Static Recrystallization Process Research Articles

Electropulsing anisotropy of cold-rolled Grade 2 titanium sheet: Effect of electric current direction on recrystallization and hardness

This study investigated the effects of electropulsing treatment (EPT) condition on the microstructure and microhardness of cold-rolled Grade 2 Ti sheet. Particular attention was paid to the dependence of the material properties on the electric current direction, referred to as electropulsing anisotropy. Direct-current (DC) EPT was applied along either the rolling direction (RD) or transverse direction (TD) of the rolled sheet. EPT along the TD resulted in a higher heating rate and maximum temperature, whereas that along the RD accelerated the static recrystallization (SRX) process under identical electropulsing parameters. Such a discrepancy was interpreted using the elongated grain structure and basal texture in the cold-rolled Ti sheet. These results were further supported by microhardness measurements, confirming the presence of electropulsing anisotropy during DC EPT in Ti alloys for the first time. In addition, the employed EPT was compared to a traditional furnace heat treatment (FHT) with rigorous temperature and time control, wherein the optimum EPT process spent only 13% of the processing time to complete SRX with a lower thermal energy requirement. This indicates a significant athermal contribution of EPT to electropulsing anisotropy.

Enhancement in interfacial bonding quality of CoCrFeMnNi high-entropy alloy vacuum hot-compression bonding via surface shot peening

Vacuum hot-compression bonding (VHCB) is an advanced solid-state bonding technique. To expand its application in high-entropy alloys (HEAs), a new strategy to enhance the interfacial bonding quality of CoCrFeMnNi HEA VHCB joints by surface shot peening (SP) was proposed, and the effects of SP treatment on the interfacial bonding behavior and mechanical properties of VHCB joints were investigated. The results revealed that SP treatment introduced a nanocrystalline layer and plastic deformation layer with non-uniform strain on the specimen bonding surface, and a hierarchical grain size gradient fine microstructure was formed by a static recrystallization process under the thermal activation condition before VHCB. During the bonding process, the fine microstructure of the SP-treated layer was retained and provided a large number of grain boundaries and dislocations, which facilitated atomic diffusion and plastic flow in the interfacial region and promoted the closure of interfacial voids and the dissolution of interfacial oxide particles. Additionally, interfacial crystallographic and tensile test analyses indicated that the SP treatment induced a multiple interfacial grain boundary (IGB) migration mechanism in the VHCB joints, which dramatically improved the IGB migration level and ultimately led to a complete recovery of the mechanical properties of the CoCrFeMnNi HEA VHCB joints.

Uncovering of the formation of rare earth texture and pseudo fiber bimodal microstructure in the high ductility Mg-2Gd-0.4Zr alloy during extrusion

The aim of this research was to elucidate the underlying mechanism involved in the formation of rare earth (RE) texture and pseudo fiber bimodal microstructure in the high ductility Mg-2Gd-0.4Zr alloy. The microstructure and texture evolution during the extrusion process were analyzed using various techniques, including optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and electron probe microanalysis (EPMA). The findings revealed that the RE texture in the extruded Mg-2Gd-0.4Zr alloy emerged during the dynamic recrystallization (DRX) process and was further strengthened during the subsequent static recrystallization and grain growth processes. The nucleation and growth of grains in the streamline region of Zr particles were delayed in comparison to other regions due to the pinning effect of Zr particles, ultimately resulting in the formation of pseudofiber bimodal microstructure in the extruded Mg-2Gd-0.4Zr alloy.

Electropulsing treatment-induced the enhanced recrystallization of deformed Al0.1CoCrFeNi high-entropy alloy at lower temperature

It is urgent to explore an efficient treatment strategy to obtain characteristic microstructures for better mechanical properties of alloys in an era of energy shortage. Here, we investigated whether electropulsing treatment (EP-treatment) could efficiently manipulate the microstructural evolution of pre-deformed Al0.1CoCrFeNi HEA. Under the condition of EP-treatment, the temperature required for complete static recrystallization of Al0.1CoCrFeNi HEA is reached earlier at 590 °C. This temperature is significantly lower than the recrystallization temperature induced by conventional heat treatment. The preferred accumulation of dislocations at grain boundaries in EP-treated Al0.1CoCrFeNi HEA provides favorable conditions for the nucleation of static recrystallization. This is due to the increased vacancy concentration and defect mobility, which are activated by the additional driving force of the electromigration effect. Moreover, the enhanced dislocation mobility leads to an increased nucleation rate within a very short electrical pulse time, which consequently facilitates accelerated recrystallization at a lower temperature. In addition, the static recrystallization process was accompanied by the accelerated annihilation of dislocations and the decreased dislocation density, which is consistent with the results of the global GND density decreasing from 2.0 × 1013 m−2 to 1.6 × 1013 m−2.

Effect of strain path change on static recrystallization behavior of AT63 magnesium alloy

The present work has examined the influence of strain path change on the static recrystallization process of Mg-6Al-3Sn (AT63) magnesium alloy. The samples were deformed at ambient temperature for three conditions; unidirectional rolling (UDR), clock cross rolling (CCR), and forging. The microstructural features of deformed and recrystallized samples at different conditions were characterized through optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), hardness testing, and X-ray diffraction (XRD). The results suggest that the recrystallization predominately started at the prior grain boundaries and within the twin boundaries. A significant reduction of the grain size from 83 µm to ∼ 7 µm was observed after recrystallization. The recrystallization kinetics was relatively slower for CCR samples. It was attributed to the level of stored energy in the material, initial deformation texture, and evolution of fine precipitates.

Mechanistic investigation on Ce addition in tuning recrystallization behavior and mechanical property of Mg alloy

Constructing bimodal grain structure is a promising approach to achieve the high strength-ductility synergy in Mg alloy. Formation of bimodal grain is closely related to the dynamic and/or static recrystallization process, which has not been fully understood in the typical Mg-RE based alloy. In this work, it is claimed for the first time that the minor Ce addition (∼0.3 wt%) into Mg matrix significantly promotes the pyramidal <c+a> and non-basal <a> dislocations at the early stage of extrusion, which consequently enhances the formation of sub-grain boundaries via the movement and recovery of pyramidal II-type <c+a> dislocations. At this stage, fine sub-grain lamellae are widely observed predominantly due to the low migration rate of sub-grain boundary caused by the limited mobility of <c+a> dislocations. At the later stage, the sub-grains continuously transform into dynamic recrystallized (DRXed) grains that have 〈101¯0〉 Taylor axis and also strong fiber texture, indicating substantial activation of pyramidal II-type <c+a> dislocation. The low mobility of <c+a> dislocations, accompanied with the solute drag from grain boundary (GB) segregation and pinning from nano-phases, cause a sluggish DRX process and thus a bimodal microstructure with ultra-fined DRXed grains, ∼0.51 µm. The resultant texture hardening and grain refinement hardening effects, originated from bimodal microstructure, result in a yield strength of ∼352 MPa, which is exceptional in Mg-Ce dilute alloy. This work clarifies the critical role of Ce addition in tuning recrystallization behavior and mechanical property of magnesium, and can also shed light on designing the other high-performance Mg alloys.

Investigation of anisotropy and structure variation of spray-formed 2195 Al-Li alloy via final temperature-controlled rolling and cold rolling

The mechanical property anisotropy is one of the critical factors of limiting extensive application of the Al-Li alloy, which is mainly influenced by the textures, grain morphology and distribution of the aging precipitates. Through the plastic deformation process, the anisotropy can be weakened by regulating the grain structure. In this work, the final temperature-controlled rolling and cold rolling were carried out to investigate the grain structure variation and its effect on the mechanical property anisotropy of the spray-formed 2195 Al-Li alloy. The results show that the yield strength is improved but the anisotropy in the mechanical properties deteriorates as the final rolling temperature is elevated from 200 ℃ to 300 ℃. This is owed to the fibrous structures and a great number of deformation textures formed in the continuous static recrystallization process. Compared with the samples through final temperature-controlled rolling, the aged sample through cold rolling possesses higher yield strength and relatively weaker anisotropy due to the fine equiaxed grain structure. In addition, intense Brass and S textures existing in the plates are proven that can result in the heterogeneous distribution of the T1 phase and the anisotropic equivalent slip system number values along the longitudinal and transverse directions, which both contribute to the strong anisotropy. To facilitate the analysis of the effect of the textures on the mechanical properties and anisotropy, the equivalent Schmid factor and slip system number were also calculated and discussed. This study looks forward to providing reliable theoretical support for the practical application of spray-formed Al-Li alloys.

Formation of an abnormal texture in Mg-Gd-Y-Zn-Mn alloy and its effect on mechanical properties by altering extrusion parameters

The Mg-8Gd-4Y–1Zn–Mn (wt%) alloys prepared by semi-continuous casting and homogenizing treatment were extruded using different extrusion ratios and subsequent cooling processes. The microstructure, texture and mechanical properties of extruded alloys were investigated. The dynamic recrystallization process is promoted by increasing the extrusion ratio, while the static recrystallization process is suppressed by subsequent water cooling rather than air cooling. When the recrystallization ratio is relatively low, the non-recrystallized region showing a fiber <101‾0>Mg//extrusion direction (ED) texture component dominates the texture characteristic. With the increase of the recrystallization ratio, the recrystallized region exhibiting an abnormal <0001>Mg//ED texture gradually dominates the texture characteristic. The grain boundary energy anisotropy and the grain boundary mobility anisotropy are assumed to be changed by the Gd and Y addition, which affect the preferential growth of the abnormal <0001>Mg//ED oriented grains and eventually lead to the formation of abnormal texture component. The as-aged alloy extruded at the extrusion ratio of 6 followed by air cooling shows the highest strength. The extruded alloys with an intensive abnormal texture component exhibit the best mechanical isotropy in both as-extruded and as-aged conditions. The differences in strength along different directions are mainly resulted from the fiber strengthening.

Evolution mechanisms of recrystallized grains and twins during isothermal compression and subsequent solution treatment of GH4586 superalloy

In present study, isothermal compression, solution treatment, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations were carried out to correlate the microstructure and deformation-solution parameters for GH4586 superalloy. And, the recrystallized mechanisms, twin evolution, γ′ precipitation and their interaction were clearly illustrated based on the analysis of the recrystallized fraction, size of γ grains, grain boundary misorientation and length fraction of twin boundaries. The results show that the process of dynamic recrystallization is controlled by the mechanisms of sub-grain rotation and strain-induced boundary migration while the process of static recrystallization is controlled by the mechanism of subgrain boundary migration. The evolution of twin boundaries during deformation is affected by two aspects: (i) the losing of twin’s identity due to higher strain; and (ii) the nucleation and growth of new twins within the DRX grains. During deformation of GH4586 superalloy, fine γ′ precipitates have a hindered effect on the recrystallization nucleated from pre-existing subgrains and twin growth. Moreover, the recrystallization process has a noticeable influence on the formation of new twins. Conversely, the existing of these twins is also beneficial for the recrystallization.

Development of weak-textured and high-performance Mg–Zn–Ca alloy sheets based on Zn content optimization

An intense texture is usually developed in conventional Mg alloys after deformation, resulting in unsatisfactory formability at room temperature. In the present work, a low-alloyed and weak-textured Mg-1.5Zn-0.2Ca (wt.%) alloy that exhibited an advantageous combination of strength and ductility was developed, with an ultimate strength of ∼270 MPa and fracture elongation of ∼30%. The role of Zn addition in the texture and microstructure evolution was systematically investigated. The Mg-xZn-0.2Ca (x = 0.5, 1.0, 1.5, and 2.0 wt%) alloys with increasing Zn content stored additional deformation energy, which accelerated the static recrystallization (SRX) process and resulted in a gradual weakening of the annealing texture. The co-addition of Zn and Ca contributed to the random SRX nucleation and the enhanced solute dragging effects by decreasing grain boundary energy, leading to a weaker texture in the Mg-1.5Zn-0.2Ca alloy than that in the Mg-1.5Zn and Mg-0.2Ca alloys after annealing. The excellent ductility of Mg-1.5Zn-0.2Ca alloy was attributed to the simultaneous increment of uniform elongation and post-uniform elongation as Zn content increases, benefited from its weakened texture and improved failure mode, respectively. This work provides valuable insights into the development of novel low-alloyed Mg sheets with a weak texture, which possess great potential for direct forming to near-net-shaped products without tearing and fracturing.

Orientations of nuclei during static recrystallization in a cold-rolled Mg-Zn-Gd alloy

The static recrystallization process of a cold-rolled Mg-Zn-Gd alloy was tracked by a quasi-in-situ electron backscatter diffraction method to investigate the orientations of nuclei. The results show that orientation distribution of nuclei is associated with nucleation mechanism. The continuous static recrystallization nuclei display similar orientations to the parent grains with TD orientation. Differently, discontinuous static recrystallization nuclei formed within the parent grains (TD-45° orientation) show random orientations and a variety of misorientation angles but preferred axes <52¯73> or <52¯70>. Interestingly, a special oriented nucleation is found. Discontinuous static recrystallization nuclei originated from boundaries of the parent grain (TD-70° orientation) show concentrated TD orientations in another side due to the preferred misorientation relationship 70° <112¯0> (∑18b). It is speculated that these two special misorientation relationships are related to the dislocation type.

Microstructural evolution and defect formation in a powder metallurgy nickel-based superalloy processed by selective laser melting

In this study, the selective laser melting (SLM) technology has been employed to manufacture a nickel-based superalloy which was conventionally prepared through powder metallurgy (PM) route. The microstructural features and defects were systematically investigated both prior to and after heat treatment and compared with the PM counterpart. Both solidification cracking and liquation cracking were observed in the SLM specimen in which the grain misorientation and low melting point (γ + γ’) eutectic played a vital role in their formation mechanism. Columnar grains oriented along building direction were ubiquitous, corresponding to strong <001> fiber texture. Solidification cell structures and melt pools are pervasive and no γ’ precipitates were detected at about 10 nm scale before heat treatment. After super-solvus solution and two-step aging treatments, high volume fraction γ’ precipitates emerged and their sizes and morphologies were comparable to those in PM alloy. <001> texture is relieved and columnar grains tend to become more equiaxed due to static recrystallization process and grain boundary migration events. Significant annealing twins formed in SLM alloy and are clarified as a consequence of recrystallization. Our results provide fundamental understandings for the SLM PM nickel-based superalloy both before and after heat treatment and demonstrate the potential to fabricate this group of alloys using SLM technology.

Effect of pre-strain levels and high temperature annealing on the formability of AZ31 Mg alloy thin sheet during stretch deformation

Pre-compression deformation at different strain levels (1.58%, 3.32%, 5.38% and 7.78%) along transverse direction (TD) are conducted to introduce (10–12) tensile twins at room temperature. The static recrystallization process is induced by annealing at 450 °C for 2 h. Then the stretch deformation is conducted at room temperature to investigate the fracture and formability of Mg sheet. The c-axis of grains tiltes from normal direction (ND) to transverse direction (TD) due to the tensile twins; however, the TD-rotated texture is inherited by the new recrystallized grains. The grains grow up and the size increases gradually as the pre-strain levels increasing. Due to the rotated orientation, the basal slips are favored and a larger Schmid factor is obtained on the pre-strained samples. The yield strength reduces, and the ductility increases. Besides, the strain hardening coefficient (n-value) increases and Lankford value (r-value) decreases at the same time. Owing to the TD-rotated orientation, the increased n-value and reduced r-value, the stretch formability is enhanced greatly. The Erichen value (IE-value) increases largestly from 2.83 mm to 5.74 mm which improves by 102.8%.

Investigation of amorphous alloys nanostructure and phase composition formed during integrated and split exposure of deformation and flash lamp annealing

Microstructure and phase composition of the Al85Ni7Fe4La4 amorphous alloy after separate and complex impacts of severe plastic deformation by shift under the pressure (SPD) with P=8 GPa, n=1-3 revolutions and flash lamp annealing (FLA) with the fluence F=10-25 J/cm2 were investigated. By X-ray structural analysis and transmission electronic microscopy (TEM) was shown, that in initially amorphous structure partial multiphase crystallization with formation of amorphousnanocrystalline composite (solid solution of crystalline Alcr and amorphous Alam and Al11La3, Al9(Fe, Ni)2 in aluminum) is developing during the SPD. FLA leads to partial nanocrystallization at F=10 J/cm2 and full crystallization after the FLA at 20-25 J/cm2. Phase transformation and static recrystallization processes are developed along with the crystallization in amorphous structural component at SPD and FLA integrated exposure.

Influence of delta phase precipitation on static recrystallization of cold-rolled Inconel 718 alloy in solid solution treatment

Due to the improvement in fatigue strength of cold-rolled blades, the static recrystallization behavior of cold-rolled Inconel 718 alloy in the solid solution treatment was investigated with the methods of optical microscopy and electron backscattered diffraction. The effect of δ phase precipitation on the static recrystallization process was also analyzed. The results show that δ phase preferentially precipitates comparing to static recrystallization. Affecting by the δ phase preferential precipitation process, sub-grain migration is the main nucleation mechanism in the static recrystallization of cold-rolled Inconel 718 alloy at 980 °C. Meanwhile, strain-induced boundary migration also exists as an aided nucleation mechanism when the rolling reduction is less than 30%.

Microstructure evolution and creep strength of new-generation oxide dispersion strengthened alloys with high volume fraction of nano-oxides

Four creep resistant Fe-based oxide dispersion strengthened (ODS) alloys with a significantly high amount of dispersed oxide nanoprecipitates have been investigated. One Fe-Al-O and three Fe-Al-Cr-Mo-Y2O3 systems with different chemical composition strengthened by yttrium nano-oxides have been prepared by mechanical alloying of powders and consolidation by hot rolling leading to ultrafine grained microstructure due to dynamic recrystallization. The thermal stability of the precipitates, effects of the processing on the microstructure (grain and precipitate size) and mechanical properties at high temperatures have been evaluated. It has been found that the rolling temperature has a significant effect on the static recrystallization process during the subsequent heat treatment and on the resulting grain size of the alloys and does not affect the size of nano-oxides and their dispersion. Tensile tests performed at a low constant rate of 10-6 s-1 allow a quick estimate of the creep strength and helps to a quick identification of optimum processing conditions. It was found that the mechanism of the fracture is changing from trans-granular to inter-granular between 600 and 800 °C which leads to a significant drop of ductility. The set of mechanical tests has been completed also for 1100 °C indicating a rather high applicability potential of the investigated system.

Quantitative metallographic characterization of welding microstructures in Ti-containing TWIP steel by means of image processing analysis

Recrystallization and grain growth phenomena in Ti-containing Twining Induced Plasticity (TWIP-Ti) steel weld joints were studied. The joints obtained through the autogenous Gas Tungsten Arc Welding (GTAW) process at different heat inputs were characterized taking into account their grain size distribution and the particle pinning. The temperature ranges at which recrystallization and grain growth occurred in TWIP-Ti steel welds were determined. The optical micrographs were analyzed by means of MATLAB-based image analysis. In-house routines were added to perform grain size distribution/classification. The statistical analysis performed on micrographs by the image processing code indicated that TWIP-Ti steel showed a bimodal grain size distribution after the solution heat treatment. The static recrystallization (SRX) process detected in the low heat input weld joint, modified its bimodal grain size distribution to normal logarithmic (lognormal). The high heat input in TWIP-Ti steel weld kept the bimodal distribution. The presence of precipitated particles like Ti(C, N) and AlN generated microstructural stability in TWIP-Ti steel at temperatures <1050 °C. At higher temperatures, particle pinning by precipitates disappeared and abnormal grain growth was produced as a result of the pinning pressure decrease.

Shape memory and superelasticity of nanograined Ti-51.2 at.% Ni alloy processed by severe plastic deformation via high-ratio differential speed rolling

A novel method of producing nanograined Ni-rich superelastic NiTi alloys in sheet form was proposed using a combination of severe plastic deformation via high-ratio differential speed rolling (HRDSR) and post-deformation annealing. The HRDSR-processed microstructure was composed of heavily deformed austenite and martensite grains, and amorphous phases. After annealing at 673 K, the severely deformed microstructure with no functional properties evolved to the nanograined structure (20–70 nm) composed of austenite and martensite nanograins and sub-nanograins through static recovery or continuous static recrystallization process. The nanograined microstructure had a high resistance to martensitic transformation upon cooling and slip deformation during straining. As a result, the HRDSR-processed alloy annealed at 673K exhibited superior superelasticity compared to the alloys with coarse grains. At the higher annealing temperature of 873 K, the micron-sized recrystallized grains with low dislocations developed through discontinuous static recrystallization process. In this case, deformation during straining was governed by the detwinning of twinned martensite, and as a result, shape memory effect was more significantly pronounced than superelasticity.

Influence of Hot Rolling and Heat Treatment on the Microstructural Evolution of β20C Titanium Alloy.

The microstructural evolution and underlying mechanism of a new high strength, high toughness near β titanium alloy, β20C, during hot deformation, and heat treatment were studied qualitatively and quantitatively. It was found that dynamic recovery occurs mainly in β phase, while α phase undergoes both a dynamic recovery and continuous incomplete dynamic recrystallization with a fraction of high-angle grain boundaries (≥15°) of 21.1% under hot-rolling. Subsequently, α phase undergoes static recrystallization with an increasing fraction of high-angle grain boundaries (21.1%→60.7%) under annealing, while the grains are equiaxed with refined grain sizes of 1.63 µm observed from the rolling direction (RD) and 1.66 µm observed from the transverse direction (TD). Moreover, the average aspect ratio of the lamellar α phase was 2.44 observed from the RD and 3.12 observed from the TD after hot rolling, but decreased to 2.20 observed from the RD, and 2.53 observed from the TD after annealing. Furthermore, the strict Burgers’ relationship between α and β phases changed after hot-rolling and remains the distortion, even after the static recrystallization process of α phase during annealing.

Influence of burnishing condition on static recrystallization of an iron sheet

Abstract This paper intends to study an effect of burnishing and annealing on static recrystallization. The result is expected to be used for a new technology to control only material surface microstructure in a steel plate. In this process, the stored strain energy by burnishing and the thermal energy by annealing lead to static recrystallization and grain growth in a material, which improve the microstructure of the material. In this paper, roller burnishing is employed to introduce the shear strain into the surface of the pure iron sheet. And effects of burnishing and annealing on static recrystallization in the finished surface layer are investigated utilizing Electron Back Scattering Diffraction (EBSD) analysis technique. The generation of shear deformation was confirmed by Kernel Average Misorientation (KAM) values at the surface layer. By conducting a series of the annealing and EBSD analyses, the static recrystallization process is observed. Since enough strain energy is accumulated by the burnishing in a specimen with large shear deformation zone, the static recrystallization occurs on the burnished surface preferentially after the annealing.