Occurrence Of Recrystallization Research Articles

Modelling Dynamic Recrystallization in FCC Metals Employing Thermostatistics

Theory for describing the conditions leading to dynamic recrystallization in FCC metals is introduced. The approach also describes stress-strain curves when this process occurs, and is unique in incorporating the effects of strain rate and temperature employing only physical parameters as input. The novelty of the approach stems from incorporating an incubation period in the equations describing the progress of dislocation density with strain; beyond such incubation dislocation free grains form. The energy barrier to ignite grain growth is expressed as a function of the strain energy stored on the material and a statistical entropy contribution due to the degrees of freedom available to a dislocation for annihilation. The incubation strain is obtained by performing an energy balance between the stored energy on the subgrain boundaries, the slip energy of boundary migration and the interfacial energy required for grain nucleation. The application of this work to pure Cu and Ni has lead to transition maps in temperature-strain rate space indicating the conditions for dynamic recrystallization occurrence.

Microstructure and microtexture in pure copper processed by high-pressure torsion

The evolution of microstructure and microtexture in high purity copper was examined after processing by high-pressure torsion (HPT). Copper disks were annealed for 1 h at 800 °C and later processed monotonously in HPT at ambient temperature for 1/4, 1/2, 1, and 5 turns under a pressure of 6.0 GPa. Electron backscattered diffraction (EBSD) measurements were taken for each disk at three positions: center, mid-radius, and near-edge. Results from EBSD for samples processed between 1/4 and 1 turn indicate the formation of Σ3 twin boundaries by recrystallization before complete microstructural refinement. The results show a gradual increase in the homogeneity of the microstructure with increasing numbers of turns, reaching a stabilized ultrafine-grained structure at 5 turns with a bimodal distribution of fine and coarse grains of 0.15 and 0.5 μm in diameter, respectively. The occurrence of recrystallization in the early straining stages was further supported by examining microtexture development with increasing numbers of turns, where this shows a gradual transition from a shear texture to a mixture of shear and recrystallization and later to a shear texture at high HPT strains. The promotion of recrystallization during HPT is probably related to the high purity of the copper.

The effect of strain-induced melt activation process on the microstructure and mechanical properties of Ti-refined A6070 Al alloy

In current research, the effect of Ti (Al–5Ti–1B) master alloy on the macrostructure of A6070 Al alloy has been investigated. The results revealed that the optimum Ti content to achieve a fine-grained structure was found to be 0.02wt.%. Then, in order to investigate the effect of strain-induced melt activation (SIMA) process on the microstructures of unrefined and 0.02% Ti-refined A6070 alloy optical and scanning electron microscopy (SEM) were used. The results showed that as the applied pre-deformation increases, the globular size decreases and the sphericity of globules increases. The results revealed that for the desired microstructure of the alloy during SIMA process, the optimum temperature, holding time and strain are 630°C, 20min and 30%, respectively. The results showed that the average globule size achieved to a minimum value of 68μm and 55μm in the unrefined and 0.02% Ti-refined alloy, respectively. The results obtained from tensile testing showed that SIMA process increases the tensile properties of the alloy significantly, although due to occurrence of recrystallization during SIMA process, tensile ductility increases more than tensile strength.

Investigations on Interaction between Recrystallization and Precipitation at Finishing Steps of Seamless Tube Rolling

Conventional controlled rolling is normally used in a hot strip mill in order to refine the microstructure and, consequently, to meet a good combination of mechanical strength and toughness in low carbon steels. Nb microalloying are used for this purpose once this rolling technique requires the occurrence of strain induced precipitation before that any recrystallization process takes place. In a hot strip mill, the transfer time between roughing and finishing steps is considered fast enough to keep all Nb, previously solubilized in plate furnace, in solid solution just before finishing. As a consequence, the thermodynamic driving force required for strain induced precipitation remains high, accelerating its kinetics and preventing the occurrence of recrystallization. On the other hand, in a seamless tube rolling, the transfer time between roughing and finishing steps is about 20 times longer than in a hot strip mill. As a consequence, Nb can precipitate in the recrystallized austenite after roughing, decreasing the driving force for strain induced precipitation at finishing. Transmission electron microscopy investigations on samples simulating the material just before finishing rolling in a seamless tube mill show that both grain boundary precipitation and co-precipitation on TiN particles takes place during transfer time. Based on these findings, this paper discusses the effect of long times after roughing steps on the driving force for strain induced precipitation at finishing rolling in seamless tube. Thermo-Calc software was used to evaluate the possible amount of Nb that remains in solid solution just before finishing. Due to previous precipitation, the driving force to be considered in the Dutta-Sellars equations for onset of precipitation decreases and a comparison between recrystallization and precipitation curves shows that recrystallization takes place before the start of strain induce precipitation.

Effect of heat treatment on stability of impact-induced adiabatic shear bands in 4340 steel

Abstract Adiabatic shear bands (ASBs) have been observed in many engineering applications, including armor plates and metal working operations subjected to both high strain rates (>10 3 S −1 ) and large strains. These bands indicate excessive deformations due to strain localization. Preventing their formation by proper choice of materials or eliminating them when formed may avert the occurrence of catastrophic failures. In this study, impacted specimens of 4340 steel, where ASBs have been observed, were subjected to post-impact annealing to determine the persistence of ASBs. Microstructural observations were made using optical and scanning electron microscopy to determine whether the physical appearance of the ASBs changes or not during the post-impact annealing. These observations were followed by X-ray diffraction analysis on the shear band regions and on the base materials to determine the influence of the heat treatment on the annealed specimens. It was found that annealing at temperatures higher than 600 °C shows significant reduction in the physical appearance of the ASB and its hardness. After 30 min annealing at 650 °C, the hardness of the shear bands and their matrixes reduced to approximately the same level as they were before deformation. The X-ray diffraction analysis also confirmed that recrystallization processes and grain growth cause the high strains of the ASBs to be reduced. Post-impact annealing at 650 °C revealed that the fine crystals in the shear bands were replaced by larger crystals compared to the surrounding material due to the early occurrence of recrystallization in the shear band regions. Post-impact annealing of the steel specimens at 750 °C and 850 °C for 30 min led to a homogenous microstructure with no trace of the shear bands. The pre-impact heat treatment properties were not affected because there was no excessive grain growth to significantly alter the strength and hardness of the impacted and annealed specimens as compared to the properties of the undeformed material. The X-ray diffraction analysis revealed that the average size of the crystals in the annealed specimens is smaller when compared to the unimpacted specimen.

Texture and Microstructural Evolution in Pearlitic Steel During Triaxial Compression

This article presents the deformation behavior of high-strength pearlitic steel deformed by triaxial compression to achieve ultra-fine ferrite grain size with fragmented cementite. The consequent evolution of microstructure and texture has been studied using scanning electron microscopy, electron back-scatter diffraction, and X-ray diffraction. The synergistic effect of diffusion and deformation leads to the uniform dissolution of cementite at higher temperature. At lower temperature, significant grain refinement of ferrite phase occurs by deformation and exhibits a characteristic deformation texture. In contrast, the high-temperature deformed sample shows a weaker texture with cube component for the ferrite phase, indicating the occurrence of recrystallization. The different mechanisms responsible for the refinement of ferrite as well as the fragmentation of cementite and their interaction with each other have been analyzed. Viscoplastic self-consistent simulation was employed to understand deformation texture in the ferrite phase during triaxial compression.

Deformation and ultrafine dynamic recrystallization of quartz in pseudotachylyte-bearing brittle faults: A matter of a few seconds

Tectonic pseudotachylytes, i.e. quenched friction-induced silicate melts, record coseismic slip along faults and are mainly reported from the brittle crust in association with cataclasites. In this study, we document the occurrence of recrystallization of quartz to ultrafine-grained (grain size 1–2 μm) aggregates along microshear zones (50–150 μm thick) in the host rock adjacent to pseudotachylytes from two different faults within quartzite (Schneeberg Normal Fault Zone, Eastern Alps), and tonalite (Adamello fault, Southern Alps) in the brittle crust. The transition from the host quartz to microshear zone interior includes: (i) formation of high dislocation densities; (ii) fine (0.3–0.5 μm) polygonization to subgrains defined by disordered to well-ordered dislocation walls; (iii) development of a mosaic aggregate of dislocation-free new grains. The crystallographic preferred orientation (CPO) of quartz towards the microshear zone shows a progressive misorientation from the host grain, by subgrain rotation recrystallization, to a nearly random CPO possibly related to grain boundary sliding. These ultrafine aggregates appear to be typically associated with pseudotachylytes in nature. We refer the crystal plastic deformation of quartz accompanied by dramatic grain size refinement to the coseismic stages of fault slip due to high differential stress and temperature transients induced by frictional heating. Microshear zones localized on precursory fractures developed during the stages of earthquake rupture propagation and the very initial stages of fault slip. Thermal models indicate that the process of recrystallization, including recovery processes, occurred in a time lapse of a few tens of seconds.

Research on Hot Deformation Behavior of 3003 Al Alloy Prepared by Different Melt-Treatment Methods

3003 Al alloy with different metallurgical quality were obtained by different melt-treatment methods, which were deformed by isothermal compression in the range of deformation temperature 300-500°C at strain rate 0.0l-10.0 s-1with Gleeble-1500 thermal simulator. The results show that the material is sensitive to positive strain rate. The hot deformation activation energy (Q) bears linear relationship with inclusion content (H) of 3003 Al alloy prepared by different melt-treatment, Q=35.62 H+171.58, the activation energy of 3003 Al alloy prepared by high melt-treatment is the lowest (174.62 KJ×mol-1), which is beneficial to the material hot plastic deformation. The critical strain of 3003 Al alloy prepared by different melt-treatment methods is investigated through the work hardening rate. Finally, the critical conditions of the investigated alloy were determined to predict the dynamic recrystallization occurrence in the paper.

Use of recalescence behavior analysis for the prediction of grain refinement in undercooled Cu–Ni alloy

Considering non-equilibrium solidification and its influence on subsequent near-equilibrium solidification together, the description of recalescence behavior in bulk undercooled Cu70Ni30 alloy was adopted to predict the corresponding microstructure transition. The thermal plateau time for near-equilibrium solidification can be deduced directly with the calculation of non-equilibrium solid fraction formed in recalescence. On the basis of quantitative description for recalescence behavior, the non-equilibrium solid fraction, residual liquid fraction, dendrite broken-up time, and thermal plateau time can be determined as functions of initial undercooling. Then, a simple and accurate application of dendrite fragmentation model was performed as the grain refinement at both low- and high-undercooling originates from dendrite breakup. The predicted undercooling regions for the double grain refinement agree well with the experimental observation. Moreover, the change of grain morphology for the second grain refinement can be ascribed to the occurrence of recrystallization produced by the enhanced residual stress upon highly undercooled solidification.

On the generation of nanograins in pure copper through uniaxial single compression

Attempts at generating nanograins through uniaxial single compression were made by deforming copper samples at 298 K and 77 K. At 298 K, dynamically-deformed samples (DDS) become softer, in contrast to quasi-statically deformed samples (QDS), which show a hardness close to the saturation value. The microstructure of DDS is characterised by deformation twins and equiaxed micron-sized grains, and the observed softening is due to the occurrence of recrystallisation (RX). At a reduced temperature of 77 K, nanograins are generated in DDS, whereas QDS show forest dislocations and twins. The generation of nanograins, which evolve through rotational DRX, is associated with the formation of shear bands with an amorphous structure. Compared with twinning, it appears that amorphisation plays a more pronounced role in high strain rate deformation at reduced temperatures (77 K). The hardness of DDS, obtained from compression at 77 K, exceeds the saturation value by 16%, whereas that of QDS corresponds approximately to saturation.

On the processing of hetero-nanostructured metals for improved strength/ductility balance by ECAE and SPS techniques

This paper has examined some recent findings concerning the processing of fully dense hetero-nanostructured materials (i.e. consisting of nano, ultrafine and micrometric grains) which can be produced by using the interplay between heavy deformation and recrystallization. By plastic deformation of bulk materials, an improved strength/ductility balance can be obtained directly by imparting high strain deformation (by equal channel angular extrusion) until the occurrence of recrystallization. Using a powder metallurgy route, the strong potential of electric field assisted sintering (ECAS) techniques for producing multi-scale microstructures when a conducting milled powder is used is demonstrated. In this case, in addition to modify the classic processing parameters (time/temperature of spark plasma sintering), altering the nature of the milled powder – by Y 2O 3 addition during the milling stage – is also a good way to delay the onset of recrystallization and, thereby, increase the fraction of ultrafine grains.

Processing Dense Hetero-Nanostructured Metallic Materials for Improved Strength/Ductility Balance through High Strain Deformation and Electrical Current Assisted Sintering (ECAS)

This paper has examined some recent findings concerning the processing of fully dense hetero-nanostructured materials (i.e. consisting of nano, ultrafine and micrometric grains) which can be produced by using the interplay between heavy deformation and recrystallization. By plastic deformation of bulk materials, an improved strength/ductility balance can be obtained directly by imparting high strain deformation (by ECAE) until the occurrence of recrystallization. Using a powder metallurgy route, the strong potential of electric field assisted sintering (ECAS) for producing multi-scale microstructures when a milled powder is used is also demonstrated. In this case, in addition to modify the classic processing parameters (time/temperature of ECAS), altering the nature of the milled powder - by Y2O3 addition during the milling stage - is also a good way to delay the onset of recrystallization and, thereby, increase the fraction of ultrafine grains.

High-speed deformation of titanium during dynamic channel-angular pressing

Abstract A titanium sample was deformed using dynamic channel-angular pressing using a shock loading technique. The structural changes and the cracking behavior occurring during the high-speed deformation were studied by metallographic inspection. A geometrical method of determining the amount of uniform shear strain upon equal-channel angular pressing is suggested. The localized deformation leads to the appearance of adiabatic shear bands. Two types of adiabatic shear bands were observed that intersect the sample in either the longitudinal or transverse directions. The occurrence of recrystallization inside the bands indicates local heating of the material to 770 – 870 K. The morphology of the adiabatic shear bands can be described as a multilayer structure. The width of bands is up to 100 m.

Tailoring structures through two-step annealing process in nanostructured aluminum produced by accumulative roll-bonding

Due to structural and textural heterogeneities and a high content of stored energy, annealing of nanostructured metals is difficult to control in order to avoid non-uniform coarsening and recrystallization. The present research demonstrates a method to homogenize the structure by annealing at low temperature before annealing at high temperature. By this two-step process, the structure is homogenized and the stored energy is reduced significantly during the first annealing step. As an example, high-purity aluminum has been deformed to a total reduction of 98.4% (equivalent strain of 4.8) by accumulative roll-bonding at room temperature. Isochronal annealing for 0.5 h of the deformed samples shows the occurrence of recrystallization at 200 °C and above. However, when introducing an annealing step for 6 h at 175 °C, no significant recrystallization is observed and relatively homogeneous structures are obtained when the samples afterwards are annealed at higher temperatures up to 300 °C. To underpin these observations, the structural evolution has been characterized by transmission electron microscopy, showing that significant annihilation of high-angle boundaries, low-angle dislocation boundaries, and dislocations characterizes the low-temperature annealing step. In a discussion, the observed annealing behavior is related to these structural changes.

Microstructure and Texture Development of Pure Aluminum and Aluminum Alloy Processed by High Speed Hot Rolling

The microstructure and texture development of pure aluminum and aluminum alloy processed by high speed hot rolling are investigated. The aluminum sheets are rolled at temperatures ranging from 410°C to 560°C at a rolling speed of 15m/s without lubrication and quenched into water at an interval of 30 ms after rolling. The redundant shear strain induced by high friction increases beneath the surface at a reduction above 50% for Al alloy (AA5052) and above 60% for pure Al (AA1050). Dynamic recrystallization occurs in the surface region when the equivalent strain exceeds a critical value that depends on rolling temperature, while only recovery occurs in the center region. The critical equivalent strain for the occurrence of recrystallization in AA5052 is lower than that in AA1050. When the large strain is introduced beneath the surface, the shear texture, the main components of which are {001} and {111} , develops. In the center region, Cu-orientation and cube orientation develop. The shear texture beneath the surface is weak when recrystallization occurs.

High-strain-rate deformation of titanium using dynamic equal-channel angular pressing

Titanium samples were deformed using equal-channel angular pressing (ECAP). Structural changes upon the uniform and localized high-strain-rate deformation and specific features of the nucleation and propagation of cracks have been studied. A geometrical method of determining the amount of uniform shear strain upon equal-channel angular pressing has been suggested. The method is based on a metallographic examination of the spatial orientation of structural components. The localized deformation leads to the appearance of adiabatic-shear bands. Two band systems are formed: longitudinal and transverse, arranged at an angle to the longitudinal. The occurrence of recrystallization inside the bands indicates local heating of the material to 770–870 K. Specific features of the structure of the adiabatic-shear bands arising in this method of deformation is their large width (to 100 μ m) and a multilayer structure.

Effect of Annealing on the Mechanical Property and Machinability of Free-Cutting Phosphor Bronze Alloy

Free-cutting phosphor bronze alloy (ASTM C54400) was used as a material for automotive parts and industrial valve parts for its high strength, machinability and excellent corrosion resistance. To obtain desired mechanical strength of the alloy, the process parameters including continuous casting speed, reduction in area and annealing temperature have to be controlled. In this study, we investigated the effect of annealing on the mechanical property and machinability of free-cutting phosphor bronze alloy fabricated by continuous casting with different casting speeds of 80, 100, and 120 m/min, respectively. After annealing, the tensile strength of the alloy decreased with increasing annealing time owing to the occurrence of recrystallization and grain growth. In the cast and swaged alloy produced at casting speed of 120 m/min, the grain growth occurred rapidly. In contrast, in the alloy produced at the casting speed of 80 m/min, the grains growth rate was low. Furthermore, the effect of Pb particle in the alloy on machinability was also investigated by using a tool dynamometer installed force sensor.

Crystallisation and Melting Behavior of Methyl Esters of Palm Oil

The methyl esters of palm oil, which consists of saturated and unsaturated esters (0.6 to 95.9% unsaturation) of the C12 to C18 fatty acids, solidify at the two temperature ranges, -52 to -45°C and -24 to 21°C, when the esters are cooled. When the esters are heated, they melt at two distinct temperatures, -25 and -33°C and a broad peak at -9 to 28°C. The heating thermograms also showed an exothermic crystallisation peak in between two endothermic melting peaks, indicating the occurrence of re-crystallisation of low melting methyl esters into higher melting point crystal and then melt again at higher temperature.

Microstructural Evolution in Pure Copper Severely Deformed by the ARB Process

An oxygen free high conductivity (OFHC) copper (99.99%) was intensely deformed by the accumulative roll-bonding (ARB) process up to equivalent strain of 4.8 at ambient temperature. The microstructure evolution during the ARB process was explained by grain subdivision. The deformed specimens revealed dislocation cell structures at low strain and elongated ultra fine grains separated by high angle boundaries at high strain. The spacing of the high angle lamellar boundary exponentially decreased as a function of strain. The fractions of high angle boundaries (HAB) and the low angle boundaries (LAB) were nearly equal even at strain of 3.2, which was significantly different from the ARB processed Al alloys and ferritic steel where the HAB fraction was above 70% at the same strain. TEM observations indicated a mixed microstructure of dislocation boundaries and cell walls with dislocation tangle at low strain of 1.6, and small recrystallized grains partly appeared above strain of 3.2. As a result, the LAB fraction due to partial recrystallization was high even at strain of 4.8. The occurrence of recrystallization is attributed to high purity of the OFHC copper, the accumulated dislocation density, and the adiabatic heating during the ARB process of one-pass large reduction without lubrication.

Annealing Behavior and Recrystallized Texture in ARB Processed Copper

The recrystallization behavior and texture development in copper accumulative roll-bonding (ARB) processed by various cycles (2, 4 and 6 cycle) were studied by differential scanning calorimetry (DSC) analysis and SEM/EBSP method. The exothermic peaks caused by recrystallization appeared at 210 ~ 253 􀍠 in each sample. The peak positions shifted to lower temperature as the number of ARB cycles increased. This result indicated that the evolution of finer microstructure with increasing number of the ARB cycles enhanced the occurrence of recrystallization at lower temperature. The stored energy calculated from the DSC curve of the ARB processed copper increased with the increasing strains. During an annealing, the preferential growth of cube-oriented grains ({100}<001>) occurred in each sample. The recystallization behavior of ARB processed copper having low stacking fault energies was distinguished from that of so-called “recovery type” materials, i.e. aluminum and low carbon steels, which shows rather continuous changes in microstructure during annealing. The accumulated strains provided the driving force for the preferential growth, which was the same mechanism as the preferential growth in normally rolled copper. The sharpest cube texture developed in ARB processed copper by 4 cycles. The difference of cube texture development between 2 cycles and 4 cycles was caused by the distribution of cube-oriented regions which corresponded to the nucleation sites of recrystallized grains before annealing. More nanocystalline layers in the vicinity of bonded interfaces were distributed in ARB processed copper by 6 cycles than 4cycles. The nanocystalline structure could grow faster than the cube-oriented grains and led to the inhibition of sharp cube texture in the ARB processed copper by 6 cycles.