DRX Grains Research Articles

Hot deformation behavior and microstructural evolutions of a Mg-Gd-Y-Zn-Zr alloy prepared by hot extrusion

The hot deformation behavior of extruded Mg-9.5Gd-4Y-2Zn-0.5Zr alloy was studied through hot compression tests conducted at deformation temperatures of 350–500 °C and strain rates of 0.001–1 s−1. Under most deformation conditions, the flow stress first reached a maximum before decreasing to a stable value, demonstrating characteristics of dynamic recrystallization (DRX). The constitutive equation of the alloy was established as ε.=1.03*1011sinh0.01287*σp2.422exp(−171150/RT) through calculations and derivation. The influences of the deformation temperature and strain rate on the microstructure and DRX of the alloy during the hot compression process were investigated using electron backscatter diffraction. It was found that at temperatures between 350 °C and 450 °C, the alloy texture exhibited multiple {0001} peak rings distributed around the normal axis of the compression direction. When the strain rate was 0.001 s−1, the average equivalent grain diameter significantly increased from 3.3 μm to 22.4 μm with the rise in deformation temperature, whereas when the deformation temperature was 450 °C, the volume fraction of DRX grains noticeably decreased from 87.6 % to 13.2 % with the increasing strain rate. Further research revealed that due to the restriction of strain rate on grain boundary migration, discontinuous dynamic recrystallization and continuous dynamic recrystallization were the predominant DRX mechanisms at low and high strain rates, respectively.

Effect of Trace Bismuth on Deformation Behavior of Ultrahigh-Purity Copper during Hot Compression

The effect of trace Bi impurities on the flow stress, microstructure evolution, and dynamic recrystallization (DRX) of the ultrahigh-purity copper was systematically investigated by a hot compression test at 600 °C. The results show that the peak stress of the ultrahigh-purity copper gradually decreases with increasing Bi content. Trace Bi impurities can refine the microstructure of ultrahigh-purity copper. However, the refinement effect of 50 wt ppm Bi is much more significant than that of 140 wt ppm Bi during the hot deformation. This effect is ascribed to the higher concentration of Bi at GBs, which induces severe GB cracks that reduces the driving force for the nucleation of DRX grains. In addition, the introduction of Bi inhibits the DRX of the ultrahigh-purity copper and transforms its DRX process from the discontinuous dynamic recrystallization (DDRX) to the coexistence of DDRX and continuous dynamic recrystallization (CDRX) mechanisms.

A study on hot deformation behaviors and microstructures of 10 vol% SiCP/Al-Cu-Mg-Ag composite prepared by spray co-deposition and extrusion

Isothermal compression test is used to investigate the hot deformation behavior of spray co-deposited and extruded 10 vol% SiCP/Al-Cu-Mg-Ag composite. In this study, an accurate strain-compensated model based on Arrhenius constitutive equation is constructed with the corresponding R value of 0.9965 and AARE of 4.05%. The experimental results are in high agreement with the predicted flow stress. Hot deformation activation energy of the SiCP/Al composite (161.685 kJ/mol) is notably higher than that of Al-Cu-Mg-Ag alloy, which is attributed to the dislocations introduced by SiC reinforcement. The microstructure evolution of compressed specimens is characterized using EBSD and TEM. DRX, including CDRX and DDRX, is the main softening mechanism under the hot deformation conditions of high temperature, low strain rate and high strain. The softening mechanism transition from DRV to DRX is due to the subgrains consuming dislocations and then transforming into DRX grains. Comparing the microstructure under different hot deformation parameters, the recrystallization degree is significantly influenced by temperature, followed by strain rate and strain. In addition, the recrystallization process is impeded by the primary Al2Cu phases, T phases and SiC particles in the composite through hindering the migration of dislocations.

Dynamic recrystallization mechanisms and microstructure evolution of a novel Al-Zn-Mg-Cu-Zr alloy by isothermal compression

The dynamic recrystallization (DRX) behavior and microstructure evolution of a novel Al-Zn-Mg-Cu-Zr alloy was investigated by isothermal compression tests at temperature of 350–470 °C and strain rate of 0.0001–1 s−1. The results showed that the flow behavior revealed typical dynamic recovery (DRV) characteristics at a high strain rate (1 s−1), while exhibited a DRX shape when the strain rate decreased. The critical strains (εc) of the DRX decreased with the decrease of strain rate or the increase of deformation temperature. A low temperature was conductive to precipitation behavior, the coarse precipitates and dispersed Al3Zr particles would pin the dislocations and substructures, which inhibit the DRX process. The DRX mechanism of the Al-Zn-Mg-Cu-Zr alloy was dominated by continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) during the hot deformation process, upon increasing the temperature and decreasing the strain rate, there was a transition point of DRX mechanism from DDRX to CDRX. Moreover, the linear relationship between the Zener-Hollomon (Z) parameter and DRX grain size was established for the further exploration of the transition of the dominant DRX mechanism.

The dependence of Zener-Hollomon parameter on softening behavior and dynamic recrystallization mechanism of a biodegradable Zn-Cu-Mg alloy

The Zn-Cu-Mg alloy exhibits good strength, ductility, anti-aging and antibacterial properties, which lays the foundation for developing high performance Zn-based biodegradable alloys. However, the constitutive equation and dynamic recrystallization (DRX) behavior of this alloy remain unclear, making the optimization of hot processing parameters almost dependent on trial and error. This work aims to address these issues by investigating the hot compression process. The calculated average activation energy Q of this alloy is 141.338 KJ⋅mol-1, exhibiting excellent heat resistance. The deformed microstructure strongly depends on the Zener-Hollomon parameter (Z=ε˙exp(QRT)). Discontinuous DRX (DDRX) dominates at low lnZ, which has a significantly different orientation from the parent grain. Continuous DRX (CDRX) occurs within the grain and at grain boundaries, and is dominant at middle lnZ, mainly through activation 〈a〉 or/and 〈c+a〉 slip systems. Additionally, the activation of prismatic slip further promote CDRX, and most CDRX grains inherit the 30°[0001] orientation from the parent grains. The volume fraction of DRX demonstrates a decreasing trend followed by an increasing trend with increasing lnZ. At high lnZ, the increase of DRX grains is conducive to weakening the texture, and twin-induced DRX (TDRX) is significantly promoted, leading to an increase in both peak stress and strain hardening rate. Furthermore, the grains with c-axis aligned parallel to the compression direction (CD) are more prone to twinning, while the c-axis perpendicular to CD are the hard orientation of basal slip and compression twins. TEM results reveal that a decrease of c/a value promotes the activation of non-basal slip near the twin boundary, and the highly active 〈c〉 and 〈c+a〉 slips contribute to the increase of strain hardening rate. The results of this study are significant for understanding the workability of Zn-Cu-Mg alloys at high lnZ due to its high efficiency and low cost.

Study on dynamic recrystallization and precipitation behavior of hot compressed Al-20Zn-0.5Mg-0.5Sc alloy

The dynamic recrystallization (DRX) behavior of the Al-20Zn-0.5Mg-0.5Sc alloy was investigated under specific conditions using a Gleeble-3500 thermal simulator. The resultant grain size, recrystallization degree and precipitated phases were studied. The results indicate that the sensitivity of DRX grain size is highly affected by temperature, feebly influenced by strain and intermediately affected by strain rate. The DRX grains are grown by an average of 3.49 μm with increasing temperature. The DRX behavior is sensitive to temperature, strain and strain rate, resulting in changes in average grain size, percentage of HAGBs and degree of recrystallization under different parameters. At 400 ℃ / 40 % / 0.1 s−1, the average grain size is refined to 12.90 μm, and the corresponding HAGBs and recrystallization degree are approximately 46.2 % and 34.3 %, respectively. The Al3Sc, Zn and η (MgZn2) phases are precipitated in the hot-compressed alloy. The Al3Sc phase promotes the precipitation of the Zn and η phases. The size, amount and morphology of Al3Sc, Zn and η phases at different parameters have an important influence on the movement of dislocations and nucleation of grains, which leads to changes in DRX grain size and the degree of recrystallization. The fine precipitated phase particles Al3Sc and η inhibit the growth of recrystallized grain by pinning grain boundaries and exerting a blocking force on dislocations. Dynamic precipitation is the dominant mechanism for the changes in DRX grain size and degree of recrystallization. To achieve the refined grains and homogeneous microstructures, the optimum parameters are recommended as 400 ℃, 0.1 s−1 and 50 %.

Microstructure control and DRX characteristics of Ni–Co–W superalloys affected by changing deformation direction on [001] columnar grain

Controlling the evolution process of columnar grains is benefits to achieve microstructure regulation during subsequent hot processing in superalloys. In present research, it takes a Ni–Co–W superalloy as an example, aims to clarify the underlying connections between the compression direction and dynamic recrystallization (DRX) behaviors in microstructure evolution. The compression direction (CD) was parallel or perpendicular to [001] columnar has been defined as CD∥[001] and CD⊥[001], respectively. The columnar evolution and DRX characteristics under two sets of experimental during hot deformation were identified deeply. The results show that complete DRX is more easily to occur when CD⊥[001], but fine DRX grains are tendency to form when CD∥[001]. DRX nucleation within CD∥[001] and CD⊥[001] deformed microstructure under dislocation energy was discussed deeply. The critical size of nucleation is decreased while the nucleation density is increased in CD∥[001] with high dislocation density, which benefits to form numerous fine DRX grains along the original columnar boundaries. In addition, according to Taylor factors (TFs) criterion, TFs difference will always existed in CD∥[001], which promotes the necklace structure gradually replaced columnar structure and some of them developed into fine DRX bands. With increasing of trues strain, TFs difference gradually decreased in CD⊥[001], DRX nucleation was inhibited, thus the existed DRX grains further grow and finally coarse DRX grains were obtained. The findings clarified the flow behaviors and DRX characteristics of [001] columnar in two directions, and then proposed a microstructure control mechanism of superalloys with [001] columnar based on deformation vector and evolution decomposition.

Hot Workability and Microstructure Evolution of Homogenized 2050 Al-Cu-Li Alloy during Hot Deformation.

For this article, hot compression tests were carried out on homogenized 2050 Al-Cu-Li alloys under different deformation temperatures and strain rates, and an Arrhenius-type constitutive model with strain compensation was established to accurately describe the alloy flow behavior. Furthermore, thermal processing maps were created and the deformation mechanisms in different working regions were revealed by microstructural characterization. The results showed that most of the deformed grains orientated toward <101>//CD (CD: compression direction) during the hot compression process, and, together with some dynamic recovery (DRV), dynamic recrystallization (DRX) occurred. The appearance of large-scale DRX grains at low temperatures rather than in high-temperature conditions is related to the particle-stimulated nucleation mechanism, due to the dynamic precipitation that occurs during the deformation process. The hot-working diagrams with a true strain of 0.8 indicated that the high strain-rate regions C (300 °C-400 °C, 0.1-1 s-1) and D (440 °C-500 °C, 0.1-1 s-1) are unfavorable for the processing of 2050 Al-Li alloys, owing to the flow instability caused by local deformation banding, microcracks, and micro-voids. The optimum processing region was considered to be 430 °C-500 °C and 0.1 s-1-0.001 s-1, with a dissipation efficiency of more than 30%, dominated by DRV and DRX; the DRX mechanisms are DDRX and CDRX.

Critical condition for initiation of dynamic recrystallisation in electron beam powder bed fused Ti-6Al-4 V Alloy

Despite the unique capabilities of Additive Manufacturing (AM) processes for producing Ti components with complex geometries, the desired properties are only achievable if a holistic scheme is devised, considering the synergistic role of post-processing steps. This work aimed to enhance the applicability of metal AM products by improving the process chain in the manufacturing industry. For this purpose, the current work demonstrates a comprehensive investigation into the hot deformation of Ti-6Al-4 V (Ti64) pre-forms produced via the Electron Beam Powder Bed Fusion (EB-PBF) process focusing on the determination of critical conditions for initiating Dynamic Recrystallization (DRX) in these components. Following a sequential evaluation procedure, hot deformation experiments were carried out at temperatures ranging from 1000 to 1200 °C and strain rates of 0.001–1 s−1 to determine the critical stress and strain required for initiating DRX in Ti64 pre-forms and compare them to their wrought counterparts. In addition, a specific Finite Element Model (FEM) was coupled with DRX kinetics equations to predict the volume percentage of DRX grains during the hot deformation of the pre-forms. The analysis of flow stress curves showed a significant peak stress at low strains, which is then followed by a period of flow softening and eventually transitions into a nearly steady-state flow at higher strains. In addition, compared to their wrought counterparts, the EB-PBF samples exhibited a significantly superior flow softening behavior, as evidenced by a more significant volume fraction of DRX and faster recrystallisation rates. It was also revealed that the normalised strain for DRX initiation in the EB-PBF Ti64 and wrought one was 0.55 and 0.67, respectively. FEM results closely matched the experimental finding, confirming its reliability in providing valuable insights into microstructural evolution and offering a time-efficient alternative for process design and property optimisation, lowering dependence on trial-and-error approaches. Through a combination of experiments, numerical analysis, and finite element simulations, this study sheds light on the macroscopic deformation and microstructural transformations occurring during hot working processes.

The dual effects of phosphorus on the hot deformation behavior in an as-cast Ni-Fe-Cr based alloy

The effect of phosphorus on dynamic recrystallization (DRX) and hot deformation behavior of an as-cast Ni-Fe-Cr based was investigated by performing hot compression deformation. The results indicate that phosphorus can play dual roles during hot deformation. The phosphorus dissolving in matrix and segregating to the dislocation can hinder the dislocation motion and DRX nucleation, which acts as the solute drag effect. And phosphorus-addition can increase the size and number of MC carbides. MC carbides increase the local dislocation density around them and promote the DRX nucleation and exhibit the particle stimulated nucleation effect. Influenced by the dual effects of phosphorus, the DRX fraction and grain size decrease first and then increase with the increase of phosphorus content. The solute drag effect of the substitutional phosphorus atoms on dislocation motion decreases the DRX nucleation rate and increase the possibility of instability at low deformation temperature in phosphorus-addition alloys. In the early stage of deformation, more MC carbides in the high-phosphorus alloy hinder the motion of dislocation and result in the stress concentration, which leads to the intergranular cracks at high temperature and high strain rate.

Hot Deformation Behavior and Microstructure Evolution of a Graphene/Copper Composite.

Graphene/copper composites are promising in electronic and energy fields due to their superior conductivity, but microstructure control during thermal mechanical processing (TMP) remains a crucial issue for the manufacturing of high-performance graphene/copper composites. In this study, the hot deformation behavior of graphene/copper composites was investigated by isothermal compression tests at deformation temperatures of 700~850 °C and strain rates of 0.01~10 s-1, and a constitutive equation based on the Arrhenius model and hot processing map was established. Results demonstrate that the deformation mechanism of the graphene/copper composites mainly involves dynamic recrystallization (DRX), and such DRX-mediated deformation behavior can be accurately described by the established Arrhenius model. In addition, it was found that the strain rate has a stronger impact on the DRX grain size than the deformation temperature. The optimum deformation temperature and strain rate were determined to be 800 °C and 1 s-1, respectively, with which a uniform microstructure with fine grains can be obtained.

Continuous Dynamic Recrystallization and Deformation Behavior of an AA1050 Aluminum Alloy during High-Temperature Compression

Continuous dynamic recrystallization (CDRX) forms a new recrystallized microstructure through the progressive increase in low-angle boundary misorientations (LAGBs) during the hot forming of metallic materials with high stacking fault energy (SFE), such as aluminum alloys. The present work investigates the effect of deformation parameters on the evolution of the dynamic recrystallization microstructures of an AA1050 aluminum alloy during compression at elevated temperatures. The alloy microstructure is investigated at deformation temperatures and strain rates in the range of 300 °C to 500 °C and 0.001 to 0.8 s−1. A well-defined substructure and subsequent DRX grains provide indication that recrystallization can proceed with continued strain under high-temperature compression. At a strain rate of 0.1 s−1, the DRX fraction is observed to be 0.25 at a temperature of 300 °C. This fraction increases to 0.32 as the temperature rises to 400 °C. The recrystallization mechanism is identified by analyzing the flow stress, the evolution of the subgrain misorientation angle, and the distribution of recrystallized grains. The observations of discontinuous dynamic recrystallization (DDRX) and CDRX under various deformation parameters are discussed. Moreover, the main substructure evolution laws observed from the high-temperature compression of an AA1050 Al alloy are summarized.

Research on hot deformation behavior and microstructure evolution mechanism of GH4169 superalloy

The hot deformation behavior and microstructure evolution of GH4169 superalloy were investigated through hot compression experiments with a temperature range of 900–1100 °C and strain rates ranging from 0.01 to 5 s−1. Concurrently, the flow stress curve of the alloy was obtained and constitutive equations were established. Based on the dynamic material model, a hot working diagram was established and the hot working window was optimized. The findings demonstrate that the high-temperature softening mechanism of this alloy primarily involves dynamic recrystallization (DRX), while both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) exist. According to the hot processing map, the process parameters corresponding to the unstable areas are low temperatures and medium-high strain rates (900–985 °C, 0.15–5 s−1), and high temperatures and high strain rates (1025–1100 °C, 2.5–5 s−1). The reason is that the appearance of deformation bands and flow localization in the microstructure. The optimal hot working parameters for GH4169 are 1000 °C/0.01 s−1 and 1050–1100 °C/0.01–0.1 s−1, where the microstructure consists of uniformly distributed DRX grains. The volume fraction of DRX in GH4169 alloy rises with higher temperatures or lower strain rates. In addition, the reduction of intergranular dislocations promotes the migration of grain boundaries from low angle grain boundaries (LAGBs) to high angle grain boundaries (HAGBs).

Inhomogeneous deformation behavior and recrystallization mechanism of Mg-Gd-Y-Zn-Zr alloy containing only intragranular lamellar LPSO phase

Inhomogeneous plastic deformation and recrystallization behavior of Mg-Gd-Y-Zn-Zr alloy containing only intragranular lamellar long-period stacking ordered (LPSO) phase were investigated by isothermal compression experiments at the temperatures of 350–450 °C, and the strain rates of 0.001 ∼ 10 s−1. Results showed that Mg-Gd-Y-Zn-Zr alloys with varying degrees of bimodal structures were obtained after compression. The lamellar LPSO phase directions in residual coarse grains were aligned in the radial direction (RD). Dynamic recrystallization (DRX) grains induced at grain boundaries, kinked bands, and lamellar shearing regions during compression synergistically promoted coarse grain refinement. Moreover, the condition of high temperature and medium strain rate (450 °C-0.1 s−1) contributed to the uniform extension of "mantle layers" of fine DRX grains. In contrast, the condition of medium temperature and high strain rate (400 °C-10 s−1) could accelerate the deflection of CD-directed (compression direction) LPSO lamellae and the formation of local high-energy regions, facilitating the generation of recrystallization band. In addition, a high strain rate (400 °C-10 s−1) is more conducive to the formation of a heavily bimodal structure than a high temperature (450 °C-0.1 s−1). Only a larger spacing of LPSO lamellae (∼1.4 μm) could meet the spatial requirements of recrystallization. Further analysis revealed that the continuous dynamic recrystallization (CDRX) mechanism characterized by the expansion of "mantle layers" and nucleation along kinked boundaries and lamellar shearing regions was the primary grain refinement mechanism. The discontinuous dynamic recrystallization (DDRX) mechanism only exhibited a limited role due to the inhibition of the highly dense lamellar LPSO phase on grain boundaries bulging. After compression, the orientation of residual coarse grains gradually deviated toward the CD. While the recrystallization with limited proportion and random orientation cannot significantly weaken the basal texture.

Influences of stress triaxiality and forming parameters on microstructural evolution and fracture mechanisms in a Ni–Cr–Mo-based superalloy

Hot tensile features in a Ni–Cr–Mo-based superalloy with different notched types are explored. Changes of microstructure and fracture mechanisms with stress triaxiality and tensile parameters are elucidated. Results manifest that the maximum load for the Ni–Cr–Mo-based superalloy in hot tensile ascends with increasing stress triaxiality. The development/interaction of dislocation clusters and subgrains is sensibly boosted, while the dynamic recrystallization (DRX) formation/coarsening behaviors is suppressed with ascending stress triaxiality or strain rate. Nevertheless, the intense generation/development in substructure and DRX grains appear at higher tensile temperature. The primary fracture mode is the ductile facture closely associated with dimples formation/coalescence and the progression of serpentine gliding. The primary fracture mechanism of Ni–Cr–Mo-based superalloy is the shear fracture characterized by the microporous polymerization. The coalescence/growing up in dimples becomes weaken with reducing stress triaxiality. Additionally, the dominant texture components are the E, F and Copper textures for the Ni–Cr–Mo-based superalloy with notched type in high-temperature tensile. With increasing stress triaxiality, the S texture sensibly increases.

The strength contribution via heterostructure in a Mg-Gd-Zn-Mn alloy

Deformation by extrusion is an effective means to enhance the performance of magnesium alloys. However, the resulting microstructure heterogeneity always lead to poor plasticity. Therefore, it is crucial to explicit the impact of heterogeneous structure on the deformation mechanism of the alloys. In this work, the microstructures evolution, mechanical properties, and fracture mechanism of Mg-xGd-1Zn-0.5Mn (x = 0, 2, 4, and 6) alloys were investigated. The elongation reached its maximum (40%) when the Gd content was 2%, which was attributed to the formation of the RE-texture and grain refinement. As the Gd content was further increased, the alloy developed a heterostructure consisting of both undynamic recrystallization (UDRX) grains and dynamic recrystallization grains (DRX). The exist of the heterostructures resulted in an increased strength which was attributed to the influences of texture strengthening derived from UDRX grains, the lamellar LPSO fiber strengthening and fine grain strengthening of DRX grains. At the initial stage of deformation in the heterostructure, the basal slip of DRX grains play a dominant role, while the UDRX grains strengthens the alloy effectively. The lamellar LPSO precipitated in the UDRX grains suppressed twinning effectively. As the concentrated stresses were enough to break through the barrier for twining of LPSO, numerous twins nucleated in the deformed grain thus resulting in the nucleation of cracks. During the process of tensile deformation, twins experienced greater basal slip resolved shear stress than that of the parent grains, resulting in significant basal slip activated due to twin. Consequently, there was a high stress concentration at the twin boundaries, leading to the nucleation of cracks. The varied deformation amount also resulted in the crack initiating at the interface of DRX grain and UDRX grain. This investigation provides valuable insights into the influence of a heterostructure on the deformation mechanism of Mg-Gd-Zn-Mn alloys.

Hot-working characteristics and microstructure analysis of Zr-Sn alloy at different temperatures and strain rates

In this work, the hot-working characteristics and microstructure evolution of Zr-Sn alloy at 600–900 °C/0.01–10 s−1 were investigated. The results indicate that the flow stress of Zr-Sn alloy decreases with the increase of compression temperature and increases with the increase of strain rate. An Arrhenius constitutive equation for the Zr-Sn alloy was established, the average deformation activation energy was calculated to be 278630.1 J/mol. The processing map of Zr-Sn alloy was obtained based on the flow stress curve, revealing that the instability zone was mainly concentrated in the low temperature zone and the medium-high strain rate zone in the high temperature zone. As the temperature increases, the proportion of dynamic recrystallization (DRX) grains increases gradually. With the increase of strain rate, the proportion of DRX grains gradually decreases. Three DRX mechanisms were observed: continuous DRX (CDRX), discontinuous DRX (DDRX), and twin induced DRX (TDRX). The secondary phase of the initial grains is mainly FeCr phase, which is not completely dissolved at 650–750°C/0.01–1 s−1.

Determining the Hot Workability and Microstructural Evolution of an Fe-Cr-Mo-Mn Steel Using 3D Processing Maps.

The Laasraoui segmented and Arrhenius flow stress model, dynamic recrystallization (DRX) model, grain size prediction model, and hot processing map (HPM) of Fe-Cr-Mo-Mn steels were established through isothermal compression tests. The models and HPM were proven by experiment to be highly accurate. As the deformation temperature decreased or the strain rate increased, the flow stress increased and the grain size of the Fe-Cr-Mo-Mn steel decreased, while the volume fraction of DRX (Xdrx) decreased. The optimal range of the hot processing was determined to be 1050-1200 °C/0.369-1 s-1. Zigzag-like grain boundaries (GBs) and intergranular cracks were found in the unstable region, in which the disordered martensitic structure was observed. The orderly packet martensite was formed in the general processing region, and the mixed structure with incomplete DRX grains was composed of coarse and fine grains. The microstructure in the optimum processing region was composed of DRX grains and the multistage martensite. The validity of the Laasraoui segmented flow stress model, DRX model, grain size prediction model, and HPM was verified by upsetting tests.

Hot Deformation Behavior and Microstructure Evolution Mechanisms of Ti6Al4V Alloy under Hot Stamping Conditions.

Through the study of the thermal rheological behavior of Ti6Al4V alloy at different temperatures (500 °C, 600 °C, 700 °C, and 800 °C) and different strain rates (0.1 s-1, 0.05 s-1, 0.01 s-1, and 0.005 s-1), a constitutive model was developed for Ti6Al4V alloy across a wide temperature range in the hot stamping process. The model's correlation coefficient reached 0.9847, indicating its high predictive accuracy. Hot processing maps suitable for the hot stamping process of Ti6Al4V alloy were developed, demonstrating the significant impact of the strain rate on the hot formability of Ti6Al4V alloy. At higher strain rates (>0.05 s-1), the hot processing of Ti6Al4V alloy is less prone to instability. Combining hot processing maps with hot stamping experiments, it was found that the forming quality and thickness uniformity of parts improved significantly with the increase in stamping speed. The phase composition and microstructures of the forming parts under different heating temperature conditions have been investigated using SEM, EBSD, XRD, and TEM, and the maximum heating temperature of hot stamping forming was determined to be 875 °C. The recrystallization mechanism in hot stamping of Ti6Al4V alloys was proposed based on EBSD tests on different sections of a hot stamping formed box-shaped component. With increasing deformation, the effect of dynamic recrystallization (DRX) was enhanced. When the thinning rate reached 15%, DRX surpassed dynamic recovery (DRV) as the dominant softening mechanism. DRX grains at different thinning rates were formed through both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), with CDRX always being the dominant mechanism.

The improved physically-mechanism constitutive model for a Ni-Mo-Cr-based superalloy with the pre-precipitation of μ phase in hot forming

High-temperature deformation features of the Ni-Mo-Cr-based superalloy with the pre-precipitation μ phase are researched utilizing thermal compression experiments. For unravelling the nucleating characteristics of substructures and interacted with μ phase in hot deformation, the transmission electron microscope (TEM) is utilized. Electron backscatter diffraction (EBSD) is employed to explore the nucleating/growth mechanisms of dynamic recrystallization (DRX). An improved physically-mechanism (IPM) model is set up for reconstituting flow stresses and microstructural evolution. The investigated results demonstrate that high-density subgrains/dislocation loops can be mainly formed by dislocations entanglement and the pinned mechanism of μ phase/grain boundaries. In addition to discontinuous/continuous DRX nucleating mechanisms, the nucleating mode induced by twins and the stimulated nucleus by μ phase are also detected. The coarsen of DRX grains is substantially repressed with reducing compressed temperature or ascending strain rate, which exacerbates the inhomogeneous scatter of grains. Meanwhile, the contents of low angle grain boundaries (LAGBs) sensibly increase, owing to the sharply increase of substructures. The superior consistence of reproducing and tested results suggests the IPM model can be employed to exactly forecast the stresses, grain size as well as DRX fraction of the investigated Ni-Mo-Cr-based superalloy with the pre-precipitation μ phase.