Isothermal Annealing Process Research Articles

Phase formation kinetics, hardness and magnetocaloric effect of sub-rapidly solidified LaFe11.6Si1.4 plates during isothermal annealing

High-temperature phase transition behavior and intrinsic brittleness of NaZn13-type τ1 phase in La–Fe–Si magnetocaloric materials are two key problems from the viewpoint of materials production and practical applications. In the present work, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation was introduced to quantitatively characterize the formation kinetics of τ1 phase in sub-rapidly solidified LaFe11.6Si1.4 plates during the isothermal annealing process. Avrami index was estimated to be 0.43 (∼0.5), which suggests that the formation of τ1 phase is in a diffusion-controlled one-dimensional growth mode. Meanwhile, it is found that the Vickers hardness as a function of annealing time for sub-rapidly solidified plates also agrees well with the JMAK equation. The Vickers hardness of τ1 phase was estimated to be about 754. Under a magnetic field change of 30 kOe, the maximum magnetic entropy change was about 22.31 J/(kg·K) for plates annealed at 1323 K for 48 h, and the effective magnetic refrigeration capacity reached 191 J/kg.

Effect of Annealing on the Curie Temperature in the Bulk Amorphous Alloys

The paper presents results of research for volumetric amorphous alloys Fe61+xCo10-XY8W1B20 where x = 0,12 in the shape of plater of 10mm x 5mm x 0.5mm diameter. Amorphous Samales were annealed at 953K for 10 minutes what caused their partial crystallization. As a result of the thermal interference in the volume of alloys, three different crystalline phases were formed aFe, Fe5Y and Fe3B. For samples after solidification and after thermal treatment thermomagnetic studiem were conducted. For amorphous alloys, only one ferro-paramagnetic transition was visible, and for the alloys after the isothermal annealing process there were two inflections on the curve .m0Ms (T) corresponding to the two Curie temperatures. These intersections came from two amorphous phases. As a result of the research, it was found that these alloys crystallized in the primal crystalization. Crystallization was the reason for the increase of the first Curie temperature of the amorphous matrix while retaining its decrease with cobalt additive. The value of the second Curie temperature in the alloys after heating was found to depend mainly on the size of the crystallization products and in particular on the Fe5Y crystallite size.

Electrical Resistance Relaxation in La<sub>55</sub>Al<sub>25</sub>Ni<sub>10</sub>Cu<sub>10</sub> Bulk Metallic Glass

Electrical resistance is always related to the electronic structure of metallic glass and sensitive to structural changes, which provides a more intuitive approach to investigate structure evolution of metallic glasses upon structural relaxation. Electrical resistance relaxation of the La55Al25Ni10Cu10 bulk metallic glass was studied using the standard four-probe method. The electrical resistance of La55Al25Ni10Cu10 bulk metallic glass decreases significantly with the structural relaxation below the glass transition temperature at 445 K. During the subsequent continuous heating, the relaxed specimen shows a reduction in the resistivity decrease at the glass transition. The relaxed electrical resistance caused by the structural relaxation during the isothermal measurement equals the changes of the electrical resistance reduction in the glass transition region in the subsequent isochronal measurements. The calculated relaxed electrical resistance as a function of the annealing time can be fitted by Kohlrausche-Williams-Watts (KWW) equation. The equilibrium value, time constant and the stretched exponent for the isochronal electrical resistance measurements at 445 K are 0.0384, 2390s and 0.66, respectively. The in-situ electrical resistance data recorded in the isothermal annealing process show the same relaxation behavior with fitting parameters 0.0362, 2033 s, and 0.74, respectively.

Influence of Cu Substitution on the Structure and Magnetic Properties of Partially Crystallized Fe62-xCo10Y8MexB20 Alloys

The paper presents results of structural and magnetic properties investigation of partially crystallized Fe62Co10Y8B20 and Fe61Co10Y8Cu1B20 alloys. Multicomponent bulk amorphous alloys exhibit a significant sensitivity to the composition changes. Even a small quantitative element substitution in composition of the initial alloy may lead to substantial changes in its various properties. Samples in form of thin plates were obtained by radial cooling method resulting in a complete amorphous structure material. Then, the one-step controlled heat treatment process led to partial crystallization of the sample causing the coexistence of amorphous matrix and embedded nanocrystalline phases. As shown in article, amorphous matrix has a noticeable influence on nano-sized grains resulting in a deformation of primary cell structure. Isothermal annealing process also had an impact on the magnetic properties of obtained samples, which was examined by the vibrating sample magnetometer (VSM).

Vpliv izotermnega žarjenja na strukturo in magnetne lastnosti masivne amorfne zlitine FeCoBYMo

Vpliv izotermnega žarjenja na strukturo in magnetne lastnosti masivne amorfne zlitine FeCoBYMo

The Effects of a High Magnetic Field on the Annealing of [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb₄ Bulk Metallic Glass.

In contrast with amorphous alloys, nanocrystalline soft magnetic materials show improved thermal stability and higher soft magnetic properties. The nanocrystalline soft magnetic composites are usually fabricated by partially crystallizing from parent amorphous alloys. This paper reports our experimental observation on the sequence of crystallization in metallic glass under a high magnetic field (HMF). An application of a HMF to bulk metallic glass (BMG) of [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 prioritizes the precipitation of α-(Fe,Co) phase separated from the subsequent precipitation of borides, (Fe,Co)23B6, upon isothermal annealing at a glass transition temperature. Furthermore, it was observed that, through the annealing treatment under a HMF, a soft magnetic nanocomposite, in which only α-(Fe,Co) phase uniformly distributes in amorphous matrix, was achieved for boron-bearing BMG. The promotion of the α-Fe or (Fe,Co) phase and the prevention of the boride phases during the isothermal annealing process help to produce high-quality soft magnetic nanocomposite materials. The mechanism by which a HMF influences the crystallization sequence was interpreted via certain changes in Gibbs free energies for two ferromagnetic phases. This finding evidences that the annealing treatment under a HMF is suitable for enhancing the soft magnetic properties of high B content (Fe,Co)-based bulk amorphous and nanocrystalline materials.

Effect of high magnetic field on crystallization behavior of Fe83B10C6Cu1 amorphous alloy

This paper aims to investigate the effect of high magnetic field on the crystallization of Fe83B10C6Cu1 amorphous ribbons. Isothermal annealing processes of the amorphous alloy were performed at some temperatures for a given time with or without an applied magnetic field. These annealed samples were characterized using differential scanning calorimeter, X-ray diffraction, and transmission electron microscopy. It has been found that a 12 T magnetic field enhances the crystallization process of the amorphous alloy, and α-Fe was precipitated from the amorphous matrix at the temperatures used. The 12 T magnetic field greatly encourages precipitation kinetics of α-Fe at appropriate annealing temperatures and time. The volume fraction and number of α-Fe crystals in the alloys annealed under the 12 T magnetic field are larger than those in the alloys annealed without the magnetic field. The mechanism of high magnetic field on the crystallization of Fe83B10C6Cu1 amorphous alloy is to enhance the nucleation and growth of α-Fe crystals.

Investigation on the overlapping bands of syndiotactic polystyrene by using 2D-IR spectroscopy

In this work, WAXD and FTIR spectroscopy were utilized to investigate the phase transition of syndiotactic polystyrene (sPS) from amorphous phase to mesophase during the isothermal annealing process at 130 °C. Two dimensional (2D) correlation infrared spectroscopy was applied to reveal the sub-bands from the highly overlapping bands. The ∼900 cm−1 band is shown to be composed of two sub-bands. One band located around 906 cm−1 corresponds to the amorphous phase, another peak that occurs around 900 cm−1 is associated with mesophase. The trans-planar conformation band at 1223 cm−1 turns out to consist of two bands which might be related to trans-planar conformation with different sequence lengths.

Formation of nanoscale titanium carbides in ferrite: an atomic study

The formation and evolution of nanoscale titanium carbide in ferrite during the early isothermal annealing process were investigated via molecular dynamics simulation. The atomic interactions of titanium and carbon atoms during the initial formation process explained the atoms aggregation and carbides formation. It was found that the aggregation and dissociation of titanium carbide occurred simultaneously, and the composition of carbide clusters varied in a wide range. A mechanism for the formation of titanium carbide clusters in ferrite was disclosed.

The mechanism of grain growth and thermal stability in Ni-1 at.% Pb alloy

The effect of undercooling and subsequent annealing on the evolution of grain growth of Ni-1 at.% Pb alloy was investigated with the help of glass fluxing technique and isothermal annealing process. To investigate the mechanism of grain growth, isothermal annealing processes of the as-quenched samples at different undercoolings were conducted at different annealing temperatures. The whole process of grain size evolution could be rationally categorized as three stages: 1. The slow growth stage; 2. The rapid growth stage; 3. The stable growth stage. With the utilization of grain boundary energy σb model, it is shown that the stability in stage 1 is due to the reduction of σb. As a result of second phase precipitation occurring at 2–3 stages, rapid grain growth occurs.

Research on the Elimination of Banded Structure in SAE8620H Gear Steel

In order to eliminating the banded structure in SAE8620H gear steel,an isothermal annealing process and its mechanism has been studied in this paper. Results showed that isothermal annealing process can effectively eliminate banded structure in SAE8620H gear steel.With isothermal temperature 640°C-a certain low temperature indeed high undercooling,and for high nucleation at this temperature resulting in the decrease of nucleation rate difference in all areas,banded structure level decreased to 1 level.When isothermal temperature decreased to 610°C and 590°C,although the undercooling is so high that various regions began to nucleate at the same time,the transition temperature is so low that alloy elements in the regions previously occupied by pearlite bands diffuse faster than other regions,thus proeutectoid ferrite grains in these regions grow faster,regenerating the banded structure .

The Influence of Heat Treatment on Irreversible Structural Relaxation in Bulk Amorphous Fe61Co10Ti3Y6B20Alloy

Amorphous materials contain structural defects, which play a key role in the magnetization process within the condition known as the approach to ferromagnetic saturation . This paper presents the results of magnetization studies, carried out on bulk Fe61Co10Ti3Y6B20 alloy when under the in uence of a strong magnetic eld. The alloy samples were obtained in the form of a rod 1 mm in diameter, and tested in the as-quenched state and after an isothermal annealing process, at a temperature below the crystallization temperature. It was observed that the heat treatment, carried out below crystallization temperature Tx, leads to irreversible structural relaxations, speci cally reorganizing the atomic con guration within the volume of the alloy into an amorphous structure.

Vaporization and Poor Wettability as the Main Challenges in Fabrication of TiB2-Cu Cermets Studied by SPS

TiB2-Cu cermets with various volume fractions of copper (from 3 to 30 vol. %) were produced via liquid phase sintering at the temperature range 1100–1200 °C in vacuum using spark plasma sintering (SPS) technique. Full densification could not be achieved as the consequence of poor wettability and vaporizing Cu. The quantitative Rietveld analysis indicated that insignificant reduction in Cu content occurred only in sample with initially 3 vol. % of Cu, but then densification was negligible. The relative density improved along with increasing volume content of Cu (10–20 vol. %), but then predominant amount of Cu introduced was reduced as the effect of vaporization or swelling, which caused that pellet with intended 20 or 30 vol. % of Cu contained respectively only 6 or 17 vol. % after sintering. Moreover, Cu droplets were released from the die at the temperature of 1000–1030 °C near the Cu melting point. The effect of vaporization was successfully reduced by increased heating rate and when isothermal annealing process was skipped, however, it could not be entirely eliminated. The experimental results on Cu vaporization are confronted with parameters that are commonly considered in the production of cermets, such as oxidation, wettability, contact angle and viscosity as well as their impact on densification.

Water-insoluble aerogels made from cellulose nanocrystals and poly(vinyl alcohol)

Cellulose nanocrystal (CNC) aerogels are attractive low-density materials, although the brittleness and/or water dispersibility can limit the usefulness of these materials. The authors report here the preparation of water-insoluble aerogels made by lyophilization of aqueous dispersions of low-aspect ratio CNCs extracted from cotton. Poly(vinyl alcohol) (PVOH) was added as a polymeric binder, and the PVOH- and the CNC-concentration ([PVOH] and [CNC]) as well as the freeze-drying conditions were systematically varied. The effect of these parameters on the morphology, mechanical properties and the specific surface area of the resulting CNC/PVOH aerogels was investigated. By increasing the PVOH-concentration, the stiffness of the aerogels increases by a factor of two, and varying the [CNC] as well as the freezing method leads to different pore morphologies. An environmentally friendly, isothermal annealing process was used to induce crosslinking without the need for auxiliary chemicals and was shown to decrease the water dispersibility and increase the stiffness by c. 50%. A more pronounced stiffness increase was achieved by chemically crosslinking the nanocomposite aerogels with sodium tetraborate (Na2B4O7), but in this case the water dispersibility was increased. This article contains supporting information that will be made available online once the issue is published.

Effect of Isothermal Annealing on Microstructural Morphology of Martensite in a Super-Martensitic Stainless Steel Subjected to Different Prior Conditions

A super-martensitic stainless steel of the Fe–Cr–Ni family was investigated for morphological changes during isothermal annealing after subjecting it to different prior conditions. The key issues during thermomechanical treatment included determination of conditions for austenite stability and reversibility and deciding the appropriate prior treatments for the cold worked alloy before subjecting it to isothermal annealing. The study evaluated the effect of isothermal annealing on the recrystallization kinetics, phase reversion, and microstructural changes in the alloy. Intercritical isothermal annealing was carried out on samples in the range 750–900 °C for short time periods in the range of 1–2.5 min. The recrystallization behavior and microstructural changes were studied by electron backscatter diffraction, X-ray diffraction, and Vickers's hardness measurements. Martensite morphology showed significant changes during the isothermal annealing process with dependence on prior matrix substrate. The tensile properties were also evaluated. The cold rolled (CR) and isothermally annealed samples provided an improved combination of strength and ductility at the optimum heat treatment parameters.

Structural characterizations and thermal analyses of Se70Te30 chalcogenide glassy alloy

The structural characterizations of Se70Te30 chalcogenide glass for the as-prepared and thermal annealed samples are identified using the X-ray diffraction (XRD) and scanning electron microscopy (SEM). The isothermal thermal annealing process results in a transformation from the amorphous to the crystalline state. The average crystallite size increases from 8.67 to 11.24nm when the temperature annealing increases from 95 to 118°C. The thermal analyses of Se70Te30 glass are investigated using a differential thermal analyzer (DTA) at five different heating rates under non-isothermal conditions.The activation energy of the glass forming (Eg) as a parameter for the glass transition is estimated by employing different approaches proposed for thermal analyses. The study of crystallization kinetics includes the estimation of the activation energy of crystallization (Ec), frequency factor (Ko) and Avrami exponent (n).

Structure Relaxations during the Isothermal Annealing of Amorphous Ca<sub>7</sub>Mg<sub>3</sub> Alloy

The molecular dynamics simulation studies on the microstructure evolution properties of amorphous Ca7Mg3 alloy during the isothermal annealing have been performed. The simulated structure factor S(q) of Ca7Mg3 is well agreed with the experimental data. Results indicate that the metallic glass of Ca7Mg3 alloy is relaxed into amorphous structure of greater stability in which much more icosahedron structures is formed; interestingly, it is also found that the short short-range order has no change, while the short-range order of the system increases during the isothermal annealing process.

Recrystallisation Characteristics of Cold Rolled Austenitic Stainless Steel during Repeated Annealing

The aim of the present study was to understand recrystallisation behaviour of a cold rolled AISI 316L austenitic stainless steel when annealed repetitively for short durations. The results were compared with isothermal annealing process. The evolving microstructure was examined by electron microscope. Electron backscattered diffraction was performed in a FEI NANO SEM 430 Field Emission Gun scanning electron microscope using an EDAX/TSL high-speed Digiview camera for Kikuchi pattern acquisition. Also progress of recrystallisation was assessed by hardness measurements at different annealing conditions. The study revealed distinct differences in the progress of recrystallisation of repeated and isothermally annealed specimens. Also the formation of ultrafine grained microstructure by repeated annealing process was noted.

Thermostability of S30432 shot peened surface layer

The recrystallisation behaviour of S30432 shot peened deformation surface layer during isothermal annealing has been investigated by X-ray diffraction line profile analysis method. The results revealed that annealing time and temperature were two key parameters for isothermal annealing process of S30432 steel. With higher annealing temperature, the recrystallisation behaviour in the shot peened deformed layer was more obvious. Based on the results of line profile analysis, the recrystallisation activation energy and microstrain relaxation energy were calculated. In addition, after isothermal annealing, the depth distribution of domain size in the deformed layer was also investigated and discussed in details. According to the analysis, the results showed that the thermostability of S30432 shot peened surface layer was relatively stable when the annealing temperature was below 650°C.

Synthesis of ultrasmall quantum dots by redirecting kinetics-based crystal growth to thermodynamics-based crystal dissolution

Ultrasmall quantum dots (< 2 nm) have been synthesised by redirecting crystal growth to dissolution during the synthetic process. The redirection is carried out with a controlled quenching–isothermal annealing process, where quantum dots are first grown and then quenched to a lower temperature, followed by thermodynamically reducing the size in an isothermal annealing process. The synthesised ultrasmall quantum dots are of high quality and their photoluminescence efficiency is generally more than 60%. With polymer overcoating, the blue-emitting ultrasmall quantum dots are water dispersible and stable for more than nine months.