Activation Energy For Recrystallization Research Articles

Thermal stability of a highly-deformed warm-rolled tungsten plate in the temperature range 1100–1250 °C

Pure tungsten is considered as armor material for the most critical parts of fusion reactors (i.e. the divertor and the first wall), among other reasons due to its high melting point (3422°C) and recrystallization temperature. The thermal stability of a pure tungsten plate warm-rolled to a high plastic strain by 90% thickness reduction was investigated by isothermal annealing for up to 190h in the temperature range between 1100°C and 1250°C. Vickers hardness testing allowed tracking the changes in mechanical properties caused by recovery and recrystallization. The hardness evolution could be rationalized in terms of a logarithmic recovery kinetics and a Johnson–Mehl–Avrami–Kolmogorov recrystallization kinetics accounting for an incubation time of recrystallization. The observed time spans for recrystallization and the corresponding recrystallization activation energy for this highly deformed plate suggest that large plastic deformations (e.g. applied during shaping) are only suitable to produce tungsten components to be used at relatively low temperatures (up to 900°C for a 2 years lifespan). Higher operation temperatures will lead to fast degradation of the microstructure during operation.

New method for analyzing recrystallization kinetics of deformed metal by differential scanning calorimeter

The drawn copper wires have been analyzed by differential scanning calorimeter (DSC) and a new method, which uses DSC measurements to determine the Johnson-Mehl-Avrami-Kolmogorov (JMAK) exponent via introducing Arrhenius behavior and modifying the baseline of DSC curves, has been proposed. The results show that JMAK exponent and recrystallization activation energy of the drawn copper wires with a strain of 2.77 are about 2.39 and 125 kJ/mol, respectively. The line linking the tangency points of DSC curve hypotenuse can be used as the baseline when calculating recrystallization fraction. The JMAK exponent obtained by the DSC method is in a good agreement with that obtained by microhardness measurements. Compared to traditional methods to measure the exponent, the proposed method is faster and less labor intensive.

Thermal behavior of copper processed by ECAP with and without back pressure

Samples of electrolytic tough pitch (ETP) pure copper were subjected to 12 passes of Equal-Channel Angular Pressing (ECAP) at room temperature with and without back pressure. Subsequent annealing was performed to evaluate the influence of back pressure during ECAP on the thermal behavior of ultrafine-grained copper. The microstructural and hardness changes caused by annealing were characterized by orientation imaging microscopy (OIM) and microhardness measurements. The application of back pressure resulted in an earlier drop in hardness upon annealing, which is believed to be associated with a lower critical temperature for the initiation of recrystallization and a rapid coarsening of microstructure. Regardless of whether back pressure was applied or not, structure coarsening during short-time annealing of ECAP-processed copper was governed by discontinuous static recrystallization. This is seen as a result of microstructure heterogeneity. Analysis of recrystallization kinetics was carried out based on observations of the microstructure after annealing in terms of the Avrami equation. The magnitude of the apparent activation energies for recrystallization in the absence of back pressure and in the case of back pressure of 100MPa was estimated to be ~99kJ/mol and ~91kJ/mol, respectively. The reasons for reduced activation energy in the case of processing with back pressure are discussed.

Determination of Recrystallization Kinetics and Activation Energy of Grain Growth Process of Cu-14Al-4Ni Shape Memory Alloy

ABSTRACTThe non-ferrous shape memory alloys have, normally, two problems that hinder its use in industrial scale: the natural aging and grains growth. The first degrades the memory effect, while the second, observed during the processing of alloy, modifies the temperatures which the transformations occur. Thus, the study of kynetic of recrystallization is important for enabling the control of hardened state in function of treatment time, without causing excessive grain growth. Therefore, the objective of this study is to determine the kinetics of recrystallization of Cu-14Al-4Ni shape memory alloy, based on an empirical law of the formation of Jonhson-Mehl-Avrami, as well as their activation energies for grain growth process according to the empirical Arrhenius law. The alloy was vacuum melted in an induction furnace. After casting, the bulk samples of the alloy were homogenized for 24 hours, solubilized and hot rolled followed by water-quenching to initiate the recrystallization. Then, different samples were annealed at temperatures close to the peak, start and end of the DSC curve. Following the heat treatments, the samples were submitted to mechanical tests and the values of the properties were correlated to the fraction transformed for determination of recrystallization’s kinetic. For the characterization of the grain growth process, analyses in optical microscopy were accomplished and all annealed samples were examined by statistical metallography and the grain sizes were measured. After measurements, the ln[-ln(1-Yrec)] x ln(t) and the ln [D-Do] x 1/T diagrams were plotted to determine the parameters of Jonhson-Mehl-Avrami equation and the activation energy of the process, respectively. The results showed that the equation of the recrystallized fraction follows the empiric law of the formation of Jonhson-Mehl-Avrami for the considered property, as well as, also showed that the alloy Cu-14Al-4Ni is extremely sensitive to temperature variation in which the alloy is treated, having a dual kinetics of grain growth. In the first domain, between 670 and 710°C, the diagram provides a value for the activation energy equal to 39.32 KJ/mol, in the second domain, between 710 and 790°C, the diagram provides a value for the activation energy equal to 9.01 KJ/mol.

Electrical Conductivity, Thermal Stability, and Lattice Defect Evolution During Cyclic Channel Die Compression of OFHC Copper

Oxygen-free high-conductivity (OFHC) copper samples are severe plastically deformed by cyclic channel die compression (CCDC) technique at room temperature up to an effective plastic strain of 7.2. Effect of straining on variation in electrical conductivity, evolution of deformation stored energy, and recrystallization onset temperatures are studied. Deformation-induced lattice defects are quantified using three different methodologies including x-ray diffraction profile analysis employing Williamson-Hall technique, stored energy based method, and electrical resistivity-based techniques. Compared to other severe plastic deformation techniques, electrical conductivity degrades marginally from 100.6% to 96.6% IACS after three cycles of CCDC. Decrease in recrystallization onset and peak temperatures is noticed, whereas stored energy increases and saturates at around 0.95-1.1J/g after three cycles of CCDC. Although drop in recrystallization activation energy is observed with the increasing strain, superior thermal stability is revealed, which is attributed to CCDC process mechanics. Low activation energy observed in CCDC-processed OFHC copper is corroborated to synergistic influence of grain boundary characteristics and lattice defects distribution. Estimated defects concentration indicated continuous increase in dislocation density and vacancy with strain. Deformation-induced vacancy concentration is found to be significantly higher than equilibrium vacancy concentration ascribed to hydrostatic stress states experienced during CCDC.

On the opposition of dynamic recrystallization and solute dragging in steels

The influence of chemical composition on the hot deformation behavior and dynamic recrystallization was studied by conducting hot compression tests on two low alloy steels, AISI 4135 and VCN200, over a temperatures range of 1000–1150°C and at strain rates of 0.001s−1 to 1s−1. The activation energy of dynamic recrystallization in 4135 and VCN200 was determined as 374kJ/mol and 435.3kJ/mol, respectively. The different apparent activation energies (about 17%) were attributed to about 30% difference between the carbon equivalents of the steels. The results confirmed that the higher the alloying elements, the higher the peak stress and strain of DRX flow curves. The results were associated with the dragging force of solute atoms on the grain boundaries. A new formula was proposed as a chemical factor to quantify the influence of solute dragging on the dynamic recrystallization behavior. Simple power equations described how the peak stress and strain depends on the Zener–Hollomon parameter. The material constants of the developed equations were related to the proposed composition factor. Unlike to dynamic recrystallization, dynamic recovery did not delayed by solute dragging.

Recrystallization behavior of cold-rolled Zr–1Nb alloy

The recrystallization behavior of cold-rolled Zr–1Nb alloy was investigated by measuring the micro-Vickers hardness of the specimens annealed for various times. Different deformation reductions and annealing temperatures were coupled to study the effects of deformation and temperature on the recrystallization behavior of Zr–1Nb alloy. The results show that both large deformation reduction and high annealing temperature accelerate the recrystallization process. The microstructural evolution during recrystallization was characterized by optical microscope (OM) and transmission electron microscope (TEM) to correlate with the variation of Vickers hardness. The TEM observation also revealed the distribution of different types of Nb-containing precipitates during recrystallization. The Vickers hardness data were fitted by using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation to derive the activation energies for recrystallization, giving the corresponding recrystallization maps. This study provides useful guidelines for the processing of a broad family of zirconium alloys based on Zr–1Nb.

Effect of back pressure on the thermal stability of severely deformed copper

The effect of back pressure during equal channel angular pressing on thermal stability of Electrolytic Tough Pitch (ETP) copper was studied. The thermal behavior was assessed by means of microhardness measurements and microstructure characterization of the deformed samples at various stages of annealing. The assessment of the variation of the recrystallized fraction of the material with annealing was also carried out using a relatively new method based on internal misorientation measurements by EBSD technique. A higher stored energy and lower activation energy for recrystallization in the case of a back pressure of 100 MPa was obtained by means of DSC analysis. As a main outcome of this work, it was found that application of back pressure reduces thermal stability of the UFG microstructure. However, the effect is relatively small and does not negate the advantages of processing with back pressure in terms of the degree of grain refinement and strength enhancement.

Recrystallization Behavior of High-Strength AA 7075 Alloy Processed by New Short-Cycled Thermo-Mechanical Processing

Recrystallization behavior under different conditions (different temperatures and times) of AA 7075 alloy processed by new short-cycled thermo-mechanical processing was investigated to design a suitable recrystallization schedule. With acquiring recrystallization activation energy by DSC, the recrystallization behavior was successfully verified by theoretical calculation. Experimental results of recrystallization response and re-dissolution of precipitates during isothermal annealing reveal excellent agreement with DSC prediction. The results show that the obtained activation energy of recrystallization can be used to establish the relationship between recrystallization temperatures and times. It is proposed that an appropriate recrystallization treatment (703-753 K/1-5 min) could be used to acquire completely recrystallized grains with size <10 μm, contributing to better formability/ductility. The coarsening rate of these fine recrystallized grains is fairly low even though extending the solution treatment times at 753 K. Therefore, it indicates that the recrystallization dynamical equation would be a useful method to adjust recrystallization temperatures and times to satisfy various requirements of structures and properties.

Modeling of the hot flow behavior of advanced ultra-high strength steels (A-UHSS) microalloyed with boron

In this research work, modeling of the hot flow behavior was carried out in a low carbon advanced ultra-high strength steels (A-UHSS) microalloyed with different amounts of boron (14, 33, 82, 126 and 214ppm). For this purpose, experimental stress–strain data of uniaxial hot-compression tests over a wide range of temperatures (1223, 1273, 1323 and 1373K (950, 1000, 1050 and 1100°C)) and strain rates (10−3, 10−2 and 10−1s−1) were used. The stress–strain relationships as a function of temperature and strain rate were successfully described on the basis of the approach proposed by Estrin, Mecking, and Bergström, together with the classical Avrami equation and the conventional hyperbolic sine function. The analysis of the modeling parameters of the hot flow curves shows that boron additions to A-UHSS play a major role in softening mechanisms rather than on hardening. The peak stress (σp) and steady-state stress (σss) values show a decreasing trend with increasing boron content, which indicates that boron additions promote a solid solution softening effect additional to that produced by DRX. The time for 50% recrystallization (t50%) tends to increase with boron additions, revealing that boron additions cause a delay of the DRX kinetics during hot deformation. Similarly, the presence of boron in the steel decreases the apparent activation energy for recrystallization (Qt), indicating that boron additions accelerate the onset of DRX. The constitutive equations developed in this way provided an excellent description of the experimental hot flow curves.

The Static Recrystallization Behavior of Fe-Mn-Si-Al Series TWIP Steel

he softening and static recrystallization behavior of typical Fe-Mn-Si-Al series TWIP steel between high temperature deformation passes was investigated by two-pass hot compress deformation experiments on Gleeble-3500 thermal simulated test machine. The dynamic model of static recrystallization was built according to the experimental results. The investigation of the effects of deformation temperature, deformation rate and pre-deformation shows that deformation rate is the most effective parameter, and the increase of deformation and pre-deformation can also promote the static recrystallization. The calculated static recrystallization activation energy of TWIP steel is about 147kJ/mol through the dynamic equation built. The results of model predict conform to the experimental results.

Investigation of relaxation processes in nanocrystalline cobalt produced by severe plastic deformation

It has been shown that the dependence of the width of X-ray diffraction lines of plastically deformed cobalt on the annealing temperature is described by the exponential function. Characteristic temperature regions corresponding to the processes of recovery and recrystallization have been established. It has been shown that the values of the activation energy of recrystallization determined from the experimental data are comparable with the activation energy of the grain-boundary diffusion in metals. The activation energy for the recovery region is considerably lower than the activation energy of migration of nonequilibrium grain boundaries in nanocrystalline metals. The X-ray diffraction data have been confirmed by the investigations of the microstructure and microhardness.

Static Recrystallization Behavior of 316LN Austenitic Stainless Steel

The static recrystallization of 316LN austenitic stainless steel was studied by double-pass hot compression tests on a Gleeble-3500 thermomechanical simulator. The specimens were compressed at the deformation temperatures of 950, 1050, 1150 °C, strain rates of 0.01, 0.1, 1s−1, strains of 0.1, 0.15, 0.2, and intervals of 1 — 100 s. The results show that the volume fraction of static recrystallization of 316LN increases with the increase of deformation temperature, strain rate, strain and interval, which indicates that static recrystallization occurs easily under the conditions of higher deformation temperature, higher strain rate and larger strain. Deformation temperature has significant influence on static recrystallization of 316LN. The volume fraction of static recrystallization could easily reach 100% at higher deformation temperatures. By microstructure analysis, it can be concluded that the larger the volume fraction of static recrystallization, the more obvious the grain refinement. The static recrystallization activation energy of 317 882 J/mol and the exponent n of 0.46 were obtained. The static recrystallization kinetics was established. The predicted volume fraction of static recrystallization is in good agreement with the experimental results.

Hot Deformation Behavior of TC21 Alloy

The dynamic recrystallization behavior of TC21 alloy during hot compression deformation was investigated at 870~990 °C and strain rate of 0.001~10 s-1 on a Gleeble-3500 thermo-simulation machine. The results show that dynamic recovery and dynamic recrystallization occurs during hot deformation. As the deformation temperature increases and strain rate decreases, the softening caused by dynamic recrystallization is more obvious. According to the relevance of flow stress, strain rate and deformation temperature, the dynamic recrystallization activation energy is obtained. The constitutive equation and dynamic recrystallization kinetics motel are set up through analyzing and calculating the data of thermo-simulation.

The Evolution of Residual Stress and Microstructure in Shot Peened S30432 Austenitic Stainless Steel at High Temperatures

The thermal relaxation behavior of residual stress and microstructure at high temperatures in S30432 austenitic stainless steel after shot peening was investigated by X-ray residual stress analyzer. The effects of exposure time and applied temperature on the residual stress and microstructure relaxation were particularly analyzed and discussed. A significant decrease of the residual stress values were observed in the first period of exposure time, followed by slowing down and then stabilization. It was also observed that a higher applied temperature produced greater relaxation. In terms of microstructure, the domain size and micro-strain were calculated by Voigt method, the results showed that the refined domain size and high micro-strain induced by shot peening were greatly relaxed at the first stage of annealing, then stabilized. With higher annealing temperature, the recrystallization behavior in the shot peened deformed layer was more obvious. Based on the results of line profile analysis, the recrystallization activation energy and micro-strain relaxation energy were calculated, respectively.

Hot deformation behavior and processing map of Cu–Ni–Si–P alloy

The high-temperature deformation behavior of Cu–Ni–Si–P alloy was investigated by using the hot compression test in the temperature range of 600-800 °C and strain rate of 0.01–5 s−1. The hot deformation activation energy, Q, was calculated and the hot compression constitutive equation was established. The processing maps of the alloy were constructed based on the experiment data and the forging process parameters were then optimized based on the generated maps for forging process determination. The flow behavior and the microstructural mechanism of the alloy were studied. The flow stress of the Cu–Ni–Si–P alloy increases with increasing strain rate and decreasing deformation temperature, and the dynamic recrystallization temperature of alloy is around 700 °C. The hot deformation activation energy for dynamic recrystallization is determined as 485.6 kJ/mol. The processing maps for the alloy obtained at strains of 0.3 and 0.5 were used to predict the instability regimes occurring at the strain rate more than 1 s−1 and low temperature (≤650 °C). The optimum range for the alloy hot deformation processing in the safe domain obtained from the processing map is 750–800 °C at the strain rate of 0.01–0.1 s−1. The characteristic microstructures predicted from the processing map agree well with the results of microstructural observations.

Kinetics of Thermally Activated Lattice Softening in Titanium: A Dynamic Calorimetry Study

The thermally-activated lattice defect relaxation kinetics in 90 % cold-rolled titanium has been investigated through differential scanning calorimetry, isothermal aging experiments, supplemented by metallography and theoretical modelling. Calorimetry results revealed two major softening thermal arrests at 755 and 944 K, respectively, marking the onset of recovery and recrystallization of deformed state. Adoption of higher heating rates resulted in overlapping of recovery and recrystallization; in fact, the excess unspent driving force due only to partial recovery accelerated the recrystallization. Standard isoconversional analysis of calorimetry data yielded 115 and 147 kJ mol−1 for apparent activation energies of recovery and recrystallization, respectively. These values agree well with the results of an independent analysis of time, temperature variation of hardness of isothermally annealed samples.

New Type Austenite Dynamic Recrystallization of Microalloyed Forging Steels 38MnVS During Forging Process

Hot compression tests of microalloyed forging steels 38MnVS were carried out on the Gleeble-3800 ther-mo-mechanical simulator at the deformation temperatures from 950 to 1150 °C with the strain rates ranging from 0.1 to 10 s−1. The effects of the deformation temperature and strain rate on the austenite dynamic recrystallization and microstructural changes were researched. The experimental results show that the dynamic recrystallization accelerated with the increase of the deformation temperatures and the decrease of the strain rate. The activation energy of dynamic recrystallization was calculated, which was about 275. 453 kJ/mol. The relation between the dynamic recrystallization and the Z-parameter was investigated, and the state chart of the dynamic recrystallization of the microalloyed forging steel 38MnVS was made according to the experimental data and the deformation parameters.

Activation Energy for Recrystallization in Fe-3wt.%Si Alloys with Different S Contents

The recrystallization behaviors of 60% rolled Fe-wt.3%Si and Fe- wt.3%Si-Mn-S alloys containing coarsen MnS particles were observed in temperature range 600°C~1000°C. The activation energy for recrystallization was determined according to an Arrhenius type of relationship. It was found that the activation energy in the temperature range 600°C~750°C was much higher than that in the temperature range 850°C~1000°C. Thermo-mechanical calculation indicates that there are hardly precipitation behaviors of MnS particles in 600°C~1000°C. Fe3C will precipitate below 650°C and Cottrell atmosphere will form just above 650°C, which induces dragging effect against the boundary migration during recrystallization and increase the activation energy. The activation energy determined was about 99kJ/mol or 217kJ/mol in Fe-3%Si alloy and 91kJ/mol or 220kJ/mol in Fe-3%Si-Mn-S alloy for the recrystallization in high temperature range of 850°C~1000°C or low temperature range of 600°C~750°C respectively.

Effect of Niobium on Static Recrystallization Characteristics of TWIP Steels

In this study, the constitutive flow behavior and static recrystallization characteristics of a Nb-microalloyed TWIP (Fe-20Mn-1.5Al-0.1Nb) steel under hot deformation conditions have been determined and results compared with those of Fe-25Mn-Al TWIP steels. Investigations using compression testing in a Gleeble simulator, including the double-hit technique, enabled the acquisition of flow stress and recrystallization data. These were analyzed to determine the powers of strain and strain rate as well as the activation energies of deformation and recrystallization (Qdef and Qrex). For given deformation and grain size parameters, the time for 50% recrystallization (t50) of the 0.1% Nb TWIP steel was significantly longer than for the Nb-free TWIP steel: it was comparable to that of Nb-microalloyed carbon steel. Qrex was higher than that of Type 304 stainless steel that has nevertheless much longer t50 times.