Isothermal Annealing Conditions Research Articles

Modeling vacancy-induced porosity in compositionally-graded complex alloys

A phase-field model of the void evolution is developed to investigate the vacancy-induced Kirkendall porosity within diffusion couples under isothermal annealing conditions. This model encapsulates a comprehensive computational framework for multicomponent diffusion, non-equilibrium vacancy diffusion, voids nucleation, and growth — integrating considerations for surface anisotropy and morphological instability. This novel thermodynamically consistent methodology combines the phase-field model with the vacancy diffusion model and uses thermodynamic and atomic mobility parameters defined by CALPHAD approach. The generalized nature of the method enables us to include both non-equilibrium and equilibrium vacancies, covering all cases of non-ideal/ideal sinks and sources for vacancies, whether at dislocations or void surfaces. Moreover, the model can deal with compositionally complex alloys such as Ni-based superalloys. Along with the full model, a fast variant is also suggested, providing long-time simulations at the experimentally relevant time and space scales valuable for both academic and industrial contexts. The model’s efficacy was demonstrated through precise simulation of voids evolution in a model AlCoCrTa/Ni diffusion couple, mirroring the experimental outcomes observed in the CMSX-10/Ni diffusion couple at 1525 K after 192 h of annealing. This encompassed accurate replication of diffusion profiles and tomographic analysis of voids. A very good agreement with the experimental data was observed, particularly in the distribution of pore volume-fractions and the delineation of void shapes.

Effect of 0.5 at. % Cu addition on the crystallization behavior and soft magnetic properties of Fe83B11C1Si2P3 alloy

In order to gain insight into how the addition of Cu element affects the crystallization mechanism of amorphous nanocrystalline alloys and soft magnetic properties, the crystallization behavior of (Fe0.83B0.11Si0.02P0.03C0.01)100-xCux (x = 0 and 0.5) amorphous ribbons was investigated based on kinetic and thermodynamic behavior under non-isothermal and isothermal annealing conditions. The results show that under non-isothermal annealing conditions, the Cu0.5 amorphous ribbon not only favors the formation of the amorphous structure, but also enlarges the temperature window for annealing and improves the thermal stability of the α-Fe(Si) phase. At the same time, the Cu0.5 amorphous ribbon shows slow kinetic and is insensitive to heating rate change during annealing treatment. Under isothermal conditions, the addition of Cu decreases the nucleation activation energy (En) and increases the growth activation energy (Eg). This further indicates that the nucleation of α-Fe(Si) grains is easier than the growth process, which is conducive to the formation of a homogeneous nanostructure and improves the soft magnetic properties of the alloy. Compared to Cu0 amorphous ribbon, it is also shown that the Cu0.5 amorphous ribbon changes the crystallization mechanism from diffusion-controlled one-dimensional to two-dimensional growth when the crystallization volume fraction x is 10–30 vol%. While three-dimensional growth become dominated when x is reaches 60 vol% for both ribbons. Therefore, the kinetics of crystallization as well as microstructure supports that the Cu0.5 ribbon is able to exhibit uniformly fine nanocrystalline structures. Further, the Cu0.5 nanocrystalline ribbon shows a higher Bs of 1.85 T and a lower Hc of 2.55 A/m, respectively, and the favorable (100) orientation of α-Fe(Si) grains formed after annealing is another factor to increase Bs.

In situ study and assessment of the phosphorus-induced solute drag effect on the grain boundary mobility of austenite

The solute drag effect of phosphorus (P) in the single-phase austenite (γ-Fe) region was studied under isothermal annealing conditions at temperatures of 1050 °C, 1150 °C, 1250 °C and 1350 °C. High-temperature laser scanning confocal microscopy (HT-LSCM) was used to observe and quantify in situ grain growth on the surface of three different samples containing 0.026 wt.-% P, 0.044 wt.-% P and 0.102 wt.-% P. The dependence of the arithmetic mean grain sizes on time and temperature were modeled mathematically using a simple ordinary differential equation (ODE) according to normal grain growth (NGG) theory. As no other major effects, i.e., Zener-pinning by precipitates, occurred under the selected experimental conditions, grain growth interference was only considered by grain boundary (GB) segregation of P. Thus, the total grain boundary mobility (M) was directly determined depending on the P content and isothermal annealing temperature. The fitted GB mobility values enabled the determination of an average binding energy value between impurity P atoms and grain boundaries (E0 = -1.1 - -0.6 eV) in the system. Finally, the GB segregation of P in γ-Fe was derived from the observed grain growth kinetics. The results showed reasonable agreement with calculations using segregation enthalpies from the literature.

Critical Examination of the Representativeness of Austenite Grain Growth Studies Performed In Situ Using HT-LSCM and Application to Determine Growth-inhibiting Mechanisms

This contribution critically addresses the potential of HT-LSCM experiments for in situ observations of austenite grain growth (AGG). By quantifying AGG for various alloys, the impact of impurity induced solute drag effects (SDE) and second phase precipitation Zener pinning forces (PZ) on AGG can be estimated. Also the grain boundary mobility (GBM) can be determined. The measured arithmetic mean of the time-resolved grain size distributions as a function of temperature and chemical composition is the most important value for quantification. The obtained data is then used to contribute to mathematical models of classical grain growth theory and to allow conclusions on parametrization of SDE and PZ. In this contribution, grain size measurements at the sample surface (in situ and ex situ) are compared with ex situ bulk measurements and experiments on grain growth in the single-phase austenite region (γ-Fe) under isothermal annealing conditions at different temperatures are presented. Grain growth results include high-purity Fe (Fe > 99.98%), binary Fe‑P, Fe‑C, and quaternary Fe-C-Nb‑N systems. For the alloys investigated, it is assumed that grain growth in high-purity Fe occurs without the influence of solute drag or precipitation mechanisms. In Fe‑P, it is shown that grain growth is inhibited by the segregation of impurity atoms at the grain boundaries (GB), which allows conclusions to be drawn about the influence of SDE. In the case of Fe-C-Nb‑N systems, the influence of Nb(C,N) precipitation on grain growth due to Zener pinning forces is presented.

Preparation of Sb2Se3-based ceramics and glass-ceramics from native thin films deposited on Kapton foil

Amorphous Sb2Se3 thin films (200–1500 nm) deposited on Kapton foil were crystallized in controlled environment using the method of direct calorimetric measurement of thin films. X-ray diffraction was used to determine the intensity of the 1D (Sb4Sb6)n ribbons preferential orientation in dependence on the processing conditions: film thickness, energy flux during the pulsed laser deposition (PLD), heating rate, low and high temperature annealing. Lower PLD energy flux combined with higher deposited film thickness (1500 nm) led to increased orientation in the [2 2 1] direction, which is favorable for thin film solar cells application. Raman spectroscopy showed that thermally induced crystallization produces oxide-free Sb2Se3 thin films. Description of the crystallization kinetics led to very accurate kinetic predictions for the isothermal annealing conditions suitable for preparation of glass-ceramics with defined amount of crystalline phase – the predictions were accurate to ∼10% of the true (experimentally determined) value.

Influence of 1.5 wt.% Bi on the Microstructure, Hardness, and Shear Strength of Sn-0.7Cu Solder Joints after Isothermal Annealing.

This manuscript reports the isothermal annealing effect on the mechanical and microstructure characteristics of Sn-0.7Cu-1.5Bi solder joints. A detailed microstructure observation was carried out, including measuring the activation energy of the intermetallic compound (IMC) layer of the solder joints. Additionally, the synchrotron µX-ray fluorescence (XRF) method was adopted to precisely explore the elemental distribution in the joints. Results indicated that the Cu6Sn5 and Cu3Sn intermetallic layers thickness at the solder/Cu interface rises with annealing time at a rate of 0.042 µm/h for Sn-0.7Cu and 0.037 µm/h for Sn-0.7Cu-1.5Bi. The IMC growth’s activation energy during annealing is 48.96 kJ mol-1 for Sn-0.7Cu, while adding Bi into Sn-0.7Cu solder increased the activation energy to 55.76 kJ mol−1. The µ-XRF shows a lower Cu concentration level in Sn-0.7Cu-1.5Bi, where the Bi element was well dispersed in the β-Sn area as a result of the solid solution mechanism. The shape of the IMC layer also reconstructs from a scallop shape to a planar shape after the annealing process. The Sn-0.7Cu hardness and shear strength increased significantly with 1.5 wt.% Bi addition in reflowed and after isothermal annealing conditions.

Shaping Microstructure and Mechanical Properties of High-Carbon Bainitic Steel in Hot-Rolling and Long-Term Low-Temperature Annealing.

Based on the research results, coefficients in constitutive equations, describing the kinetics of dynamic, meta-dynamic, and static recrystallization in high-carbon bainitic steel during hot deformation were determined. The developed mathematical model takes into account the dependence of the changing kinetics in the structural size of the preliminary austenite grains, the value of strain, strain rate, temperature, and time. Physical simulations were carried out on rectangular specimens. Compression tests with a flat state of deformation were carried out using a Gleeble 3800. Based on dilatometric studies, coefficients were determined in constitutive equations, describing the grain growth of the austenite of high-carbon bainite steel under isothermal annealing conditions. The aim of the research was to verify the developed mathematical models in semi-industrial conditions during the hot-rolling process of high-carbon bainite steel. Analysis of the semi-industrial studies of the hot-rolling and long-term annealing process confirmed the correctness of the predicted mathematical models describing the microstructure evolution.

Self-assembly of block copolymers under non-isothermal annealing conditions as revealed by grazing-incidence small-angle X-ray scattering

An accurate knowledge of the parameters governing the kinetics of block copolymer self-assembly is crucial to model the time- and temperature-dependent evolution of pattern formation during annealing as well as to predict the most efficient conditions for the formation of defect-free patterns. Here, the self-assembly kinetics of a lamellar PS-b-PMMA block copolymer under both isothermal and non-isothermal annealing conditions are investigated by combining grazing-incidence small-angle X-ray scattering (GISAXS) experiments with a novel modelling methodology that accounts for the annealing history of the block copolymer film before it reaches the isothermal regime. Such a model allows conventional studies in isothermal annealing conditions to be extended to the more realistic case of non-isothermal annealing and prediction of the accuracy in the determination of the relevant parameters, namely the correlation length and the growth exponent, which define the kinetics of the self-assembly.

Deformation Behavior of a VT18U Titanium Alloy in the Oxidized State

The influence of the isothermal annealing conditions in air (at 560 and 625°C for 1000 h) on the deformation behavior of a high-temperature VT18U titanium alloy under tension is studied. The plasticity of the alloy at room temperature decreases from 14 to ≈5% after annealing at the investigated temperatures. The thickness of the gas-saturated layer is 17 and 42 μm after annealing at 560 and 625°C, respectively. The main factor determining the plasticity of the alloy in the oxidized state is the oxygen content gradient in the gas-saturated layer.

Annealing twin evolution in the grain-growth stagnant austenitic stainless steel microstructure

Twin evolution mechanism in the growing and otherwise growth-stagnant grains of austenitic stainless steel was studied here during isothermal annealing and repetitive anneal-quench cycle. Specimens were subjected to two different types of annealing treatment; a) isothermal annealing for different durations at 1200 °C and b) also repetitively annealed at 1200 °C and then water quenched. Under isothermal annealing conditions a critical grain growth kinetics was found responsible for the occurrence of high twin frequency. Interestingly, almost negligible twinning tendency was seen after grain growth becomes stagnant. However, reoccurence of twinning activity was seen in growth-stagnant grains when subjected to the repetitive anneal-quench cycles. Reoccurrence of twinning, in otherwise growth stagnant grains, behaviour is attributed to thermo-elastoplastic strain generation due to repetitive thermal volume expansion and contraction. It is proposed in this study that quenching from high temperature resulted in interface stress generation due to rigid body rotation of grains and also local modified stacking fault energy sites at grain boundary.

Microstructure, Solidification Texture, and Thermal Stability of 316 L Stainless Steel Manufactured by Laser Powder Bed Fusion

This article overviews the scientific results of the microstructural features observed in 316 L stainless steel manufactured by the laser powder bed fusion (LPBF) method obtained by the authors, and discusses the results with respect to the recently published literature. Microscopic features of the LPBF microstructure, i.e., epitaxial nucleation, cellular structure, microsegregation, porosity, competitive colony growth, and solidification texture, were experimentally studied by scanning and transmission electron microscopy, diffraction methods, and atom probe tomography. The influence of laser power and laser scanning speed on the microstructure was discussed in the perspective of governing the microstructure by controlling the process parameters. It was shown that the three-dimensional (3D) zig-zag solidification texture observed in the LPBF 316 L was related to the laser scanning strategy. The thermal stability of the microstructure was investigated under isothermal annealing conditions. It was shown that the cells formed at solidification started to disappear at about 800 °C, and that this process leads to a substantial decrease in hardness. Colony boundaries, nevertheless, were quite stable, and no significant grain growth was observed after heat treatment at 1050 °C. The observed experimental results are discussed with respect to the fundamental knowledge of the solidification processes, and compared with the existing literature data.

Dissolution Kinetics of Spheroidal-Shaped Precipitates in Age-Hardenable Aluminum Alloys

As a first attempt, a mathematical model is proposed to predict the dissolution kinetics of non-spherical secondary phase precipitates during solution heat treatment of age-hardenable aluminum alloys. The model uses general spheroidal geometry to describe the dissolution process of the alloys containing needle/disc-shaped particles with different size distributions in a finite matrix. It is found that as the aspect ratio deviates from unity, the dissolution rate is accelerated. Also, the dissolution rate of the particles in the alloy containing the particle size distribution is lower than that of mono-sized particles system. The modeling results for dissolution of θ′ precipitates in an Al-Cu alloy are compared with experiments, and a good agreement was found between the modeling and the experimental results. The proposed model can be applied to different isothermal and non-isothermal annealing conditions.

The Influence of Grain Structure on Intermetallic Compound Layer Growth Rates in Fe-Al Dissimilar Welds

The thickness of the intermetallic compound (IMC) layer that forms when aluminum is welded to steel is critical in determining the properties of the dissimilar joints. The IMC reaction layer typically consists of two phases (η and θ) and many attempts have been made to determine the apparent activation energy for its growth, an essential parameter in developing any predictive model for layer thickness. However, even with alloys of similar composition, there is no agreement of the correct value of this activation energy. In the present work, the IMC layer growth has been characterized in detail for AA6111 aluminum to DC04 steel couples under isothermal annealing conditions. The samples were initially lightly ultrasonically welded to produce a metallic bond, and the structure and thickness of the layer were then characterized in detail, including tracking the evolution of composition and grain size in the IMC phases. A model developed previously for Al-Mg dissimilar welds was adapted to predict the coupled growth of the two phases in the layer, whilst accounting explicitly for grain boundary and lattice diffusion, and considering the influence of grain growth. It has been shown that the intermetallic layer has a submicron grain size, and grain boundary diffusion as well as grain growth plays a critical role in determining the thickening rate for both phases. The model was used to demonstrate how this explains the wide scatter in the apparent activation energies previously reported. From this, process maps were developed that show the relative importance of each diffusion path to layer growth as a function of temperature and time.

Production and characterization of CuCrO2 nanofibers

Composite nanofibers were prepared from homogeneously mixed copper chloride, chromium nitrate and polyvinylpyrrolidone solution by electrospinning and were subsequently heat treated under isochronal and isothermal annealing conditions. In the isochronal experiments, CuCrO2 formation began at 773 K. Annealing at 973 K yielded single-phase, highly porous CuCrO2 nanofibers accompanied by the formation of large quasi-hexagonal grains. In contrast, isothermal annealing at 973 K resulted in the formation of phase-pure CuCrO2 nanofibers with belt-like morphology and relatively higher density. A substantial decrease was observed in the size and number of plate-like grains in these nanofibers. When the fibers were annealed isothermally at 973 K for 1 h, the average width and specific surface area of the synthesized fibers were 150 nm and 7.85 m2 g−1, respectively, and the direct bandgap was 2.82 eV. The CuCrO2 behaved like a semiconductor, and the thermally activated energy was approximately 0.157 eV. A simple formation mechanism was proposed for the quasi-hexagonal grains.

Phase separation of a supersaturated nanocrystalline Cu–Co alloy and its influence on thermal stability

The thermal decomposition behaviour, the microstructural evolution and its influence on the mechanical properties of a supersaturated Cu–Co solid solution with ∼100nm average grain size prepared by severe plastic deformation is investigated under non-isothermal and isothermal annealing conditions. Pure fine-grained Cu and Co exhibit substantial grain growth upon annealing, whereas the Cu–Co alloy is thermally stable at the same annealing temperatures. The annealed microstructures are studied by independent characterisation methods, including scanning electron microscopy, electron energy loss spectroscopy and atom probe tomography. The phase separation process in the Cu–Co alloy proceeds by the same mechanism, but on different length scales: a fine scaled spinodal-type decomposition is observed in the grain interior, simultaneously Co and Cu regions with a larger scale are formed near the grain boundary regions. Subsequent grain growth at higher annealing temperatures results in a microstructure consisting of the pure equilibrium phases. Such mechanisms can be used to tailor nano structures.

Role of applied uniaxial stress during creep testing on precipitation in Mg–Nd alloys

The coupled effects of an applied stress and temperature, during creep testing at 90MPa/177°C, on intragranular second phase (orthorhombic β′ and fcc β1 phases) precipitation in a binary Mg–0.5at% Nd alloy were investigated by comparing and contrasting with second phase precipitation under isothermal (177°C) annealing conditions. In the early stages of creep deformation, after 5h, homogeneous precipitation of fine scale β′ was observed within the α-Mg matrix, while on the other hand, only small pockets of Nd enrichment were observed in the isothermally annealed sample after 5h, indicating that β′ precipitation is accelerated during creep testing. With respect to β1 precipitation, these precipitates appeared to decorate dislocation lines primarily, which is indicative of a dislocation stress-field assisted precipitation mechanism, in both creep tested and isothermally annealed samples. Interestingly, the β1 precipitates appear to grow and coarsen more rapidly in the case of the isothermally annealed sample than in the creep tested sample. The differences between the precipitation behavior of β′ and β1 phases under the influence of stress and temperature (creep testing) vs. only under isothermal annealing conditions have been rationalized based on the crystal structures of these phases and their elastic interaction energies with the stress-fields associated with dislocations as well as bulk applied stress.

KINETICS OF GRAIN NUCLEATION IN SUPERCOOLED QUASI-TWO-DIMENSIONAL Cu

Grain nucleation kinetics of nanocrystalline Cu in a quasi-two-dimensional system was investigated using molecular dynamics. Nanoscale grains nucleate rapidly from supercooled Cu under isothermal annealing conditions. An Avrami exponent from the JMAK equation implies that the kinetics follows the continuous nucleation model. Observation of grain growth during the nucleation stage shows that the kinetics follows the classical grain growth model with a low growth exponent. The lower value of the exponent illustrates the rapid growth of grains during this stage. Using the growth exponent, the activation energy for nucleation in Cu was obtained and compared with the values obtained in earlier studies.

Unraveling structural evolution of LiNi0.5Mn1.5O4 by in situ neutron diffraction

The electrochemical properties of the spinel LiNi0.5Mn1.5O4 cathode material are influenced by the synthesis processes, which determines the impurity phase and the distribution of Ni and Mn in the spinel structure. Taking advantage of neutron's high sensitivity to Ni and Mn, in situ neutron diffraction has been employed to quantify the phase formation/structural evolution process under continuous heating/cooling and isothermal annealing conditions. The results show that the subtle Ni and Mn ordering process occurs slowly at 700 °C and the degree of ordering can be controlled by the annealing time. At temperatures above 750 °C, the LiNi0.5Mn1.5O4 spinel phase starts to decompose into the rock-salt impurity phase accompanied by the release of O2. The rock-salt phase reverts back to the spinel phase upon cooling along with the oxygen uptake. The dynamic process of structural evolution of LiNi0.5Mn1.5O4 that was unraveled by in situ neutron diffraction is valuable for guiding the synthesis of cathode materials with desirable properties.

Dynamics of crystallization with fractal geometry: Extended KJMA approach in glasses

AbstractAlthough the kinetics of crystallization under isothermal annealing conditions in glassy materials is fairly well understood by the Kolmogorov–Johnson–Mehl–Avrami (KJMA) formalism, there is still a matter of debate on its applicability. By introducing a fractal geometry and the Meyer–Neldel rule on thermally activated processes into the KJMA theory, we can overcome some of the difficulties with the classical KJMA mode, with maintaining its intuitive simplicity. Very reasonable physical parameters are deduced from the extended KJMA theory

New Method of Evaluating the Crystallization Activation Energy of Ge2Sb2Te5 by In situ Resistance Measurement

A new method of evaluating crystallization activation energy of Ge2Sb2Te5 is proposed by in situ resistance measurement under isothermal annealing conditions. Linear relationship between logarithmic time and reciprocal temperature in modified Johnson–Mehl–Avrami–Kolmogorov equation is derived under the assumption that proportion of resistance drop from the initial value is closely related to crystal fraction. Crystallization activation energy thus obtained is 2.67 eV. Numerical calculation was conducted to manifest the validity of this analysis based on percolation model. Moreover, crystallization behavior of patterned single-line structure of Ge2Sb2Te5 was evaluated, and the scaling effect of increasing activation energy with decreasing line width was observed.