Secondary Recrystallization Research Articles

Splitting of needle-like precipitates in grain-oriented silicon steel manufactured by the acquired inhibitor method

Needle-like precipitates are formed in high-permeability grain-oriented (HGO) silicon steel manufactured by the acquired inhibitor method after nitriding. A sawtooth splitting of the needle-like precipitate was observed in high-temperature annealing. To understand this, the needle-like precipitates at different temperatures have been analyzed using scanning electron microscope (SEM) and transmission electron microscope (TEM). The needle-like precipitate is defined as (Al,Si,Mn)N having a hexagonal crystal structure similar to AlN, and its longer direction correspond to the [0001] (c-axis) direction. Its needle shape is closely related to the growth conditions including high supersaturation of nitrogen and low nitriding temperature. With the increase of temperature, the needle-like (Al,Si,Mn)N gradually transforms to AlN, and it splits along the gaps at c-axis edges into 6–8 hexagonal regular particles finally. In this process, the crystal structure and orientation stay the same. The evolution of morphology shows the sawtooth unique characteristic because the dissolution of unstable (Al,Si,Mn)N precipitate occurs simultaneously with the formation of regular AlN precipitates under the near-equilibrium condition. The splitting results in the dispersing and increasing of precipitates in steel, which is significant for studying secondary recrystallization

Avoiding abnormal grain growth when annealing selective laser melted pure titanium by promoting nucleation

In this study, the recrystallization process of selective laser melted (SLM) commercially pure titanium (CP-Ti) was followed by a quasi-in-situ electron backscatter diffraction (EBSD) method at 800 °C and 700 °C. Abnormal grain growth (AGG) initially emerges at the beginning of annealing at 800 °C and is primarily caused by strain-induced boundary migration (SIBM). However, at the beginning of annealing at 700 °C, many small equiaxed grains are formed due to more recrystallization nucleation, followed by the formation of AGG resulting from the secondary recrystallization. Based on these findings, appropriate control of the annealing time at 700 °C can mitigate AGG to obtain fine equiaxed grains with weak texture to enhance the tensile properties of SLM CP-Ti.

Microstructural aspects of new grade ODS alloy consolidated by rotary swaging

In the present paper, the authors have investigated the microstructural aspects of recrystallization of new ODS alloy (Fe-14Cr-10Al-4Y2O3) consolidated by hot rotary swaging. Microstructural characterization of samples was performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). Insight of mechanical properties was performed by Vickers hardness testing and measurement of Young's modulus. A thorough analysis of microstructure concerning grains, oxide particles and dislocations was performed for two states of the material, i.e. in the as-received state after rotary swaging exhibiting the ultra-fine-grained microstructure and in the state after annealing provoking the desirable coarse-grained structure. Different conditions of rotary swaging followed by the same annealing conditions induced different structures, either coarse-grained or bimodal when secondary recrystallization did not happen in the entire sample's volume. The hardness measurements revealed no significant difference between the coarse-grained and the bimodal microstructures, however, the coarse-grained material exhibited systematically higher values of the temperature-dependent Young's modulus in comparison with the bimodal counterpart. Detailed microscopic analysis of the material helped to clarify and discuss the measured properties of the newly developed ODS alloy.

Anorthite-reducing glass-crystalline materials synthesized in plasma

The paper presents the experimental results of the glass-ceramic material production using the low-temperature plasma. The dependences are suggested for the main physical-and-mechanical properties (compressive and flexural strengths, density, linear thermal expansion coefficient) of products and the mixture compositions. The centers of secondary recrystallization are identified for the anorthite phase (CaAl2Si2O8). These inclusions chaotically locate on the surface of the synthesis products and resemble dendritic microinclusions up to 90 nm in size. A comparative analysis is given to the properties of glass-ceramic materials produced by the low-temperature plasma and traditional methods.

Thermal deformation behavior and microstructure evolution of GH4169 superalloy under the shear-compression deformation conditions

The shear-compression deformation of GH4169 superalloy at 1000–1200 °C and 0.01–1 s−1 was investigated by physical and numerical simulation based on a specific shear-compression sample (SCS). OM, EBSD, and TEM analysis revealed the three typical regions in SCS. Dynamic recrystallization (DRX) preferred to occur in the slot region due to the strain concentration, where the Mises equivalent strain was about 4–6 times higher than the pre-set ones. Mixed grains were observed in the transition region because of insufficient deformation. In contrast, equiaxial grains were preserved in the cylindrical region with negligible deformation. Experimental stress decreased with the increase of temperature and decrease of strain rate. The maximum stress was 180 MPa, which was only 30% of that under single compression deformation. Furthermore, four DRX mechanisms were observed in GH4169. Discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) were the dominant and auxiliary mechanisms, respectively. Twinning dynamic recrystallization (TDRX) occurred in the annealing twins with a fast nucleation rate, and the intersected nano-twins provided favorable nucleation sites for DRX. Lastly, secondary dynamic recrystallization (SDRX) with unique nucleation mechanism of triangularly intersecting subgrain boundaries emerged in SCS, which nucleated in the DRX grains and further refined the microstructure of GH4169.

Effect of Holding Time of Decarbonization Annealing on Recrystallization in Fe-3.2%Si-0.047Nb% Low-Temperature Oriented Silicon Steel

In this study, the effects of decarburization annealing time on the primary recrystallization microstructure, the texture and the magnetic properties of the final product of 0.047% Nb low-temperature grain-oriented silicon steel were investigated by means of OM, EBSD and XRD. The results show that when the decarburization annealing condition is 850 °C for 5 min, the uniform fine primary recrystallization microstructure can be obtained, and the content of favorable texture {111} <112> is the highest while that of unfavorable texture {110} <112> is the lowest, which is mostly distributed near the central layer. At the same time, there are the most high-energy grain boundaries with high mobility in the primary recrystallization microstructure of the sample annealed at 850 °C for 5 min, and the ∑9 boundary has the highest percentage of grain boundaries. The samples with different decarburization annealing time were annealed at high temperature. It was found that perfect secondary recrystallization occurred after high-temperature annealing when the decarburization annealing condition was 850 °C for 5 min. The texture component was characterized by a single Goss texture, and the size of the Goss grain reached 4.6 mm. Under such annealing conditions, the sample obtained shows the optimal soft magnetic properties of B800 = 1.89 T and P1.7/50 = 1.33 w/kg.

Complete Goss Secondary Recrystallization by Control of the Grain Size and Texture of Primary Recrystallization in Grain-Oriented Silicon Steel.

Matrix microstructure and texture controlling is an important way to optimize Goss ({110}<001>) abnormal grain growth (AGG) in high magnetic induction grain-oriented silicon (Hi-B) steel during primary recrystallization. In the present work, a matrix with homogeneous grain size and favorable texture components was obtained through two-stage normalized annealing followed by primary recrystallization. Furthermore, secondary recrystallization was performed for sharp Goss orientation by slow heating and purified annealing. It was found that plenty of island grains, which occurred and disappeared gradually, accompanied the process of AGG. Through analyzing the evolution of microstructure and texture, we realized that the formation of island grains was related to the large-size grains in matrix, and the elimination of that was attributed to the special grain boundaries which satisfied both coincident site lattice (CSL) and high-energy (HE) models. It was essential to control grain size and favorable orientations in matrix comprehensively for the high-efficient abnormal growing of sharp Goss orientation, through which excellent magnetic properties could be obtained simultaneously.

Rapid Secondary Recrystallization of the Goss Texture in Fe81Ga19 Sheets Using Nanosized NbC Particles.

Herein, a simple and efficient method is proposed for fabricating Fe81Ga19 alloy thin sheets with a high magnetostriction coefficient. Sharp Goss texture ({110}<001>) was successfully produced in the sheets by rapid secondary recrystallization induced by nanosized NbC particles at low temperatures. Numerous NbC precipitates (size ~90 nm) were obtained after hot rolling, intermediate annealing, and primary recrystallization annealing. The relatively higher quantity of nanosized NbC precipitates with 0.22 mol% resulted in finer and uniform grains (~10 μm) through thickness after primary recrystallization annealing. There was a slow coarsening of the NbC precipitates, from 104 nm to 130 nm, as the temperature rose from 850 °C to 900 °C in a pure nitrogen atmosphere, as well as a primary recrystallization textured by strong γ fibers with a peak at {111} <112> favoring the development of secondary recrystallization of Goss texture at a temperature of 850 °C. Matching of the appropriate inhibitor characteristics and primary recrystallization texture guaranteed rapid secondary recrystallization at temperatures lower than 950 °C. A high magnetostriction coefficient of 304 ppm was achieved for the Fe81Ga19 sheet after rapid secondary recrystallization.

Microstructure and magnetic properties of ultra-thin grain-oriented silicon steel: Conventional process versus strip casting

Ultra-thin grain-oriented silicon steels were processed via the conventional process and strip casting process, and the mechanisms of texture inheritance and inhibitor induced secondary recrystallization were comparatively studied in detail. In the conventional route with commercial grain-oriented silicon steel as the starting material, exact {111} 〈112〉 dominated the deformation texture after cold rolling with ~ 63% reduction, and strong Goss texture was formed in 0.08–0.10 mm thin-gauge primary recrystallized sheet. Meanwhile, weak {210} 〈001〉 and {h11} < 1/h12 > textures were also observed because of the inhomogeneous deformation with initial deviated Goss texture. After high temperature annealing, the sheet experienced incomplete secondary recrystallization with abnormal growth of Goss grains accompanied by normal growth of {210} 〈001〉 and {411} 〈148〉 grains, considering the absence of pinning force on grain boundary migration. In the strip casting process, the intensity of γ-fiber texture was increased, while grain size decreased from ~ 13.3 to ~11.4 μm and the area fraction of Goss grains decreased from 2.8% to 0.6% in the primary recrystallized sheet with decrease of thickness from 0.15 to 0.08 mm. During high temperature annealing process, MnS, (Nb,V)N and AlN provided continuous and sufficient inhibition force for normal grain growth, which guaranteed the formation of complete secondary recrystallization of exact Goss grains. Superior magnetic induction B8 was up to 1.97 T in the strip casting process, which was 0.09 T higher than the conventional route. Meanwhile, the iron losses P1.7/50 and P1.0/1000 were as low as 0.85 and 20.7 W/kg, respectively, which were comparable to the reported ultra-thin grain-oriented silicon steels. The present study provided a novel method to produce high efficiency ultra-thin grain-oriented silicon steel, which may significantly contribute to energy saving in high-frequency electrical devices.

In situ growth of the ZIF‐8 on the polymer monolith via CO2‐in‐water HIPEs stabilized using metal oxide nanoparticles and its photocatalytic activity

AbstractHere, the ZnO/polyacrylamide (ZnO/PAM) macroporous composite monolith was first prepared through the radical polymerization of acrylamide in ZnO‐costabilized CO2‐in‐water Pickering high internal phase emulsions (HIPEs). An efficient and green synthesis method was used to prepare the ZnO/PAM porous precursor materials, and ZnO nanoparticles (NPs) were used as a metal ion source to grow the metal organic frameworks (MOF) phase through secondary recrystallization, thereby synthesizing the target MOF/PAM porous monolith. The effects of the contents of ZnO NPs on the formation of HIPEs and properties of the target MOF/PAM composites for the photocatalytic degradation of cationic dye were studied. As the content of ZnO NPs increased to 3.75 wt%, the composite monolith had high mechanical stress (0.24 MPa) at 80% strain and recovered their shape. The as‐synthesized ZIF‐8/PAM composites were characterized using X‐ray diffraction, scanning electron microscope (SEM), Brunauer Emmett Teller (BET), energy dispersive X‐ray spectrometer (EDS) mapping, and TG methods. SEM images showed that the hybrid materials were interconnected, and the sizes of the open macropore and interconnected pore throats were 30–125 and about 2–20 μm, respectively. In addition, the ZIF‐8/PAM material could be used as a high‐efficiency photocatalyst to remove the cationic dye at a fast rate (88.6%).

Improvement of texture and magnetic properties of strip-cast grain-oriented electrical steel by trace Bi addition

The improvement of recrystallization texture and magnetic properties in grain-oriented electrical steel with trace Bi addition processed by strip casting process was studied in this study. Low casting temperature and sufficient liquid steel flow during strip casting process enabled relatively homogeneous distribution of Bi in the as-cast strip, which accelerated the precipitation during solidification and cooling process. As a result, relatively high density of duplex MnS + AlN, MnS and AlN was formed at grain boundaries and inside grain. Due to the synergistic effect of continuous precipitation and fragmentation of coarse precipitates during cold rolling, the number density of precipitates was increased and mean size was decreased along the processing route. High density of precipitates is attributed to severe inhomogeneous plastic deformation, which promoted the formation of high density in-grain shear bands during cold rolling and recrystallization Goss texture during annealing. Homogeneous microstructure with average grain size of ~ 11.6 μm, strong Goss texture with intensity of 2.5 multiples of random distribution, fine precipitates with number density of ~ 1.9 × 109 /cm2 and mean size of ~ 49 nm were formed after primary recrystallization annealing. Furthermore, grain-oriented electrical steel with Bi addition experienced complete secondary recrystallization of exact Goss grains during high-temperature annealing, which is attributed to beneficial optimization of Goss texture and strong inhibition effect, and superior magnetic induction B8 of 1.91 T was obtained. It is expected that this study paves a new way for fabrication of high-performance grain-oriented electrical steel.

Avoiding abnormal grain growth in thick 7XXX aluminium alloy friction stir welds during T6 post heat treatments

Abstract Friction stir welding (FSW) is becoming an essential welding technique in the aerospace industry. However, friction stir welds are usually weaker than the parent material. The dissipated heat causes a softening of the 7XXX series aluminium alloys. Post-welding heat treatments are used to restore the highest strength after welding. The reference treatment is composed of two-steps heating: a solution heat treatment typically at 470 °C for 30 min, quench in water followed by artificial aging 120 °C for 24 h. These treatments are responsible for very significant grain coarsening in the weld nugget zone known as secondary recrystallization, sometimes leading to abnormal grain growth (AGG). This is particularly true for thick plates. This secondary recrystallization phenomenon is detrimental for tensile performances. A detailed analysis of the conditions leading to secondary recrystallization during post-welding heat treatments is provided. A procedure to avoid secondary recrystallization in FSWed joints is then highlighted. Thermal cycles during welding are controlled by welding on a low thermal conductivity backing plate (i.e. stainless steel) which avoids very fine grains in the bottom of the weld nugget. This new procedure opens the path to excellent welds presenting at least the strength and ductility of the T6 base material.

Secondary recrystallization behavior in magnetostrictive Fe-Ga thin sheets induced by nano-sized composite precipitates

Strengthening η texture (⟨001⟩//rolling direction) through secondary recrystallization can significantly improve the magnetostriction coefficient of Fe-Ga thin sheets and expand the use of Fe-Ga alloys in high-frequency applications. However, the secondary recrystallization of Goss texture in Fe-Ga thin sheets deteriorates significantly with the thinning of the sheet thickness under the current inhibitors of micron-sized NbC particles or precipitates. In this paper, a new composite inhibitor system, composed of nano-sized MnS and Nb(C,N) precipitates, is proposed to induce the secondary recrystallization Goss texture in Fe-Ga thin sheets with thicknesses less than 0.3 mm. The density and size of MnS and Nb(C,N) is designed based on the effect of alloy composition and hot rolling parameters on the precipitation kinetics of MnS and Nb(C,N), including the relative nucleation rate-temperature (NrT) and precipitation-time-temperature (PTT) curves. It is found that the stored deformation energy increases the maximum nucleation rate and the fastest precipitation temperature. A large number of lamellar and rectangular precipitates (MnS and Nb(C,N)) with a size of 5∼20 nm were precipitated after hot-rolling at a temperature of 800∼1,050 °C and a total thickness reduction of 80%. These nano-sized inhibitors provide a strong pinning effect for the primary recrystallized grains, and induce abnormal grain growth of Goss grains at an annealing temperature of 950∼1,050 °C. Resultingly, centimeter-sized Goss grains with a small deviation angle and maximum magnetostriction coefficient of 264 ppm are successfully achieved in Fe-Ga alloy thin sheets with a thickness of 0.25 mm.

Secondary recrystallization in grain-oriented silicon steel

Grain-oriented silicon steel is a soft magnetic material widely used for transformer cores. Magnetic properties are closely related to the sharpness of Goss texture, which is evolved by secondary recrystallization. An overview of our results will be presented; secondary recrystallization behaviour has been investigated by in situ observation utilizing synchrotron x-ray topography. The results support the selective growth mechanism. Migration of Goss grains is controlled by inhibitor and sharper Goss grains, which have higher frequency of CSL-boundaries to the matrix, start to grow preferentially while the other grains are stagnated by inhibitor. Secondary recrystallization behaviour can be explained based on this CSL-model. At the initial stage, inhibitor gradient through the thickness promotes Goss selective growth from the subsurface layer and at the later stage, inhibitor drop rate dominates the Goss selective growth. Inhibitor and CSL-boundaries synergistically affect Goss selective growth and Goss grain evolves to a coarse grain.

Tribological Properties of AI2O3 Coatings Obtained by Detonation

In the present work, the effect of the detonation-spraying mode on the tribological properties of Al2O3 coating was studied. It was found that a decrease in the delay time between shots is leading to an increase in the hardness and elastic module of Al2O3 coatings. The obtained values of the coefficient of friction showed that a decrease in the delay time between shots leads to a slight reduction in the coefficient of friction. It is determined that reducing the delay time between shots to 0.25 seconds leads to an increase in the resistance of coatings to abrasive wear. It was found that the main reason for the increase in hardness and wear resistance with a decrease in the delay time between shots to 0.25 s is associated with an increase in the volume fraction of the α-Al2O3 phase. It was found that increase in the volume fraction of the α-Al2O3 phase is caused by the secondary recrystallization γ → α, which occurs due to the heating of particles during coating formation, i.e. due to increase in temperature above 1100 °C in single spots of the coating when they are put each other.

Secondary Recrystallization of Grains with Cube Orientation for Pure Iron Tape

The formation mechanism of secondary recrystallized grains with cube orientation has been investigated for a pure iron tape prepared by a three-stage cold-rolling method. The crystal orientation before the final cold-rolling affected the appearance of secondary recrystallized grains with cube orientation. The {0 ī 2}<1 0 0> orientation before the final cold-rolling led to the formation of secondary recrystallized grains with cube orientation on the tape surface.

Correlative Synchrotron X-ray Imaging and Diffraction of Directed Energy Deposition Additive Manufacturing

The governing mechanistic behaviour of Directed Energy Deposition Additive Manufacturing (DED-AM) is revealed by a combined in situ and operando synchrotron X-ray imaging and diffraction study of a nickel-base superalloy, IN718. Using a unique DED-AM process replicator, real-space imaging enables quantification of the melt-pool boundary and flow dynamics during solidification. This imaging knowledge was also used to inform precise diffraction measurements of temporally resolved microstructural phases during transformation and stress development with a spatial resolution of 100 µm. The diffraction quantified thermal gradient enabled a dendritic solidification microstructure to be predicted and coupled to the stress state. The fast cooling rate entirely suppressed the formation of secondary phases or recrystallisation in the solid-state. Upon solidification, the stresses rapidly increase to the yield strength during cooling. This insight, combined with the large solidification range of IN718 suggests that the accumulated plasticity exhausts the ductility of the alloy, causing liquation cracking. This study has revealed the mechanisms that govern the formation of highly non-equilibrium microstructures during DED-AM.

Recent advances in the kinetics of normal/abnormal grain growth: a review

Recent progress in the kinetics of grain coarsening and abnormal grain growth (AGG) is presented in this overview article. The factors affecting the kinetics of grain growth is reviewed with the emphasis on the recent findings on the solute drag and Zener pinning effects as well as the special case of duplex alloys, where the latter is discussed for the behavior of dual-phase steels during intercritical annealing. The common isothermal kinetics models for grain growth are listed, which is followed by the critical discussion on the simplifications and the commonly used methods for the determination of grain growth exponent (n) and activation energy (Q). The obtained values of n and Q for several classes of important engineering alloys such as microalloyed steels, stainless steels, magnesium alloys, aluminum alloys, titanium alloys, and high-entropy alloys are summarized with the discussion on the obtained values of kinetics parameters and their deviation from the theoretical expectations. Finally, the factors leading to AGG (such as the coarsening and dissolution of pinning particles and the crystallographic texture), the proposed mechanisms (such as the solid-state wetting and the grain boundary faceting/defaceting phenomena), and the kinetics of AGG (based on the empirical power law and the similarity of AGG to primary recrystallization in the form of secondary recrystallization) are reviewed. This overview can shed light on the understanding of grain growth and its effects.

Hot Rolling vs. Forging: Newly Developed Fe-Al-O Based OPH Alloy

Two oxide precipitation hardened (OPH) Fe-Al-O-based steels were prepared by mechanical alloying. In addition to the variant using yttria nano-precipitates to improve the mechanical properties, a variant using only alumina precipitates for strengthening was also prepared. Therefore, a more economically acceptable variant of these steels was developed. Hot consolidation is a significant production step for achieving the required mechanical properties. Hot consolidation was performed by either hot rolling or forging. The heat treatment was subsequently performed on both variants (0.85Fe–0.11Al–0.03Y2O3–0.01Y and 0.87Fe–0.11Al–0.02O2) of the alloys to support secondary recrystallization. The paper describes the influence of the consolidation method on grain size, material recrystallization, and mechanical properties. The difference in the consolidation method was reflected in the grain size after the heat treatment, where the material consolidated by hot rolling reached a grain size of almost 200 μm, while after forging the maximum grain size was around 30 μm. A higher ultimate tensile strength was achieved with forged states, both with and without the heat treatment.

Goss Texture Evolution in a Grain-Oriented Fe-6.5wt%Si Steel Processed by Strip-Casting

Fe-6.5wt%Si steel is an excellent soft magnetic material due to the near-zero magnetostriction and low core losses. In this study, a 0.3 mm-thick grain-oriented 6.5wt%Si steel sheet was produced by a novel strip casting and two-stage rolling. The microstructure and texture evolution were investigated with a special emphasis on the nucleation and growth of Goss grains. The thin normalized strip was composed of large columnar grains and small equiaxed grains. During intermediate annealing, Goss grains nucleated in the shear bands of the deformed &lt;111&gt;//ND grains, and the deformed {111}&lt;112&gt; grains provided most of the nucleation sites. After primary annealing, the Goss grains distributed across the entire thickness, which was different from the conventional rolling route. The fraction of high-angle boundaries (20°-45°) surrounding the Goss grains was apparently higher than those of the matrix grains, which promoted the abnormal growth of the Goss grains during secondary recrystallization.