Isothermal Annealing Process Research Articles

The thermal stability and degradation mechanism of Cu/Mo nanomultilayers

ABSTRACT The microstructural evolution of Cu/Mo nanomultilayers upon annealing was investigated by X-ray diffraction and transmission electron microscopy. The isothermal annealing process in the temperature ranges of 300–850°C was conducted to understand the thermal behavior of the sample and follow the transformation into a nanocomposite. Annealing at 600°C led to the initiation of grain grooving in the investigated nanomultilayer, and it degraded into a spheroidized nanocomposite structure at 800°C. The sample kept the as-deposited Cu {111}//Mo{110} fiber texture up to 850°C. The residual stress was investigated to explain microstructure changes. The activation energy of degradation kinetics of Cu/Mo nanomultilayers was determined to understand the rate-determining mechanism for the degradation of nanolaminate structures.

Evolution of coupling modes between α and β relaxations in metallic glass-forming liquids revealed by nano-calorimetry

The coupling behavior of α and β relaxations in supercooled liquids (SLs) is a key scientific issue that relates to the essence of glass transition and the modulation of glass properties. Here, we observe the evolution of α and β relaxations in Au50Ag7.5Cu17.5Si25 alloys from glass to SLs by nano calorimetric technique. We find that all the β participates in α relaxation in these metallic glasses (MGs) upon heating. Two typical coupling (or decoupling upon cooling) modes of α and β relaxations in Au-based MG SLs have been discovered, dependent on the degree of coupling between α and β relaxations in glasses. The coupling degree increases during the isothermal annealing process as the energy state of the glass decreases. Above a critical degree the decoupling mode changes from “β splits from α” to “α splits from β”. A survey including metallic glasses, polymer glasses and oxide glasses shows the general existence of the critical degree of coupling in glasses. The differences in local structure feature in glasses with different coupling degree is elucidated by analysis on atomic nonaffine displacements using molecular dynamics simulations. Further our work demonstrates that the plasticity of MGs can be effectively modulated by altering the coupling behavior between α and β relaxation.

Phase-Field Modeling of Spinodal Decomposition in Fe–Cr–Co Alloy under Continuous Temperature-changing Conditions

Fe-Cr-Co alloys are becoming important as a half-hard magnet for their novel applications, including non-contact electromagnetic brakes, because of the controllability of its magnetic hardness depending on the modulated structure formed by spinodal decomposition. However, the experimental optimization of the complicated heat-treatment process to control the microstructure significantly increases the development cost, and microstructure prediction by computational simulation is desired. In this study, we first developed the method of phase-field simulation for spinodal decomposition in Fe-Cr-Co alloy during various heat treatments, including isothermal heat treatment, multistep continuous fast and slow cooling, which allows us to conduct a simulation of spinodal decomposition under conditions close to the condition of practical heat treatment. The simulation results revealed that the morphology of the modulated structure is predominantly determined by the cooling rate and does not change significantly during the subsequent isothermal annealing process, while the difference between the concentrations of the FeCo-rich magnetic phase and the Cr-rich non-magnetic phase increases. Continuous cooling at rates higher than 140 K/h demonstrates the maximum number densities of the ferromagnetic particles of α1-phase seemingly almost reaching saturation, which is expected to give rise to exhibiting the largest coercive force of the Fe-Cr-Co magnet. Moreover, this method can be extended to other materials for designing a modulated structure to show a desired property.

Crystallization analysis and determination of Avrami exponents during isothermal annealing and the effect of cooling rate on the evolution of the atomic structure of Pd78Si16Cu6 alloy: A molecular dynamics simulation study

In this study, molecular dynamics simulations have been carried out to investigate the effect of cooling rate on the atomic structure of the Pd78Si16Cu6 alloy and the isothermal annealing process on the evolution of its glassy structure. It is observed that the atomic structure, glass transition temperature and crystallization temperature of the system are affected significantly by the cooling rate. While the liquid system cooled at slower cooling rates than 1 K/ps transitions to a crystalline structure, a glassy structure is observed for faster cooling rates. The crystallization kinetics of the annealing process are explained by the Johnson, Mehl and Avrami model using Honeycutt–Andersen crystalline-type bonded pairs. The findings show that the system transitions from a metastable phase to a stable crystalline phase during isothermal annealing of the amorphous Pd78Si16Cu6 alloy, and increasing the annealing time for lower temperature increases the thermal stability of the amorphous phase.

Manipulating Conjugated Polymer Backbone Dynamics through Controlled Thermal Cleavage of Alkyl Side Chains.

The morphological stability of an organic photovoltaic (OPV) device is greatly affected by the dynamics of donors and acceptors occurring near the device's operational temperature. These dynamics can be quantified by the glass transition temperature (Tg ) of conjugated polymers (CPs). Because flexible side chains possess much faster dynamics, the cleavage of the alkyl side chains will reduce chain dynamics, leading to a higher Tg . In this work, the Tg s for CPs are systematically studied with controlled side chain cleavage. Isothermal annealing of polythiophenes featuring thermallycleavable side chains at 140°C, is found to remove more than 95% of alkyl side chains in 24h, and raise the backbone Tg from 23to 75°C. Coarse grain molecular dynamics simulations are used to understand the Tg dependence on side chain cleavage. X-ray scattering indicates that the relative degree of crystallization remains constantduring isothermal annealing process. The effective conjugation length is not influenced by thermal cleavage; however, the density of chromophore is doubled after the complete removal of alkyl side chains. The combined effect of enhancing Tg and conserving crystalline structures during the thermal cleavage process can provide a pathway to improving the stability of optoelectronic properties in future OPV devices.

Investigation into Changes of Microstructure and Abrasive Wear Resistance Occurring in High Manganese Steel X120Mn12 during Isothermal Annealing and Re-Austenitisation Process.

Hadfield steel, under unit pressure conditions, strengthens itself by forming a high density dislocation structure, which results in increased resistance to dynamic impact wear. However, under abrasion conditions, the homogeneous microstructure of the cast steel is insufficient to achieve the expected service life. The aim of the research is to conduct a comparative analysis of the material in its as-delivered state and after two-stage heat treatment (isothermal annealing followed by re-austenitisation). It was found that after isothermal annealing of X120Mn12 grade steel at a temperature of 510 °C, a microstructure with a complex morphology consisting of colonies of fine-grained pearlite, (Fe,Mn)3C carbides distributed along the grain boundaries of the former austenite and needle-like (Fe,Mn)3C carbides was obtained in the austenite matrix. The subsequent thermal treatment of the steel with the use of supersaturating annealing at 900 °C resulted in a heterogeneous microstructure consisting of evenly distributed globular carbide precipitations in a matrix of considerably finer austenite grains in comparison with the as-delivered original state. As a result of the final microstructural changes achieved, a 16.4% increase in abrasion resistance was obtained compared to the delivered condition.

Isothermal annealing of selenium (Se)-implanted silicon carbide: Structural evolution and migration behavior of implanted Se

Isothermal annealing studies of selenium-implanted silicon carbide (SiC) were conducted at temperatures >1200 °C. Implantation were performed using Se ions of 200 keV to a fluence of 1 × 1016 cm−2at room temperature, 350 °C and 600 °C. After implantations, samples were then subjected to an isothermal annealing process at 1300 °C, 1350 °C and 1400 °C for 10 h cycles up to 80 h. The radiation damage in SiC and its morphological change were characterized using Raman spectroscopy and scanning electron microscopy (SEM), respectively. The migration of implanted Se was monitored by Rutherford backscattering spectrometry (RBS). Implantation at RT amorphized SiC while implantation at 350 and 600 °C retained crystallinity with defects. Isothermal annealing led to significant recrystallization during the first annealing cycle in all annealing temperatures. The broadening of the Se RBS profile was observed in the RT implanted samples only during the first and second annealing cycles at all annealing temperatures. The diffusion coefficients at 1300 °C, 1350 °C and 1400 °C were estimated to be 1.4 × 10−20 m2s−1, 2 × 10−20 m2s−1 and 2.5 × 10−20 m2s−1, respectively, which yielded to an activation and pre-exponential factor of 2 × 10−22 J and 1.7 × 10−16 m2s−1 respectively. No measurable diffusion of the Se implanted into SiC was observed in the isothermally annealed hot implanted samples (at implantation temperature of 350 °C and 600 °C) confirming the radiation enhanced migration of Se in the RT implanted samples.

Single-mode helical Bragg grating waveguide created in a multimode coreless fiber by femtosecond laser direct writing

We demonstrate the fabrication of single-mode helical Bragg grating waveguides (HBGWs) in a multimode coreless fiber by using a femtosecond laser direct writing technique. This approach provides a single-step method for creating Bragg grating waveguides. Specifically, the unique helical structure in such an HBGW serves as a depressed cladding waveguide and also generates strong Bragg resonance due to its periodicity. Effects of pulse energy, helical diameter, and helical pitch used for fabricating HBGWs were studied, and a single-mode HBGW with a narrow bandwidth of 0.43 nm and a Bragg wavelength of 1546.50 nm was achieved by using appropriate parameters, including a diameter of 10 μm and a helical pitch of 1.07 μm. The measured cross-sectional refractive index profile indicates that a depressed cladding waveguide has been created in this single-mode HBGW. Moreover, five single-mode HBGWs with various Bragg wavelengths were successfully fabricated by controlling the helical pitch, and this technique could be used for achieving a wavelength-division-multiplexed HBGW array. Then, the temperature and strain responses of the fabricated single-mode HBGW were tested, exhibiting a temperature sensitivity of 11.65 pm/°C and a strain sensitivity of 1.29 pm/με, respectively. In addition, the thermal stability of the single-mode HBGW was also studied by annealing at a high temperature of 700°C for 15 h. The degeneration of the single-mode waveguide into a multimode waveguide was observed during the isothermal annealing process, and the peak reflection and the Bragg wavelength of the fundamental mode exhibited a decrease of ∼ 7 dB and a “blue” shift of 0.36 nm. Hence, such a femtosecond laser directly written single-mode HBGW could be used in many applications, such as sapphire fiber sensors, photonic integrated circuits, and monolithic waveguide lasers.

High-strength carbon nanotube fibers with near 100% purity acquired via isothermal vacuum annealing

The Fe and carbonaceous particles are the main impurities in a carbon nanotube fiber (CNTF) fabricated by the floating catalyst chemical vapor deposition (FCCVD) method, which have an adverse effect on various applications, leading to strong demand for purification process to obtain high-purity CNTFs. However, the commonly used purification methods combining oxidation and acid refluxing cannot remove the impurities thoroughly, and will introduce undesired damage onto CNT wall at the same time. Herein, we propose an efficient purification strategy involving an isothermal annealing process under high vacuum to achieve mass preparation of CNTFs with high purity and mechanical/electrical properties. The Fe and carbonaceous impurities can be almost completely removed after vacuum annealing at 1800 °C. The purified CNTFs exhibit enhanced oxidation resistance due to the removal of Fe catalyst particles. The diffusion and evaporation of Fe atoms are most likely responsible for the removal of Fe particles enclosed with graphitic shells. The purified CNTF shows a high specific tensile strength of 1.82 ± 0.04 N/tex due to the high alignment of the CNT bundles within fibers, which can retain 80% of the value for pristine CNTF. The purified CNTF exhibits similar trends of electrical conductivity as the specific tensile strength and has a high conductivity of 3.65 ± 0.11 × 105 S/m. The large-scale preparation of high-purity CNTFs with high-performance will promote the real application of CNTFs.

Development of a technology of isothermal annealing with the use of the forging heat for chromium‑molybdenum steel

The article discusses the results of investigations performed during a thermo-mechanical treatment of forgings made of chromium‑molybdenum 42CrMo4 grade steel. The treatment was realized during a regular series production. The forging process was combined with a heat treatment carried out directly after forging on a specially adapted station. Such a production technology will make it possible to eliminate the step of repeated heating of the forgings. On the example of an element of a steering gear, it was demonstrated how it is possible to perform an isothermal annealing process starting from the temperature at which the trimming of the forgings ends. During the cooling of the forgings, it is enough to maintain the temperature at the proper level in order for the exothermal phase transformation of austenite into pearlite to take place. The test results show that, with the properly selected temperature of isothermal annealing, it is possible to obtain an equilibrial ferritic-pearlitic structure in the required hardness scope. Introducing such a solution into the industrial practice would allow savings more than 80% of the energy used for the heat treatment of forging in this case.

Precipitation of α-Fe from Fe84−xSi4B12+x (x = 1, 3) Amorphous Alloys Under High Magnetic Field Annealing

This work aims to investigate the effects of high magnetic field annealing (HMFA) on the precipitation of α-Fe from Fe84−xSi4B12+x (x = 1, 3) amorphous precursors. Isothermal annealing process for Fe81Si4B15 and Fe83Si4B13 amorphous ribbons has been performed with and without the magnetic field. The magnetic field shows the effects of increasing the nucleation rate and decreasing the grain size of α-Fe crystals simultaneously, during the crystallization processes of the investigated amorphous alloys. By applying HMF, α-Fe crystals with more homogeneous distribution and smaller grain size are achieved in the amorphous matrix, which is crucial helpful to improve the magnetic properties for Fe–Si–B amorphous alloys. The mechanism of HMFA affecting the crystallization microstructure is also discussed in the present work.

Effect of a high magnetic field on the macro- and microstructures of Zn-Mg alloy during semi-solid isothermal annealing process

Zn-Mg alloy was isothermally annealed in the semi-solid range under a high magnetic field (HMF). Results showed that, regardless of the HMF, the specimens exhibit inhomogeneous structures, i.e., primary Zn-rich dendrites and granular (nearly spherical or platelike in three dimensions) particles are distributed in the upper and lower parts, respectively. Applying the HMF and decreasing the annealing temperature significantly contribute to the structural homogeneity, which are ascribed to the induced magnetic viscosity resistance force and the higher viscosity of the interdendritic liquid, respectively. Moreover, the HMF aligns the primary dendrites and platelike particles with their longer axes parallel to the HMF direction B. In addition, the HMF preferentially orients all the granular particles with their c-axis perpendicular to B, regardless of their sizes and shapes. The regular alignment and preferential orientation are related to the magnetocrystalline anisotropy of the primary Zn-rich crystals.

High-order fiber Bragg grating fabricated by femtosecond laser pulses for high-sensitivity temperature and strain sensing

We have extensively investigated the characteristics of temperature and strain sensing for two high order (the 3rd and 4th order) fiber Bragg Gratings (FBGs). The FBGs were manufactured in a single mode fiber (SMF-28) without hydrogen-loading utilizing femtosecond laser based on a point-by-point direct writing method. The annealing process was studied under the response of temperature up to 900℃. With regarding to the 3rd-order gratings, the experimental sensitivity of temperature (T-sensitivity) was 15 pm/℃ in the range of 25–900N℃ and that of longitudinal strain (S-sensitivity) was 3.7 pm/ ℃ in the range of 0–1824 με. For the 4th-order gratings, T-sensitivity was 13 pm/℃ in the range of 22−900 ℃ and S-sensitivity was 1.1 pm/με in the range of 0–1824 με, the experimental value of which were lower than that of the 3rd-order FBG. During the isothermal annealing process, we achieved that the peak reflectivity of 98 % (-17.3 dB transmission loss) with stability in high-temperature at Bragg wavelength of 1548.5 nm for the 3rd-order gratings and the peak reflectivity of 93 % (-11.6 dB transmission loss) at Bragg wavelength of 1411.3 nm for the 4rd-order gratings. It indicated that the sensitivity of the FBGs depends on the mode order. The proposed FBGs have potential for high-temperature sensing and high-resolution strain sensing. The multi-parametric detection and hazardous environment monitor based on the FBGs can be accomplished in the near future.

Influence of Cu Content on Structure and Magnetic Properties in Fe86-xCuxB14 Alloys.

Influence of Cu content on thermodynamic parameters (configurational entropy, Gibbs free energy of mixing, Gibbs free energy of amorphous phase formation), crystallization kinetics, structure and magnetic properties of Fe86-xCuxB14 (x = 0, 0.4, 0.55, 0.7, 1) alloys is investigated. The chemical composition has been optimized using a thermodynamic approach to obtain a minimum of Gibbs free energy of amorphous phase formation (minimum at 0.55 at.% of Cu). By using differential scanning calorimetry method the crystallization kinetics of amorphous melt-spun ribbons was analyzed. It was found that the average activation energy of α-Fe phase crystallization is in the range from 201.8 to 228.74 kJ/mol for studied samples. In order to obtain the lowest power core loss values, the isothermal annealing process was optimized in the temperature range from 260 °C to 400 °C. Materials annealed at optimal temperature had power core losses at 1 T/50 Hz—0.13–0.25 W/kg, magnetic saturation—1.47–1.6 T and coercivity—9.71–13.1 A/m. These samples were characterized by the amorphous structure with small amount of α-Fe nanocrystallites. The studies of complex permeability allowed to determine a minimum of both permeability values at 0.55 at.% of Cu. At the end of this work a correlation between thermodynamic parameters and kinetics, structure and magnetic properties were described.

Deformation behavior of annealed Cu64Zr36 metallic glass via molecular dynamics simulations

In the present work, we investigate the isothermal annealing process of Cu64Zr36 metallic glass (MG) by means of molecular dynamics (MD) simulations, and characterize the compression properties under different temperatures and compressive rates. The pair distribution functions, the bond-orientational order, and the cluster-type index method are modeled and analyzed to characterize changes in the structure. Results of the modeling and analysis reveal that the MgZn2-type Laves, MgCu2-type Laves and five-fold local symmetry structures can be formed during the isothermal holding at 950 K. Study on the compression properties shows that at a fixed strain rate, the yield strength and yield drop all increase with decreasing temperature. At a fixed temperature, however, the increase of the strain rate leads to a noticeable increase in the yield strength at 600 K, but has little effect on yield strength at 10 K. Moreover, the modeling and analysis of the structure at a temperature of 10 K and strain-rate of 5 × 107 s−1 demonstrate that the order-disorder transformation initiates the shear band.

Analysis of the Possibilities of Using Forging Heat in Isothermal Annealing Processes for AISI 4140 Alloy

Analysis of the Possibilities of Using Forging Heat in Isothermal Annealing Processes for AISI 4140 Alloy

Investigation of kinetic recovery process in low dose neutron-irradiated nuclear graphite by thermal annealing

ABSTRACT To investigate the kinetic recovery process of low dose neutron-irradiated graphite, nuclear-grade isotropic graphite IG-110U and ETP-10 were neutron irradiated using the JMTR up to 1.38 × 1023 n/m2 (En > 1 MeV) at ~473 K. In-situ measurement of macroscopic length was conducted during the isothermal and isochronal annealing process from room temperature up to 1673 K. From room temperature to 773 K for IG-110U, and to 1023 K for ETP-10, macroscopic lengths, lattice parameters, and unit cell volumes of both specimens recovered to their pre-irradiation values, and this recovery process subdivided into two stages. The activation energies of macroscopic volume recovery at 523–673 K and 673–773 K were determined to be ~0.22 eV and ~0.57 eV for IG-110U, respectively; ~0.13 eV and ~2.59 eV at 523–923 K and 923–1023 K for ETP-10, respectively. The migration of not only single interstitials but also interstitials dissociated from submicroscopic interstitial groups along basal planes followed by vacancy-interstitial recombination play a dominant role in the first stage. The second stage is suggested to proceed via the motion of carbon groups along basal planes for IG-110U, and migration of single interstitials along the c-axis for ETP-10. During 773 K or 1023 K up to 1673 K, macroscopic length continuously shrank with decreasing shrinking rate, even with a turnaround to swell at 1173 K for IG-110U.

Microstructure and fracture toughness of a WC-Fe cemented carbide layer produced by a diffusion-controlled reaction

To improve the mechanical properties of the surface of iron-based alloys, a tungsten carbide-iron (WC-Fe) cemented carbide layer is produced on an alloy by adopting an isothermal annealing process, which was performed at 1050 °C for 4 h. By deeply etching the obtained sample, the morphologies of the WC ceramic grains in the WC-Fe hardmetal layer are characterized via scanning electron microscopy. The present results reveal three distinct morphologies consisting of rectangular, triangular prism and multi-layered shapes. Furthermore, the mechanical properties and fracture toughness of the WC-Fe layer are investigated through combined nanoindentation and Vickers indentation techniques. Nanoindentation testing is performed in a load range of 100 to 450 mN. Based on the data collected from the nanoindentation results, the average values of the hardness, Young' modulus and deformation ratio are evaluated, and the fracture toughness is determined to have a value of 3.08 MPa·m½ at 450 mN. In the Vickers indentation technique, however, by identifying the crack type and choosing the appropriate model, the fracture toughness is calculated to be 1.85–3.44 MPa·m½ at applied loads ranging from 0.98 to 4.9 N. The obtained fracture toughness results exhibit good consistence between the nanoindentation and Vickers indentation methods.

Evaluation of microstructure and growth kinetics of tungsten carbide ceramics at the interface of iron and tungsten

The microstructure evolution and growth kinetics of tungsten carbide (WC or W2C) ceramics at the interface of iron and tungsten during isothermal annealing process from 1100 °C to 1150 °C were investigated by X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and scanning electron microscopy (SEM), respectively. The results show that carbide formation that involves W2C, WC and a small quantity of Fe2W2C, is strongly dependent on a diffusion-controlled reaction. During the evolution process of tungsten carbides, W2C emerges as an initial phase by the reaction of W and C, and WC is formed as the second phase, because the value of Gibbs free energy of formation of W2C is more negative than that of WC at the annealing temperature. The initial W2C transforms into WC until complete consumption of the W2C. Additionally, Fe2W2C between W2C and WC was formed at a relatively low temperature (1100 °C and 1125 °C) with a long annealing time (60 min). After the W2C is consumed completely, WC grows by C diffusion, where the growth kinetics of a dominant WC ceramic exhibits a parabolic growth law before metal tungsten is completely consumed. The growth activation energy of the WC layer is calculated to be achieved with a value of 208.68 kJ/mol. When the metal tungsten is completely consumed after annealing at 1150 °C for 60 min, the W2C and Fe2W2C disappear, and only the WC phase exists in the final product.

Microstructure, texture and mechanical properties evolution of extruded fine-grained Mg-Y sheets during annealing

The microstructure, texture and mechanical properties evolution of the extruded fine-grained Mg-(1, 5) Y (wt%) alloy sheets were investigated during annealing. The as-extruded sheets exhibited a fully dynamic recrystallized (DRXed) microstructure consisting of uniform and fine equiaxed grains (5.7–8.7 µm) and a small amount of YH2 second phase particles. Both sheets exhibited significant thermal stability at 300 °C annealing and remarkable grain growth at temperatures higher than 400 °C. The measured grain growth activation energy (Q) of Mg-1Y at all temperatures tested was 91 ± 3 kJ/mol, suggesting that the growth was controlled by grain boundary diffusion. Meanwhile, for Mg-5Y alloy at lower temperatures (300–400 °C), the Q value (59 kJ/mol) indicated that the grain growth was controlled by grain boundary diffusion while the Q value (175 kJ/mol) at higher temperatures (400–450 °C) implied that lattice self-diffusion controlled the process. A representative rare-earth texture was present for both sheets in the as-extruded condition, and remarkable basal texture weakening was observed with the grain growth during each isothermal annealing process. The texture weakening can be ascribed to the preferential growth of non-basal oriented grains based on the EBSD analysis, which was likely related to the segregation of Y atoms at grain boundaries. The microhardness and grain size relationship can be described well by the Hall-Petch relation for all the annealed sheets, while the yield stress was not the case indicating that the macroscopic strength was more sensitive to the texture than the localized microhardness.