Recrystallization Nuclei Research Articles

Twins in the rolled Mg–Zn–Ca alloy with high formability

Abstract

Graininess variation research of steel 20 welding joints at deformation-thermal treatment

It is known that the metal grains’ size affect on the metal physical-and-mechanical properties. At the fusion welding the metal grains irregular growth takes place, graininess is increasing. It affects negatively on physical-and-mechanical properties on the welding joint metal. For decreasing graininess variation thermal and thermoplastic treatment of welding joints (annealing, thermoplastic treatment) is used. In was carried out the comparative analysis of the results of steel 20 welding joints structure research after deformation at the temperature-rate regime of super plastic deformation and after annealing and after thermo cyclical treatment. At superplastic deformation during hot deformation, became possible processes of dislocation redistribution, including vacancy creeping, forming recrystallization nuclei and their growth, migration of the large angular boundaries. In this research a thermo mechanical treatment at super plasticity regime deformation is studied. In this case the effect of super plasticity deformation is not revealed in full, because for that there must present fine grain structure. The results of the research showed the positive effect of the thermoplastic treatment at super plasticity regime deformation. Slow rate of deformation in combination with definite temperature affects on the thermal activated processes of thermal loss of strength. Microstructure research was carried out on Neophot-30 microscope. For studying the graininess structure variation changing the parameters of grains dimensions distribution were calculated by using the industry system of images analysis SIAMS 600. It was determined that in the zone of the welding joint thermal effect the increasing of graininess variation in metal took place. After annealing of the welding joint the graininess variation in the zone of thermal effect almost had not changed. After isothermal rolling with heated rollers at the regime of super plastic deformation and thermo cyclical treatment the graininess variation in the zone of thermal effect decreased and approached to the value of the basic metal graininess variation.

Microstructure evolution at the onset of discontinuous dynamic recrystallization: A physics-based model of subgrain critical size

• Evolution of subgrains to recrystallization nuclei is possible at very low strains. • Dynamic recrystallization nuclei increases on a sigmoidal curve with strain. • Nucleation rate increases up to a maximum and decreases to zero at peak strain. • Growth of recrystallized nuclei diminishes to zero at or over the peak. • Site saturation is the mechanism of dynamic recrystallization up to the peak. 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.

Dynamic recrystallization in high-purity aluminum single crystal under frictionless deformation mode at room temperature

Abstract

Texture transformations in Ag thin films

A thickness-dependent texture transformation during annealing of initially (111) fiber-textured face-centered-cubic metal thin films is phenomenologically well known: sufficiently thin films retain the (111) texture, while sufficiently thick films transform to a (100) fiber texture. This transformation has been explained based on minimization of strain and interface energies, but recent work calls into question the roles of both of these driving forces. A high-throughput experimental method for the study of this texture transformation has been developed and applied to thin silver films with and without Ti adhesion layers. More than 150 individual samples spanning a range of thicknesses and interface conditions were prepared in a single deposition run. The texture evolution of these samples was characterized using X-ray diffraction as a function of time and temperature during annealing. The transformation proceeds despite the fact that the stresses are too low according to the strain/interface energy model. For films with Ti adhesion layers, the transformation kinetics and extent of transformation depend on the film thickness in a surprising way with intermediate thickness films showing initially fast transformations and stable mixed textures, while thicker films show an incubation time but transform fully. The results are consistent with reduction in defect energy (e.g. dislocations or point defects) as the driving force for secondary grain growth in an environment in which only (100) recrystallization nuclei can form. The driving force increases with film thickness so the nonmonotonic variation in transformation rate implies that the density of (100) nuclei decreases with thickness.

Linking recovery and recrystallization through triple junction motion in aluminum cold rolled to a large strain

Recovery mechanisms and kinetics have been studied in commercial purity aluminum (AA1050) cold rolled to a true strain of 5.5 (99.6% thickness reduction) and annealed at low temperatures from 140 to 220°C. Transmission electron microscopy, electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) are used to characterize the microstructural evolution during annealing. The microstructural characterization shows that a deformed lamellar structure coarsens uniformly during annealing by triple junction motion while maintaining the lamellar morphology, leading to a gradual transition into a more equiaxed structure, where recrystallization nuclei start to evolve. The apparent activation energy for the microstructural coarsening is estimated separately for different stages characterized by an increase in the lamellar boundary spacing measured by EBSD and ECC. The apparent activation energy increases during annealing, from 110kJmol−1 at the beginning to 230–240kJmol−1 at the end of uniform coarsening, linking the recovery stages to recrystallization. The increase in activation energy underpins operation of different diffusion mechanisms for migration of boundaries and their junctions during coarsening, and solute drag may become increasingly important as the structure coarsens. These findings form the basis for a discussion of the thermal behavior of a fine lamellar structure produced by cold rolling to a large strain of both scientific and applied interest.

Effect of initial grain sizes on the grain boundary network during grain boundary engineering in Alloy 690

Abstract

Influence of the initial concentration of hydrogen and modes of vacuum annealing on the structure, phase composition, and mechanical properties of sheet billet of the VT6 alloy: Part 1. Influence of the initial hydrogen concentration and modes of vacuum annealing on the phase composition and the average diameter of grain of the VT6 alloy

The possibilities of using reversible hydrogen doping in combination with warm rolling for milling the structural components and the formation of a heterophase structure are shown by the example of the VT6 alloy. Alloy doping with 0.3–0.7 wt % hydrogen and the formation of a α + β + α2 structure is accompanied by the enhancement of the local nonuniformity of deformation and an increase in the number of recrystallization nuclei during warm sheet rolling. The relation between the initial hydrogen concentration and the temperature of vacuum annealing (VA) with the phase composition and the size of structural components is established.

Influence of rolling temperature on static recrystallization behavior of AZ31 magnesium alloy

Poor formability of rolled magnesium (Mg) alloys extremely restricts applications in form of sheets originating from formation of strong basal texture. Recently, we found that increasing rolling temperature from 723 to 798 K for a AZ31 Mg alloy can significantly improve stretch formability due to remarkable texture weakening after annealing. In this study, static recrystallization behaviors of AZ31 alloy sheets rolled at 723 and 798 K were investigated by electron backscattered diffraction analyses at different annealing stages in order to understand the origin of high temperature rolling on texture weakening. For both sheets, similar deformation microstructures with approximately the same types and fractions of twins exist in the as-rolled condition and recrystallized grains are mainly formed at pre-existing grain boundaries due to discontinuous recrystallization during subsequent annealing. However, only the basal texture of the latter remarkably weakens due to the formation of new recrystallized grains with well-dispersed orientations. Non-basal slips enhanced during high temperature rolling at 798 K are most likely responsible for the texture randomization as a result of rotations of recrystallization nuclei.

Annealing Behavior of RAFM ODS-Eurofer Steel

Oxide-dispersion-strengthened (ODS) ferritic-martensitic steels are candidates for applications in fusion power plants where microstructural long-term stability at temperatures of ˜650°C to 700°C are required. The microstructural stability of 80% cold-rolled reduced-activation ferritic-martensitic 9% Cr ODS-Eurofer steel was investigated within a wide range of temperatures (300°C to 1350°C). Fine oxide dispersion is very effective to prevent recrystallization in the ferritic phase field. The low recrystallized volume fraction (<0.1) found in samples annealed at 800°C is associated with the nuclei found at prior grain boundaries and around coarse M23C6 particles. The combination of retarding effects such as Zener drag and concurrent recovery decrease the local stored energy and impede further growth of the recrystallization nuclei. Above 900°C, martensitic transformation takes place with consequent coarsening. Significant changes in crystallographic texture are also reported.

Dynamic Recrystallization Mechanisms Operating under Different Processing Conditions

Dynamic recrystallization (DRX) is one of the most important mechanisms for microstructure evolution during deformation of various metals and alloys. So-called discontinuous DRX usually develops in structural materials with low to medium stacking fault energy during hot working. The local migration, i.e. bulging, of grain boundaries leads to the formation of recrystallization nuclei, which then grow out consuming work-hardened surroundings. The cyclic character of nucleation and growth of new grains during deformation results in a dynamically constant average grain size. The dynamic grain size is sensitively dependent on temperature and strain rate and can be expressed by a power law function of flow stress with a grain size exponent of about-0.7 under conditions of hot working. Recent studies on DRX phenomenon suggest that a decrease in deformation temperature changes the structural mechanism for new grain formation. As a result, the grain size exponent in the relationship between the dynamic grain size and flow stress approaches about-0.25 under warm working conditions.

Microstructure and mechanical property of rolled-weld magnesium alloy AZ31

In this paper, a combined cold-rolling and welding technique was applied to magnesium alloy AZ31 in order to investigate the effect of cold rolling and static recrystallization on microstructures evolution and mechanical properties of the welded joints. The results showed that the 7% rolled-weld sample obtained the highest tensile strength (252MPa) and strength coefficient (87.6%). With the increase of rolling strain, the average grain size decreased in heat affected zone due to the effect of static recrystallization. The recovery, recrystallization nuclei, and grain growth processes in heat affected zone during welding were analyzed by thermodynamic theories and models, respectively. However, abnormal voids and intergranular fracture behavior were found in the 10% rolled-weld joints after tensile test, which was mainly attributed to the gas inclusions, initial micro-voids and high residual stress in base metal.

The Effect of Cold Rolling Direction on the Secondary Recrystallization in Fe-3%Si Steel

In order to verify the origin of Goss nuclei for secondary recrystallization in Fe-3%Si steel, the effect of cold-rolling direction on the secondary recrystallization was examined in this study. The cold-rolling direction was rotated through 0 ~ 90 degrees about the hot-rolling direction on normal direction axis of hot-rolled sheet. In spite of the different initial texture before cold rolling, the 88% cold rolled texture was formed by similar α and γ fiber regardless of the rotation of cold rolling direction. Likewise, regardless of the cold rolling direction, the primary recrystallized sheets had a similar texture. In particular, the area fraction of Goss component (tolerance angle within 15º) in the primary recrystallized sheets was increased in the cold rolling condition of rotating through 60, 90 degrees from the hot rolling direction. After high temperature annealing at 1200°C, the secondary recrystallized grain was fully evolved in the all conditions. The sharpness of Goss texture in secondary recrystallized sheet was decreased as increasing the rotation angle of cold rolling direction.

SEM and TEM Investigations of Recovery and Recrystallization in Technically Pure Molybdenum

Abstract Beside traditional applications of refractory metals, e.g. in high temperature furnace construction, lighting or glass industry, one of the most important molybdenum products nowadays are large plates which are frequently used as targets for the sputtering of molybdenum layers in thin-film transistor liquid crystal displays. For the hot rolling of the sintered pre-material, the control over the recovery and recrystallization behavior is of particular importance. Molybdenum tends to a very recovery controlled behavior during hot deformation, at which the dislocations arrange into subcell boundaries instantaneously. These pronounced recovery processes seem to consume a large amount of the stored deformation energy for the actual recrystallization. On the other hand, recovery provides the future recrystallization nuclei. For a comprehensive characterization of these microstructural processes, electron microscopy appears to be the most proper means. The aim of this study is to evaluate the significance of electron channeling contrast imaging, electron back scatter diffraction and transmission electron microscopy with regard to recovery and recrystallization processes in molybdenum. Furthermore, appropriate specimen preparation procedures for scanning and transmission electron microscopy are described.

Recrystallization kinetics and microstructure evolution during annealing of a cold-rolled Fe–Mn–C alloy

The recrystallization behavior of a 50% cold-rolled Fe–21.6% Mn–0.38% C alloy during annealing at temperatures between 560 and 700 °C was investigated. Microhardness tests were used to characterize the recrystallization kinetics. X-ray diffraction, optical microscopy, electron back-scatter diffraction measurements and transmission electron microscopy were used to characterize the grain microstructure and texture evolution during annealing. The obtained experimental data were evaluated in terms of the Johnson–Mehl–Avrami–Kolmogorov model. The obtained values of the Avrami exponent ranged from 0.70 to 1.37. The nucleation of new grains was found to be site saturated. The growth rate of nuclei decreased with annealing time and was mainly responsible for the low Avrami exponents. This decrease in the growth rate was due to a reduction in the driving force essentially because of recovery. On the basis of the experimentally obtained growth rate, the recovery behavior as well as the grain boundary migration was analyzed. The activation energies found suggest pipe-diffusion-controlled dislocation climb and grain boundary diffusion as recovery and boundary migration mechanisms, respectively. Shear bands, grain boundaries and triple junctions were identified as preferential nucleation sites. The inhomogeneous grain microstructure after recrystallization is attributed to non-randomly distributed nuclei and an inhomogeneous distribution of the stored energy. The annealing texture above 630 °C was very weak and determined by the orientation of recrystallization nuclei, whereas after annealing at 560 °C, the α-fiber (〈1 1 0〉//RD) character of the texture was enhanced, apparently as a consequence of selective growth of grains with α-fiber orientations.

Precipitation and Recrystallization Behaviors of a High-Nitrogen Austenitic Stainless Steel Aging at 90°C

The behaviors of precipitation and recrystallization aging at 900 after cold work in a Fe-18Cr-12Mn-0.48N high-nitrogen austenitic stainless steel were investigated by means of optical microscopy (OM) and transmission electron microscopy (TEM). The results show that precipitation and recrystallization occur simultaneously after aging at 900 for 10min. The precipitates firstly appear at dislocation, sub-boundary and inside the subgrain. Precipitation at these positions hinders the formation of recrystallization nucleus. With aging time increasing, precipitates appear in the recrystallized grain boundary and inside the grain. Precipitation at these positions hinders the growth of the recrystalized grains.

A continuum based microstructure model of inhomogeneous hardening and recovery as a pre-stage of recrystallization nucleation

AbstractStarting from the fundamental relation between the diffusion flux of point defects (vacancies, interstitials) and the gradient of the diffusion potential the diffusion equation is set up and applied to the calculation of the concentrations of both types of point defects. Thereby, all possible sources and sinks and their mutual effects are taken into consideration. These equations are coupled with the equations for the temporal derivatives of dislocation density, porosity, pore size and temperature. For the close-up region of spherical inhomogeneities (pores, hard inclusions) and a grain boundary this system of six partial differential equations is solved numerically. For simplicity a one dimensional model in space is used and uniaxial tension or pure shear load is assumed. Hardening and the simultaneous recovery are characterized by the local and temporal development of the point defect concentrations and especially of the dislocation density, which is proportional to the intrinsic strain energy. The gradients of both strain energy and lattice rotation determine the formation and growth of recrystallization nuclei. It is the aim of this work to present a new mathematical model and also to give a perceptual general view on the concerted action of the fundamental physical processes, which necessarily lead to hardening, recovery (softening) and recrystallization nucleation. Moreover it seems to be a basis of a future general theory of microstructure changes.

Evolution of Microstructure and Texture during Annealing of Aluminum AA1050 Cold Rolled to High and Ultrahigh Strains

The microstructure and texture of commercial purity aluminum (AA1050) have been investigated after cold rolling to von Mises strains of 3.6 to 6.4 followed by recovery and recrystallization during annealing. The evolution of structural parameters of the deformed microstructure, such as boundary spacing and fraction of high-angle boundaries (HABs), did not reach saturation in the given strain range. Recovery was accompanied by structural coarsening and by a decrease in the fraction of HABs. The coarsening rate increased with increasing strain prior to annealing. Recrystallization nuclei were found to form both in deformation zones around coarse particles and in recovered lamellar structures. The process of recrystallization in the present material can thus be characterized as discontinuous recrystallization. In recrystallized conditions, the average grain size was related to the grain orientation: the mean size of grains having orientations of the rolling texture was smaller than the size of grains with other orientations. The orientation dependence of the recrystallized grain size was more pronounced in the samples rolled to ultrahigh strains.

Recrystallization of plane strain compressed Al–1 wt.% Mn alloy single crystals of typical unstable orientations

A systematic study of crystal lattice reorientation in early stages of recrystallization has been carried out to correlate the orientations of recrystallization nuclei with the deformation microtexture and with slip systems. Microstructure and texture of Al-1 wt.% Mn single crystals of unstable initial orientations of {112}111, {100}001 and {001}110 have been examined by high-resolution field-emission gun scanning electron microscope local orientation measurements. All single crystals were channel-die deformed at room temperature and then annealed for a short time. It was shown that often observed presence of the 112 directions as rotation axes in the formation of new nuclei orientation directly suggested a close link with the deformation process.

A Phase-Field Simulation of Nucleation from Subgrain and Grain Growth in Static Recrystallization

A new recrystallization phase-field model is proposed, in which the three stages of static recrystallization phenomena, i.e., recovery, nucleation and nucleus growth are sequentially taken into account in a computation. From the information of subgrain patterns and crystal orientations in a polycrystal that are obtained by a dislocation-crystal plasticity FE analysis based on a reaction-diffusion model, subgrain groups surrounded by high angle boundary are found out. Next, subgrains in the group are coalesced into a nucleus by rotation of crystal orientation and migration of subgrain boundaries through a phase-field simulation. Then a computation of nucleus growth is performed also using the phase-field model on account of an autonomic incubation period of nucleation, in which stored dislocation energy assumes a role of driving force. It is shown that the present method can numerically reproduce the three stages of static recrystallization as a sequence of computational procedure. (author)