Geometric Dynamic Recrystallization Research Articles

The Evolution of Grain Microstructure in Friction Stir Welding of Dissimilar Al/Mg Alloys with Ultrasonic Assistance.

The process of grain refinement during welding significantly influences both the final microstructure and performance of the weld joint. In the present work, merits of acoustic addition in the conventional Frictions Stir Welding (FSW) process were evaluated for joining dissimilar Al/Mg alloys. To capture the near "in situ" structure around the exit hole, an "emergency stop" followed by rapid cooling using liquid nitrogen was employed. Electron Backscatter Diffraction analysis was utilized to characterize and examine the evolution of grain microstructure within the aluminum matrix as the material flowed around the exit hole. The findings reveal that two mechanisms, continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX), jointly or alternatively influence the grain evolution process. In conventional FSW, CDRX initially governs grain evolution, transitioning to GDRX as material deformation strain and temperature increase. Subsequently, as material deposition commences, CDRX reasserts dominance. Conversely, in acoustic addition, ultrasonic vibration accelerates GDRX, promoting its predominance by enhancing material flow and dislocation movements. Even during the material deposition, GDRX remains the dominant mechanism.

Grain refinement and strain delocalization in TRIP high-entropy alloys during hot deformation

Transformation-induced plasticity (TRIP) high-entropy alloy (HEAs) have shown promising mechanical properties, making them potential candidates for various applications. The preparation of structures with fine grains and heterogeneous orientation is important to improve the strength and ductility of TRIP-HEAs. However, the evolution and mechanism of grain refinement with heterogeneously oriented structures of TRIP-HEAs during thermal deformation and its effect on strain delocalization are not clear. In this paper, the microstructure evolution and strain delocalization of a TRIP-HEA, Co35Cr25Ni15Mn15Fe10 (atomic percent, at %) was examined through hot compression tests conducted at temperatures ranging from 900 ℃ to 1000 ℃, and strain rates ranging from 0.001 s−1 to 0.1 s−1. The results revealed that the average grain size of the alloy decreases with increasing deformation temperature and reducing strain rate due to the appearance of dynamic recrystallization (DRX) and annealed twins. After deformation at 1000 °C-0.001 s−1, the average grain size of the alloy decreases from 169.85 μm (initial sample) to 21.5 μm, and the volume fraction of DRX and twin boundaries (TBs) increases to 81.9 % and 37.6 %, respectively. The DRX mechanism of the TRIP-HEA transformed from discontinuous dynamic recrystallization (DDRX) at 900 °C-0.1 s−1/0.01 s−1 to geometric dynamic recrystallization (GDRX) at 900 °C-0.001 s−1 and then to continuous dynamic recrystallization (CDRX) at 950 °C/1000 °C-0.001 s−1. Furthermore, the occurrence of DRX promotes the formation of annealed twins, resulting high volume fraction of high angle boundaries (HAGBs) and twin-matrix pairs, and contributing to grain refinement. Encouragingly, these HAGBs and twin-matrix pairs not only contributes to grain refinement, but also facilitates strain delocalization via dispersing strain and forming strain gradient, thus leading to coordinated deformation. This result is of great significance for understanding and improving the mechanical properties of TRIP-HEAs, and it can help in the design of high-performance TRIP-HEAs for various applications.

Microstructure, Corrosion and Mechanical Properties of Medium-Thick 6061-T6 Alloy/T2 Pure Cu Dissimilar Joints Produced by Double Side Friction Stir Z Shape Lap-Butt Welding

A novel double side friction stir Z shape lap-butt welding (DS-FSZW) process was proposed to achieve excellent mechanical properties of Al/Cu medium-thick dissimilar joints. The influence of welding parameters on weld microstructure and properties of DS-FSZW joint were systematically investigated. It indicated that defect-free medium-thick Al/Cu DS-FSZW joint could be achieved under an optimal welding parameter. DS-FSZW joint was prone to form void defects in the bottom of the second-pass weld. The recrystallization mechanisms at the top and middle of the weld nugget zone (WNZ) were continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX). While the major recrystallization mechanism at the bottom of the WNZ was GDRX. DS-FSZW joint of the optimal welding condition with 850 r/min-400 mm/min was produced with a continuous thin and crack-free IMCs layer at the Al/Cu interface, and the maximum tensile strength of this joint is 160.57 MPa, which is equivalent to 65.54% of pure Cu base material. Moreover, the corrosion resistance of Al/Cu DS-FSZW joints also achieved its maximum value at the optimal welding parameter of 850 r/min-400 mm/min. It demonstrates that the DS-FSZW process can simultaneously produce medium-thick Al/Cu joints with excellent mechanical performance and corrosion resistance.

Low-temperature superplastic deformation mechanism of ultra-fine grain Ti–6Al–4V alloy by friction stir processing

The ultra-fine grain Ti–6Al–4V alloy sheet was successfully achieved by friction stir processing (FSP). The low-temperature superplastic deformation mechanism of Ti–6Al–4V alloy under the strain rate of 3 × 10−4 s−1 at 600 °C was studied by scanning electron microscope, electron backscatter diffraction and superplastic tensile test. It is found that the grain size of Ti–6Al–4V alloy by FSP is refined from 7.48 μm to 0.82 μm, and there is no preferential crystallographic orientation of the uniform grain. After FSP, the β phase of the base material (BM) in the grain boundary is refined and homogenized. During the superplastic tensile processing, with the increase of strain, the grain is refined continuously, and the α→β phase transition is induced by the dislocations pile-up at the β grain boundary, and the content of β phase increases. The grain near the fracture was refined to 0.35 μm, and the stress concentration led to the formation of cavities near the β phase, which eventually caused the fracture. The dynamic recrystallization in superplastic tensile processing is mainly geometric dynamic recrystallization and discontinuous dynamic recrystallization. The elongation of superplastic tensile (1033%) is 19.7 times higher than that of BM (50%). The order of superplastic deformation mechanism in the low-temperature superplastic deformation of the ultra-fine grain Ti–6Al–4V alloy sheet is as follows: intragranular dislocation slip and diffusion, grain boundary sliding, and phase boundary sliding. Phase boundary sliding is the main deformation mechanism for low-temperature superplastic deformation.

Formation mechanisms and mechanical properties in aluminum cables and copper terminals ultrasonic welded joint

To fabricate highly reliable Al cables/Cu terminals components and elucidate the microstructure evolution of the joint during ultrasonic welding, ultrasonically welded structures of 50 mm2 Al cables and 2 mm-thick copper terminals were conducted at welding energies from 1200 J to 2800 J and an amplitude of 54.6 μm in this paper. As the welding energy increased, the degree of plastic deformation of the Al wires gradually increased, the microstructure of the Cu side remained basically unchanged, and the effective contact area of the Cu/Al interface was gradually enlarged, enhancing the coherence between the Al wires, and the Cu-Al interfacial structure changed from an unbonded area to continuous mechanical interlocking. Parallelly, the Al grains transformed from fine serrated to equiaxed and lamellar crystals driven by geometric dynamic recrystallization (GDRX), continuous dynamic recrystallization (CDRX), and dynamic recovery (DRV), forming distinct shear textures of B/B̅{111}<110>, Rt Goss{110}<110>, Rt Cube{001}<110> and the recrystallization texture F{111}<112>, and the content of shear textures increased from 2.9% to 17.4%. In addition, the tensile shear failure load of the joint reached 2962 N, which met the requirements of cable installations in the field of new energy vehicles (>1650 N). Fracture analysis showed the fracture location transitioned from the Cu-Al interface to the Al-Al interface with an increase in welding energy from 1600 to 2400 J. This work has theoretical guidance for the continuous promotion of vehicle light-weighting.

Effect of Sc on the recrystallization behavior and mechanical properties of Al-Zn-Mg-Cu-Zr alloys under isothermal multidirectional compression

To better achieve grain refinement and further optimize the comprehensive mechanical properties of Al-Zn-Mg-Cu-Zr alloys, this paper investigates the effect of trace Sc on the recrystallization behavior of Al-Zn-Mg-Cu-Zr alloys under isothermal multidirectional compression (IMC) and the mechanical properties in two-stage aging state. It was discovered that the addition of Sc enhanced the tensile and yield strengths of the alloys by 3.8% and 4.3%, respectively, and simultaneously increased the elongation from 13.4% to 15.4%. The OM, EBSD and TEM techniques were used to characterize the microstructure of the alloy. The results showed that the Al3(Sc,Zr) phase produced by the addition of 0.25% Sc not only refines the grains, but also changes the recrystallization behavior of the alloy by pinning the dislocations and (sub) grain boundaries in the combination of IMC deformation. It inhibits the generation of geometric dynamic recrystallization (GDRX) grains and the nucleation of discontinuous dynamic recrystallization (DDRX) grains but promotes the formation of continuous dynamic recrystallization (CDRX) as the predominant mode of dynamic recrystallization. The alloy gains a finer grain structure by simultaneously promoting the formation and inhibiting the growth of recrystallized grains. As a result, the improvement in the overall mechanical properties of Al-Zn-Mg-Cu-Sc-Zr alloys brought about by the addition of Sc can be attributed primarily to the refinement of the initial as-cast grains, as well as fine-grain strengthening due to the inhibition of recrystallized grain growth and the Orovan strengthening of the nanoscale secondary phase Al3(Sc,Zr) itself.

Mechanism of dynamic recrystallization of a FeCrAl alloy during hot compression

FeCrAl alloys have the opportunity to replace current Zr-based cladding materials in light water reactors, and dynamic recrystallization (DRX) plays an important role on microstructural controlling of FeCrAl alloys during hot or warm deformation, such as forging, hot rolling, extrusion. In this research, dynamic recrystallization mechanism of FeCrAl alloy has been systematically investigated by microstructure characterization after isothermal compression. Results show that deformation strain rate and temperature could significantly influence microstructure and DRX mechanism of FeCrAl alloy. When deformed at 0.1s−1, prior grains become long strip with serrated boundaries and are refined by geometric dynamic recrystallization (GDRX). Prior grains are not separated by high angle grain boundaries (HAGB) even deformed at 1200 °C, because strain is too small to impingement of HAGB. Whereas, when deformed at 10s−1 strain rate, there are lots of sub-boundaries with about 2° misorientation within grains, and several strain-free grains are found located at boundaries when deformed at 900 °C or 1000 °C, while the prior grains are divided by HAGB and equiaxed grains are formed at 1100 °C. It was found that mechanism of DRX changes from discontinuous dynamic recrystallization (DDRX) to continuous dynamic recrystallization (CDRX) with increasing temperature from 900 °C to 1100 °C. The phenomenon results from the dislocation movement behaviors changes with increasing temperature. This research will provide meaningful guidance for industrial application of FeCrAl alloys.

Effects of recrystallization and element diffusion behavior on interfacial bonding quality and mechanical properties of aluminum laminated composites

For aluminum laminated composites (ALCs), interlayer bonding strength plays a crucial role in its performance. In this work, 7050/Al/7A52 and 7A62/Al/7A52 ALC were studied to explore the effects of recrystallization and element diffusion on interfacial bonding quality and mechanical properties. The results show that attributed to the deformation localization occurs at the 7A52/pure Al interface, continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX) and geometric dynamic recrystallization (GDRX) coexist, while only DDRX exists at the bonding interfaces of 7050/pure Al and 7A62/pure Al. Among them, 7A62/pure Al interface has the most obvious element diffusion behavior among the three kinds of interfaces. Notably, recrystallization would have a higher effect on improving bonding quality, which causes the 7A52/pure Al interface of the composites more difficult to delaminate during the tensile test. Besides, the enhancement of element diffusion depth at the 7A62/pure Al interface promotes the bonding strength of 7A62/Al/7A52 ALC to increase by 21.0% compared with that of 7050/Al/7A52 ALC. After the dynamic impact test, the peak flow stress and energy absorption value of 7A62/Al/7A52 ALC reached 567.1 MPa and 72.2 MJ/m3, and the interface remains intact. However, due to the existence of huge strength deviation in the 7A62/pure Al interface, it is more prone to fail during the tensile process, resulting in a decrease in the elongation of the composites.

Insight on ultrasonic effect of Al[sbnd]Cu aluminum alloy joints fabricated by ultrasonic assisted non-thinning and penetrating friction stir welding

AlCu alloy joints of ultrasonic assisted non-thinning and penetrating friction stir welding (U-NTPFSW) and NTPFSW were prepared. The microstructure and mechanical properties of the joints were studied comparatively to elucidate the effect of ultrasonic. This study provided an in-depth understanding of the microstructure formation mechanism and strengthening mechanism of joints under the ultrasonic effect. Ultrasonics broadened the width of the stir zone (SZ) and stationary shoulder affected zone (SSAZ) by promoting the material plastic flow ability and changing the material flow at the weld root. The ultrasonic addition enhanced plastic heat production and promoted grain growth in SZ. Compared with NTPFSW, the SZ of both joints was dominated by continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms, and geometric dynamic recrystallization (GDRX) grains were also present in U-NTPFSW. The mechanical properties of the joint were improved from 339 MPa to 348 MPa. Ultrasonics introduced numerous vacancies into the weld, which promoted the precipitation of the Al2Cu phase and increased the precipitated phase density of the thermal-mechanical affected zone (TMAZ). The interaction mechanism between the precipitated phase and dislocations was analyzed. The geometrically necessary dislocation (GND) density and residual stress of TMAZ were reduced due to ultrasonic vibration.

Hot Deformation Behavior and Microstructural Evolution of the As-rolled 7050Al Alloy

The hot deformation behavior of the as-rolled 7050 aluminum alloy was investigated using isothermal hot compression experiments with temperatures ranging from 300 to 460 °C and strain rates ranging from 0.001 to 1 s−1, up to a total strain of 1.1. A strain-compensated Arrhenius (SCA) model was established based on the experimental flow stress data. The predictive accuracy of the model was numerically evaluated using the Correlation Coefficients (R) and the Average Absolute Relative Error (AARE). Electron backscatter diffraction (EBSD) was used to characterize the effects of strain rate and compressive temperature on the microstructure evolution and recrystallization behavior. The results indicated that the geometric dynamic recrystallization (GDRX) occurred at high strain rates or low temperatures, while continuous dynamic recrystallization (CDRX) became the dominant recrystallization mechanism with increasing deformation temperature and decreasing strain rate. The work hardening effect was enhanced during the later stage of deformation, and discontinuous dynamic recrystallization (DDRX) became the main recrystallization mechanism.

Heterogeneous microstructural evolution during hydrodynamic penetration of a high-velocity copper microparticle impacting copper

Microparticle hydrodynamic penetration (HDP) may be associated with the erosion regime in cold spray processing and other high-velocity impact events. Here, in an experimental approach where we can individually launch particles and study the impact sites, we explore copper microparticles impacted on copper substrates at velocities above 900 m/s where HDP begins. We lift cross-sectional lamellae from the impact sites with a focused-ion beam for further microstructural characterization using electron backscatter diffraction and scanning transmission electron microscopy. Due to the gradients of strain, strain rate, and temperature associated with HDP, heterogeneous microstructures result. The structural evolution processes observed include deformation twinning and multiple dislocation-mediated grain recrystallization mechanisms—geometric dynamic recrystallization (gDRX), discontinuous DRX (dDRX), and meta DRX (mDRX). The higher strains at the interface lead to the most significant structural changes and complex mechanisms. In contrast, there is a gradient to more conventional dislocation plasticity away from the interface (on either the particle or substrate side). These microstructural observations are consistent with the deformation map for copper and extend the observations of impact-induced recrystallization across new regimes of behavior.

Dynamic recrystallization behavior of selective laser melted 316L stainless steel

The present work deals with the hot deformation behavior of selective laser melted 316L stainless steel with an emphasize on the involved recrystallization mechanisms. Toward this end, a series of hot compression test have been conducted in the temperature range of 700–1000 °C under the various strain rates of 0.001, 0.01 and 0.1s−1. The flow pattern analysis through Jonas-Poliak method verifies the occurrence of dynamic recrystallization in whole investigated temperature range. The intensified substructure development and continuous recrystallization is identified as the main softening mechanism in the course of thermomechanical processing. This causes appearance of a broad peak and low softening fraction in corresponding flow curves. The cellular substructure in the as-printed state facilitates low-angle into high-angle boundaries conversion, which provide a proper condition for the occurrence of continuous mechanisms and increases the rate of recrystallization. However, detailed investigation of the sub-boundary maps indicates the contribution of discontinuous and geometric dynamic recrystallization mechanisms. Dynamic recrystallization causes substantial refinement; and the percentage of the grain holding size lower than 10 μm increases from 2 % in printed microstructure to 16.8 % and 24 % in the microstructure deformed at 700 and 1000 °C.

Mechanisms of macrozone elimination and grain refinement of near α Ti alloy via the spheroidization of the Widmannstätten structure

Regions with sharp local textures, i.e., macrozones, are common microstructural inhomogeneities in titanium (Ti) alloys that significantly reduce the fatigue resistance of Ti alloy parts. In this study, a two-stage thermomechanical operation was developed to eliminate macrozones and optimize microstructure in a near α Ti alloy. Multiscale microstructure characterization and full-field crystal plasticity (CP) modeling were conducted to uncover the underlying mechanisms. Spheroidizing through dynamic recrystallization (DRX) and superplasticity are indispensable ingredients. The alloy exhibited multiple DRX phenomena, which are strongly associated with grain morphologies. The discontinuous DRX and the main spheroidizing mechanism of the equiaxed primary α phase failed to eliminate macrozones but exacerbated microstructure heterogeneities due to the strong anisotropy of the hexagonal close-packed α phase. The Widmannstätten structure (WS), fabricated through the β heat treatment that generated abundant nano-defects and serrated grain boundaries, significantly promoted the occurrence of geometric dynamic recrystallization (GDRX). A complete GDRX of lamellar α grains can generate a sufficiently fine-grain structure to achieve superplastic deformation in the subsequent hot compression at a moderate deformation. The full-field CP simulations considering slip only versus slip + superplasticity confirmed that the dislocation slip dominated deformation aggravated deformation heterogeneity and the formation of macrozones. In contrast, the superplastic deformation suppressed the heterogeneous deformation and randomized the grain rotation, which in turn homogenized the microstructure and eliminated macrozones. Based on these observations, the β heat treatment followed by α hot compression is an effective method to achieve grain refinement and macrozone elimination of Ti alloys.

Optimization of numerical parameters and microstructure evolution on 2195 Al–Li alloy extrusion process

The present study designed a set of extrusion dies with unequal bearing lengths to investigate the extrusion deformation behavior of 2195 Al–Li alloy. With the standard deviation of the velocity field (SDV) and extrusion force as optimization objectives, the extrusion process parameters (billet temperature, die temperature and extrusion speed) were optimized using Taguchi method and orthogonal test. The results indicate that billet temperature is the most significant factor affecting SDV and extrusion force, with a range of 1.287 and 7.076, respectively. Furthermore, the optimal extrusion parameters were determined as billet temperature 420 °C, die temperature 410 °C and extrusion speed 0.2 mm/s, resulting in the SDV value of 0.3345 and the extrusion force of 17.059 MN. The grains on the extruded profile were flattened and elongated both in the extrusion direction and cross-section, showing obvious <111>∥TD and <100>∥TD preferred orientation, and discontinuous dynamic recrystallization (DDRX) and geometric dynamic recrystallization (GDRX) mostly occurred, but no significant differences in grain size were observed, thus verifying the accuracy of the numerical simulation. Additionally, the ultimate tensile strength, yield strength and elongation of the extruded profile reached 437.1 MPa, 345.9 MPa and 11.7%, respectively.

Microstructure evolution and texture development during production of homogeneous fine-grained aluminum wire by friction extrusion

This study aims to understand the microstructure evolution and texture development during friction extrusion of aluminum alloys, focusing on AA7075 as exemplary alloy system. Electron backscatter diffraction technique has been employed to obtain crystallographic data from various regions in front of the die and in the wire. It can be deduced that the combination of continuous dynamic recrystallization and geometric dynamic recrystallization mainly govern the formation of a fine-grained structure, however discontinuous dynamic recrystallization may also play a role at high temperature. The global shear deformation during the process was characterized as a simple shear deformation with dominant B/B¯ simple shear texture components. The material flow is mainly driven by the in-plane shear strain and the extrusion-induced shear strain that are determined by die rotational speed and extrusion force, respectively. The in-plane shear strain strongly affects the formation of a homogeneous fine-grained microstructure in the aluminum wire. In this regard, a novel material flow model for friction extrusion has been proposed.

Hot Deformation Behavior and Dynamic Softening Mechanism in 7B50 Aluminum Alloy.

The hot deformation behavior and dynamic softening mechanism of 7B50 aluminum alloy were studied via isothermal compression experiments in the range of 320-460 °C/0.001-1.0 s-1. According to the flow curves obtained from the experiments, the flow behavior of this alloy was analyzed, and the Zener-Hollomon (Z) parameter equation was established. The hot processing maps of this alloy were developed based on the dynamic material model, and the optimal hot working region was determined to be 410-460 °C/0.01-0.001 s-1. The electron backscattered diffraction (EBSD) microstructure analysis of the deformed sample shows that the dynamic softening mechanism and microstructure evolution strongly depend on the Z parameter. Meanwhile, a correlation between the dynamic softening mechanism and the lnZ value was established. Dynamic recovery (DRV) was the only softening mechanism during isothermal compression with lnZ ≥ 20. Discontinuous dynamic recrystallization (DDRX) becomes the dominant dynamic recrystallization (DRX) mechanism under deformation conditions of 15 < lnZ < 20. Meanwhile, the size and percentage of DDRXed grains increased with decreasing lnZ values. The geometric dynamic recrystallization (GDRX) mechanism and continuous dynamic recrystallization (CDRX) mechanism coexist under deformation conditions with lnZ ≤ 15.

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.

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.

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.