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Microstructure characterization and dynamic recrystallization in an Alloy 800H
Abstract
The nucleation mechanism of dynamic recrystallization of an austenitic steel Alloy 800H at intermediate temperatures (900, 950, 1000 and 1050°C) and strain rates from 10−1 s−1 to 10−3 s−1 was studied by transmission electron microscopy (TEM). It is found that dynamic recrystallization (DRX) starts in the region of grain boundaries without bulging. Detailed investigations show that the development of the microstructure is highly inhomogeneous, consisting of dense dislocation walls and microbands in which dynamic recrystallization occurred near the grain boundaries due to an orientation gradient. Correlation between microstructural evolution and nucleation mechanism is analysed on the basis of substructure observations and measurements.
... With the increase of temperature, LAGBs transform into sub-grains by consuming dislocations, and sub-grains grow up to form HAGBs [27]. With the continuous formation of HAGBs in grains during hot deformation, the new RDX mechanism is promoted, which is consistent with the characteristics of CDRX mechanism [39,40] . It is noteworthy that when the deformation temperature increases to 1150 °C, the proportion of HAGBs decreases to 30.0%. ...
... With the increase of temperature, LAGBs transform into sub-grains by consuming dislocations, and sub-grains grow up to form HAGBs [27]. With the continuous formation of HAGBs in grains during hot deformation, the new RDX mechanism is promoted, which is consistent with the characteristics of CDRX mechanism [39,40] . It is noteworthy that when the deformation temperature increases to 1150 • C, the proportion of HAGBs decreases to 30.0%. ...
In this study, isothermal single-pass forming doformation of forged 18CrNiMo7-6 alloy steel was carried out by Gleeble-3500 thermal simulation testing machine. The constitutive equations and processing maps with parameters of deformation temperature and strain rate were established. The results show that the optimum hot deformation parameters are temperature 1050 °C, strain rate 0.1 s–1 with the peak power efficiency being 0.432. The mechanism of grain refinement during hot compression was also characterized by electron backscatter diffraction (EBSD). The results show that continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX) and grain growth are the main microstructure evolution mechanisms during hot working. The rotation of sub-grains under CDRX mechanism is the main factor for the formation of new grains. In addition, the DDRX mechanism is formed by the bulging of HAGBs at the grain boundary triple junction of the original grains, and the CDRX mechanism forms finer grains. The study also found that temperature affected the organization evolution mechanism, the DDRX mechanism plays a leading role when the temperature is low. With the increase of deformation temperature, CDRX begins to play a leading role and forms finer grains. When the deformation temperature rises to 1150 °C, the grains continue to grow at a higher temperature.
... A remarkable fraction of annealing twins can be observed in DRX microstructure of different low or medium SFE materials, which play a crucial role to promote the expansion of the recrystallized grains during DRX. In earlier studies [10][11][12][13], DRX was studied at low strain rates (10 À 4 -10 -1 s À 1 ). Irrespective of previous work, origin of twin boundaries (i.e. ...
... As illustrated in Fig. 9b, the HAGBs can also originate from grain fragmentation in the course of deformation near the initial boundaries or in the interior of the grains. The parent grains are divided into different parts producing new boundaries due to activation of different sets of slip systems (indicated by white dashed lines) in order to ensure a compatible deformation of the adjacent grains [12]. ...
The objective of the study described here is to evaluate the effect of temperature, strain rate, and strain on the microstructure of dynamically recrystallized nickel-chromium alloy (800H) subjected to hot compression over a wide range of strain rates. The microstructural evolution was studied by electron backscattered diffraction (EBSD) and the effect of adiabatic heating on hot deformation was analyzed to correct the flow curves at high strains. The grain orientation spread (GOS) approach was used to distinguish the dynamic recrystallization (DRX) grains from the deformed matrix. The nucleation mechanism of DRX and the role of Sigma 3(n) CSL boundaries during DRX were explored. Additionally, the influence of carbides on the DRX behavior was studied within the temperature of 850-950 degrees C. The results indicated that the DRX can be stimulated by adiabatic heating and strong dislocation-dislocation interaction occurring with increase in the strain rate in the range of 1-30 s(-1). The threshold value of GOS (1.2 degrees) separated the DRX grains from the deformed matrix. The recrystallized grains nucleated at preexisting grain boundaries by extensive bulging associated with grain fragmentation. The Sigma 3(n) CSL boundaries play an important role in DRX and they can be generated through interaction among them after the initiation of DRX. The precipitation of Cr23C6 and Ti(C, N) at the parent grain boundary could restrain or even inhibit the occurrence of DRX in the temperature range of 850-950 degrees C.
... In the initial grains and twin boundaries, new very small grains form in a specific crystallographic orientation due to DRX [21]. Aqeel Abbas et al. (2020) employed AZ91 magnesium alloy as the base material [22], added 1 wt.% tungsten disulfide (WS2) as a reinforcement phase, and used mechanical stirring casting to prepare the material. ...
This study investigates the effects of zinc (4 wt.%) and severe plastic deformation on the mechanical properties of AZ61 magnesium alloy through the stir-casting process. Severe plastic deformation (Equal Channel Angular Pressing (ECAP)) has been performed followed by T4 heat treatment. The microstructural examinations revealed that the addition of 4 wt.% Zn enhances the uniform distribution of β-phase, contributing to a more uniformly corroded surface in corrosive environments. Additionally, dynamic recrystallization (DRX) significantly reduces the grain size of as-cast alloys after undergoing ECAP. The attained mechanical properties demonstrate that after a single ECAP pass, AZ61 + 4 wt.% Zn alloy exhibits the highest yield strength (YS), ultimate compression strength (UCS), and hardness. This research highlights the promising potential of AZ61 + 4 wt.% Zn alloy for enhanced mechanical and corrosion-resistant properties, offering valuable insights for applications in diverse engineering fields.
... In the initial grains and twin boundaries, new very small grains form in a specific crystallographic orientation due to DRX [21]. Aqeel Abbas et al. (2020) employed AZ91 magnesium alloy as the base material [22], added 1 wt.% tungsten disulfide (WS2) as a reinforcement phase, and used mechanical stirring casting to prepare the composite material. ...
This study investigates the effects of Zinc (4wt.%) and severe plastic deformation on the mechanical properties of AZ61 magnesium alloy through the stir-casting process. The severe plastic deformation (equal channel angular pressing (ECAP)) has been done followed by T4 heat treatment. The microstructural examinations revealed that the addition of 4wt.%Zn enhances the uniform distribution of β-phase, contributing to a more uniformly corroded surface in corrosive environments. Additionally, dynamic recrystallization (DRX) significantly reduces the grain size of as-cast alloys after undergoing ECAP. The attained mechanical properties demonstrate that after a single ECAP pass, AZ61 + 4wt.%Zn alloy exhibits optimal yield strength (YS), ultimate compression strength (UCS), and hardness. This research highlights the promising potential of AZ61 + 4wt.%Zn alloy for enhanced mechanical and corrosion-resistant properties, offering valuable insights for applications in diverse engineering fields.
... Equation (1) implies that the stored energy increases as dislocation density. Wang et al. [42] reported that the large nucleation driving force in the non-recrystallized microstructures where the large lattice distortions are observed. According to Kenta [43], it is considered that the DRX behavior is explained by the enhanced driving force resulting from the curvature-induced local storage of energy in the lattice. ...
Mg–8.7Gd–4.18Y–0.42Zr magnesium alloys are subjected to hot compressive tests at a strain rate of 0.002–2 s⁻¹ and a compression temperature of 300–450 °C. Compared with strain rate, compression temperature appears to have a greater impact on dynamic precipitation. By virtue of the pinning effect, precipitates located immediately adjacent to the dynamic recrystallized (DRX) grain boundaries may greatly affect grain growth. As the compression temperature increases, DRX grains multiply rapidly. Nevertheless, the number of DRX grains is decreased owing to the increase in strain rate. However, further increasing the strain rate to 2 s⁻¹, the increased energy stored generates stronger driving forces, causing dislocations to move, thereby raising the number of DRX grains by a considerable amount. With an increase in compression temperature and a decrease in strain rate, substructure development has been described as high-density dislocation → VLAGBs (sub-GBs) → LAGBs → HAGBs → DRX grains. It is DRX that is responsible for the predominant softening mechanism in this studied alloy, accompanied by a continuous dynamic recrystallization (CDRX) process characterized by transforming low-angle grain boundaries into high-angle grain boundaries, as well as discontinuous dynamic recrystallization (DDRX), which is characterized by bulging grain boundaries.
Graphical abstract
... Fig. 5(d) shows the dislocation cells which are formed due to multiple-slip in the HS zone, as indicated by white dashed lines. Wang et al. [32] considered that grains would be divided into different parts due to multiple-slip and resulted in the formation of small grains during deformation. Besides, there exhibits a very slight grain growth in the SF zone compared with the non-tested sample, and it keeps consistent with that the proportion of low CSL boundaries in the SF zone has a moderate decrease compared with the non-tested sample. ...
Samples cut from a 700 ℃/322 MPa/9117 h creep-ruptured specimen as well as a non-tested sample were characterized by electron backscatter diffraction to investigate the evolution of low ∑ (∑≤29) coincident site lattice boundaries during strain at elevated temperature for Haynes 282 superalloy. Results show that the proportion of ∑3 boundaries, including ∑9 and ∑27 boundaries, decreases sharply with increasing strain. The two-mode phase-field crystal method is applied to simulate the dynamic evolution process of a ∑3 boundary. During deformation at ε´=5.62e-6, a twin embryo grows toward the initial ∑3 boundary with increasing strain and impinges onto it finally to form a ∑3-∑3-∑9 triple junction. In addition, part of the initial ∑3 boundary transforms into random grain boundary when the strain is large enough. Large numbers of dislocations are detected nearby ∑3 boundaries. They cause severe lattice rotations near ∑3 boundaries. In addition, some straight random boundaries can be found in strain zones only. Therefore, the transformation of ∑3 boundaries into random grain boundaries is the critical reason for the sharp decrease of ∑3 boundaries in strain zones.
... In case of low SFE materials such as austenitic steels, once the critical condition in terms of strain or dislocation density is satisfied, DRX is triggered [21,22]. It is well established that "Strain Induced Grain Boundary Migration" (SIBM) or bulging process is responsible for the nucleation of DRX grains especially at low strains [23]. It signifies that the nucleation of dynamically recrystallized grains occurs at the locally bulged boundaries of initial grains and increases until a layer of DRX grains covers the boundaries [24]. ...
Hot deformation behavior of the superaustenitic stainless steel type 1.4563 was investigated by conducting hot compression tests at the temperatures of 900–1050 °C and at strain rates in the range of 0.001–1 s−1. The microstructural changes were then characterized using optical and scanning electron microscopy as well as energy dispersive X-ray (EDX) microanalyses. The results showed that hot deformation at low temperatures, i.e. 900–950 °C, and at low and medium strain rates, i.e. 0.001–0.1 s−1, can lead to the formation of wormlike precipitates on grain boundaries resulting in the restriction or even inhibition of dynamic recrystallization. At higher strain rates or higher temperatures when respectively the time was too short or the driving force for dynamic precipitation was rather low, dynamic recrystallization occurred readily. Further, at low strain rates and high temperatures, where the occurrence of dynamic precipitation is difficult, there was no sign of particles. In this case, the interactions between solute atoms and mobile dislocations resulted in tiny serrations in the flow curves instead. The EDX analyses indicated that the chemical composition of the observed precipitates was (Cr, Fe, Mo)23C6.
... The fine and serrated grains were considered as evidence of dynamic recrystallization (DRX). DRX is the strengthening mechanism during hot deformation caused by the generation and migration of high angle boundaries inside the sub-grains [19,20]. At areas farther away from the rupture region, no DRX was observed because the strain was not high enough. ...
Nickel-base superalloys are considered as materials for piping and structural materials in a very high temperature gas cooled reactor (VHTR). They are subjected to the environmental degradation caused by a continuous process for oxidation due to small amount of impurities in He coolant during long term operation. In the present study, the oxidation behaviors of several nickel-base superalloys such as Alloy617, Haynes214 and Haynes230 in particular, were studied at the temperature of 900°C and 100°C in air, the high purity He environment. Oxide layers were analyzed by SEM and EDX. The differences in oxidation behaviors of these alloys were mainly caused by different protective oxide layers on surface. In the case of Alloy617 and Haynes230, Cr 2O3 layer formed on the surface which is not stable at 1100°C. Therefore, the weight increased significantly due to oxidation at the initial stage, which followed by a decrease due to the spoiling and volatilization of Cr2O3 layer. On the other hand, since Haynes214 has mainly Al2O3 oxide layer on surface which is more stable and dense structure at the higher temperature, the weight gain eventually reaches to parabolic. Microstructural characteristics of internal carbides and carbide depletion zone were analyzed. With oxidation time, continuous grain boundary carbides of M23C6 type were getting thin or it disappeared partially. Especially, carbides on grain boundary disappeared entirely below oxide layer (carbide depletion zone). It was getting wide with oxidation time. For Haynes214, the size of carbide depletion zone was smaller than other alloys because Al2O3 layer acted as a diffusion layer prevented effectively to penetration of oxygen into base metal.
... DRX nucleation preferentially occurred in local regions with large orientation gradients and high dislocation density, such as original GB region, deformation bands, (distorted) twin boundaries, etc. [33][34][35][36][37][38]. At super-solvus deformation, the combined effects of the absence of γ′ precipitates and the easier activation of slip system resulted in the promotion of GB DRX manifested by faster formation of new HAGBs than at sub-solvus deformation, as illustrated in Figs. ...
... Different nucleation mechanisms of DRX grains were also observed at lower and higher temperature ranges, respectively. X. Wang et al. [12] studied the microstructure evolution taking place below T r during hot deformation and indicated that orientation gradients near grain boundary make the initial high angle grain boundaries (HAGBs) the preferential nucleation sites. Additionally, grain fragmentation due to activation of different sets of slip systems induces HAGBs within the original grains, which provides additional nucleation sites. ...
Considering the pinning effect of fine carbides on grain boundaries, hot compression tests were performed above the dissolution temperature of Cr23C6 to investigate the hot deformation behavior of a Fe–Ni–Cr alloy (800H). The results show that the single peak stress associated with dynamic recrystalization (DRX) became more distinct at higher temperature and lower strain rate. The process of DRX was thoroughly stimulated when deformed above 1000 °C. Constitutive equations for hot deformation were established by regression analysis of conventional hyperbolic sine equation. The relationships between Zener–Hollomon parameter (Z) and the characteristic points of flow curves were established using the power law relation. Furthermore, kernel average misorientation (KAM) and grain orientation spread (GOS) were used to map the distribution of local misorientation and estimate the fraction of DRX, respectively. The critical strain and peak strain were used to predict the kinetics of DRX with the Avrami-type equation.
... Diffusion modeling was carried out using DICTRA Version 25 software (a " sister " program of THERMO-CALC), which allows the exploration of diffusion-controlled phase transformations, including such phenomena as diffusion couples, both single-phase and heterogeneous [25,26]. In the heterogeneous case, it is always assumed that the amount of the second phase (e.g., the c 0 strengthening precipitates or the equilibrium carbide TiC constituent particles in Alloy 800H) is small and that the average concentration in the strengthening phase could be defined by the conditions of local equilibrium for the average concentrations of components272829. A detailed exposition of the numerous metallurgical problems that could be solved with DICTRA is given in the DICTRA manual [25]. ...
The goal of next generation reactors is to increase energy ef?ciency in the production of electricity and provide high-temperature heat for industrial processes. The ef?cient transfer of energy for industrial applications depends on the ability to incorporate effective heat exchangers between the nuclear heat transport system and the industrial process. The need for ef?ciency, compactness, and safety challenge the boundaries of existing heat exchanger technology. Various studies have been performed in attempts to update the secondary heat exchanger that is downstream of the primary heat exchanger, mostly because its performance is strongly tied to the ability to employ more ef?cient industrial processes. Modern compact heat exchangers can provide high compactness, a measure of the ratio of surface area-to-volume of a heat exchange. The microchannel heat exchanger studied here is a plate-type, robust heat exchanger that combines compactness, low pressure drop, high effectiveness, and the ability to operate with a very large pressure differential between hot and cold sides. The plates are etched and thereafter joined by diffusion welding, resulting in extremely strong all-metal heat exchanger cores. After bonding, any number of core blocks can be welded together to provide the required ?ow capacity. This study explores the microchannel heat exchanger and draws conclusions about diffusion welding/bonding for joining heat exchanger plates, with both experimental and computational modeling, along with existing challenges and gaps. Also, presented is a thermal design method for determining overall design speci?cations for a microchannel printed circuit heat exchanger for both supercritical (24 MPa) and subcritical (17 MPa) Rankine power cycles.
... There have been numerous research efforts aimed at the microstructural characterization of Alloy 800H under different heat treatment conditions, 20,21 c. Structurbericht designation L12 corresponds to the Pearson symbol cP4 with prototype Cu3Au; while the D022 designation -to the Pearson symbol tI8 with prototype TiGa3. ...
The U.S. Department of Energy selected the high temperature gas-cooled reactor as the basis for the Next Generation Nuclear Plant (NGNP). The NGNP will demonstrate the use of nuclear power for electricity, hydrogen production, and process heat applications. The NGNP Project is currently investigating the use of metallic, diffusion welded, compact heat exchangers to transfer heat from the primary (reactor side) heat transport system to the secondary heat transport system. An intermediate heat exchanger will transfer this heat to downstream applications such as hydrogen production, process heat, and electricity generation. The channeled plates that make up the heat transfer surfaces of the intermediate heat exchanger will have to be assembled into an array by diffusion welding. This report describes the preliminary results of a scoping study that evaluated the diffusion welding process parameters and the resultant mechanical properties of diffusion welded joints using Alloy 800H. The long-term goal of the program is to progress towards demonstration of small heat exchanger unit cells fabricated with diffusion welds. Demonstration through mechanical testing of the unit cells will support American Society of Mechanical Engineers rules and standards development, reduce technical risk, and provide proof of concept for heat exchanger fabrication methods needed to deploy heat exchangers in several potential NGNP configurations.1 Researchers also evaluated the usefulness of modern thermodynamic and diffusion computational tools (Thermo-Calc and Dictra) in optimizing the parameters for diffusion welding of Alloy 800H. The modeling efforts suggested a temperature of 1150 C for 1 hour with an applied pressure of 5 MPa using 15 m nickel foil as joint filler to reduce chromium oxidation on the welded surfaces. Good agreement between modeled and experimentally determined concentration gradients was achieved
... The fine and serrated grain was considered as the evidence of the dynamic recrystallization (DRX). DRX is the strengthening mechanism during hot deformation caused by the generation and migration of high angle boundaries inside the sub-grains [19,20]. At areas farther away from the rupture region, no DRX was observed because the strain was not high enough. ...
The oxidation characteristics of Alloy 617, a candidate structural material for the key components in the very high-temperature gas-cooled reactor (VHTR), were investigated. High-temperature oxidation tests were conducted at 900 and 1100°C in air and helium environments and the results were analysed. Alloy 617 showed parabolic oxidation behaviour at 900°C, but unstable oxidation behaviour at 1100°C, even in a low oxygen-containing helium environment. The SEM micrographs also revealed that the surface oxides became unstable and non-continuous as the temperature or the exposure time increased. According to the elemental analysis, Cr-rich oxides were formed on the surface and Al-rich discrete internal oxides were formed below the surface oxide layer. After 100h in 1100°C air, the Cr-rich surface oxide became unstable and non-continuous, and the matrix elements like Ni and Co were exposed and oxidized. Depletion of grain boundary carbides as well as matrix carbides was observed during the oxidation in both environments. When tensile loading was applied during high-temperature oxidation, the thickness of the surface oxide layer, the internal oxidation, and decarburization were enhanced because of the increase in diffusion of oxidizing agent and gaseous reaction products. Such enhancement would have detrimental effects on the high-temperature mechanical properties, especially the creep resistance of Alloy 617 for the VHTR application.
Motivation for this study was the absence of any report of stacking fault energy (SFE) estimate and post-dynamic recrystallization (PDRX) effects at temperatures >1000°C in γ′-free Co-base superalloys. Therefore, the substructural evolution during hot compression of Co-22Cr-22Ni-14W-2Fe-0.1C superalloy (H188) at temperatures (1050–1150°C) and at 10⁻¹s⁻¹ is studied. Unusual substructural features viz. extrinsic stacking faults, dense dislocation walls, microbands, and subgrains are observed with an increase in strain (0.01–0.70), even at 1100°C. Denoised electron backscattered diffraction data revealed an enormous increase in overall recrystallized (PDRX+DRX) fraction from 30 to 75% with increase in temperature from 1050 to 1100°C at ε=0.7. Comprehensive microstructural analyses suggest that continuous dynamic recrystallization is the dominant flow softening mechanism in low SFE (estimated to be 14 ± 2.0 mJ/m²) alloy H188. However, the PDRX effects triggered by the presence of higher stored energy and recrystallized nuclei contributing to a massive increase in recrystallized fraction at 1100°C.
Dynamic recrystallization (DRX) occurs during high-temperature deformation in metals and alloys with low to medium stacking fault energies. Previous simulations and experimental research have shown the effect of temperature and grain size on DRX behavior, but not the effect of the grain boundary character distribution. To investigate the effects of the distribution of grain boundary types, experimental testing was performed on stainless steel 316L specimens with different initial special boundary fractions (SBF). This work was completed in conjunction with computer simulations that used a modified Monte Carlo method which allowed for the addition of anisotropic grain boundary energies using orientation data from electron backscatter diffraction (EBSD). The correlation of the experimental and simulation work allows for a better understanding of how the input parameters in the simulations correspond to what occurs experimentally. Results from both simulations and experiments showed that a higher fraction of so-called “special” boundaries (e.g., Σ3 twin boundaries) delayed the onset of recrystallization to larger strains and that it is energetically favorable for nuclei to form on triple junctions without these so-called “special” boundaries.
Single-pass compression tests of an alumina-forming austenite (AFA) alloy (Fe–20Cr–30Ni–0.6Nb–2Al–Mo) were performed using a Gleeble-3500 thermal–mechanical simulator. By combining techniques of electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM), the dynamic recrystallization (DRX) behavior of the alloy at temperatures of 950–1100 °C and strain rates of 0.01–1.00 s⁻¹ was investigated. The regression method was adopted to determine the thermal deformation activation energy and apparent stress index and to construct a thermal deformation constitutive model. Results reveal that the flow stress is strongly dependent on temperature and strain rate and it increases with temperature decreasing and strain rate increasing. The DRX phenomenon occurs more easily at comparably higher deformation temperatures and lower strain rates. Based on the method for solving the inflection point via cubic polynomial fitting of strain hardening rate (θ) versus strain (ε) curves, the ratio of critical strain (εc) to peak strain (εp) during DRX was precisely predicted. The nucleation mechanisms of DRX during thermal deformation mainly include the strain-induced grain boundary (GB) migration, grain fragmentation, and subgrain coalescence.
In spite of many advantages in wrought magnesium alloys, the limited number of slip and twinning systems limited their application in industry due to their poor plastic deformability and their strong anisotropic mechanical properties at low temperature. An important approach to control the process and improve the properties of magnesium alloys could be provided by the study on the deformation mechanisms in the condition of initial anisotropy. The micro-texture evolution of samples with initial textures was investigated under hot and warm deformation by means of orientation mapping based on EBSD technique to reveal the mechanism of dynamic recrystallization (DRX) and the effect of plastic slip. In addition (mis-)orientation evolutions and characteristics of microstructure of twins and shear bands are investigated to analyze the influence of twinning and shear banding on the properties of magnesium alloys.
To understand the mechanism of DRX in AZ31 magnesium alloy, samples with initial textures were analyzed by plane-strain compression at 340ºC using a strain rate of 10-2s-1. The results indicate that DRX proceeded in a continuous mode irrespective of initial textures. In addition, plastic slip plays an important role causing various initial orientations approaching basal orientation (<0001>||ND). Viscous flow was observed in the sample with basal initial texture, which contributes further to the deformation ability of materials.
Magnesium alloys show predominant superplasticity at low strain rate, grain boundary slide is the main mechanism of superplastic deformation; in addition, dislocation slip and diffusion at grain boundaries are the assistant mechanisms for superplasticity. To explore the contribution of plastic slipping at high temperature in AZ31 magnesium alloy, quasi-plane-strain compression was taken on a Gleeble thermal simulator at a temperature of 340ºC and a low strain rate of 410-4s-1(which is in the superplastic deformation condition). The experimental results from EBSD measurement reveal that the plastic slipping occurs concurrently with superplastic deformation, which exhibits a slow orientation change compared with forenamed higher strain rate; the basal slipping is the dominating mechanism, and no significant effect of c+a non-basal slipping activity was found in the EBSD experiment.
Compression twinning assorts with the strain compressed along c axis of the hexagonal structure of magnesium alloys with strong basal texture during further compressing or rolling at low temperature. Furthermore, compression twins are very closely related with the formation of shear bands and cracks. A clear morphological difference was observed between the tension twins and the compression twins. The compression twinning occurs in the grains with basal orientation, the compression twins lose their exact twin relationship easily which should be resulted from the instability of twin orientation and immobility of twin boundaries on further deformation. Double-twinning of {1011}-{1012} types is detected. In contrary the boundaries of the tension twins migrate easily and the twin relationship of 86°<1120> can retain easily.
The dependency of twinning on grain orientation in magnesium alloys was investigated by EBSD analyses and Schmid factor calculation and compared with that in an FCC metal of high manganese TWIP (Twinning Induced Plasticity) steel. A close orientation dependence on deformation mechanism and distinct transformation kinetics of twinning and slipping evidently differentiate under tension and compression deformation respectively.
Because of the immobile compression twin boundaries, new compression twins were easily initiated beside existed compression twins forming shear bands at further strain, therefore the width of shear band depends on the number of twins and the space between twins. EBSD analyses reveal that relative sliding within compression twin bands (cluster) increased along with the widening of the bands and the amount of twins increasing within the twin bands, which increases the misorientation in deformed matrix grains on both sides of twin band. Furthermore, orientation changes along the axes of <1120> were detected within the grains with basal orientation. Shear bands containing twin variants converge together and penetrated each other, leading to the increase of misorientation and finally the formation of cracks along shear bands.
In order to optimize the hot working technology of Incoloy 800H, the dynamic recrystallization (DRX) behavior of Incoloy 800H at temperatures ranging from 850 V to 1100 V and strain rates from 0.01 s(-1) to 10 s(-1) was investigated by single-pass compression tests on MMS-300 thermo-mechanical simulator. The evolutions of microstructure and nucleation mechanisms of DRX were analyzed combined with the technique of EBSD and TEM. The results show that when the deformation temperature is below 950 degrees C, the behavior of DRX is obviously restrained by precipitation of Cr23C6 and Ti(C,N). Therefore the hot deformation constitutive equations in two temperature intervals (from 850 degrees C to 950 degrees C and from 950 degrees C to 1100 degrees C) were established by regression analysis with the deformation activation energy of 465.394 kJ/mol and 427.360 kJ/mol respectively. The inflection points were determined by fitting a third order polynomial to the ln theta-epsilon curves, which makes the prediction for the ratios of critical stress to peak stress and critical strain to peak strain more accurately. Accordingly, the mathematical models of critical stress and critical strain vs Z parameter were deduced. The DRX nucleation mechanisms of Incoloy 800H during hot deformation mainly include strain induced grain boundary migration, grain fragmentation and subgrain coalescence.
Based on the stress-strain data of thermal simulation compression tests, the processing maps of the annealed GH4169 alloy with coarse grains were established according to the dynamic materials model (DMM). Combined with the optical microscopy (OM) observations and the electron backscatter diffraction (EBSD) technique analysis, the stable and instable regions of the annealed GH4169 alloy with coarse grains were determined and the mechanisms of deformation under different conditions were investigated. The suggested processing parameters of the annealed GH4169 alloy with coarse grains were also studied. The results show that the flow instability of the annealed alloy with coarse grains occurring at the strain rates of 10 -0.25~1 s -1 and the temperatures of 950~1100°C is mainly related to the cracking induced by the local plastic flow. Three classic zones of dynamic recrystallization for GH4169 alloy with coarse grains are found in the region with moderate and low strain rates. The maxium energy dissipation efficiency occurring at the strain rate of 10 -3s -1 and the temperature of 950°C is closely associated with the promotion of dynamic recrystallization due to the δ phases precipated along the grain boundaries as well as the local nucleation of dynamic recrystallization inside the grain. Comprehensively considering the energy dissipation efficiency, elongation and microstructure, the process parameters of the starting forging for the alloy with coarse grains are suggested to be the strain rates of 10 -2.7~10 -1.5 s -1 and temperatures of 1087.5~1100°C, and those of the final forging are suggested to be the strain rates of 10 -2.5~10 -1.5 s -1 and temperatures of 1000~1065°C.
In order to optimize the hot working technology of Incoloy 800H, the dynamic recrystallization (DRX) behavior of Incoloy 800H at temperatures ranging from 850 �to 1100 � and strain rates from 0.01 s−1 to 10 s−1 was investigated by single–pass compression tests on MMS–300 thermo–mechanical simulator. The evolutions of microstructure and nucleation mechanisms of DRX were analyzed combined with the technique of EBSD and TEM. The results show that when the deformation temperature is below 950 �, the behavior of DRX is obviously restrained by precipitation of Cr23C6 and Ti(C,N). Therefore the hot deformation constitutive equations in two temperature intervals (from 850 �to 950 �and from 950 �to 1100 �) were established by regression analysis with the deformation activation energy of 465.394 kJ/mol and 427.360 kJ/mol respectively. The inflection points were determined by fitting a third order polynomial to the ln�–" curves, which makes the prediction for the ratios of critical stress to peak stress and critical strain to peak strain more accurately. Accordingly, the mathematical models of critical stress and critical strain vs Z parameter were
deduced. The DRX nucleation mechanisms of Incoloy 800H during hot deformation mainly include
strain induced grain boundary migration, grain fragmentation and subgrain coalescence.
A physical model based on the evolution of subgrains size is proposed to describe the nucleation and growth processes during discontinuous dynamic recrystallization. The evolution of subgrains to viable recrystallization nuclei was found possible at very low strains. Afterwards, the number of stable nuclei considerably increased on a sigmoidal trend with strain and reached a saturated state at about 0.6 times the peak strain. The dependence of nucleation rate on strain was modeled using an Avrami-type equation and the driving force for the growth of recrystallized nuclei was similarly modeled in terms of strain. It is also shown that “site saturation” is the governing mechanism for the initiation of the discontinues dynamic recrystallization at the grain boundaries. The flow stress of the material was calculated using the law of mixture of recrystallized and unrecrystallized regions with fractional softening as the stress-partitioning factor. Satisfactory agreement between predicted and experimental results was obtained, thereby confirming the validity of the proposed model.
In the present study, hot deformation behavior of AISI 410 martensitic stainless steel was investigated and modeled after conducting compression tests at the temperature range of 900-1150 °C and strain rate range of 0.001-1 s−1. At the studied temperature and strain rates, the flow curves were typical of dynamic recrystallization (DRX) showing a hardening peak followed by a softening one, and a steady state. The flow curves up to the peaks were modeled using the Estrin and Mecking equation. The softening due to DRX was also considered to increase the consistency of the developed model. The experimental equation proposed by Cingara and McQueen was also used to model the work hardening region. The results showed that the phenomenological model based on the Estrin and Mecking equation resulted in a better model for the work hardening region. Based on the Avrami equation, a model was developed to estimate the flow softening due to DRX between the peak and the starting point of steady state. The average value of the Avrami exponent was determined as 2.2, and it decreased with the increasing Zener-Hollomon parameter.
The present work examines in detail the substructure and texture characteristics in a Ni–30Fe austenitic model alloy subjected to deformation by plane strain compression (PSC) at temperatures between 700 and 900 °C to strains between 0.2 and 1 using a strain rate of 1 s−1. The flow curves display characteristics typical of limited dynamic recrystallization. The deformed matrix texture is similar to that expected for rolling/PSC deformation of face-centred cubic metals while the comparatively weaker dynamic recrystallization texture is dominated by the Cube component. The non-Cube deformed matrix grains contain “organized”, self-screening arrays of microbands aligned along the slip planes with high Schmid factors. By contrast, the Cube deformed matrix grains exhibit more “random” cell substructure with a low density of superimposed larger-angle dislocation walls. These walls are related to {1 1 1} slip planes at low strains, but tend to follow the rigid body rotation toward the compression plane at large strains.
The dynamic recrystallization (DRX) behavior of a coarse grain sized Nb microalloyed austenite (∼ 800 μm), typical of thin slab casting processes, has been studied. Continuous torsion tests were carried out at different, Z, Zener-Hollomon parameter values. It has been observed that as Z increases the curves move to higher values of stress and both the peak, εp, and steady state, εss, occur at larger strains, with an increase in the εss - εp strain difference. Consequently, an increment in Z produces a delay in the beginning and the progression of the DRX process. In the present work, it has been found that the pre-existing grain boundaries are the most favorable nucleation sites for DRX for all Z values, the nucleation mechanisms being related to strain induced migration of high angle grain boundaries. However, in the case of high Z values, intragranular nucleation on defects generated during deformation is also observed. The microstructure analysis denotes also that dynamic recrystallization is a process dominated by repeated nucleation with limited growth.
The tensile properties of austenitic Alloy 800H have been evaluated at temperatures ranging from ambient to 1000°C. The results indicate that the tensile strength was gradually reduced with increasing temperature. Between ambient temperature and 200°C, the failure strain was reduced possibly due to the occurrence of dynamic strain aging followed by its enhancement at temperatures up to 500°C. Alloy 800H did not exhibit any failure in an acidic solution at constant-load. However, the true failure stress in this environment was reduced at elevated temperatures under a slow-strain-rate condition. The magnitude of electrochemical potentials became more active with increasing temperature. A detrimental effect of more noble controlled potential on the cracking susceptibility was noted in terms of the true failure stress. A combination of ductile and intergranular brittle failures was seen in the primary fracture surface of the tested cylindrical specimens.
The hot deformation behavior of AISI 410 martensitic stainless steel was investigated by conducting hot compression tests
between 1173 K (900 °C) and 1423 K (1150 °C) and between strain rates of 0.001 s−1 to 1 s−1. The hyperbolic sine function described the relation well between flow stress at a given strain and the Zener–Hollomon parameter
(Z). The variation of flow stress with deformation temperature gave the average value of apparent activation energy as 448 kJ/mol.
The strain and stress corresponding to two important points associated with flow curve (i.e., peak strain and the onset of steady-state flow) were related to the Z parameter using power-law equations. A model also was proposed based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation
to estimate the fractional softening of dynamic recrystallization at any given strain. This model can be used readily for
the prediction of flow stress. The values of n and k, material constants in the JMAK equation, were determined for the studied
material. The strains regarding the peak and the onset of steady-state flow were formulated in term of applied strain rate
and the constants of the JMAK equation. A good agreement was found between the predicted strains and those obtained by the
experimental work.
A research effort was made to evaluate the usefulness of modern thermodynamic and diffusion computational tools, Thermo-Calc{copyright} and Dictra{copyright}, in optimizing the parameters for diffusion welding of Alloy 800H. This would achieve a substantial reduction in the overall number of experiments required to achieve optimal welding and post-weld heat treatment conditions. This problem is important because diffusion welded components of Alloy 800H are being evaluated for use in assembling compact, micro-channel heat exchangers that are being proposed in the design of a high temperature gas-cooled reactor by the US Department of Energy. The modeling was done in close contact with experimental work. The latter included using the Gleeble 3500 System{reg_sign} for welding simulation, mechanical property measurement, and light optical and Scanning Electron Microscopy. The modeling efforts suggested a temperature of 1150 C for 1 hour with an applied pressure of 5 MPa using a 15 μm Ni foil as a joint filler to reduce chromium oxidation on the welded surfaces. Good agreement between modeled and experimentally determined concentration gradients was achieved, and model refinements to account for the complexity of actual alloy materials are suggested.
The effect of rolling and annealing on the microstructure and high temperature creep properties of alloy 617 were investigated. Two types of foil specimens with different thickness reductions were prepared by thermo-mechanical processing. Recrystallization and grain growth were readily observed at specimens annealed at 950 and 1100°C. The uniform coarse grains increase resistance against creep deformation. The grain size effect in creep deformation was dominant up to 900°C, while dynamic recrystallization effect became dominant at 1000°C. Dynamic recrystallization was observed in all the creep deformed foils, even though some specimens had already been (statically) recrystallized during annealing. Steady state creep rates decreased with increasing annealing temperature in the less rolled foils. The apparent activation energy Qapp for the creep deformation increased from 271 to 361kJ/mol as the annealing temperature increased from 950 to 1100°C.
The microstructure of thermally grown oxides (TGO) and the creep properties of alloy 617 were investigated. Oxidation and creep tests were performed on 100 μm thick foils at 800–1000 °C in air environment, while the thickness of TGO was monitored in situ. According to energy dispersive X-ray (EDX) mapping micrographs observation, superficial dense oxides, chromia (Cr2O3), which was thermodynamically unstable at 1000 °C, and discrete internal oxides, alumina (α-Al2O3), were found. Consequently, the weight of the foil specimen decreased due to the spalling and volatilization of the Cr2O3 oxide layer after an initial weight-gaining. Secondary and tertiary creeps were observed at 800 °C, while the primary, secondary and tertiary creeps were observed at 1000 °C. Dynamic recrystallization occurred at 800 °C and 900 °C, while partial dynamic recrystallization at 1000 °C. The apparent activation energy, Qapp, for the creep deformation was 271 kJ/mol, which was independent of the applied stress.
The microstructure and crystallographic texture development in an austenitic Ni-30 pct Fe model alloy was investigated within
the dynamic recrystallization (DRX) regime using hot torsion testing. The prominent DRX nucleation mechanism was strain-induced
grain boundary migration accompanied by the formation of large-angle sub-boundaries and annealing twins. The increase in DRX
volume fraction occurred through the formation of multiple twinning chains. With increasing strain, the pre-existing Σ3 twin
boundaries became gradually converted to general boundaries capable of acting as potent DRX nucleation sites. The texture
characteristics of deformed grains resulted from the preferred consumption of high Taylor factor components by new recrystallized
grains. Similarly, the texture of DRX grains was dominated by low Taylor factor components as a result of their lower consumption
rate during the DRX process. The substructure of deformed grains was characterized by “organized,” banded subgrain arrangements,
while that of the DRX grains displayed “random,” more equiaxed subgrain/cell configurations.
In order to examine the microstructural evolution during hot-compression deformation of the biomedical Co-29Cr-6Mo (weight
percent) alloy without the addition of Ni, hot-compression tests have been conducted at deformation temperatures ranging from
1050°C to 1200°C at various strain rates of 10−3 to 10s−1. The grain refinement due to dynamic recrystallization (DRX) was identified under all deformation conditions by means of
field-emission scanning electron microscopy/electron backscattered diffraction (FESEM/EBSD) and transmission electron microscopy
(TEM) observations. Although the DRX grain size (d) of the deformed specimens considerably decreased with an increasing Zener–Hollomon (Z) parameter at strain rates ranging from 10−3 to 0.1s−1, a grain size coarser than that predicted from the d-Z relation was obtained at strain rates of 1.0 and 10s−1. An ultrafine-grained microstructure with a grain size of approximately 0.6μm was obtained under deformation at 1050°C at 0.1s−1, from an initial grain size of 40μm. The grain refinement to a submicron scale of biomedical Co-Cr-Mo alloys has been achieved with hot deformation by ~60pct
due to DRX, in which the bulging mechanism is not operative. The ultrafine grains obtained due to DRX without bulging is closely
related to the considerably low stacking-fault energy (SFE) of the Co-Cr-Mo alloy at deformation temperatures.
Hot deformation behavior of AISI 410 martensitic stainless steel was investigated by conducting hot compression tests at a
temperature range of 900 °C to 1150 °C and a strain rate of 0.001-1s−1. The relation between flow stress and Zener-Hollomon (Z) parameter was successfully modeled via the hyperbolic sine function
under a wide range of deformation conditions, and the values were determined for the apparent activation energy, Q, and the
empirical material constants A and n. Power-law equations show that the values of the peak and steady-state stress are related
to the dimensionless parameter Z/A. The results also show that the flow stress deviates from linear dependence on the Z/A
parameter at ασ < 0.8 and ασ > 1.2. The deviations reflect the power and exponential equations, respectively, for low and
high stress levels. The way the peak and steady-state strains vary with the Z parameter was studied and best models were developed.
A model based on the Avrami equation was developed to estimate the kinetics of the dynamic recrystallization (DRX) for different
deformation conditions. The results of the model show that the Avrami exponent decreases as Z rises. In addition, the Avrami
constant of k bears no distinct relation with Z. The optical microscopy observations of prior austenite grains confirm that
the DRX grain size has an adverse relation with the Z parameter. The best relation between the DRX grain size and the Z/A
ratio is proposed on the basis of the grain size measurements.
Key wordsmetals-deformation-thermomechanical processing-recrystallization-compression test
The influence of prior grain boundaries on nucleation of recrystallisation and associated texture development in iron has been investigated using deformed bicrystal specimens. The samples were designed to simulate as closely as possible the conditions of orientation and geometry existing in heavily cold rolled polycrystals. In all cases nucleation occurred preferentially at grain boundaries and resulted in well defined recrystallisation textures. The most significant finding was that components of the 〈111〉//ND γ-fibre texture gave rise to new orientations within the same fibre spread rotated ~30° around the normal direction. No evidence was found of systematic lattice rotations adjacent to the grain boundaries in the deformed state. The texture evolution is interpreted in terms of a compromise between the frequency of potential nuclei and their respective growth velocities. Present results permit rationalisation of several important aspects of texture development in conventional low carbon steels.
Electron microscope observations on some polycrystalline metals suggest that after small to moderate deformation, recrystallization occurs by the migration of the original grain boundaries. A theory based on this mechanism can account for the known form of the recrystallization kinetics without necessarily introducing any anisotropy of grain boundary mobility. For this mechanism the so-called recrystallization activation energy is identical to the activation energy for grain boundary migration.
Structural changes taking place under warm deformation of pure copper were studied in compression at temperatures ranging from 473 to 673 K (0.35 to 0.50 T-m) under strain rates of 10(-3)-10(-1) s(-1). Dynamic recrystallization (DRX) takes place fully or partly at temperatures higher than 523 K, while no fine grains are evolved in the pan-caked original grains with serrated boundaries even after high strains at 473 K. The new grains are evolved by bulging mechanism associated with local migration of the original grain boundaries and the evolution of twins and dislocation boundaries. The relationship of peak flow stress to the new grain size evolved under warm deformation can be expressed by a power law function with a grain size exponent of about -0.35, which is different from that for DRX taking place under hot deformation (i.e. -0.75). The correlation between the mechanisms of plastic deformation and the structural evolution, such as dislocation densities, cell sizes, and DRX grain sizes, and the mechanisms of low and high temperature DRX are discussed in combination with the analysis of deformation behaviour at moderate temperatures.
The relation between polycrystal deformation and single crystal deformation has been studied in this work. A pure aluminium polycrystal having an average grain size of 300μm has been strained in tension at room temperature. The flow stress has been determined at four different strains (0.05, 0.14, 0.22 and 0.34) and deformation microstructures have been characterized qualitatively and quantitatively by transmission electron microscopy. Improved experimental techniques have allowed large foil areas to be characterized and in total 89 grains have been examined. At the four strains examined a classification of the deformation microstructures into three different types has shown a correlation between the grain orientation and the type of deformation microstructure which develops during straining. The dislocation density has been calculated at each strain and by assuming that the shear stress is proportional to the square root of the dislocation density the shear stress–strain relationship has been derived for each of the three groups of grains showing different deformation microstructures. The stress–strain curves show a strain hardening behaviour which depends on the orientation of the grain. The behaviour of the grains embedded in the polycrystal is compared with the behaviour of single crystals and the stress–strain curve of the polycrystal is estimated with good accuracy from single crystal data, which are weighted based on a quantitative texture analysis of the polycrystal.
The aim of the current study was to investigate the nucleation mechanisms of new grains during DRX and the deformation behavior of a necklace structure. The investigations were conducted on Ni3Al, because Ni3Al develops a distinct necklace structure during dynamic recrystallization DRX. Local orientation measurements were conducted to determine misorientations between new recrystallized grains and their parent grains. DRX was set off by strain induced bulging of prior grain boundaries. Additionally, the formation of new grains by recrystallization twinning was observed. With progressing DRX the orientation coherency of DRX grains with the matrix grains diminished rapidly, and the texture tended to randomize. The strain rate sensitivity indicated superplastic flow in the recrystallized volume. The deformation behavior changed significantly, when these soft regions formed a contiguous 3D network along the original grain boundaries. A new model for the flow curve is proposed that accounts for the percolation character of necklace structures.
Many metals and alloys with low and intermediate stacking fault energy undergo dynamic recrystallization (DRX). Due to the growing importance of hot deformation in metal forming there is an increasing interest in the understanding and modeling of microstructure evolution during DRX and its effect on flow behavior. However, despite extensive research in this field and numerous data on a variety of materials the physical understanding of DRX still remains very qualitative. Especially the nucleation of DRX lacks a detailed physical understanding and experimental evidence, due to the difficulties of investigating the micromechanisms of dynamic processes during high temperature deformation. The improved techniques of single grain orientation measurements by using EBSD (electron backscatter diffraction) in the SEM allow to measure the local orientation arrangement and thus identify the orientations of individual nuclei. The current report focuses on the examination of the substructure evolution during dynamic recrystallization with particular attention to the role of continuous subgrain rotation or instabilities of the subgrain structure near the grain boundary with regard to nucleation during DRX.
The deformation pattern at grain boundaries and at triple junctions in polycrystalline high purity aluminium (99.999%) has been studied by electron back scattering pattern (EBSP) observations. Specimens of two different grain sizes rolled to give 5% and 30% reductions have been examined by these different EBSP scans: (i) scans across grain boundaries, (ii) scans along grain boundaries and (iii) two-dimensional scans near triple junctions. These scans are carried out in small steps (1–5 μm) over long distances (up to 50 μm). The EBSP measurements show that the level of perturbations increases with strain and that enhanced zones of perturbations are observed at grain boundaries and especially near triple junctions. In specimens deformed by 30%, such zones of large perturbation are observed at most of the grain boundaries, and in the specimens deformed by 5%, at some triple junctions. The EBSP measurements are compared to previous microstructural observations by transmission electron microscopy and the effect of the zones of enhanced perturbations is discussed with relation to the mechanical and thermal behaviour at highly deformed polycrystalline metals.
The critical strain criterion εp = εx for the transition from cyclic to single peak recrystallization is demonstrated to be invalid for the high temperature deformation of f.c.c. metals in tension and compression. The role of the strain and strain rate gradients present in solid torsion bars in raising the apparent torsion peak strain εp above the εp values obtained from homogeneous tension or compression testing is clarified. A similar, and larger, effect is shown to cause discrepancies in the torsion values of the recrystallization strain εx. An alternative criterion for the transition is described, based on grain size considerations. The latter indicate that cyclic flow curves are associated with grain coarsening and that single peak flow curves are associated with grain refinement. The critical condition is D0 = 2Ds, where D0 and Ds are the initial and stable grain sizes respectively. The transition in flow curve shape under strain rate change conditions is also analyzed. It appears that after an increase in strain rate, the flow curve displays a single peak, whereas, after a strain rate decrease, multiple peaks are observed. The critical condition at which the shape of the stress-strain curve changes from the multiple to the single peak type is Ds1 = Ds2, where Ds1 and Ds2 are the stable dynamically recrystallized grain sizes before and after the change in strain rate, respectively. The results indicate that single peak behaviour is caused by the “necklace” or “cascade” recrystallization of coarse-grained materials, which produces a large spread in the nucleation strain εc, and accordingly a highly unsynchronized form of local recrystallization. The growth process (and consequently the grain size) in this case appears to be deformation limited. By contrast, recrystallization is nearly completely synchronized in fine-grained materials, because the high density of grain nuclei leads to a small spread in the nucleation strain. The grain size under these conditions is determined by impingement, and is thus nucleation not growth controlled. Finally, it is concluded that the interpretation given to the transition in flow curve shape by the relative grain size model, expressed in terms of the spread Δεc in nucleation strain εc, is in broad agreement with the one derived by earlier workers on the basis of computer simulations, and in the absence of grain size considerations.RésuméNous démontrons que le critère de la déformation critique εp = εx pour la transition entre les recristallisations cyclique et à pic unique n'est pas valable pour la déformation à haute température des métaux c.f.c. en traction et en compression. Nous clarifions le rôle des gradients de déformation et de vitesse de déformation présents dans barres de torsion solides, dans l'augmentation de la déformation apparente au pic de torsion εp au-dessus des valeurs εp obtenues pour des essais de traction ou de compression homogènes. Nous montrons qu'un effet analogue et plus grand provoque des différences dans les valeurs de la déformation de recristallisation en torsion εx. Nous présentons un autre critère pour la transition, critère qui repose sur des considérations concernant la taille des grains. Il montre que des courbes d'écoulement cycliques sont associées à un grossissement des grains et que des courbes d'écoulement à pic unique sont associées à un affinage des grains. La condition critique est D0 = 2Ds, oùD0 et Ds sont respectivement les tailles de grains initiale et stable. Nous avons également analysé la transition de la forme des courbes d'écoulement en fonction de variations de la vitesse de déformation. Lorsqu'on augmente la vitesse de déformation, la courbe d'écoulement présente un pic unique, alors qu'on observe des pics multiples après une diminution de la vitesse de déformation. La condition critique pour laquelle la forme des courbes contrainte-déformation passe du type à pics multiples au type à pic unique est: Ds1 = Ds2, où Ds1 et Ds2 sont les tailles des grains stables après recristallisation dynamique, respectivement avant et après le changement de la vitesse de déformation. Ces résultats montrent que le comportement à pic unique est provoqué par la recristallisation en “collier” ou en “cascade” des matériaux à gros grains, qui conduit à une grande dispersion dans la déformation de germination εc et donc à une forme très désynchronisée de recristallisation locale. Le processus de croissance (et par suite la taille des grains) est limité, dans ce cas, par la déformation. Au contraire, la recristallisation est pratiquement entièrement synchronisée dans les matériaux à grains fins, car la forte densité de germes de grains conduit à une faible dispersion dans les valeurs de la déformation de germination. La taille des grains est déterminée dans ces conditions par leur rencontre; elle est ainsi contrôlée par la germination et non par la croissance. Pour conclure, nous remarquons que l'interprétation que nous avançons pour la transition dans la forme des courbes d'écoulement à partir d'un modèle de taille relative des grains, exprimée par la dispersion Δεc de la déformation de germination εc, est en bon accord général avec celle qu'ont obtenue antérieurement d'autres auteurs à partir de simulations sur ordinateur et en l'absence de toute considération concernant la taille des grains.ZusammenfassungEs wird gezeigt, daß das Kriterium für die kritische Dehnung εp = εx für den Übergang der Rekristallisation mit zyklischem zum einfachen Maximum ungültig ist für den Fall der Hochtemperaturverformung von kfz. Metallen im Zug- und im Druckversuch. Außerdem wird die Rolle geklärt, die die Gradienten der Verformung und der Verformungsrate in massiven. Torsionsstäben für die Erhöhung der maximalen Torsionsdehnung εp über die εp-Werte für homogene Zug- oder Druckverformung hinaus haben. Ein ähnlicher, aber größerer Effekt führt zu Diskrepanzen in der Rekristallisationsdehnung εx in Torsion. Ein alternatives Kriterium, welches von der Korngröße ausgeht, wird für den Übergang beschrieben. Die Korngrößen zeigen, daß zyklische Fließkurven mit Kornvergröberung, Fließkurven mit einem Maximum mit Kornverfeinerung einhergehen. Die kritische Bedingung ist D0 = 2Ds (D0: anfängliche, Ds: stabile Korngröße). Der Übergang in der Form der Fließkurve nach einer änderung der Dehnungsrate wird ebenfalls untersucht. Es scheint, als ob nach einem Anstieg der Dehnungsrate die Fließkurve ein einzelnes Maximum aufweist, wohingegen nach Erniedrigung der Dehnungsrate mehrfache Maxima beobachtet werden. Die kritische Bedingung, bei der sich die Form der Verfestigungs-kurve vom Typ mit mehrfachem zum Typ mit einfachem Maximum ändert, ist Ds1 = Ds2 (Ds: stabile dynamisch rekristallisierte Korngröße vor (Ds1) und nach (Ds2 der Änderung in der Dehnungsrate). Die Ergebnisse legen nhe, daß das Verhalten mit einzelnem Maximum durch eine “Kaskaden-” oder “Ketten-” artige Rekristallisation grobkörnigen Materials verursacht wird. Ein solches Material weist eine weite Spanne in der Keimbildungsdehnung εc auf und hat dementsprechend ein lokal sehr wenig synchron ablaufendes Rekristallisationsverhalten. Der Wachstumsprozeß und folglich die Korngöße scheinen in diesem Falle verformungsbegrenzt zu sein. Dagegen ist die Rekristallisation in feinkörnigem Material nahezu vollständig synchronisiert, da die hohe Dichte an Keimen zu einer kleinen Spanne in der Nukleationsdehnung führt. Die Korngröße ist unter diesen Umständen bestimmt durch Aufeinandertreffen und daher keimungs- und nicht wachstumskontrolliert. Schließlich wird gezeigt, daB die Interpretation der änderungen in der Fließkurvenform mit dem Modell der relativen Korngrößen, ausgedrückt als als die Spanne Δεc in der Nukleationsdehnung εc in breiter übereinstimmung steht mit dem Modell, welches frühere Autoren auf der Basis von Rechnersimulationen ohne Rücksicht auf die Korngrößen abgeleitet hatten.
A criterion for the initiation of dynamic recrystallization during hot working is proposed. This is based on the contention that the reduced driving force due to concurrent deformation modifies the normal energy balance which defines the conditions for nucleation of new grains. A principal conclusion from the analysis is that the initiation of dynamic recrystallization requires a critical dislocation density irrespective of whether dynamic recovery is occurring simultaneously or not. The theory forecasts behaviour in reasonable accord with experiment particularly as regards the temperature and strain rate dependence of the stress maximum which characterizes hot working stress-strain curves from materials undergoing dynamic recrystallization. In addition, the present predictions, combined with a treatment of grain boundary nucleated reactions due to Cahn, may be used to calculate the dynamically recrystallized grain size.