Recrystallization Theory Research Articles

Grain refinement mechanism in adiabatic shear band of solution treated Ti–10V–2Fe–3Al alloy under high strain rate

The grain refinement mechanism in the adiabatic shear band (ASB) of solution treated Ti–10V–2Fe–3Al alloy was investigated under high strain rates using a split Hopkinson pressure bar and hat-shaped specimens. The TEM results show that elongated and parallel grains are the major characteristics of the transition region between the matrix and the centre of the ASB. The central region of the ASB consists of ultrafine and equiaxed recrystallized grains with a typical size of 100 nm. According to the classical one-dimensional Fourier heat conduction equation and recrystallization theory calculations, it is proved that ultrafine and equiaxed recrystallized grains did not undergo significant growth during the cooling stage after deformation. The thermodynamics and kinetics calculated results indicate that instant grain refinement within the ASB is due to the rotational dynamic recrystallization (RDR) that occurs during the deformation process.

Abnormal Recrystallization Behavior of the Cold‐Rolled AlZnMgZr (7046A) Sheets during Annealing

The recrystallization behavior and microstructures’ evolution of cold‐rolled AlZnMgZr (7046A) sheets with various annealing treatments are investigated. Compared with the conventional hardness versus annealing temperature curves, the abnormal hardening occurs above 375 °C, which triggers the contemplation about abnormal recrystallization behavior and abnormal hardening behavior herein. In accordance with texture evolution with diverse annealing temperatures or time, recovery and recrystallization processes are more sensitive to temperature than holding time, and the rolling texture also transforms into recrystallization texture at 350–375 °C. Afterward, microstructure analysis and the recovery and recrystallization theory are combined to obtain information about the dislocation density, the grain features, and the solid‐solution ability, explaining the annealing‐induced hardening phenomenon. In addition, the recovery process means that the dislocations are consumed and subgrains with low misorientation angle are transformed into subgrains with high misorientation angle, whereas the high‐angle subgrains are transformed into recrystallized grains during the recrystallization process. Combined with the grain distribution and abnormal hardness evolution, the abnormal recrystallization behavior is determined to occur at 375 °C. Due to poor thermal activation, recrystallization occurs preferentially in favorable sites, and some grains grow preferentially, resulting in abnormal recrystallization behavior.

Nucleation Mechanism of Discontinuous Dynamic Recrystallization with Restrained Grain Boundary Sliding

The discontinuous dynamic recrystallization (DDRX) nucleation process was investigated during the TMP of a nickel-based superalloy with a coarse columnar-grain structure. Special attention was focused on the formation mechanism of the ‘newborn boundary’ which separates the free-strain volume from deformed grain. The special grain morphology and large grain size restrained the grain boundary sliding of the studied alloy, and hence made it difficult to form a subboundary through strain induction or twining to separate the free-strain volumes from the deformed grains. This is very different with the classical DDRX theory, where the grain boundary sliding has less restraint and hence leads to grain boundary shearing and the following strain induced boundary or twin boundary. However, the experiment results showed that DDRX took place and developed well during the studied deformation process with restrained grain boundary sliding. It was found that the ‘newborn boundary’, which separates the free-strain volume from deformed grain to close this area, was formed through dislocation piling-up and rearrangement instead of strain induction or twinning caused grain boundary shearing. Hence, a potential nucleation theory was then proposed and discussed, which is expected to enrich the discontinuous dynamic recrystallization theory.

A novel method to study recrystallization behavior: Continuous heating stress relaxation

The characteristics of recrystallization determine the hot working process, the final properties, and the limits of the high-temperature service of metal materials. Metal recrystallization theory is also one of the most basic scientific issues in materials science. In this paper, a novel continuous heating stress relaxation method was proposed to determine the recrystallization temperature of metal sheets, and the recrystallization behavior of H65 brass with a cold rolling reduction rate of 70% is studied by this method. The results show that the stress relaxation method can accurately obtain the recrystallization temperature characteristic points of metal samples. The effects of external load and heating rate on stress relaxation are analyzed. Compared with the hardness method, the stress relaxation method can continuously monitor the mechanical properties of materials during recrystallization. The stress relaxation method can be used to determine the recrystallization activation energy Qr of metal through the less experimental workload. The Qr of the brass samples determined by the stress relaxation method is 127 KJ/mol, which is very close to the 131 KJ/mol determined by the hardness method.

Interface formation and bonding mechanisms of hot-rolled stainless steel clad plate

Since the 1980s, vacuum hot rolling has been developed to fabricate the stainless steel clad plates by the Iron and Steel Institute of Japan. Herein, hot rolling is a widely used solid-state bonding process to join the carbon steel substrate and stainless steel cladding. In this paper, we provide a brief overview of the vacuum hot rolling process and effective parameters on the interface characteristics and shear strength of stainless steel clad plate. The effects of surface preparation condition, atmosphere condition, vacuum degree, rolling temperature, rolling reduction ratio, interlayer, heat treatment on the microstructure, interface characteristics and mechanical properties of stainless steel clad plate have been analyzed in detail. It is shown that the interface transition zone is formed due to the carbon diffusion, and the strong interface bonding is attributed to the sufficient alloy elements diffusion of Fe, Cr and Ni. Moreover, the interface shear strength and toughness are also affected by interfacial precipitation phase and multiple oxides. Finally, the present work concluded the bonding mechanism of hot-rolled stainless steel clad based on the oxide film theory, diffusion theory, recrystallization theory and three stage theory.

Boundary migration in a 3D deformed microstructure inside an opaque sample

How boundaries surrounding recrystallization grains migrate through the 3D network of dislocation boundaries in deformed crystalline materials is unknown and critical for the resulting recrystallized crystalline materials. Using X-ray Laue diffraction microscopy, we show for the first time the migration pattern of a typical recrystallization boundary through a well-characterized deformation matrix. The data provide a unique possibility to investigate effects of both boundary misorientation and plane normal on the migration, information which cannot be accessed with any other techniques. The results show that neither of these two parameters can explain the observed migration behavior. Instead we suggest that the subdivision of the deformed microstructure ahead of the boundary plays the dominant role. The present experimental observations challenge the assumptions of existing recrystallization theories, and set the stage for determination of mobilities of recrystallization boundaries.

Sheared lherzolite xenoliths revisited

The microstructures of sheared lherzolite xenoliths from South African kimberlites are investigated using new microstructural analysis techniques and new rheological data. By applying electron backscatter diffraction (EBSD) methods and the M‐index technique for quantifying the strength of lattice preferred orientation (LPO), it is demonstrated that olivine and orthopyroxene, after an initial stage of dynamic recrystallization, deformed by different mechanisms: olivine deformed by dislocation creep, while orthopyroxene deformed by a grain‐size sensitive mechanism that randomized the preexisting LPO. New rheological data, which incorporate the effects of pressure on deformation, are applied to these xenoliths. The effects of pressure are shown to dramatically reduce the inferred strain rates of these xenoliths in comparison to previous estimates. The contrasting microstructural features of olivine and orthopyroxene are analyzed using the Avrami theory of recrystallization, coupled with microstructural observations and experimental data. This analysis suggests that (1) the difference in the recrystallized grain‐size and (2) the difference in the completeness of recrystallization, can be largely explained by the contrast in grain boundary mobility between olivine and orthopyroxene.

Determination of Texture and Microstructure of Recrystallized Metals Using High-Energy Synchrotron Radiation

Measurements of textures and microstructures of materials with very high angular and local resolving power can be achieved using high-energy synchrotron radiation in combination with the sweeping-detector technique. Also in-situ measurements of the changes of orientations and microstructures during heat treatment and/or deformation processes are possible. These grain-resolved methods show, impressively, many details e.g. of the recrystallization process which can otherwise not be seen. The results of these measurements can be the basis for comprehensive recrystallization theories.

Evolution mechanism of grain refinement based on dynamic recrystallization in multiaxially forged austenite

Austenite grain refinement based on continuous dynamic recrystallization can be realized through multiaxial forging Fe–32%Ni alloy at 550 °C and 2×10−2 s−1, which is generally considered not to take place. It not only greatly reinforces the recrystallization theory but also is of great directory significance to the development of new generation iron and steel materials. As a result, the evolution of the continuous dynamic recrystallization during multiaxial forging can be summarized as such a process that deformed bands crossing each other subdivide an austenite grain into several subgrains and these subgrains are gradually angled to new independent grains with their boundaries being transformed into big angle boundaries in subsequent deformation.

Measurement of High-Resolution Recrystallization Textures in Nickel Sheets Using High-Energy Synchrotron Radiation

The new developed “sweeping detector” techniques using high energy synchrotron radiation allow to measure textures and microstructures of materials and their change during heat treatment with high location and orientation resolution. Here we show these new methods applied to cold rolled and subsequently annealed nickel samples. The grain-resolved measurements show, impressively, many details of the recrystallization process which can otherwise not be seen. The results of these measurements can be the base for omprehensive recrystallization theories.

Recrystallization Texture and Microstructure in Ni and AlMg1Mn1 Determined with High-Energy Synchrotron Radiation

The newly developed “sweeping detector” technique with high energy synchrotron radiation allows to measure textures and microstructures of materials with high location and orientation resolution. This method was applied to hot rolled aluminium manganese alloys and to rolled nickel samples in different recrystallization stages. The grain-resolved measurements show, impressively, many details of the recrystallization process which can otherwise not be seen. That can be the base for comprehensive recrystallization theories.

Development of dynamic recrystallization theory

Although hot working had been defined as deformation above the recrystallization temperature (determined after cold working), it was only about 1965 that dynamic recrystallization (DRX) was confirmed to be occurring during the deformation; two decades were required to clarify the similarities to, and the differences from, static recrystallization. In classical discontinuous DRX in Cu, Ni, and γ-Fe, successive necklaces of new grains cause work softening; however in steady-state, the nuclei are uniformly distributed as reestablished dislocation structure limits growth. In high recovery metals at high strains, the grain boundary (GB) serrations meet across the elongated thinned grains thus pinching them off into almost equiaxed grains containing a substructure, thus geometric DRX.

Dynamic recrystallization behavior of 35CrMo structural steel

The dynamic recrystallization behavior of 35CrMo steel was studied with compression test in the temperature range of 1 223–1 423 K and the strain rate range of 0.01–10.00 s−1. The initiation and evolution of dynamic recrystallization were investigated with microstructure analysis and then the critical strain ɛ c for dynamic recrystallization initiation, the strain for maximum softening rate ɛ* and the steady strain ɛs were obtained to be 2.92×10−3 Z 0.1381, 1.60 × 10−3 Z 0.1780 and 3.26 × 10−2 × Z 0.0972 respectively by analysis of work-hardening rate-strain θ-ɛ curves, where Z is the Zener-Hollomon parameter. The dynamic recrystallization fraction was determined using recrystallization theory, and the effects of initial grain size, strain rate and deformated temperature on the dynamic recrystallization kinetics were investigated. The results show: \(X_{DRX} = 1 - \exp (\frac{{\varepsilon - \varepsilon _c }}{{\varepsilon _s - \varepsilon _c }})^{2.28} )\), the dynamic recrystallization fraction is slightly delayed due to the somewhat larger initial grain size and markedly delayed with the decrease of temperature. On the other hand, it is significantly accelerated with the increase of the strain rate. Finally, the relationships between the initiation time, ending time of dynamic recrystallization and the deformed temperature were analyzed in detail.

Kinetics of Ice Recrystallization in Aqueous Fructose Solutions

ABSTRACTRecrystallization rates of ice in fructose solutions were determined over a range of temperatures and ice phase volumes. Accretive and migratory recrystallization both occur, the dominant mechanism being strongly dependent on the mean size of the crystals. Accretion dominated when crystals were small and close together. Recrystallization rates decreased with decreasing ice phase volume and decreasing temperature. The temperature dependence of the rates was consistent with Williams‐Landell‐Ferry kinetics. With a mean field correction for ice phase volume the observed data fit the recrystallization theory of Lifshitz, Slyozov and Wagner reasonably well.

Crystallographic texture in mechanically alloyed oxide dispersion-strengthened MA956 and MA957 steels

The crystallographic textures of two mechanically alloyed, oxide dispersion-strengthened steels have been studied in the as-deformed and recrystallized conditions. The MA956 steel, which has relatively large chromium and aluminum concentrations, is found to exhibit a strong 〈110〉 fiber texture in the extruded condition. Despite the larger degree of deformation imparted to MA957 during extrusion, its crystallographic texture is found to be rather weak. Significant differences have also been found in the texture after recrystallization heat treatments. These differences could not be explained on the basis of deformation or recrystallization theory. However, the discovery that under certain specific conditions MA957 partially transforms to austenite has helped rationalize the texture results. In particular, the austenite is found to form at the temperature where the extrusion process is usually carried out. The observation of austenite is found to be consistent with thermodynamic phase stability calculations and metallographic experiments.

Crystallographic orientation of a recrystallized grain grown in a strained single crystal of ice

Abstract A rectangular solid sample, made from a single crystal of ice, with a grid of grooves cut on its top face, was uniaxially compressed and then annealed to allow recrystallization. Crystallographic orientations were determined by the transmission Laue method, and additional studies were made of the orientation distribution of the recrystallized grains and the growth process. Many fine grains recrystallized at the grooved face at the beginning of annealing but only a few grew into large grains invading the lower portion of the crystal on continued annealing. Although the fine grains were randomly oriented, the large grains appeared to show some preferred orientation to the 〈0001〉 axis of the matrix. The results are interpreted using a kinetic theory of recrystallization.

Crystallization of amorphous Fe- P- C alloys

The transformation of amorphous Fe-P-C alloys from the amorphous state to crystalline state was studied by differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Compositions included iron and phosphorus rich alloys and alloys with compositions near the eutectic composition. The crystallization of Fe-P-C alloys proceeded by the following processes: for the iron rich alloy, bcc α Fe precipitated first; for the phosphorus rich alloy, Fe2P and iron carbide precipitated first; for the alloys near the eutectic line, the eutectic reactions dominated, although some α Fe or Fe2P plus iron carbide crystals were observed. The composition dependence of the transformation can be explained successfully by a free energy model with the help of the phase diagram. From DSC and TEM results, the nucleation rate of the α Fe precipitate decreased with time in the Fe rich alloys according to the recrystallization theory of Avrami.

Crystallization of triglycerides at low supercooling

Recrystallization was studied for a number of dispersions consisting of crystals of saturated triglycerides in oil by measuring the specific surface area of the crystals as a function of time. Since the crystals had a stable modification, the main driving force of the recrystallization process was due to the smallness of the crystals. The specific surface area was measured by means of permeametry. In most cases it was found that it decreased by about 30% over a period of, e.g., 10 days, depending on composition and temperature of storage. The results were compared with recrystallization theories for diffusion-controlled processes. It is concluded that the experimental results cannot be explained in this way, but that the rate-determining step is the incorporation of the molecules into the growing crystal.

The ledge theory of recrystallization in polycrystalline metals

Abstract When a polycrystalline metal specimen is deformed, ledges must be produced on grain boundaries in the interior of the specimen, corresponding to dip lines on the external surface, but much less pronounced. During recrystallization, grain boundary migration occurs and a ledge would tend to straighten out so as to reduce the total energy by decreasing the area of grain boundary. If the volume swept by the migrating boundary remains stress free, and the height of the ledge is greater than a critical value dependent on the energy density in the deformed material, it is shown that the grain boundary will continue to migrate until the nucleus of a new grain capable of indefinite growth has been produced. The critical height h is given by h=0.1r=0.1 γ/E. where E is the energy density of the deformed materiel, γ is the grain boundary energy per unit area, and r is the equilibrium radius of curvature of the grain boundary between a recrystallized grain and the deformed matrix. On this basis it is shown th...

Recrystallization Studies of High Purity Aluminum Single Crystals

AbstractThe two basic recrystallization theories, that of oriented growth and tliat of oriented nucleation, have been reviewed in light of data obtained from growth of single grains into strained single crystals of high purity (99.99%) aluminum. It appears that a compromise of these two theories would best explain the results obtained under this investigation. Basically, the proposed theory states that a stress is developed between coherent embryo nuclei and the strained matrix crystal. The relative orientation of the embryo nucleus and the matrix coupled with the residual stress in the strained matrix crystal will determine the magnitude of the stress between the embryo arid the matrix. When this stress is of sufficient magnitude, the coherent bonds between the embryo and the matrix will rupture and the embryo will become a nucleus which can then grow and consume the matrix.Competing with this process is the phenomenon known as polygonization. Partial relief of the residual stress in the sprained crystal takes place as polygonization occurs. This partial stress relief is often sufficient to prevent the formation of nuclei and the occurrence of subsequent recrystallization.