Continuous Dynamic Recrystallization Research Articles

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 0.001s-1 to 0.1s-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-0.001s-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-0.1s-1/0.01s-1 to geometric dynamic recrystallization (GDRX) at 900 °C-0.001s-1 and then to continuous dynamic recrystallization (CDRX) at 950 °C/1000 °C-0.001s-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.

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.

Microstructure evolution and local mechanical properties of friction stir additively manufactured (FSAM) AA5083/AA6061/AA7075 gradient composite.

The current study is focused on the successful fabrication and characterization of microstructure and properties gradient composite made of 3 mm thick sheets of AA5083-O, AA6061-T6, and AA7075-T6 alloys using friction stir additive manufacturing (FSAM) technique. Two combinations of four layered FSAM builds (6756 and 7567) were fabricated successfully without any major defects at the optimized process parameters of 850 rpm (tool rotational speed) and 55 mm/min (tool transverse speed). The material mixing, interfacial bonding features, microstructure evolution, and mechanical performance within the stir zones (SZ) were thoroughly examined using advanced characterization techniques. Electron backscatter diffraction (EBSD) analysis confirmed a gradient microstructure along the build depth in both FSAM builds, with average grain sizes decreasing from 52 μm (AA5083), 46 μm (AA6061), and 40 μm (AA7075) in base metals to ∼ 4 to 7 μm within the SZ. The remarkable grain refinement within the SZ was mainly attributed to the dominant presence of the continuous dynamic recrystallization (CDRX) mechanism. Texture analysis corresponding to pole figures (PFs) and orientation distribution function (ODF) plots reveals the crystallographic texture evolution along the build depth. The prevalent existence of B/ and C textures throughout all regions of the FSAM builds affirms the presence of ample shear strain during the FSAM procedure. Through thickness miniature tensile and microhardness tests depict the gradient in mechanical performance for both the FSAM builds along the build depth. Microhardness gradients in the range from 80 HV0.1 to 145 HV0.1 were observed along the build depth. The axial sample in the build direction shows appreciable strength (265 MPa and 224 MPa) and uniform elongation (28% and 24 %) for 6756 and 7567 FSAM build, respectively. Furthermore, remarkable strain hardening behaviour corresponding to hardening capacity (Hc) and hardening exponent (n) was noticed for both the axial build samples compared to AA6061-T6 and AA7075-T6 base metals. These findings underscore the potential of FSAM in fabricating functionally gradient composite materials tailored to specific functional requirements.

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.

Effects of Er addition on the hot deformation behavior and microstructure evolution of AZ31 alloy

AZ31 magnesium alloys typically have poor room temperature ductility after initial deformation. To address this disadvantage, rare earth element Er is introduced to regulate the structure, weaken texture, and promote non-basal slip to achieve plasticization. In this study, the deformation behavior and microstructure evolution during the hot compression process of Er added AZ31 alloy were systematically investigated. Hot compression stress-strain curves showed that the addition of Er reduced the deformation resistance. The constitutive equation and processing map were established. By combining the processing map with the microstructure analysis and texture evolution, an optimal processing area was determined to be 350–400 °C at a strain rate of 0.01 s−1. The influence of Er addition on the twinning behavior, recrystallization mechanism, and slip system of the AZ31 alloy were discussed. During the deformation process, the twinning behavior violates the maximum SF criterion, which is related to severe local stress. Continuous dynamic recrystallization, discontinuous dynamic recrystallization, and twinning induced recrystallization are all detected in the alloy. Through the in-grain misorientation axis and transmission electron microscopy, it can be confirmed that the addition of Er promoted the occurrence of pyramidal <c+a> slip. This is related to the modification of stacking fault energy by Er and the hindrance effect of the second phase containing Er on the basal dislocation.

Optimization of hot-rolling parameters for a powder metallurgy Ti–47Al–2Cr–2Nb-0.2W alloy based on processing map and microstructure evolution

In this work, a two-stage rolling method with varied temperature and strain rate was proposed for powder metallurgy (PM) Ti–47Al–2Cr–2Nb-0.2W alloy sheets based on the guidance of processing map and microstructure evolution. Compared with conventional rolling at a high temperature and a low strain rate, the TiAl alloy exhibited enhanced plastic deformability through the implementation of the two-stage rolling method, resulting in a higher cumulative true strain. Microstructure analysis revealed significant grain refinement in the rolled sheet. During the first stage of hot rolling with high temperatures and low strain rates, the softening behavior caused by dynamic recovery (DRV) and dynamic recrystallization (DRX) is conducive to the plastic deformation in the second stage rolling (low temperatures and high strain rates). In addition, the formation of mechanical twins further promotes DRX behavior. The softening mechanism of α2 phase is DRV, and that of γ phase is continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX).

Effect of initial grain size on hot deformation behavior and recrystallization mechanism of Al-Zn-Mg-Cu alloy

Grain size is industrially important for improving mechanical properties and formability. In order to investigate the effect of grain size on the hot workability and deformation mechanisms, Al-Zn-Mg-Cu alloys with different initial grain sizes, 241.2, 100.4 and 48.4 μm, were subjected to hot compression tests. Using the established constitutive equation, the rheological behavior is described and the influence of the initial grain on the thermal deformation behavior is analyzed. Electron back-scatter diffraction (EBSD) analysis was used to study the dynamic recrystallization (DRX) mechanisms of Al-Zn-Mg-Cu alloys with different initial grain sizes. The microstructures obtained under various deformation conditions were analyzed. The results show that decreasing the initial grain size enhances both the peak stress and the activation energy for thermal deformation. Meanwhile, the dynamic recrystallization mechanism varies depending on the initial grain size. Continuous dynamic recrystallization (CDRX) is the primary DRX nucleation mode for coarse-grained alloys. Nonetheless, discontinuous dynamic recrystallization (DDRX) is predominant for alloys with fine grains.

Suppressing anisotropy in additive manufacturing by shear crystallographic texture development during multi-layer friction stir deposition

The solid-state additive manufacturing (AM) approach based on a strategy of sheet lamination (SL) using friction stir deposition/friction surfacing, and was investigated for multi-layer metal deposition in the present study. This resulted in synthesis of fully dense parts with a homogeneous and thermally stable fine-grain microstructure with isotropic properties by avoiding solidification-related defects and exploiting dynamic recrystallized shear crystallographic texture component. A non-heat-treatable AA5083 aluminum-magnesium alloy exhibiting serration yielding behavior was used for friction stir additive manufacturing (FSAM) solid-state deposition using a rotational speed of 1000 rpm with two working windows for the consumable rod feed rate in the range of 35-45 mm/min and a traverse speed in the field of 300-400 mm/min. The microstructural features and mechanical properties were assessed across different sections of the solid-state deposited AA5083 alloy structure to verify a pore-free structure with homogeneous and isotropic behavior. Metallography revealed the formation of uniform and refined equiaxed grains derived from continuous dynamic recrystallization (CDRX) with an average size of ∼4 μm over the entire region of the deposited wall through outstanding layers diffused adhesion. Mechanical testing of the multi-layer structure indicates isotropic tensile properties along the building direction (BD) and transverse direction, due to the shear crystallographic texture formed by thermo-mechanical processing during additive deposition process. An excellent combination of ultimate tensile strength (of 370 MPa) and elongation to fracture at ∼33% was achieved enhanced by serration yielding tensile flow behavior due to solute element/dislocation interactions during plastic straining.

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.

Substructure heterogeneity during hot deformation of ferritic stainless steels - Experimental characterization and discussion assisted by a mean-field model

The microstructure evolution of ferritic stainless steels during hot deformation is complex and needs to be finely described. Quantifying the evolution of the misorientation of low-angle boundaries depending on the thermomechanical path is a key point in controlling the recrystallization of such steels. This paper proposes an experimental methodology based on large-scale electron back-scattered diffraction (EBSD) mapping characterization using a Symmetry 2 camera to track the low-angle boundary evolution. Different thermomechanical paths, from 900 to 1100 °C with strain from 0.3 to 0.9, were studied using uniaxial compression tests. Flow stress follows a usual hardening stage, followed by a slight softening regime, mostly attributed to continuous dynamic recrystallization. Distributions of low-angle boundary populations (density vs sub-grain size) are quantified by two populations either near grain boundaries or inside grain bulk; the proportion of sub-grains at grain boundaries increases with strain. This bimodal distribution is made with a probabilistic Gaussian mixture model. These results are discussed using a modified mean-field model of Gourdet-Montheillet. This model is insufficient to capture the transient stage of continuous dynamic recrystallization unless its set of parameters is adjusted to accommodate the saturation of the density of low-angle boundaries near grain boundaries. This saturation is a result of the low-angle boundary density gradients in the grains.

Effect of welding speed on microstructural evolution and strengthening mechanism of friction-stir welded 7075 aluminum

In present study, we aimed to analyze the microstructural characteristics and strengthening mechanism of AA7075-T6 alloy after the friction-stir welding speeds from 150 to 350 mm min-1. The continuous dynamic recrystallization underwent actively with higher welding speed, leading to equiaxial and fine grains, as evidenced by misorientation distribution, grain orientation spread, and aspect ratio. Regarding the textural evolution of the stir zone, the weld at lower welding speed retained the deformation texture (e.g., Brass), while the higher welding speed showed the development of recrystallization texture (e.g., Goss). The weld at 250 mm min-1 revealed superior weldability for strength of over ∼80% owing to both grain refinement and accumulated dislocations despite the dissolution of precipitate such as ηˊ.

Microstructure, mechanical properties, and strengthening mechanisms of ultra-high strength Al-Zn-Mg-Cu alloy prepared by continuous extrusion forming process

In recent years, the strategy of obtaining ultra-high strength bulk Al-Zn-Mg-Cu alloys materials through implementing severe plastic deformation (SPD) has emerged as the prominent trend in structural lightweight development in aerospace and transportation fields. However, the inherent poor formability makes the SPD process of high-alloying Al-Zn-Mg-Cu alloys a great challenge. In the present work, a continuous SPD process named the Conform, utilizing a rotational speed of 4 rpm and a mold preheating temperature of 300 °C, was performed on the high-alloying 7055 Al alloy and prepared rod materials with excellent mechanical properties. The results indicated that violent shear deformation accompanied by rapid temperature rise contributes to the specific grain refinement mechanism of continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). The grain configuration formed by this cooperative mechanism is advantageous for the alloy's compatible deformation capability. As a result, a synergistic improvement in the strength and ductility of the Conformed 7055 Al alloy was obtained. Additionally, the strengthening mechanisms induced by the Conform process were quantitatively calculated, revealing that solid solution strengthening and dislocation strengthening were the primary reasons for the enhanced strength of processed alloys.

Thermal deformation behavior and microstructural evolution of the rapidly-solidified Al–Zn–Mg–Cu alloy in hot isostatic pressing state

Isothermal uniaxial compression experiments were performed in the temperature range of 340–460 °C and the deformation rate range of 0.001–1 s−1 to analyze the hot deformation behavior of the rapidly-solidified Al–Zn–Mg–Cu alloy in hot isostatic pressing state. A strain-compensated constitutive model was established to determine the flow stress in the alloy (based on the true stress-true strain data), and the average activation energy for the hot deformation was calculated as Q = ∼146 kJ/mol. The proposed model exhibited a high predictability with an average absolute relative error of 2.68% and a correlation coefficient of 0.99724. The processing map revealed that the alloy in the hot isostatic pressed state offers better workability than the spray-deposited alloy, and its optimal workable ranges at a strain of 0.78 are 370–390 °C/0.004–0.01 s−1 and 400–460 °C/0.005–0.06 s−1, respectively. The microstructural evolution shows that the main dynamic softening mechanism changes from dynamic recovery to continuous dynamic recrystallization with the increase in temperature and the decrease in strain rate.

Dynamic strain aging and recrystallization mechanisms in laser powder bed melt printing of highly textured Hastelloy X

Hastelloy X (HX) is used in a wide range of aerospace applications due to its excellent high-temperature properties, Dynamic strain aging (DSA) and Dynamic recrystallization (DRX) are of great interest as they affect the mechanical behavior at high temperatures. In this study, the serrated flow behavior of highly textured Hastelloy X fabricated by laser powder bed fusion (LPBF) is investigated during uniaxial straining over a wide range of temperatures (20–800 °C) and strain rates (5×10−2 s−1 to 5×10−4 s−1). The alloy exhibits different types of tensile serrated flow in the temperature range of 200–600 °C. The alloy has been shown to have a very good tensile flow behavior. The presence of the Portevin-Le Chatelier effect (PLC) due to dynamic strain failure was observed at temperatures lower than 600 °C, respectively, with an average activation energy value of 45 kJ/mol calculated using the quasi-static aging (McCormick) model, and an average activation energy value calculated using the solute resistance (Cottrell) model of 41.8 kJ/mol, which is significantly lower than the 80–130 kJ/mol of conventionally fabricated HX. The effects of temperature and strain rate on fracture morphology and microstructure were analyzed by SEM, and the dynamic recrystallization behavior and nucleation mechanism were characterized by electron backscatter diffraction (EBSD). The changes in the content of recrystallized texture Cube and deformed texture Goss at different deformation temperatures are explained, and the dynamic recrystallization mode changes, from continuous dynamic recrystallization (CDRX) to discontinuous dynamic recrystallization (DDRX) as the deformation temperature increase. Verification of the different DRX mechanisms contributes to a comprehensive understanding of the microstructural evolution of the hot deformation of HX fabricated by LPBF.

Mechanisms for dynamic recrystallization in a β-quenched Zr-1Nb-1Sn-0.1Fe alloy during hot compression

In this paper, uniaxial hot compression tests of β-quenched Zr-1Nb-1Sn-0.1Fe alloy under a strain of 0.91 were conducted at various temperatures ranging from 600 °C to 750 °C. The microstructure, texture evolution, deformation mode and dynamic recrystallization (DRX) mechanism were investigated using electron backscatter diffraction (EBSD). The results of the In-Grain Misorientation Axis (IGMA) method showed that the prismatic <a> was the dominant deformation mode when the temperature was 600 °C. The prismatic <a> and pyramidal <a + c> were the dominant deformation modes during hot compression between 650 and 750 °C. To study the mechanism of DRX, the geometrically necessary dislocation (GND) density, grain orientation spread (GOS) and grain reference orientation deviation (GROD) maps were calculated. The results showed that both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) coexisted during hot compression. When deformed at 600 °C, the recrystallization mechanism was mainly DDRX, while it was mainly CDRX above 600 °C. The change in the DRX mechanism can be attributed to the activation fraction of different slip systems and the resulting development of misorientation.

Microstructure Evolution and Recrystallization Mechanisms of a Cu-Cr-Sn Alloy during Thermal Deformation Process.

Thermal deformation behavior of Cu-Cr-Sn alloy ingots under deformation temperatures ranging from 600 °C to 950 °C and strain rates from 0.01 s-1 to 10 s-1 was investigated in detail. The thermal deformation constitutive equation and thermal processing map of the alloy were established, respectively. The activation energy Q was determined as 430.61 KJ/mol. The optimal deformation system corresponding to the hot working diagram was a deformation temperature of 900 °C and strain rate of 0.1 s-1. Under these deformation conditions, twin dynamic recrystallization (TDRX), continuous dynamic recrystallization (CDRX), and discontinuous dynamic recrystallization (DDRX) occurred simultaneously, with the twinning process causing the stress-strain curve to exhibit a wavy change. The thermal deformation microstructure of the alloy is co-regulated by different recrystallization mechanisms, with DDRX occurring mainly at low deformation temperatures, and both CDRX and DDRX occurring at high deformation temperatures.

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.

Mesoscale modeling of continuous dynamic recrystallization in Ti-10V-2Fe-3Al alloy

Continuous dynamic recrystallization (CDRX) is the dominant restoration mechanism of near-β titanium alloys during hot deformation. But till now, there is seldom visual modeling of the process which limits its industrial application. Based on CDRX mechanisms characterized by low-angle grain boundaries (LAGBs) formed and partially transformed into high-angle grain boundaries (HAGBs), the present study established a cellular automata (CA) model to investigate the substructure evolution and mechanical response of Ti-10V-2Fe-3Al alloy. Besides, a new set of equations for subgrain rotation considering the effect of the α-phase was integrated into the CA model to improve its applicability in titanium alloy. The model was validated by experimental results and then applied to predict the substructure evolution during hot working. Simulation analysis revealed that, except for high strain rates and temperatures, a high fraction of α/β phase boundaries could also promote the development of CDRX. The proposed CA model provided an effective method for microstructure optimization in hot forging of the Ti-10V-2Fe-3Al alloy.

Hot Compression Behavior of 18Cr–11Mn–0.06Ni–0.19N High‐Mn Ultralow Nickel Duplex Stainless Steel under Large Deformation

To enhance the rolling production efficiency of high‐manganese low‐nickel duplex stainless steel (DSS) and optimize hot compression parameters, hot compression with a range of 0.01–10 s−1/850–1150 °C and deformations of 70% is used to investigate the hot deformation behavior of 18Cr–11Mn–0.06Ni–0.19 N DSS, which reveals the differences in the recrystallization mechanism of two phases and their thermoplastic properties. The results show that strain partitioning and softening occur first in the ferrite phase, with the austenite phase undergoing complex and incomplete softening. Most ferrite experiences continuous dynamic recrystallization and develops a &lt;001&gt;//ND texture when deformed at 0.1–10 s−1/950 °C, with higher temperatures facilitating the merging of high‐angle grain boundaries. A deformation‐induced phase transformation from δ to γ occurs in ferrite when deformed at 0.01 s−1/850–950 °C. However, austenite softening is dominated by discontinuous dynamic recrystallization, enhanced by twinning‐induced strain‐induced boundary migration when deformed at 0.1 s−1/950 °C; dynamic recovery controlled by dislocations is enhanced when deformed at 0.1 s−1/1150°C and 10 s−1/950 °C. A strain‐compensated constitutive model with an average absolute relative error value of 7.8% is established to better predict the flow stresses. Experimental DSS displays optimal hot workability in the parameter ranges of 0.1 s−1/950–1050 °C.

Dynamic Recrystallization Nucleation Mechanism and Precipitation Behavior of Homogeneous Al-Zn-Mg-Cu Alloy during Hot Deformation

The hot deformation behavior of Al-Zn-Mg-Cu alloy was investigated by flow stress curves in isothermal hot compression experiments with deformation temperatures of 350–450 °C and strain rates of 0.01 s−1 to 1 s−1, and the constitutive equation of homogeneous alloy was obtained. At the same time, the dynamic recrystallization and precipitation behavior during hot deformation and the relationship between them and the Z parameters were studied by using EBSD and TEM. DRV is the main mechanism of dynamic softening. With the decrease in Z parameter, the softening mechanism changes from dynamic recovery to discontinuous dynamic recrystallization or continuous dynamic recrystallization. At a higher Z parameter, the dislocation density and precipitated phase density are also higher because the high dislocation density provides heterogeneous nucleation sites of the precipitated phase. A large number of precipitates in the alloy also inhibit the nucleation and growth of dynamic recrystallization by hindering dislocation movement and grain boundary migration.