Early Stages of Recrystallization in Equal-Channel Angular Pressing (ECAP)-Deformed AA3104 Alloy Investigated Using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Orientation Mappings

Early stages of recrystallization in alloys containing complex structure of second phase particles are of considerable practical interest. They were observed for the AA3104 alloy in which large particles occur against the background of randomly distributed dispersoids. The samples were deformed by equal channel angular pressing and then slightly annealed to obtain the state of partial recrystallization. The highly deformed alloy contained a structure of flat grains with the spacing between high-angle grain boundaries ranging from 100 nm to 1 ??m. On annealing, the structure coarsened and got transformed into nearly equiaxed grains by both discontinuous and continuous recrystallization. The nucleation of new grains was observed in statically recrystallized bulk samples using scanning electron microscopy, and during in-situ recrystallization in a transmission electron microscope. Special attention was paid to the nucleation of new grains in areas close to large second phase particles, where a relatively high stored energy was expected to stimulate nucleation. A particular role in the rise of nuclei is attributed to migration of low angle boundaries. During recrystallization at 623 K (350?°C), in most of the observed cases, the growth of grains occurred by coalescence of neighbouring cells and by migration of high-angle grain boundaries. These processes led to nearly equiaxed grains of similar size. Orientation mappings showed that although orientations of new grains were widely scattered, they were not completely random.     

Microstructural evolution during the austenite-to-ferrite transformation from deformed austenite

It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures below the nonrecrystallization temperature (T _nr ). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation. In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined T _nr for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence the mechanical properties of the steel.     

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.     

Aspects of orientation-dependent grain growth in extra-low carbon and interstitial-free steels during continuous annealing

The present work concentrates on the application of orientation imaging microscopy (OIM) based on the electron backscattered diffraction (EBSD) technique to the investigation of the microstructural evolution of an extra-low carbon (ELC) steel and a Ti-Nb-bearing interstitial-free (IF) steel, during continuous annealing. Aspects like the nucleation, the evolution of the recrystallized volume fraction and grain size of grains with different orientations, the interface area limiting recrystallized {111} regions, and the apparent growth rates have been considered. Different criteria have been applied in order to identify crystallites produced during annealing. During the first stages of annealing, a network of grain boundaries with misorientations higher than 10 deg is produced, mainly inside the deformed γ-fiber grains. The crystallites formed within this network, free from cells or subgrains at their interiors, can be considered as potential nuclei. However, among all, only some of them become effective due to an important selection. The {111} recrystallized grains have a significant size and number advantage as compared with other texture components, and a hard impingement between clusters of {111} grains is produced during grain growth. The effect of grain growth behind the recrystallization front seems to be negligible as compared with the grain coarsening produced by the migration of this front, driven by the cold-work stored energy.     

Recrystallization and formation of austenite in deformed lath martensitic structure of low carbon steels

The effect of prior deformation on the processes of tempering and austenitizing of lath martensite was studied by using low carbon steels. The recrystallization of as-quenched lath martensite was not observed on tempering while the deformed lath martensite easily recrystallized. The behavior of austenite formation in deformed specimens was different from that in as-quenched specimens because of the recrystallization of deformed lath martensite. The austenitizing behavior (and thus the austenite grain size) in deformed specimens was controlled by the competition of austenite formation with the recrystallization of lath martensite. In the case of as-quenched (non-deformed) lath martensite, the austenite particles were formed preferentially at prior austenite grain boundaries and then formed within the austenite grains mainly along the packet, block, and lath boundaries. On the other hand, in the case of lightly deformed (30 to 50 pct) lath martensite, the recrystallization of the matrix rapidly progressed prior to the formation of austenite, and the austenite particles were formed mainly at the boundaries of fairly fine recrystallized ferrite grains. When the lath martensite was heavily deformed (75 to 84 pct), the austenite formation proceeded almost simultaneously with the recrystallization of lath martensite. In such a situation, very fine austenite grain structure was obtained most effectively.     

Recrystallization Behavior of a Heavily Deformed Austenitic Stainless Steel During Iterative Type Annealing

The study describes evolution of the recrystallization microstructure in an austenitic stainless steel during iterative or repetitive type annealing process. The starting heavily cold deformed microstructure consisted of a dual phase structure i.e., strain-induced martensite (SIM) (43 pct in volume) and heavily deformed large grained retained austenite. Recrystallization behavior was compared with Johnson Mehl Avrami and Kolmogorov model. Early annealing iterations led to reversion of SIM to reversed austenite. The microstructure changes observed in the retained austenite and in the reverted austenite were mapped by electron backscatter diffraction technique and transmission electron microscope. The reversed austenite was characterized by a fine polygonal substructure consisting of low-angle grain boundaries. With an increasing number of annealing repetitions, these boundaries were gradually replaced by high-angle grain boundaries and recrystallized into ultrafine-grained microstructure. On the other hand, recrystallization of retained austenite grains was sluggish in nature. Progress of recrystallization in these grains was found to take place by a gradual evolution of subgrains and their subsequent transformation into fine grains. The observed recrystallization characteristics suggest continuous recrystallization type process. The analysis provided basic insight into the recrystallization mechanisms that enable the processing of ultrafine-grained fcc steels by iterative type annealing. Tensile properties of the processed material showed a good combination of strength and ductility.     

Phase-field model prediction of nucleation and coarsening during austenite/ferrite transformation in steels

A phase-field simulation is performed to study the kinetics of austenite to ferrite (γ → α) transformation in a low-carbon steel during continuous cooling. Emphasis is placed on the influence of nucleation, along with ferrite grain coarsening behind the transformation front, on microstructural evolution. Results show that grain coarsening is significant even before all nucleation has been completed and occurs via two different coarsening mechanisms, grain boundary migration and ferrite grain crystallographic rotation, both of which can be clearly observed occurring as the simulated microstructure evolves. For some grains, sudden growth jumps are predicted by the model—a phenomenon that has been observed before by synchrotron X-ray diffraction. This study quantitatively demonstrates the phenomenon that increasing cooling rate leads to nucleation off initial austenite grain boundaries, which is also verified by studying the morphology of ferrite grains as predicted using different nucleation mode assumptions. A relationship between nucleation site distribution and the nucleation rate is demonstrated by computer simulation.     

Aspects of Orientation-Dependent grain growth in extra-low carbon and interstitial-free steels during continuous annealing

The present work concentrates on the application of orientation imaging microscopy (OIM) based on the electron backscattered diffraction (EBSD) technique to the investigation of the microstructural evolution of an extra-low carbon (ELC) steel and a Ti-Nb-bearing interstitial-free (IF) steel, during continuous annealing. Aspects like the nucleation, the evolution of the recrystallized volume fraction and grain size of grains with different orientations, the interface area limiting recrystallized {111} regions, and the apparent growth rates have been considered. Different criteria have been applied in order to identify crystallites produced during annealing. During the first stages of annealing, a network of grain boundaries with misorientations higher than 10 deg is produced, mainly inside the deformed γ-fiber grains. The crystallites formed within this network, free from cells or subgrains at their interiors, can be considered as potential nuclei. However, among all, only some of them become effective due to an improtant selection. The {111} recrystallized grains have a significant size and number advantage as compared with other texture components, and a hard impingement between clusters of {111} grians is produced during grain growth. The effect of grain growth behind the recrystallization front seems to be negligible as compared with the grain coarsening produced by the migration of this front, driven by the cold-work stored energy. J.L. Bocos, formerly Researcher with CEIT     

Austenite Formation in Plain Low-Carbon Steels

In this study, austenite formation from hot-rolled (HR) and cold-rolled (CR) ferrite-pearlite structures in a plain low-carbon steel was investigated using dilation data and microstructural analysis. Different stages of microstructural evolution during heating of the HR and CR samples were investigated. These stages include austenite formation from pearlite colonies, ferrite-to-austenite transformation, and final carbide dissolution. In the CR samples, recrystallization of deformed ferrite and spheroidization of pearlite lamellae before transformation were evident at low heating rates. An increase in heating rate resulted in a delay in spheroidization of cementite lamellae and in recrystallization of ferrite grains in the CR steel. Furthermore, a morphological transition is observed during austenitization in both HR and CR samples with increasing heating rate. In HR samples, a change from blocky austenite grains to a fine network of these grains along ferrite grain boundaries occurs. In the CR samples, austenite formation changes from a random spatial distribution to a banded morphology.     

The effect of small deformation on abnormal grain growth in bulk Cu

The effect of small deformation below the level (about 8 pct) required for primary recrystallization on abnormal grain growth (secondary recrystallization) has been investigated in bulk polycrystalline Cu. The starting microstructure, without any texture and with a nearly uniform grain size of 168 μm, has been obtained by compressing a cylindrical Cu specimen and recrystallizing at 800 °C. The fully recrystallized specimen shows distinct abnormal grain growth (AGG) after heat treatment at 800 °C for 12 hours. Most of the grain boundaries are faceted when observed under transmission electron microscopy (TEM), and most of the faceted segments are expected to be singular. A singular grain boundary free of defects will migrate by two-dimensional nucleation of new layers, with its velocity varying nonlinearly with the driving force arising from the grain-size difference. Such a growth mechanism is analogous to the well known process for the growth of crystals with singular surfaces from liquid or vapor. The grains slightly larger than the average size will hardly grow, because the driving force for their growth is not sufficient for nucleation of new crystal layers at the boundaries. Those grains larger than a certain critical size will, however, grow at ever-increasing rates with their increasing size, because of the sufficient driving force for two-dimensional nucleation. Such a selective accelerated growth of large grains results in overall AGG behavior. The specimen deformed to 2 pct shows AGG after heat treatment for only 5 minutes at 800 °C, and after 1 hour, large impinged grains are obtained. The grain boundaries show many extrinsic dislocations even after the heat treatments. As proposed earlier by Gleiter, Balluffi, Smith, and their colleagues, the extrinsic grain-boundary dislocations increase the grain-boundary mobilities even at low driving forces, and, hence, even those grains slightly larger than the average size can rapidly grow at the early stages of the heat treatment, in agreement with the observation. In the specimens deformed to 4 to 8 pct, below the level for primary recrystallization, all grains grow steadily without producing distinct AGG. With high densities of extrinsic dislocations at the grain boundaries even after long heat treatments, all grains can readily grow, resulting in overall growth patterns resembling the normal growth. When deformed to 20 and 50 pct, primary recrystallization occurs, and the subsequent AGG behavior depends on the grain size obtained at the completion of the primary recrystallization. Similar small-deformation effects are observed with heat treatment at 600 °C.     

Computer simulation of recrystallization—III. Influence of a dispersion of fine particles

Two-dimensional Monte Carlo simulations of recrystallization have been carried out in the presence of incoherent and immobile particles for a range of different particle fractions, a range of stored energies and a range of densities of potential nuclei (embryos). For stored energies greater than a critical value (H/J > 1) the recrystallization front can readily pass the particles leading to a random density of particles on the front and a negligible influence of particles on the recrystallization kinetics. At lower stored energies the particles pin the recrystallization front leading to incomplete recrystallization. However at very low particle fractions, when the new grain has grown much larger than the matrix grains, before meeting any particles, the new grains can complete the consumption of the deformed grains giving complete “recrystallization” by a process that appears to be similar to abnormal grain growth. Particles are, as reported previously, very effective at pinning grain boundaries, both of the deformed and recrystallized grains, when boundaries migrate under essentially the driving force of boundary energy alone. Such boundaries show a density of particles that rises rapidly from the random value found at the start of the simulation. As a consequence, particles very strongly inhibit normal grain growth after recrystallization. Such growth can only occur if the as-recrystallized grain size is less than the limiting grain size seen in the absence of recrystallization. Under these circumstances a small increment of grain growth occurs until the grain boundaries once again acquire a higher than random density of particles.     

Transmission pseudo-Kossel (TK) studies of recrystallized hot-deformed (dynamically recovered) polycrystalline aluminium

The early stages of recrystallization of new grains in previously hot-deformed polycrystalline aluminium samples have been studied using the transmission pseudo-Kossel technique to establish the orientational relationship between the new strain-free grains and the adjacent deformed grains. The aluminium samples were deformed to true strains of 0.22 and 0.51 at 300°C with a strain rate of 4.61 × 10⁻⁴ s⁻¹. The hot-deformed samples were quenched in water after the deformation and the delay time was less than 10 s. Recrystallization studies were carried out at 300°C. The orientational measurements showed that at the early stages of recrystallization the orientations of the new grains were within the orientational spread of the adjacent deformed grains. The predominant mechanism of nucleation was found to be strain induced boundary migration (SIBM).     

Interactions between recrystallization and precipitation in hot-deformed microalloyed steels

The static recrystallization and precipitation characteristics of low alloy steels containing Nb were studied following high temperature compression in the austenite range. The recrystallization kinetics in the Nb-steels were slower than those in the plain carbon steels by approximately an order of magnitude when Nb was in solution. However, much greater retardation of recrystallization was observed in the Nb-steels when precipitates were formed. The initiation of both recrystallization and precipitation appeared to be localized in the early stage of their nucleation. Recrystallization initiated predominantely at the prior austenite grain boundaries. Boundaries were also the preferential nucleation sites for NbCN precipitation. Hence, the NbCN particles were distributed in a highly localized fashion, sometimes delineating what may have been the prior austenite grain or subgrain boundaries. The heterogeneous nature of the nucleation process for recrystallization and precipitation suggested that the critical factor which determined the retardation of recrystallization was the local pinning effect of precipitation. The local pinning force was estimated from the local distribution of precipitation and was found to be of magnitude comparable to that of the driving force for recrystallization. The precipitation pinning force increased in the beginning of precipitation, showed a peak in the intermediate stage, and finally decreased as particles coarsened and were distributed more uniformly.     

Kinetics of Delta-Ferrite to Austenite Phase Transformation in a Two-Phase Fe-Al-C Alloy

The kinetics of delta-ferrite to austenite phase transformation was investigated using a quenching dilatometer in a Fe-Al-C alloy. The results showed that the austenite phase nucleated along the delta grain boundaries. The transformed austenite morphology changed from cellular to Widmanstätten pattern when the holding temperature decreased from 1398 K to 1123 K (1125 °C to 850 °C). Full partitioning of the substitutional alloying elements was observed and the spacing of the austenite plates was controlled by the diffusing distance of the substitutional elements. Interestingly, growth of the austenite front was controlled by the long-range diffusion of carbon from the center of the delta grains to the growing front. Deformation of the delta phase enhanced the nucleation of austenite at existing grain boundaries and newly formed subgrain boundaries.     

变形晶界对低碳钢显微组织的影响

对不同温度下变形和变形后再加热到奥氏体区的低碳钢SS400的显微组织进行了研究，结果表明：变形使奥氏体和铁素体晶界呈锯齿状，锯齿状的奥氏体晶界优先成为铁素体的形核位置，锯齿状的铁素体晶界有利于铁素体再结晶核心的形成。     

Fe-6.5%Si合金温轧后退火过程中再结晶行为

用电子背散射技术观察了700℃温轧板在退火过程中的组织及织构演变以了解其再结晶行为.结果表明,温轧织构由强的（111）〈112〉、较弱的〈110〉∥RD及Goss组成,再结晶织构与之相似.〈110〉∥RD及（111）〈112〉新晶粒首先形成于与之构成小角度晶界的形变晶粒的晶界附近,而在角隅及组织不均匀区等位置孕育出与周围晶粒构成大角度晶界的晶核,择优取向不明显.退火过程中（111）〈112〉在形变组织中累积,最终转化为（111）〈112〉再结晶晶粒.分析认为,温轧后退火是不均匀组织在低储存能驱动下的再结晶过程.（112）〈110〉及（111）〈112〉形变拉长晶粒多发生连续再结晶从而退火织构与形变态相似.在角隅区形成核心进而发生不连续再结晶,核心取向的统计性及不连续晶核的长大弱化再结晶织构,其中Goss晶粒多以此方式形成于（111）〈112〉晶粒内部.      

变形对微合金钢在两相区变形时显微组织的影响

通过热处理试验和单道次压缩热模拟试验,研究了微合金钢加热到两相区变形时的组织演变规律,并分析了变形量的影响,使用OM、SEM和EBSD技术分析了试验钢的微观组织和取向分布。结果表明,试验钢加热到两相区保温后,奥氏体相变在原铁素体晶界上发生,变形时晶界上的奥氏体发生应变诱导相变,形成细小的仿晶界铁素体,变形铁素体发生动态回复或动态再结晶。随变形量和变形温度的提高,硬度下降,800℃下增加变形量,动态回复向动态再结晶发展,动态再结晶形核机制是亚晶转动生长,名义变形量为70%时得到均匀的超细晶组织,有效晶粒平均等效直径为2.7μm,大角度晶界的体积分数达到92.8%。     

Recrystallization following hot-working of a high-strength low-alloy (HSLA) steel and a 304 stainless steel at the temperature of deformation

The recovery and recrystallization behavior of two commercial quality steels, a Cb(Nb) strengthened high-strength low-alloy (HSLA) steel and a 304 stainless steel, was studied following hot-working. Specimens were deformed in tension at a constant head velocity of 2 in.Js to reductions-in-area of 30 to 50 pct at temperatures in the austenite range from 1600° to 1900°F. The subsequent annealing behavior was observed at the temperature of deformation. Decreasing recrystallization rates with decreasing temperature andJor deformation were observed. It is suggested that CbC precipitation occurred during annealing of the HSLA steel and accounted for an arrest in the softening behavior. For the 304 stainless steel it is concluded that dynamic recrystallization took place during deformation, that thermal microtwinning was an active recovery mechanism during annealing, and that there was a preference for grain boundaries as nucleation sites for recrystallized grains. These conclusions regarding the annealing behavior of 304 stainless steel were supported by metallographic analysis of specimens water quenched from the temperature of deformation. Formerly with Rensselaer Polytechnic Institute,Troy, New York. Formerly Professor, Rensselaer Polytechnic Institute. This paper is based upon a thesis submitted by T. L. CAPELETTI in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Rensselaer Polytechnic Institute.     

Modeling of recrystallization after inhomogeneous deformation

Recrystallization in an inhomogeneously predeformed material (a cold-drawn cylindrical rod) is described by an analytical model and simulated with the Monte-Carlo technique. For this purpose, an equation for the nucleation rate during recrystallization as a function of local deformation has been derived. The analytical model considering the derived nucleation equation is capable of predicting the progress of the recrystallization front as observed in experiments with Titanium Grade 2. The Monte-Carlo model has been developed on the basis of the analytical model. Different functions for the local deformation were introduced, and recrystallization and subsequent grain growth were simulated. With the aid of simulation, the formation of both a grain size gradient and large elongated grains in the region of critical deformation can be understood. The graded microstructure is a consequence of the combined effect of inhomogeneous nucleation and anisotropic growth of the recrystallizing grains. Experimental grain size gradients were reproduced quantitatively by the present simulations. Agreement was also found for the grain elongation that forms during the recrystallization and grain growth stages.     

Microstructures developed during thermomechanical treatment of HSLA steels

One purpose of thermomechanical treatment of steels, for example the control rolling of plate, is to produce the finest uniform microstructure in the product to optimize its strength and toughness. To achieve this end requires control of the structual changes that occur during reheating, high-temperature (roughing) deformation, lower temperature (finishing) deformation, and austenite transformation. A study has been made of the effects of the deformation processing variables on the microstructural changes that occur in high-strength low-alloy (HSLA) steels in the temperature ranges in which complete, partial, or no recrystallization occurs. The experimental technique comprised a sequence of plane-strain compressions of specimens being cooled at rates controlled to simulate the rate of cooling of slabs being rolled to plates. The results show that in the complete recrystallization range a fixed pass schedule refines the initial grain sizes in steels of a wide range of compositions and initial grain sizes to about the same final size; the final recrystallized grain size decreases with deformation temperature (within the complete recrystallization range), increasing strain rate, and increasing draft,i.e., with any deformation parameter that increases flow stress; the solute content and initial grain size, which are fixed by reheat temperature, control the temperature at which complete recrystallization stops during hot rolling. Deformation in the “partial recrystallization” range causes duplexing in the austenite that cannot be removed by subsequent rolling, consequently rolling in this range should be avoided. During finishing, the height of the flattened austenite grains decreases with increasing finishing reduction, but at a rate less than the rate of decrease of specimen height, indicating that some recovery is occuring. The grain diameter of the ferrite formed from fine, elongated austenite is about half the austenite grain-boundary separation (measured in the throughthickness direction) indicating that the elongated boundaries are the primary nucleation sites for ferrite.