Dynamic Recrystallization Grains Research Articles

Influence of microstructure evolution on hot ductility behavior of a Cr and Mo alloyed Fe-C-Mn steel during hot deformation

Cr and Mo alloyed Fe-C-Mn steel was conducted by hot tensile experiments under different strain rates and deformation temperatures. The influence of microstructure evolution on fracture mechanism was studied. The principal failure mechanism was attributed to intergranular crack propagation and microvoid aggregation when deformed at 800-1100 ℃, changing to a mixed ductile-brittle intergranular fracture when deformed at 1200 ℃. After hot deformation, the microstructure transformed from a mixed phase of pearlite and ferrite to a lath martensite structure. Moreover, the lath martensite structure transitioned from disorder to order as the deformation temperature increases. The study revealed that in case of lower temperature when dynamic recovery (DRV) was dominated, the growth of voids was impeded by fine dynamic recrystallization (DRX) grains, resulting in an irregular void morphology. In case of higher temperature, the degree of DRX was high, voids were surrounded and impeded by newly formed DRX grains, which were gradually elongate with deformation. On the other hand, when there were DRX grains present between voids, void coalescence was less likely to occur, thereby inhibiting damage propagation.

Study of the Dynamic Recrystallization Behavior of Mg-Gd-Y-Zn-Zr Alloy Based on Experiments and Cellular Automaton Simulation

The exploration of the relationship between process parameters and grain evolution during the thermal deformation of rare-earth magnesium alloys using simulation software has significant implications for enhancing research and development efficiency and advancing the large-scale engineering application of high-performance rare-earth magnesium alloys. Through single-pass hot compression experiments, this study obtained high-temperature flow stress curves for rare-earth magnesium alloys, analyzing the variation patterns of these curves and the softening mechanism of the materials. Drawing on physical metallurgical theories, such as the evolution of dislocation density during dynamic recrystallization, recrystallization nucleation, and grain growth, the authors of this paper establish a cellular automaton model to simulate the dynamic recrystallization process by tracking the sole internal variable—the evolution of dislocation density within cells. This model was developed through the secondary development of the DEFORM-3D finite element software. The results indicate that the model established in this study accurately simulates the evolution process of grain growth during heat treatment and the dynamic recrystallization microstructure during the thermal deformation of rare-earth magnesium alloys. The simulated results align well with relevant theories and metallographic experimental results, enabling the simulation of the dynamic recrystallization microstructure and grain size prediction during the deformation process of rare-earth magnesium alloys.

Improved hardness of Mg-0.5Ni-xY alloys via grain refinement and formation of LPSO structures

The effect of yttrium addition on the grain refinement, formation of long period stacking ordered (LPSO) structures, and improved hardness in hot extruded Mg99.5-xNi0.5Yx (x = 0, 0.5, and 1 at%) alloys was investigated. It was revealed that with increasing Y content, the volume fraction of the LPSO phase (VLPSO) increased, and the average dynamic recrystallization (DRX) grain size (D) was refined. It was shown and quantified that both VLPSO and D contribute to the improvement of hardness, where the deviation from the Hall-Petch relationship was ascribed to the strengthening effect of VLPSO. The equation of H = 36.7 + 0.64 VLPSO + 35/√D was proposed to quantify the synergistic effects of hardening mechanisms to predict the hardness of Mg99.5-xNi0.5Yx alloys after deformation by hot extrusion.

Microstructure evolution of ATI718 plus alloy during high-speed machining: Experiments and a combined FE-CA approach

Excellent surface integrity is an eternal pursuit in high performance manufacturing, with microstructure being a crucial component of the surface integrity dataset and a key factor controlling surface properties such as fatigue and creep. The multi-physical fields generated by thermo-mechanical loads during high-speed machining act on the processed surface layer, influencing the evolution of microstructures. To investigate the microstructural evolution mechanisms of ATI718 plus during high-speed machining, cutting experiments and techniques such as Electron back scatter diffraction (EBSD), Transmission Kikuchi diffraction (TKD), and Precession electron diffraction (PED) is conducted to quantitatively analyze the microstructures in the chip shear zone and the machined surface. Subsequently, a combined finite element (FE) and cellular automata (CA) model is developed to simulate the microstructure evolution during the cutting process. The discontinuous dynamic recrystallization (DDRX) mechanism is employed to demonstrate the nucleation and growth of grains under the influence of multiple physical fields. The simulation and experimental results show similar dynamic recrystallization (DRX) grain sizes, indicating acceptable accuracy of the CA model in terms of DRX grain size. The comparison between experimental and simulation results confirms the occurrence of both continuous dynamic recrystallization (CDRX) and DDRX during the cutting process. The synergistic competition between CDRX induced grain lamellar refinement and DDRX induced grain growth emerge as the primary mechanism driving microstructural evolution. A layer of ultrafine grains, with a thickness within 20 μm, is formed on the machined surface. Results under different parameters demonstrate that the temperature has a more significant impact on the thickness of the ultrafine grain layer and the diameter of grains within the layer compared to the strain rate.

Improved ductility of hot extruded Mg-0.5Zn-0.5Zr-1RE alloy by Li addition

Mechanical properties of lean Mg-xLi-1RE-0.5Zn-0.5Zr alloys after hot working by the extrusion process were systematically investigated. By increasing Li content, the coarsening of the dynamic recrystallization (DRX) grain size (D) was recorded. This was ascribed to the decrease of the solidus temperature (TSolidus) by increasing Li content, and hence, the increase of the homologous temperature during hot deformation (T/TSolidus). As a result, the tensile yield stress (TYS), ultimate tensile strength (UTS), compressive yield stress (CYS), and ultimate compressive strength (UCS) decreased by increasing Li content. Accordingly, the Hall-Petch relationships of TYS = 145.8 + 69/√D and CYS = 69.3 + 100/√D were proposed, where TYS, CYS, and D are expressed in MPa, MPa, and μm, respectively. However, total elongation in the tension test and fracture strain in the compression test were significantly improved and the crystallographic texture intensity decreased by increasing the Li content. These improvements were found to be quite beneficial for overcoming and breaking the strength–ductility trade-off. The mechanical behavior of these alloys was compared to other competitive alloys for lightweight applications.

Dissolution of η phase and evolution of dynamically recrystallized grains in ATI 718Plus superalloy during hot compressive deformation

The dissolution mechanism of the η-Ni3(Al0.5Nb0.5) phase during hot compression in Ni-based ATI718Plus superalloy has been investigated. The results indicate that with increasing deformation, the amount of dynamic recrystallization grains in the γ matrix increases, and the dissolution of the η phase tends to be accelerated. At the precipitation temperature of the η phase of 950 °C, the η phase area fraction decreased from 9.8% to 3.0% after deformation. The deformation promotes dislocation accumulation at the boundaries of the η phase, developing into subgrains and then evolving into DRX grains due to subgrain rotation. With increased strain, the subgrain rotations and DRX grain growths result in extrusion and subsequent fracture of the η phase. Due to strain incompatibility between the η phase and γ matrix, dislocation sustains are accumulated at their interfaces, which disrupts the η/γ semi-coherent interface, leading to localized amorphization and the decomposition of the η phase. Furthermore, dislocation surrounding the η phase can act as diffusion channels for solute elements, promoting the dissolution of the η phase and resulting in the element concentration gradient distribution from the η phase to the γ matrix.

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.

Microstructure and mechanical behavior of coal-derived reduced graphene oxide reinforced AA7075 fabricated via friction stir processing

Owing to its distinctive combination of physical and mechanical properties, coal-derived reduced graphene oxide (rGO) can be an effective and sustainable reinforcement for high strength aluminium alloys. This study explores the mechanical and metallurgical properties of friction stir processing (FSP) fabricated surface composite of AA7075 using coal-derived rGO as reinforcement. Detailed examination of reinforcement dispersion and metallurgical characteristics in the developed surface composite is carried out through optical and scanning electron microscopy (SEM). The reinforced rGO particles resulted in enhanced formation of dynamic recrystallization (DRX) grains by providing more nucleation sites as confirmed by electron back-scattered diffraction (EBSD). Moreover, the dilution of Cu-rich coarse particles and the presence of η phase finer particles in the processed zone observed through energy dispersive x-ray spectroscopy (EDS) led to a finer grain microstructure. The rGO reinforced stir zone demonstrated a significant increase of 56.97 % in microhardness value, while its Charpy impact strength increased by 200 %. The enhancement in the mechanical properties is attributed to the rGO supported grain refinement through DRX. The findings of this experimental investigation provide insights into the utilization of coal-derived rGO as reinforcement for surface composite development with an exceptional combination of mechanical and metallurgical properties.

Investigation of Microstructure and Mechanical Properties of Extruded Mg-6Bi and Mg-6Bi-1Ag Alloys.

The extruded Mg-6Bi alloy and Mg-6Bi-1Ag alloy subjected to extrusion at 300 °C with the extrusion ratio of 25:1 and die-exit speed of 2 m/min were used to investigate microstructure characteristics and mechanical behavior. The experimental results demonstrate that the bimodal microstructure, composed of coarse dynamic unrecrystallized (unDRXed) grains and fine dynamic recrystallized (DRXed) grains, was obtained after extrusion. The Ag addition can obviously promote dynamic recrystallization and average grain size. It also indicates that the dynamic precipitation is significantly promoted by Ag addition during extrusion, obtaining more fraction of the Mg3Bi2 precipitates. Moreover, the extruded Mg-6Bi-1Ag alloy has a high tensile yield strength of 304 ± 2.0 MPa, which is increased by 19% compared to the extruded Mg-6Bi alloy, and elongation of 11.0 ± 1.7%, almost the same as 11.9 ± 0.9% of the extruded Mg-6Bi alloy. This result also shows that the extruded Mg-6Bi-1Ag alloy exhibits better strain hardening capacity. Therefore, Ag exhibits an effective role in promoting dynamic recrystallization and dynamic precipitation, resulting in the enhancement of strength and strain hardening capacity of the extruded Mg-6Bi-1Ag alloy, as well as keeping high ductility.

Study on interfacial healing mechanism of a Ni-Co base dual-phase superalloy during hot-compression bonding

Hot-compression bonding (HCB) as an advanced technique for preparation of large-size homogenized components is widely applied. Despite the fact that interfacial healing mechanisms during HCB have been extensively investigated, the interfacial healing mechanisms in duplex superalloys still need to be clarified. This article investigates effect of deformation temperature on multiple interface (γ′-γ′, γ-γ, γ-γ′) healing mechanism of dual-phase superalloys. The healing mechanism of γ-γ interface at different temperatures involves a necklace-like distribution of discontinuous dynamic recrystallization (DDRX) grains occupying original interface. With increasing deformation temperature, the healing mechanism of γ′-γ′ interface transforms from DDRX to diffusion bonding due to weakening of strain concentration. Simultaneously, as deformation temperature increases, the deformation mechanism of primary γ′ phase transforms from shearing by stacking fault to dislocation pairs. At deformation temperature 1100 °C, the healing mechanism of γ′-γ interface is heterogeneous epitaxial recrystallization (HERX) induced by primary γ′ phase driven by diffusion of Cr element in dislocation pairs. However, as deformation temperature decreases, healing mechanism in γ′-γ interface transforms into DDRX. Although interfacial healing mechanism is transformed at different deformation temperatures, the synergistic action of different recrystallization mechanisms ensures that mechanical properties of joints reached the same level as those of base material.

Study on the hot deformation behavior and microstructure evolution of as-forged GH3625 alloy

GH3625 alloy is widely used in aviation, nuclear power and other fields of hot section components. However, the high alloying level, significant deformation resistance, and narrow hot working window of the alloy bring great challenge to the plastic deformation of the alloy. The hot deformation behavior and microstructure evolution of as-forged GH3625 alloy were studied by hot compression tests with deformation temperatures of 930–1180 °C, strain rates of 0.001–1 s−1 and true strain of 0.92. The results show that the flow stress of the alloy exhibits noticeable characteristics of temperature-negative sensitivity and rate-positive sensitivity. The true stress-strain curves are of two types: dynamic recovery (DRV) and dynamic recrystallization (DRX). The strain-compensated Arrhenius constitutive model was used to predict the alloy's flow behavior, with a linear fitting correlation coefficient (R) of 0.9872 and an average relative error (AARE) of 8.92%. The hot processing map of the alloy based on Prasad was constructed and superimposed. It was found that there were two instability regions, which were located in the low temperature and high strain rate region and the high temperature and low strain rate region respectively. The optimal processing region was 1154∼1180 °C/0.75–1 s−1 combined with the true stress-strain curves and the microstructure. Continuous dynamic recrystallization (CDRX) at subgrains rotation and discontinuous dynamic recrystallization (DDRX) at grain boundaries were identified during thermal deformation. As the η value decreases, the dynamic recrystallization grain size decreases under the deformation condition of 930–1180 °C/1 s−1, and the microstructure evolution mechanism changes from DDRX→ DDRX + CDRX→ DDRX + CDRX + DRV → CDRX + DRV, and ∑3 TBs also reduces.

Influence of Processing Temperature and Strain Rate on the Microstructure and Mechanical Properties of Magnesium Alloys Processed by Single-Pass Differential Speed Rolling

Magnesium-based alloys show significant promise for widespread applications owing to their lightweight nature and improved mechanical properties achieved through grain refinement via hot rolling. This investigation focuses on Mg-xAl-yCa-zMn (AXM alloys), pre-heated to temperatures of 350, 400, and 450 °C and processed via both single-pass differential speed rolling (DSR) and conventional rolling (CR). The key findings reveal the interplay between processing temperature, strain rate during single-pass rolling, and an innovative approach for incorporating varying amounts of Ca, influencing grain size, quantity of dynamic recrystallization (DRX) grains, and overall mechanical properties, including strength and ductility. A noteworthy observation is the positive correlation between an increase in the total reduction during hot rolling and a higher fraction of DRXed grains. This leads to a significant reduction in average grain size, diminishing from 60.3 ± 54.3 μm to 19.5 ± 14.2 μm at 40%, nearly a third the size of T4 grains (the initial homogenized microstructure of the AXM alloys). The resultant material strength experiences a doubling from an average of 125 ± 10.2 MPa (T4) to 260 ± 25.8 MPa (DSR rolled at 40%) for the AXM alloys with potential improvement in the ductility depending on rolling speed conditions. This study also aims to analyze the combination of rolling temperature, rolling speed, thickness reduction, speed difference and Ca content implemented across a wide range of temperatures and strain rates to provide a holistic approach to the processing parameters affecting the microstructure and mechanical properties of AXM alloys. Furthermore, this study provides a deeper understanding of DRX mechanisms, including continuous DRX (CDRX), discontinuous DRX (DDRX), and twinning induced DRX (TDRX), while each of these mechanisms plays a distinct role in the overall enhancement of formability and performance of magnesium alloys.

Improving strength-ductility of Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr magnesium alloy due to bimodal LPSO and dislocations

Rare-earth (RE) magnesium alloys have attracted lots of attention due to their excellent mechanical properties. In this work, the microstructure and mechanical properties of as-extruded 8.5Gd-4.5Y-0.8Zn-0.4Zr magnesium alloy under different solution treatment were examined with the optical microscope (OM), scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM), electron back-scattered diffraction (EBSD) and Instron testing machine. The results show that the ES12 alloy (solution treatment for 12 h at 520 °C) has the highest ultimate tensile strength (UTS) of 390 MPa with a fracture elongation of 24.5% at the cost of a minor drop in yielding strength (YS) compared to the as-extruded alloy. During solution treatment, the block-shaped long period stacking ordered (LPSO) in as-extruded alloy evolves into plate-shaped LPSO, which disperses at grain boundaries (GBs), and lamellar LPSO, which distributes in grains. The coexistence of plate-shaped and lamellar LPSO, which impedes the dislocations movement, and the activated <c + a> dislocations are regarded as the primary reasons for mechanical properties improvement. Furthermore, the (11–21) <1–100> texture in as-extruded alloy transforms into the (11–20) <0001> texture in ES12 alloy. The average grain size increases from 3.45 μm in as-extruded alloy to 18.70 μm in ES12 alloy. The Schmid factors of {0001} <11–20>, {10-10} <11–20>, {10–11} <11–20>, and {11–22} <11–23> increase, which indicate that slip systems are more easily activated in plastic deformation. The dynamic recrystallization (DRX) grains fraction increase to 92.8% for ES12 alloy due to the particle-stimulated nucleation (PSN) mechanism triggered by block-shaped and plate-shaped LPSO. The freshly DRXed grains further weaken the texture, and reduce the dislocation density. All of these factors increase elongation of RE magnesium alloy.

Research on the Hot Deformation Process of A100 Steel Based on High-Temperature Rheological Behavior and Microstructure.

To obtain the optimal hot deformation process, the rheological and dynamic recrystallization behaviors of A100 steel were researched through isothermal compression tests. Firstly, a Hensel-Spittel constitutive model was established based on the stress-strain curves. Secondly, dynamic recrystallization percentage and grain size models were established to identify the necessary conditions for complete dynamic recrystallization. Finally, microstructural analysis was employed to validate the accuracy of the recrystallization model. The results indicate that the flow stress is highly sensitive to both the strain rate and the temperature, and the HS model demonstrates a high predictive accuracy, with a correlation coefficient of 0.9914. There exists a contradictory relationship between decreasing the average grain size and increasing the recrystallization percentage. The higher the percentage of dynamic recrystallization, the larger the average grain size tends to be. This situation should be avoided when devising the actual processing procedures. The optimal hot working processes for achieving complete dynamic recrystallization and a smaller average grain size are as follows: a strain equal to or greater than 0.6, a temperature between 1193 and 1353 K, and a strain rate between 0.1 and 1 s-1.

Revealing the microstructure evolution mechanism and mechanical responses of a novel Al–Zn–Mg–Cu alloy by hot deformation process

The effect of hot rolling with different rolling deformations on the microstructure evolution and improvement of mechanical properties of the novel Al–Zn–Mg–Cu alloy are investigated via optical microscope (OM), scanning electron microscope (SEM), electron backscattered diffusion (EBSD) and transmission electron microscope (TEM). The findings showed that the equiaxed grains with low dislocation density gradually elongate to become fibrous with high dislocation density as hot rolling deformation growing, and the dynamic dissolution of precipitations occurs at the grain boundaries of small dynamic recrystallization (DRX) grains. Dislocation entanglement is formed when a high density of dislocation is clearly visible following various rolling deformations. Dislocation cells could evolve into sub-grains, and ultimately become recrystallized grains as a result of continuous energy absorption. The percentage of low angle grain boundaries (LAGBs) with varying rolling deformation increases swiftly then declines slightly which linked to more deformed grains and sub-grains, as well as less recrystallized grains. High angle grain boundaries (HAGBs), however, show the opposite pattern. Ultimate tensile strength (UTS) and yield strength (YS) of the alloys reach peaks at deformation of 90% (UTS: 620.0 MPa, YS: 583.7 MPa). Which is predominantly attributed to the texture enhancement, grain refinement, and pinning effect of Al3(Sc, Zr, Ti) particles of L12 structure on the grain boundaries and dislocations.

Elucidating the warm compression of CoCrCuFeNi high entropy alloy: Modeling and microstructural evolution

The warm compression tests were carried out at various temperatures of 400, 550, and 700 °C and strain rates of 0.01, 0.001, and 0.0001 s−1 to unravel the warm deformation behavior of CoCrCuFeNi high entropy alloy (HEA). The microstructural evolutions were characterized using x-ray diffraction (XRD), differential scanning calorimetry (DSC), a field emission gun scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS), and electron backscattered diffraction (EBSD) detector. These microstructure studies show a dendritic structure containing a bright phase rich in Cu and a darker phase rich in Fe, Cr, and Co. The solidification and melting points of the bright phase are 1136 °C and 904 °C, respectively and the dark phase nucleates at 1365 °C based on DSC results. A Zener-Hollomon parameter was employed to model and predict the warm compression behavior by calculating the activation energy (∼451 kJ/mol), developing a precise constitutive equation (correlation coefficient: R = 0.99), and constructing a processing map for determining the stable temperature and strain rate (strain rates lower than 10−2.75 s−1 and temperatures between 400 and 700 °C) of the plastic flow behavior of the alloy. The FESEM results confirmed the stability and instability domains predicted by the processing map. Moreover, the EBSD analysis indicated that the continuous dynamic recrystallization (CDRX) grains nucleated in the strain rate of 0.0001 s−1 at temperatures higher than 400 °C. Larger grains were formed at 550 and 700 °C due to lower Z parameters in higher temperatures.

Effect of induced screw twisted strain on the twin orientation regulation and the ductility of pre-twined ZM21 Mg alloys

A novel twist strain-induced twin orientation regulation (TITOR) has been developed to modify the basal texture of Mg alloys. Firstly, the {10–12} tensile twins are pre-set by pre-compression on the ZM21 Mg alloy along the extrusion direction (ED), and then the TITOR process is adjusted to regulate the twins’ orientation. The results show that the orientation of some twins deviated by 30° from the ED direction to the transverse direction (TD) direction after introducing twist strain. The introduction of multiple passes of twist strain will result in almost all grains transforming into dynamic recrystallization (DRX) grains. And the generation of DRX behavior will be updating the microstructure. The mechanism in the TITOR process is continuous dynamic recrystallization (CDRX), which does not alter the orientation of the grains. When the PT samples undergo multiple passes TITOR treatments, the orientation of the DRX grains shifts towards the twisted plane (grain c-axis deviates by ∼ 30° from ED to TD direction). The formation of twist texture results in a larger value of SF, which is beneficial for the basal slip. Compared with the extruded Mg alloys, the TITOR-8P sample fracture elongation (FE) has been increased from 14.3 % to 30.6 % due to the promotion of basal slip activities and grain refinement. The ultimate tensile strength (UTS) has been increased from 243.5 MPa to 375.5 MPa. The newly proposed TITOR scheme is of great significance for promoting the application of Mg alloys.

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.

Fracture behavior of AZ31B magnesium alloy under high-temperature ultrasonic-assisted shearing with related ductile-brittle transformation mechanism

A wide width is one of the fundamental requirements for magnesium alloy sheets to fulfill the lightweight needs of the aerospace and rail transportation industries. Nevertheless, ensuring the dimensional integrity of wide-format magnesium alloy sheets is a challenge in most manufacturing facilities. Simultaneously, processing operations are susceptible to encountering production issues, including but not limited to significant loss of raw materials, elevated costs, environmental contamination, and safety risks. Therefore, to fulfill the operational demands of the hot rolling finishing line for wide sheets made of magnesium alloy, the present study employed ultrasonic-assisted shearing (UAS) to carry out a high-temperature shearing test investigation on the AZ31B magnesium alloy sheet. In this research, we employed scanning electron microscopy, optical microscopy, and a micro-hardness tester to investigate the relationship between ultrasonic amplitude and fracture behavior, deformation mechanism, and mechanical properties of magnesium alloy sheets following exposure to ultrasonic vibration. The fracture morphology of the sheet fracture demonstrates that the behavior of the magnesium alloy sheet fractures under ultrasonic vibration changes from ductile fracture under conventional shearing to brittle fracture under ultrasonic-assisted shearing. The primary cause of this transformation between ductile and brittle is the effect of ultrasonic vibration on the material's internal energy. High-energy ultrasonic amplitude action causes magnesium to absorb a significant amount of ultrasonic energy. This results in an increase in the number of dynamic re-crystallization grains in the material organization, a decrease in the number of twins, and significant refinement of the grains. The sheet's hardness also changes along the rolling and normal directions, with a uniform distribution. The magnesium alloy sheet's fracture quality is optimal when the ultrasonic amplitude is 10 μm in the UAS process. This treatment results in notable enhancements in the section's straightness and brightness. Our study aims to advance the mass manufacturing of wide-format magnesium alloy sheets by investigating a cost-effective and high-yield technique for shearing magnesium alloy sheets.

Grain boundary migration behavior and microstructure evolution during multilayer additive hot-compression bonding

Multilayer additive hot-compression bonding (MAHCB) was a new technology to prepare large-size homogeneous materials, the strain hardening at the bonding interface was an urgent issue for its application. In this paper, the effect of grain boundary migration on strain hardening was studied by the MAHCB process of pure copper. The results show that prolonging the holding time can significantly promote both interfacial grain boundary migration (IGBM) and matrix grain boundary migration (MGBM). The bonding interface became tortuous due to the promotion of IGBM, accompanied by the reduction in dislocation density at the interface and the gradual elimination of interfacial defects. It occurred mainly by the evolution of the triple junction grain boundary from T-type to equal type, which was driven by the decrease in the interface energy. On the other hand, the growth of dynamic recrystallization (DRX) grains was improved with MGBM, and enlarged the low dislocation density region. With the combined effect of both, strain hardening was completely eliminated, when the strain was 0.15 and the holding time was 3 h. Then the optimal bonding properties were obtained, the average tensile-shear strength and elongation were 191.2 MPa and 212 %, and the recovery rates were 99.7 % and 98.9 %, respectively, compared to the as-cast pure Cu samples.