Continuous Dynamic Recrystallization Research Articles

Dynamic recrystallization mechanisms of as-forged Al–Zn–Mg-(Cu) aluminum alloy during hot compression deformation

The thermal deformation behavior of as-forged Al–Zn–Mg-(Cu) aluminum alloy at strain rate 0.001–1 s−1, temperature 663–783 K and deformation degree 0–60% was studied by isothermal compression test, and an accurate strain-compensated Arrhenius constitutive equation was established. Based on the microstructure characteristics of compressed specimens, the effects of strain rate, temperature and strain on dynamic recrystallization (DRX) behavior and substructure evolution were determined. The results show that the alloy undergoes discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX) simultaneously at high temperature and low strain rate (743 K, 0.001–0.01 s−1), where CDRX includes microshear band assist (MSBA), homogeneous increase of misorientation (HIM) and progressive lattice rotation near grain boundaries (LRGB). However, with decreasing temperature and increasing strain rate, GDRX basically disappears, and DDRX becomes the main DRX mechanism. At medium to high strain rate (0.1∼1 s-1), high-density dislocations accumulate at the grain boundaries and numerous microshear bands form inside grains during the early deformation stage. The misorientation angles of these pre-existing geometrically necessary boundaries gradually increase with temperature and strain, ultimately transforming into subgrains and DRX grains.

Effect of rotation speed on microstructure and mechanical properties of bobbin tool friction stir welded T2 copper

In this study, the effects of different rotation speeds (600, 800, 1000 and 1200 rpm) on the microstructure, mechanical properties, electrochemical corrosion properties and resistivity of T2 pure copper bobbin tool-friction stir welded joint at a fixed welding speed of 50 mm/min were investigated. The results show that the grain size of the microstructure of welded joints with different rotational speeds has a large difference, and the recrystallization mode are continuous dynamic recrystallization, discontinuous dynamic recrystallization and geometric dynamic recrystallization. The fracture location is at the thermo-mechanically affected zone of the retreating side with the largest difference in microstructure gradient and grain size. Due to the combined effect of grain size and low angle grain boundaries distribution, the welded joints at 1000 rpm have higher tensile strength, elongation, microhardness values, and electrical conductivity, and the welded joints at 600 rpm have higher yield strength. The impedance curve and polarization curve fitting results in electrochemical corrosion are consistent, and the corrosion is mainly intergranular corrosion and full-scale corrosion, and has good corrosion resistance at 600 rpm and 1000 rpm rotational speed.

Hot deformation behavior and dynamic recrystallization mechanism of Ti-48Al-2Nb-2Cr alloy with near-γ microstructure

Hot processing is extremely effective in optimizing the microstructure of γ-TiAl alloy, and hot deformation mechanism is the decisive factor in the microstructure evolution. In this paper, hot compression tests under different conditions were carried out to investigate the characteristics of hot deformation behavior and deformation mechanism of Ti-48Al-2 Nb-2Cr alloy with near-γ microstructure. The Arrhenius constitutive model was established, and the predicted flow stress agreed well with the experimental value. Hot processing maps at different strains were constructed. High power dissipation efficiency (η) region which was suitable for hot working appeared in the temperature range of 1180–1210 ℃ and the strain rate range of 0.001–0.01 s−1. At the strain rates from 0.001 to 0.1 s−1, dynamic recrystallization (DRX) served as the primary softening mechanism of the alloy, which was closely related to the disordered transformation of α2→α. At the temperature above Tα2→α and the strain rate of 1 s−1, flow softening of the alloy was mainly controlled by dynamic recovery (DRV). The occurrence of DRX in the lamellar structure can be promoted by the elevated temperatures and reduced strain rates. Discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) appeared in the γ and α/α2 phases of the alloy deformed at the conditions corresponding to high η region. With the decrease of η value, DRX nucleation of the alloy under the relevant conditions was chiefly dominated by DDRX.

Revealing hot deformation behavior of inconel X-750 superalloy: A novel hot processing map coupled with grain boundary engineering

In this work, a series of isothermal hot compression tests were performed at the temperature of 900–1150 °C with the strain rate ranging from 0.01 to 10s−1 in Inconel X-750 alloy (referred as Alloy X-750). Based on the flow stress data, the back-propagation artificial neural network model was utilized to accurately predict the flow stress at elevated temperatures. Subsequently, by constructing a modified hot processing map coupled with the variation of Σ3n (n = 1, 2, and 3) twin boundaries fraction, the optimal processing parameters were determined to be in the range of 1100–1150 °C and 1-10s−1 for Alloy X-750. Moreover, the possibility of grain boundary engineering via hot deformation were also evaluated by using some critical quantification descriptors of twin related domains (TRDs), such as the number of grains in the TRDs, the lengths of the longest chain, and the average TRDs size. All the results can further verify the validity of modified hot processing map. Finally, the high-temperature flow behavior combined with the relevant microstructure characterization showed that discontinuous dynamic recrystallization is considered as the predominant softening mechanism, while the continuous dynamic recrystallization and particle-stimulated nucleation also additionally contribute to the recrystallization nucleation during the hot deformation of Alloy X-750.

Microstructural evolution and mechanical properties of cooling medium assistant friction stir processed AZ31B Mg alloy

AZ31B magnesium alloy prepared by conventional friction stir processing (FSP) usually exhibits an intense basal texture, resulting in an unsatisfactory strength and ductility. In this work, cooling medium-assisted FSP was conducted on the AZ31B magnesium alloy. The effects of a liquid CO2 coolant on the thermal cycle, microstructure, and mechanical properties of the stir zone (SZ) were evaluated. The adoption of a liquid CO2 coolant resulted in markedly decreased peak temperature and increased cooling rate. The SZ exhibited a fine grain structure with abundant dislocations and twins. The grain refinement mechanism was attributed to the combinational effect of discontinuous dynamic recrystallization, continuous dynamic recrystallization, and twinning-induced geometric dynamic recrystallization. The SZ showed a best combination of ultimate tensile strength of 293 MPa and fracture elongation of 18.6%. The interaction of dislocations and twins rendered the plastic deformation more stable during tensile testing.

Revealing the deformation behavior and microstructure evolution in Mg-12Y–1Al alloy during hot compression

In this study, the hot uniaxial compression of the heat-treated Mg-12Y–1Al alloy containing long-period stacking ordered (LPSO) phase was conducted by a Gleeble-3500 simulator. Compressive mechanical properties at elevated temperatures and the effects of deformation temperature (300–450 ℃) and strain rate (0.001–1 s−1) on the hot deformation behavior of the alloy were systematically investigated. Combining with an optical microscope (OM), electron backscatter diffraction (EBSD), and in-grain misorientation axes (IGMA) analysis, the microstructure and texture evolution, dislocation slip, and dynamic recrystallization (DRX) mechanism of the alloy was studied in detail. The results showed that the flow stress decreased with the increase in temperature and the decrease in strain rate. At a strain rate of 1 s−1, the compressive strength at 300 ℃ and 350 ℃ was basically the same. After deformation, the morphology of Al2Y had almost no change, while the LPSO underwent apparent kinking deformation. The increase in temperature was conducive to DRX. The DRX mechanism of the alloy mainly included discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), and DDRX was dominant. The dense lamellar LPSO at the grain boundary was detrimental to the occurrence of DDRX. In addition, the deformed microstructure presented a typical [0001]//CD (compression direction) texture. With the increase in temperature, the texture intensity decreased progressively due to the activation of multiple slip systems and the increased DRX fraction. At a strain rate of 0.01 s−1, when the temperature increased to 450 ℃, the number of deformed grains dominated by pyramidal Ⅱ<c+a> slip increased substantially.

Deformation behavior and dynamic recrystallization mechanism of a novel high Nb containing TiAl alloy in (α + γ) dual-phase field

Dynamic recrystallization (DRX), microstructure evolution and texture of high Nb containing TiAl alloys with a composition of Ti-43Al-6Nb-1Mo-1Cr (at.%) during the isothermal deformation are investigated in this paper. The recrystallized γ grains with L10 structure dominate the deformation in condition 1150°C/0.005 s-1. Meanwhile, the deformation-induced γ/α2→α phase transformation occurs in present TiAl alloy. Except the continuous dynamic recrystallization (CDRX) characterized by crystal orientation accumulation, lots of grains with specific 89±3° misorientation angles related to discontinuous dynamic recrystallization (DDRX) are observed in the γ phase. The specific misorientation angles might be caused by grains rotation, which is related to the activation of {111}<110> slipping system in γ phase. As regards as texture, the texture of α2 phase mainly maintains transverse texture during the deformation. And γ phase exhibits typical hot-rolled texture at low strain. With the strain increasing to 40%, the scattered texture appears, due to the nucleation of many dynamic recrystallized grains. The variety of texture is mainly caused by different DRX behaviors at various strains.

Hot compression deformation behavior and dynamic recrystallization of a precipitate-free Mg–Bi–Sn alloy

The addition of Sn improves the properties of Mg–Bi-based alloys, and can potentially overcome the high-temperature limitations of magnesium (Mg) alloys. However, high-Sn alloying leads to stress concentration during hot extrusion. Hence, in this study, the dynamic recrystallization (DRX) mechanism and hot-deformation behavior of a novel precipitate-free Mg–0.25Bi–0.25Sn alloy during hot compression conducted at 250–400 °C with strain rate of 0.001–1 s−1 were examined in detail by constructing processing maps and deriving constitutive equation. The obtained correlation coefficient was 0.99113, indicating that the proposed method can adequately predict the hot deformation behavior of the studied alloy. The calculated average activation energy was 143.0336 kJ/mol. According to the processing maps, the ideal processing areas corresponded to the conditions of 300–400 °C/0.001 s−1/1 s−1 and 400 °C/0.1 s−1. In addition, a transformation of prismatic <a> slip to pyramidal II <c+a> slip was observed at 350 °C, and the continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) processes occurred during hot compression. The classification of the dominant DRX mechanisms was based on the Zener–Hollomon parameter (Z). Under high-Z deformation conditions, CDRX and DDRX occurred simultaneously; however, CDRX was the predominant process. Under medium-Z deformation conditions, DDRX was the only DRX process. Under low-Z deformation conditions, DDRX dominated over the DRX mechanism and was partially accompanied by CDRX.

Hot deformation behavior and dynamic recrystallization mechanism of Ti2ZrTa0.75 refractory complex concentrated alloy

Hot-deformation behavior and dynamic recrystallization (DRX) mechanism of Ti2ZrTa0.75 refractory complex concentrated alloy were systematically studied via uniaxial compression tests at various deformation temperatures from 800 °C to 1100 °C and different strain rates from 10−3 s−1 to 1 s−1. Stress-strain curves showed an increase in flow stress with decreasing deformation temperature and/or increasing strain rate, and there was continuous strain softening after the peak stress. Microstructure analysis based on electron backscattering diffraction (EBSD) indicated that deformation bands occurred in the hot deformation microstructure with a strain rate of 1 s−1, which is the appropriate location for DRX. Microstructure analysis revealed that DRX occurred during the hot deformation process and DRX fraction and grain size increase with increasing deformation temperature and/or decreasing strain rate. The grain boundary bulging nucleation mechanism and necklace-like structure of DRX grains demonstrate a typical discontinuous dynamic recrystallization (DDRX) process. Cumulative misorientation analysis of the initial grain interior and new grains formed without relying on the initial grain boundaries proved the occurrence of continuous dynamic recrystallization (CDRX).

Enhancing the strength and ductility in an Al–5Mg alloy via accumulative continuous extrusion forming at room and cryogenic temperatures

In this work, the microstructural evolution of Al–5Mg (wt%) alloy fabricated by accumulative continuous extrusion forming (ACEF) at room temperature (RT) and cryogenic temperature (CT) was investigated. The grain refinement mechanism during ACEF was revealed. The strengthen and ductility enhancements of the alloy were discussed. The results showed that good grain refinement effect was achieved in the alloy after multi-pass RT-ACEF, and was further enhanced in the alloy fabricated by 1 pass CT-ACEF. High-density dislocations, as the driven force of continuous dynamic recrystallization, were achieved in the 1 pass CT-ACEFed alloy, and the growth of recrystallized grains was limited at CT. As a result, superior grain refinement effect was obtained in the alloy after CT-ACEF. Simultaneous strength and ductility improvements were found in the ACEFed alloy. The ultimate tensile strength and elongation were enhanced to 294.3 MPa and 55.1 % of the alloy after 1 pass CT-ACEF. Grain boundary strengthening and dislocation strengthening dominated the strength enhancement of the ACEFed alloy. The high work hardening exponent and wide strain interval were responsible for the increase in ductility of the CT-ACEFed alloy. The purpose of this study is to provide a high-efficiency strategy to achieve Al–Mg alloys with superior mechanical performance and overcome the strength-ductility trade-off.

Development and analysis of a powder bed friction stir (PBFS) additive manufacturing process for aluminum alloys: A study on friction-stirring pitch (ω/v) and print location

Metallic components fabricated with powder bed fusion additive manufacturing (AM) processes are known to have defects arising from liquid-solid phase transformation. Friction stirring can be an effective alternative source of heat, as evidenced in some of the recently developed powder deposition-based solid-state AM processes. However, they possess practical issues such as making compacted powder feedstock beforehand, consolidation of powder filler inside the tool during deposition followed by necessary extrusion, and significant flash formation, among a few. This paper proposed a novel powder bed friction stir (PBFS) additive manufacturing process inside a specifically designed mould that could potentially mitigate such issues in the existing technologies. The process was successfully demonstrated by building single-track and parallel double-track 3D structures layer-by-layer from an AA6061 powder bed using a rotating non-consumable friction stirring tool. Furthermore, for a better understanding of the underlying process mechanics, deposits were analyzed for their surface topography, microstructural evolution, and mechanical performance under different heat inputs as well as different print locations. Different heat inputs were applied by varying the friction-stirring pitch (FS pitch) from ‘Low’ to ‘High’. The print locations analysis was carried out in a parallel double-track fabricated at the optimal FS pitch. The results revealed that the ‘High’ FS pitch yielded an increased percentage of recrystallized grains and a fraction of high-angle grain boundaries as well as a larger average grain size due to the high heat input, better material intermixing, and the dominance of continuous dynamic recrystallization. As a result, an improved mechanical performance, except for the microhardness, was observed. The ‘High’ FS pitch also exhibited an improved surface topography. The results also revealed that the relative dominance of the re-stirring-induced recrystallization and the reheating-induced plastic deformation was the key factor for the variation of the microstructure in the longitudinal, transverse, and short-transverse directions. A gradual decrease in the tensile strength was observed from the ‘Top’ to the ‘Bottom’ segments of the overlapped region, contrary to the central region of a track.

Hot Deformation Behavior and Processing Maps of an As-Cast Al-5Mg-3Zn-1Cu (wt%) Alloy.

One of the key issues limiting the application of Al-Mg-Zn-Cu alloys in the automotive industry is forming at a low cost. Isothermal uniaxial compression was accomplished in the range of 300-450 °C, 0.001-10 s-1 to study the hot deformation behavior of an as-cast Al-5.07Mg-3.01Zn-1.11Cu-0.01Ti alloy. Its rheological behavior presented characteristics of work-hardening followed by dynamic softening and its flow stress was accurately described by the proposed strain-compensated Arrhenius-type constitutive model. Three-dimensional processing maps were established. The instability was mainly concentrated in regions with high strain rates or low temperatures, with cracking being the main instability. A workable domain was determined as 385-450 °C, 0.001-0.26 s-1, in which dynamic recovery (DRV) and dynamic recrystallization (DRX) occurred. As the temperature rose, the dominant dynamic softening mechanism shifted from DRV to DRX. The DRX mechanisms transformed from continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX), and particle-stimulated nucleation (PSN) at 350 °C, 0.1 s-1 to CDRX and DDRX at 450 °C, 0.01 s-1, and eventually to DDRX at 450 °C, 0.001 s-1. The eutectic T-Mg32(AlZnCu)49 phase facilitated DRX nucleation and did not trigger instability in the workable domain. This work demonstrates that the workability of as-cast Al-Mg-Zn-Cu alloys with low Zn/Mg ratios is sufficient for hot forming.

Friction Stir Welding of Dissimilar Al 6061-T6 to AISI 316 Stainless Steel: Microstructure and Mechanical Properties.

The friction stir welding (FSW) process was recently developed to overcome the difficulty of welding non-ferrous alloys and steels. In this study, dissimilar butt joints between 6061-T6 aluminum alloy and AISI 316 stainless steel were welded by FSW using different processing parameters. The grain structure and precipitates at the different welded zones of the various joints were intensively characterized by the electron backscattering diffraction technique (EBSD). Subsequently, the FSWed joints were tensile tested to examine the mechanical strength compared with that of the base metals. The micro-indentation hardness measurements were conducted to reveal the mechanical responses of the different zones in the joint. The EBSD results of the microstructural evolution showed that a significant continuous dynamic recrystallization (CDRX) occurred in the stir zone (SZ) of the Al side, which was mainly composed of the weak metal, Al, and fragmentations of the steel. However, the steel underwent severe deformation and discontinuous dynamic recrystallization (DDRX). The FSW rotation speed increased the ultimate tensile strength (UTS) from 126 MPa at a rotation speed of 300 RPM to 162 MPa at a rotation speed of 500 RPM. The tensile failure occurred at the SZ on the Al side for all specimens. The impact of the microstructure change in the FSW zones was significantly pronounced in the micro-indentation hardness measurements. This was presumably attributed to the promotion of various strengthening mechanisms, such as grain refinement due to DRX (CDRX or DDRX), the appearance of intermetallic compounds, and strain hardening. The aluminum side underwent recrystallization as a result of the heat input in the SZ, but the stainless steel side did not experience recrystallization due to inadequate heat input, resulting in grain deformation instead.

Role of geometric dynamic recrystallization in nanocrystalline alloys

Nanocrystalline and near-nano-grained metals are plagued with an overall lack of ductility and formability. This is in part due to their inability to support the dislocation mechanisms or undergo the same types of transformations that conventional coarse-grained metals access inorder to increase their workability. Therefore, despite their advanced properties such as high strength, the manufacturing of such fine-grained metals, which exposes them to multi-axial states of stress in operations such as rolling, drawing, extrusion, forging, and bending, has not been possible. In this work, we address this limitation and highlight an important aspect of geometric dynamic recrystallization (GDRX) in nanocrystalline materials as a way to improve manufacturability. In particular, using two different alloy compositions and two different processing techniques, this study points to the fact that if nanocrystalline or near nano metals can be truly stabilized, they can repeatedly undergo the same types of processes that conventional coarse-grained metals endure, increasing both their formability and hence manufacturability. The results reveal that due to a large number of initial high-angle grain boundaries and large applied compressive strains, GDRX is activated in the absence of any discontinuous dynamic recrystallization (DDRX) or continuous dynamic recrystallization (CDRX). This is a unique aspect of stabilized nanocrystalline materials, as traditionally fine-grained metals are not microstructurally stable enough to accommodate such an event.

Revealing the ultra-high high-temperature compressive mechanical properties and deformation mechanism of a heterostructured AlNp/Al nanocomposite

Heterostructured materials have attracted increasing attention owing to their superior mechanical properties. In this work, the high-temperature mechanical property and deformation mechanism of a heterostructured AlNp/Al composite were revealed by isothermal compression tests. The results showed that the superior compressive strength of the AlNp/Al composite could be maintained at both room temperature and high temperature of 300–500 °C during the whole deformation process. The strong pinning effect of the dispersive AlN nanoparticle (AlNp) on the matrix grains could prevent grain boundary softening and grain growth, as well as the special layered heterogeneous structure and network structure could avoid high temperature softening. The hot deformation mechanism of the AlNp/Al composite could be affected by the strain rate and deformation temperature. It was confirmed that continuous dynamic recrystallization (CDRX) dominated the whole deformation process when deformed at low strain rates (0.0001–0.01 s−1) and high temperatures (400–500 °C); while dynamic recovery (DRV) was the main mechanism at high strain rate (0.1–1 s−1) and low temperature (300–350 °C) Furthermore, the strain rate sensitivity exponent (m) was used to assess the relationship between peak stress and deformation temperature and strain rate, which could predict the instability region of the composite. It shows that the simultaneous action of high temperatures and high strain rates should be avoided during hot deformation. This work shed light on the microstructure design of the high-strength heat-resistant aluminum matrix composite.

Effect of rotational speed and feed rate on microstructure and mechanical properties of 6061 aluminum alloy manufactured by additive friction stir deposition

Additive friction stir deposition (AFSD) is attractive for its ability to create freeform and fully dense structures without melting and solidification. Hence, additive friction stir deposition is an alternative to fusion-based additive manufacturing technology. In this study, the influence of the AFSD parameter (i.e., rotational speed and feed rate) on the mechanical properties and microstructure of 6061 aluminum alloy is investigated. The as-deposited 6061 aluminum alloy exhibits a relatively homogeneous microstructure with extensive equiaxed grains. Compared to the feedstock material, the ultimate tensile strength of the as-deposited 6061 aluminum alloy decreased to 65% from 320 to 210 MPa. The results of electron backscatter diffraction indicate that continuous dynamic recrystallization occurs during the AFSD process. Furthermore, it is evidence that the grain size and ultimate tensile strength are positively correlated with feed rate and rotational speed, whereas the elongation at break decreases with the increase in feed rate and rotational speed.

Modeling energy consumption inside cutting deformation zone to predict the stress distributions, temperature and microstructure by micro irreversible entropic thermodynamics

It is important to analyze the process of chip formation by means of the energy consumption during metal cutting, which is related to the cutting power (or cutting force), the cutting temperature and the machined surface integrity. In this study, the consumed energy field inside the cutting deformation zone is established by considering four micro irreversible processes including the dislocation generation, annihilation, movement, and continuous dynamic recrystallization. A new shear-angle solution is proposed on the basis of the steady conditions of irreversible processes (i.e., the entropy production rate reaches the minimum), whose adequacy is verified by the experimental results of the shear angle and friction coefficient along the tool-chip interface. Based on the predicted consumed energy field inside the cutting deformation zone, the deformation mechanism, microstructure, stress and temperature are predicted and analyzed. The results show that three stages including the loading stage, unloading stage and steady stage are divided during chip formation according to the consumed energy field. The consumed energy and statistically stored dislocation density are both of gradient distribution in the primary shear zone, and continuous dynamic recrystallization is the main mechanism of grain refinement during chip formation, which is dominated in the secondary deformation zone. The stress and temperature inside the cutting deformation zone as well as the hardness of the machined surface are predicted according to the energy balance relationship and consumed energy field, indicating the feasibility of using energy consumption to comprehensively describe the internal changes of workpiece material inside the cutting deformation zone.

DRX mechanisms of a Ni-Co-W type superalloy with typical columnar grains during hot compression

Columnar evolution and DRX mechanisms of a Ni-Co-W type superalloy with typical columnar grains during hot deformation were investigated processed at the γ′ sub/super-solvus temperatures. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) was employed to characterize the nucleation at columnar boundary. The results show that columnar grains gradually replaced by new dynamic recrystallization (DRX) grains and nucleation was prioritized at original columnar boundary or/and folding position. Meanwhile, DRX mechanisms of the columnar structure associated with γ′ phase were identified deeply. Discontinuous dynamic recrystallization (DDRX) was the main mechanism when deformation processed at γ′ sub/super-solvus temperatures. Bulging columnar grain boundaries are first as the nucleation sites of the new DDRX grains under various conditions. High angle grain boundaries (HAGBs: θ > 15°) bends to the side with γ′ phase, and then the DRX grains formed at the bulging grain boundary. In addition, high dislocation density zone occurred around the large size γ′ phases (more than 1 µm), leading to the misorientation generated rapidly and then the HAGBs formed. The formation of DRX grains by this way is also called particle stimulated nucleation (PSN) mechanism. Continuous dynamic recrystallization (CDRX) occurs when hot deformation processed at γ′ sub-solvus temperature with high strain level, which is accompanied by dislocation accumulation and an obviously increase in the fraction of grain boundaries with 10–15° misorientation angle. The findings help to understand different DRX mechanisms of Ni-based superalloy with columnar structure during microstructure evolution under hot forming, and are also beneficial for optimizing the microstructure to control the required properties of the Ni-based superalloys for manufacturing turbine disc.

A new microstructural refinement mechanism of ultra-large β grains in near-β titanium alloys during primary hot-working

In this work, the multi-step hot-working of a near-β titanium alloy, Ti-55511 alloy with centimeter-sized grains, was performed at 1170 °C. It was confirmed that the static/metadynamic recrystallization during interpass annealing was the primary mechanism responsible for the microstructural refinement. Meanwhile, a new breakdown mechanism of the ultra-large β grains with 〈001〉 preferential orientation was observed during deformation, which was essentially different from the traditional discontinuous or continuous dynamic recrystallization. That is, new grain boundaries and even boundary networks were produced interconnecting the corners/protrusions of the 〈001〉 -orientated grains through a low-angle grain boundary merging mechanism, which progressively severed the large β grains into smaller ones. Such a breakdown mechanism may be an important complement for microstructural refinement during primary hot-working, which allowed the breakdown of the ultra-large β grains under fairly low strain level.

Investigation on microstructure evolution of Mg-Gd-Zn-Zr alloys during friction stir processing by liquid CO2 cooling assisted stop action

The microstructure evolution of Mg-14Gd-2.5Zn-0.5Zr (wt.%) alloys during friction stir processing (FSP) were investigated through liquid CO2 cooling assisting stop action technology. In particular, solutionizing at 480 °C (LT4) and 520 °C (HT4) was carried out to clarify the evolution of different second phases. The results showed that grain size decreased from ∼20 μm to ∼1–3 μm in the stir zone, and a weak texture intensity could be found after FSP. Although the type of the second phases in as-cast, LT4 and HT4 samples did not change during FSP, different evolution sequences about the second phase and grain refinement could be found. Bulk X phase was broken into finer one later than recrystallization and refinement of α-Mg grain in LT4 alloy, while β-phase fragmentation and formation of recrystallized grains occurred simultaneously in HT4 alloy. In addition, the intragranular 14H-LPSO structure retained in three samples after FSP. Due to the different types and volume fractions of the second phases, the as-cast and LT4 alloy were mainly refined by continuous dynamic recrystallization (CDRX), while both CDRX and geometric dynamic recrystallization (GDRX) occurred in HT4 alloy. After FSP, the yield strength increased by 74%, 110% and 90% for as-cast, LT4, and HT4 alloy, respectively.