Occurrence Of Recrystallization Research Articles

Microstructures and Mechanical Properties of Microalloyed and Equal Channel angular Pressed AZ91 Mg Alloy

Commercial AZ91 Mg alloys were equal channel angular pressed (ECAP) for up to 6 passes at temperature as low as 473K. Microstructures and mechanical properties of as-received and as-pressed samples were investigated. It is shown that ECAP applied successfully to these materials at elevated temperatures leads to grain refinement due to the occurrence of recrystallization during the pressing process and to significant improvements in the strength and ductility of these materials. The microstructure was obviously refined and the average grain size was considerably reduced from over 28μm to below 1μm. From tensile testing at room temperature (RT), the ultimate tensile strength (UTS) increases obviously from 318MPa to 365MPa, in contrast, the uniform elongation increased from 12% of the as-received to 26% of the pressed up to 4 passes. These observations may be attributed to the effects combined grain refinement and texture development.

Effect of Cold Rolled on Microstructure and Mechanical Properties of TiNi Shape Memory Alloy

The effect of cold rolled reductions on microstructure and mechanical properties of TiNi alloys was investigated by means of tension test and TEM. From the results it can be determined that when the annealing temperature reached 550℃, the recrystallization and grain growth began to occur after the cold deformation, and the tensile strength of the alloy decreased greatly and the elongation was more than 50%. The stress platform of stress-strain curves was decided by cold deformation and annealing temperature, which meets the Clasusius-Clapeyron equation. When the alloy rolled above 37%, large deformation energy storage is conducive to the occurrence of recrystallization. The microstructure evolves form recovery to nucleation of amorphous structures, recrystallization and grain growth as the rising of annealing temperature.

Influence of consolidation methods on the recrystallization kinetics of a Fe–14Cr based ODS steel

The recrystallization behavior during thermal annealing with or without prior cold work has been investigated in a 14%Cr ODS steel consolidated by two different methods, hot extrusion (HE) and hot isostatic pressing (HIP).We show that a 1400°C-1 h annealing induces an increase of the oxide nanoparticles radius from 1.3 to 3 nm, however the grain size remain stable despite a recovery of sub-grain boundaries for the hot extruded material.When pre-deformation is applied before annealing, almost full recrystallization can be achieved on the HE ODS steel. In this study, we show recrystallization after 40% cold deformation and annealing 30 min at 1450 °C together with coarsening of oxide particles. At lower temperature and higher pre-deformation (70%-1150 °C/30 min), we show that recrystallization can be achieved without change of the oxide size distribution.We show that due to a lower initial dislocation density, recrystallization is strongly delayed, by at least 250 °C, in the HIP material. Finally, we show that the evolution of the size of the oxide precipitates is controlled by the time and temperature of annealing and are independent on the pre-deformation and occurrence of recrystallization.

Microstructure study of a severely plastically deformed Mg-Zn-Y alloy by application of low angle annular dark field diffraction contrast imaging

Microstructural investigation of extremely strained samples, such as severely plastically deformed (SPD) materials, by using conventional transmission electron microscopy techniques is very challenging due to strong image contrast resulting from the high defect density. In this study, low angle annular dark field (LAADF) imaging mode of scanning transmission electron microscope (STEM) has been applied to study the microstructure of a Mg-3Zn-0.5Y (at%) alloy processed by high pressure torsion (HPT). LAADF imaging advantages for observation of twinning, grain fragmentation, nucleation of recrystallized grains and precipitation on second phase particles in the alloy processed by HPT are highlighted. By using STEM-LAADF imaging with a range of incident angles, various microstructural features have been imaged, such as nanoscale subgrain structure and recrystallization nucleation even from the thicker region of the highly strained matrix. It is shown that nucleation of recrystallized grains starts at a strain level of revolution (earlier than detected by conventional bright field imaging). Occurrence of recrystallization of grains by nucleating heterogeneously on quasicrystalline particles is also confirmed. Minimizing all strain effects by LAADF imaging facilitated grain size measurement of nm in fully recrystallized HPT specimen after .

Modeling dynamic recrystallization of L-605 cobalt superalloy

The evolution of the microstructure of L-605 cobalt superalloy during high-temperature deformation is investigated using mean-field models combined with a reverse-engineering method. This method provides the unknown material parameters such as mobility and nucleation frequency, and it helps to quantify the occurrence of recrystallization. The effect of alloying elements on the mobility is quantified by comparison with the pure metal. Despite the large concentration of elements, the solute-drag effect can still be properly described by the Cahn model. A comparison with experimental results sheds new light on the occurrence of joint mechanisms that are usually neglected, such as self-heating and meta-dynamic recrystallization. The inclusion of these phenomena is not significantly more complicated when compared to the basic model, but it significantly improves the predictability for large strain rate.

Microstructural characteristics of a commercially pure Zr treated by pulsed laser at different powers

A commercially pure Zr sheet was treated by pulsed laser at two different powers (25 and 400W) and microstructural features in various laser-modified zones were characterized by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECC) techniques. Results show that in the 400W specimen, there are three laser-modified zones with distinct microstructural characteristics: melting zone (MZ), solid state phase transformation zone (SSPTZ), and recrystallized zone (RXZ). Both the MZ and the SSPTZ are mainly composed of Widmanstätten structures with fine precipitates decorating coarse α plate boundaries. Based on measured mean widths of α plates, cooling rates experienced by both zones are calculated to be a few hundred degrees per second. For the RXZ, second phase particles (SPPs) are found to distribute linearly inside bulk grains, suggesting the occurrence of recrystallization from earlier formed plate structures. Orientation examination by EBSD on plate structures in the MZ reveals presence of all 12 α variants transformed from the same β parent grain according to the Burgers orientation relationship, which may shed some light on suppressing α-variant selection in Zr alloys. For the 25W specimen, the decreased laser power cannot melt the substrate and only solid state phase transformation occurs at very high cooling rates (>6000°C/s). The laser-modified zones could be distinguished as SSPTZ-1 or SSPTZ-2 for cooling from β or α+β domains accordingly. Plate structures in both zones are produced by diffusionless martensitic transformation, as suggested by dense inner-plate nanotwins and absence of SPPs. For those untransformed α grains in the SSPTZ-2, although their initial bulk morphologies are maintained, slight subdivision by many subgrains in their interiors is noticed. In addition, aggregates of three α plates with the “indentation mark” morphology are often observed for the martensitic plate structures in the 25W specimen and boundary misorientations between the α plates are found to be 60°/<112̅0> (the second Burgers misorientation).

Post-treatment Process of Semi-solid Powder Rolling

Semi-solid powder rolling is a strip manufacturing process, which combines powder rolling with semi-solid rolling in one step to prepare high-performance metallic strips. A total of 7050 aluminum alloy strips were prepared by semi-solid powder rolling and then used to study the post-treatment process and evaluate qualities of post-treated strips. Three post-treatment processes involving sintering, pre-sintering—cold deformation—sintering, and hot rolling were used to process the semi-solid powder rolled strips. The results show that both swelling of pores and densification occur during solid-state sintering while microhardness increases. The decrease in density is due to that the swelling of pores has a larger effect on the density change than that of densification. The relative density reaches 92.6% with the maximum microhardness of 300 HV after “pre-sintering—cold deformation—sintering” process. High microhardness is due to the ultra-fine secondary particles uniformly distributed within the grains. The microhardness and relative density of a hot rolled strip are 176 HV and 99%, respectively, and the secondary particles disappear. Occurrence of recrystallization in these three post-treatment processes makes the grains finer.

The Requirements for Superplasticity with an Emphasis on Magnesium Alloys

Superplasticity is defined formally as a tensile elongation of at least 400% and a strain rate sensitivity of ≈0.5. It is now well established that superplasticity occurs only in materials having very small grain sizes, typically less than ≈10 μm. These small grains may be achieved in two different ways in magnesium alloys: either by thermomechanical processing or through the occurrence of recrystallization in the early stages of deformation in alloys tested with coarse grain sizes.

Microstructures and properties of Al–Zn–Mg–Mn alloy with trace amounts of Sc and Zr

Abstract Microstructures and properties of Al–Zn–Mg–Mn alloy with trace amounts of Sc and Zr have been investigated. The results show that the addition of minor Sc and Zr to Al–Zn–Mg–Mn alloy can effectively refine grain size caused by the formation of Al 3 (Sc, Zr) particles with cubic L1 2 structure, which is coherent with α(Al) matrix. After solution treatment at 470 °C for 1 h, more homogenous recrystallization grains are observed in Al–Zn–Mg–Mn alloy; partial recrystallization occurs in Al–Zn–Mg–Mn–0.12Sc–0.12Zr alloy. However, Al–Zn–Mg–Mn–0.24Sc–0.12Zr alloy still remains an unrecrystallized fiber-like structure, which reveals that minor Sc and Zr can remarkably inhibit the occurrence of recrystallization. After aging treatment at 24 h for 120 °C, the tensile strength of Al–Zn–Mg–Mn–0.12Sc–0.12Zr alloy increased by 40 MPa as compared to Al–Zn–Mg–Mn alloy. When the Sc content increased to 0.24 wt%, the tensile strength of Al–Zn–Mg–Mn–0.24Sc–0.12Zr alloy reached a maximum. The main strength mechanisms are grain refinement strengthening with combined Sc and Zr additions and precipitation strengthening of Al 3 (Sc, Zr) particles. Compared with Al–Zn–Mg–Mn alloy, Al–Zn–Mg–Mn alloy with trace amounts of Sc and Zr exhibits higher corrosion resistance due to the discontinuity distribution of η precipitates along the grain boundary and the resistance effect on recrystallization behavior by Al 3 (Sc,Zr) particles.

The effect of SiC nanoparticles on deformation texture of ARB-processed steel-based nanocomposite

In this study, the influence of SiC nanoparticles on deformation texture of steel-based nanocomposite fabricated by accumulative roll bonding process was investigated. It was found that there was a texture transition from the rolling texture to the shear texture for both pure interstitial free steel and steel-based nanocomposite. However, the texture transition occurred in different cycles for the pure steel (the third cycle) and steel-based nanocomposite (the first cycle). It was realized that the fraction of low misorientation angle grain boundaries was decreased and the fraction of high misorientation angle grain boundaries was increased by the number of cycles. Also, recrystallization occurred in the pure steel and steel-based nanocomposite samples after the third and first cycles, respectively. In addition, the occurrence of recrystallization in steel-based nanocomposite was sooner than that of pure steel. At the early stage of dynamic recrystallization in processed steels, the {011}<100>-oriented grains were evolved and the fraction of grains with α-fiber and γ-fiber orientations was slightly decreased. The formation of the rolling texture in the steel-based nanocomposite samples was different from the typical rolling texture for the pure steel samples, due to the presence of the SiC nanoparticles in the nanocomposite. The weak rolling texture was attributed to the high stored energy of deformation, which was, in turn, due to low deformation temperature.

Comparison of microparticles and nanoparticles effects on deformation texture of steel-based composite and nanocomposite fabricated by the ARB process

In this study, the effect of SiC microparticles and nanoparticles on deformation texture of steel-based composite and nanocomposite fabricated by the accumulative roll bonding (ARB) process was investigated. Textural evolution during the ARB process was evaluated using X-ray diffraction. It was found that the presence of microparticles and nanoparticles in the IF steel matrix changed the intensity of texture components and fibers. There was a texture transition from rolling texture to shear one for pure IF steel, steel-based composite, and steel-based nanocomposite. However, the texture transition occurred in different cycles for these samples. It was realized that recrystallization occurred in pure IF steel, steel-based composite, and steel-based nanocomposite after the third, second, and first ARB cycles, respectively. In addition, occurrence of recrystallization in the steel-based composite and nanocomposite was sooner than in pure IF steel. Finally, it was found that with increasing the number of cycles, the fraction of low misorientation angle grain boundaries decreased and the fraction of high misorientation angle grain boundaries increased.

The Influence of the Induced Ferrite and Precipitates of Ti-bearing Steel on the Ductility of Continuous Casting Slab

AbstractIn order to investigate the loss of the ductility of Ti-bearing ship plate steel under 1000 °C, where the ductility begins to reduce rapidly, so the hot ductility of Ti-bearing ship plate steel has been obtained using the Gleeble 1500 thermal-mechanical simulator and also the studies about the effect of grain boundary ferrite films and precipitates containing Ti on the ductility has been carried out. The result showed that the TiN particles precipitating at 950 °C with a larger size and smaller volume fraction cannot effectively suppress the occurrence of recrystallization and the ductility still retains at a high level, although R.A. value presents a certain degree of decline compared with 1000 °C. A large number of smaller Ti(C,N) particles precipitate at 900 °C and can induce the formation of a very small amount of fine grain boundary ferrite, which deteriorates the adhesion strength of the grain boundary, so the R.A. value rapidly reduces to less than 50%. When the temperature falls to close Ae3 (827 °C), the amount of the grain boundary ferrite films increase due to the ferrite phase transformation, but the ferrite film thickness becomes more uneven at the same time, which results in the increase of strain concentration and plays a leading role in causing the decrease of ductility, so the R.A. value has been kept less than 40% as the temperature cooling to 800 °C from 850 °C. When the temperature further decreases, the ductility starts to recover due to the increase of average ferrite film thickness to a greater degree which greatly reduces the strain concentration of the grain boundary.

A thermostatistical theory for solid solution effects in the hot deformation of alloys: an application to low-alloy steels

The hot deformation of low-alloy steels is described by a thermostatistical theory of plastic deformation. This is based on defining a statistical entropy term that accounts for the energy dissipation due to possible dislocation displacements. In this case, dilute substitutional and interstitial atom effects alter such paths. The dislocation population is described by a single parameter equation, with the parameter being the average dislocation density. Solute effects incorporate additional dislocation generation sources. They alter the energy barriers corresponding to the activation energies for dislocation recovery, grain nucleation and growth. The model is employed to describe work hardening and dynamic recrystallization softening in fifteen steels for a wide range of compositions, temperatures and strain rates. Maps for dynamic recrystallization occurrence are defined in terms of processing conditions and composition.

Effects of deformation temperature on the microstructure, ordering and mechanical properties of Fe–6.5 wt% Si alloy with columnar grains

The effects of deformation temperature on the microstructure, ordering and mechanical properties of Fe−6.5wt%Si alloy with columnar grains compressed at 300–900°C were studied. The results show that under the conditions of temperature ≤700°C and compression reduction of 50%, the columnar grains remain intact without occurrence of recrystallization in the samples. After deformation at 800°C, the columnar grain boundaries become serrated and subgrains form in the sample during dynamic recovery. With further increasing the temperature to 900°C, dynamic recrystallization takes place obviously. High-density of deformation twins form in the samples deformed at 500°C or below, while deformation twins are not observed in the samples deformed at 550°C or above. The Fe−6.5wt%Si alloy has the characteristic of deformation disordering and reordering at intermediate temperatures. The degree of deformation disordering is larger than that of reordering at the deformation temperature of 400–600°C, while this relationship is opposite at 700–800°C. At the deformation temperature lower than 700°C, deformation resistance of the Fe−6.5wt%Si alloy decreases drastically with the increase of the temperature, while it decreases slowly as the temperature increased to 800–900°C. The appropriate deformation temperature of the Fe−6.5wt%Si alloy with columnar grains is suggested at 400–700°C.

Tailoring Dynamic Recrystallization in Ni- and Fe-Base High Temperature Alloys

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

Modelling Dynamic Recrystallization in FCC Metals Employing Thermostatistics

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

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

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

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

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

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

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

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

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