, the crystal grain growth rates have almost the same value of about 3.20×10 −7 m/s for the crystallized phases in both alloys. Based on these results, the compositional effect on the stability of undercooled melt is discussed in the present study.