Effect of Initial Grain Size on the Dynamic Recrystallization of Hot Deformation for 3003 Aluminum Alloy

The 3003 aluminum alloys with four different initial grain sizes were deformed by isothermal compression in the range of deformation temperature 300–500 °C at strain rate 0.01–10.0 s^?1 with Gleeble-1500 thermal simulator. The results show that the smaller the initial grain size of the alloy, the greater the required deformation resistance, and the smaller the peak strain, which is conducive to the occurrence of dynamic recrystallization (DRX). The DRX critical strain increases with the decrease of the deformation temperature or the increase of the strain rate, and the DRX volume fraction increases with the decrease of the strain rate and the increase of the deformation temperature. The average grain size of 3003 aluminum alloy after deformation is smaller than that before deformation. The smaller the initial grain size, the lower the critical recrystallization strain. So the DRX is carried out more fully, contributing to the thermoplastic deformation of the alloy.     

Effect of Hot Deformation on Texture and Microstructure in Fe-Mn Austenitic Steel During Compression Loading

Hot compression tests of a new high-Mn austenitic steel were carried out at deformation temperatures of 700, 800, 900, and 1000 °C under strain rate of 0.01 s^?1. The hot deformation behavior was investigated by the analyses of flow curves, texture, and deformed microstructures. Microstructures of the deformed specimens and macrotexture were examined using electron backscatter diffraction and x-ray diffraction methods, respectively. The results showed that the flow stress depended strongly on the deformation temperature and decreased by increasing deformation temperature. The microstructural evidence indicated that the dynamic recrystallization (DRX) process of experimental steel was initiated at 800 °C with necklace structure. The volume fraction of DRX grains was considerably increased by increasing deformation temperature to 1000 °C. Texture of the DRX grains tended to become a weak texture and was associated with the formation of Goss and R-Cube components. Meanwhile, martensitic transformation was detected in the hot-deformed austenite. The martensitic transformation was the most difficult in the DRX grains because of the effect of small grain size. The tendency of transformation was decreased after compression at 1000 °C.     

Processing maps for hot working of a P/M iron aluminide alloy

The hot working behavior of a Fe–24 wt.% Al iron aluminide alloy processed by the powder metallurgy route has been studied in the temperature range 750–1150°C and strain rate range 0.001–100 s⁻¹ by establishing processing maps at different strains in the range 0.1–0.5. The features in the processing maps have changed with strain suggesting that the mechanisms of hot deformation are evolving with strain. Early in the deformation (strain of 0.1), the map exhibited a single domain with a peak efficiency of power dissipation of about 44% occurring at about 1100°C and a strain rate of about 0.03 s⁻¹. This domain represents dynamic recrystallization (DRX) of the initial material possibly causing a substantial grain refinement. With increasing strain, a bifurcation has occurred giving rise to two domains: (1) at strain rates lower than about 0.1 s⁻¹ and temperatures above 1000°C, superplastic deformation has occurred, and (2) at strain rates higher than about 10 s⁻¹ and temperatures above 1125°C, DRX has occurred. The material exhibited flow localization at lower temperatures and higher strain rates. On the basis of the processing maps, the optimum processing routes available for hot working of this material are outlined.     

Hot deformation mechanisms in Ti-5.5Al-1Fe alloy

The mechanisms of hot deformation in the alloy Ti-5.5Al-1Fe have been studied in the temperature range 750 to 1150 °C and with the true strain rate varying from 0.001 to 100 s⁻¹ by means of isothermal compression tests. At temperatures below β transus and low strain rates, the alloy exhibited steady-state flow behavior, while, at high strain rates, either continuous flow softening or work hardening followed by flow softening was observed. In the β region, the deformation behavior is characterized by steady-state behavior at low strain rates, yield drops at intermediate strain rates, and oscillations at high strain rates. The processing maps revealed two domains. (1) In the temperature range 750 to 1050 °C and at strain rates lower than 0.01 s⁻¹, the material exhibits fine-grained superplasticity. The apparent activation energy for superplastic deformation is estimated to be about 328 kJ/mole. The optimum conditions for superplasticity are 825 °C and 0.001 s⁻¹. (2) In the β region, a domain occurs at temperatures above 1100 °C and at strain rates from 0.001 to 0.1 s⁻¹ with its peak efficiency of 47% occurring at 1150 °C and 0.01 s¹. On the basis of kinetic analysis, tensile ductility, and grain size variation, this domain is interpreted to represent dynamic recrystallization (DRX) of β phase. The apparent activation energy for DRX is estimated to be 238 kJ/mole. The grain size (d) is linearly dependent on the Zener-Hollomon parameter (Z) per the equation

Superplastic behaviour and microstructure evolution of a fine-grained TA15 titanium alloy

The fine-grained microstructure of TA15 titanium alloy was prepared through two-step forging technology combined with high and low temperatures, and a transnormal superplastic elongation of more than 2000% was obtained. The superplastic behaviour and microstructure evolution were systematically researched at different temperatures and strain rates through superplastic tensile test. The results indicate that the fine-grained TA15 alloy exhibits superplasticity at temperatures of 760–980°C and initial strain rates from 1.1 × 10⁻² to 5.5 × 10⁻⁵ s⁻¹. The optimal superplastic conditions are 940°C and 3.3 × 10⁻⁴ s⁻¹, in which the average elongation is 2526% and the maximum elongation is 2743%. During superplastic deformation, dynamic recovery and recrystallization occur obviously, and the corporate effect of strain hardening and recrystallization softening decides the superplastic ability directly.     

Dynamic Recrystallization Kinetics and Microstructural Evolution for LZ50 Steel During Hot Deformation

The dynamic recrystallization (DRX) behavior of LZ50 steel was investigated using hot compression tests at a deformation temperature of 870-1170 °C and a strain rate of 0.05-3 s^?1. The effects of deformation temperature, strain, strain rate, and initial austenite grain size on the microstructural evolution during DRX were studied in detail. The austenite grain size of DRX was refined with increasing strain rate and decreasing temperature, whereas the initial grain size had no influence on DRX grain size. A model based on the Avrami equation was proposed to estimate the kinetics of the DRX under different deformation conditions. A DRX map, which was derived from the DRX kinetics, the recrystallized microstructure, and the flow stress analysis, can be used to identify optimal deformation conditions. The initiation of DRX was lower than Z _c (critical Zener-Hollomon parameter) and higher than ε_c (critical strain). The relationship between the DRX microstructure and the Z parameter was analyzed. Fine DRX grain sizes can be achieved with a moderate Z value, which can be used to identify suitable deformation parameters.     

Characteristics of High-Temperature Deformation Behavior of Ti-45Al-2Cr-3Ta-0.5W Alloy

High-temperature deformation behavior tests of as-cast Ti-45Al-2Cr-3Ta-0.5W alloy were conducted over a wide range of strain rates (0.001-1.0 s^?1) and temperatures (1150-1300 °C). The flow curves for the current alloy exhibited sharp peaks at low strain levels, followed by pronounced work hardening and flow localization at high strain levels. Phenomenological analysis of the strain rate and temperature dependence of peak stress data yielded an average value of the strain rate sensitivity equal to 0.25 and an apparent activation energy of ~420 kJ/mol. Processing maps were established under different deformation conditions, and the optimal condition for hot work on this material was determined to be 1250 °C/0.001 s^?1. The stable deformation region was also found to decrease with increasing strain. Dynamic recrystallization (DRX) was the major softening mechanism controlling the growth of grains at the grain boundary. Meanwhile, local globularization and dynamic recovery (DR) were the main softening mechanisms in the lamellar colony. When deformed at higher temperatures (~1300 °C), the cyclic DRX and DR appeared to dominate the deformation. Moreover, the evolution of the β phase during hot deformation played an important role in the dynamic softening of the alloy.     

Constitutive analysis and processing map for hot working of a Ni-Cu alloy

The hot deformation behavior of a Ni-Cu alloy was studied using hot compression testing in the temperature range of 950 °C–1150 °C and at strain rates of 0.001 s^?1-1 s^?1. Flow curves at low strain rates, up to 0.01 s^?1, were typical of DRX characterized by a single peak, while at higher strain rates, the typical form of a DRX flow curve was not observed. The power-law constitutive equation was used to correlate flow stress to strain rate and temperature, and the apparent activation energy of hot deformation was determined to be about 462.4 kJ/mol. The peak strain and stress were related to the Zener-Hollomon parameter and the modeling formula was proposed. The dependence of flow stress to the Z changed at ln Z=38.5, which was considered to be a critical condition for the change in the mechanism of dynamic recrystallization. The efficiency of power dissipation was determined to be between 10–35 percent at different deformation conditions. According to the dynamic material model, stable flow was predicted for the studied temperature and strain rate ranges. Highly serrated grain boundaries at low strain rates were considered to be a reason for the occurrence of continuous dynamic recrystallization. On the contrary, at high strain rates, equiaxed grain structure was attributed to the typical discontinuous dynamic recrystallization.     

Dynamic Recrystallization Mechanism of Inconel 690 Superalloy During Hot deformation at High Strain Rate

The hot deformation characteristics of Inconel 690 superalloy were investigated on the Gleeble-3800 thermal-mechanical simulator. The testing temperatures were in the range of 1000-1200 °C, the strain rate was 10 s^?1, and the maximum true strain was 0.9. Optical microscopy, transmission electron microscopy, and electron backscatter diffraction techniques were employed to analyze the microstructure evolution and nucleation mechanisms of dynamic recrystallization (DRX). The results show that multiple-cycle discontinuous dynamic recrystallization (DDRX) occurs in the process of hot deformation under the conditions above. DRX grain size decreases with decreasing temperature and increasing strain. DDRX with sub-grains directly transforming into grains is the dominating nucleation mechanism of DRX. And, the nucleation mechanism of bulging of the original grain boundaries can only be considered as an assistant nucleation mechanism of DRX, which mainly occurs in the beginning of the deformation.     

Hot deformation behavior and microstructure evolution of a Mg–Gd–Nd–Y–Zn alloy

The hot deformation behavior of homogenized Mg–6.5Gd–1.3Nd–0.7Y–0.3Zn alloy was investigated during compression at temperatures of 250–400 ℃ and at strain rates ranging from 0.001 to 0.100 s~(-1). Microstructure analyses show that the flow behaviors are associated with the deformation mechanisms. At the lower temperatures(250–300 ℃), deformation twinning is triggered due to the difficult activation of dislocation cross-slip. Dynamic recrystallization(DRX) accompanied by dynamic precipitation occurs at the temperature of 350 ℃ and influences the softening behavior of the flow.DRX that develops extensively at original grain boundaries is the main softening mechanism at the high temperature of 400 ℃ and eventually brings a more homogeneous microstructure than that in other deformation conditions. The volume fraction of the DRXed grains increases with temperature increasing and decreases with strain rate increasing.     

Recrystallization in hot vs cold deformed commercial aluminum: a microstructure path comparison

Static, isothermal recrystallization at a temperature of 400 °C was studied by means of quantitative microscopy in a well-characterized, commercial purity aluminum-alloy AA1050 that had undergone plane strain deformation at 400 °C at a strain rate of 2.5 s⁻¹ to an equivalent strain of 2. The microstructural properties, V_v, the volume fraction recrystallized, S_v, the interfacial area density separating recrystallizing grains from deformed volumes and <λ>, the mean recrystallized grain free length, were all measured stereologically as a function of time and the reaction kinetics, microstructural path, grain boundary migration rates and nucleation characteristics of the recrystallization were quantified experimentally. The results are compared to a recently published study of recrystallization in the identical pre-deformation starting material but after room temperature deformation by rolling to a comparable strain. Recrystallization kinetics differences between the two materials include: the hot deformed material had a higher, by at least 120 °C, recrystallization temperature; had many fewer recrystallization nuclei leading to a factor of about three larger as-recrystallized grain size; lacked a Cahn-Hagel growth rate transient like the cold deformed exhibited; and required a slightly different impingement model for the microstructural path analysis. In both cases particle stimulated nucleation (PSN) was thought to be operative but it seemed to be much more potent after cold deformation.     

Electron backscattered diffraction study on superplastic properties of coarse-grained Fe-27 at% Al

An electron backscattered diffraction technique has been used to investigate crystallographic features of a superplastic coarse-grained Fe-27 at% Al alloy. Alloy samples studied have been tensile tested in a temperature range between 600 and 800°C in air under an initial strain rate of 1×10⁻⁴ s⁻¹. As a result of dynamic recovery and recrystallization, the grain structure undergoes four major transitions: subgrain-boundary formation, grain-boundary migration, formation and growth of recrystallized grains. A model based on the microstructural evolution is described. Subgrains form during an initial stage of high-temperature deformation when deformation is conducted at low temperature (600°C). Upon further deformation at 700°C, grain boundaries migrate, resulting in the formation of new grains. When deformation is made further to a larger elongation or at even higher temperature (800°C), dynamic recovery and recrystallization occur significantly, resulting in grain refinement and hence superplasticity. Refined grains thus formed maintain crystallographic relationships with parent grains.     

Microstructure evolution of 7050 aluminum alloy during hot deformation

Hot compression of 7050 aluminum alloy was performed on Gleeble 1500D thermo-mechanical simulator at 350 ℃ and 450 ℃ with a constant strain rate of 0.1 s-1 to different nominal strains of 0.1, 0.3 and 0.7. Microstructures of 7050 alloy under various compression conditions were observed by TEM to investigate the microstructure evolution process of the alloy deformed at various temperatures. The microstructure evolves from dislocation tangles to cell structure and subgrain structure when being deformed at 350 ℃, of which dynamic recovery is the softening mechanism. However, continuous dynamic recrystallization (DRX) occurs during hot deformation at 450 ℃, in which the main nucleation mechanisms of DRX are subgrain growth and subgrain coalescence rather than particle-simulated nucleation (PSN).     

Deformation Behavior of AZ31 Magnesium Alloy During Tension at Moderate Temperatures

The mechanical properties and deformation mechanisms have been studied by tension testing at temperatures between 373 and 523 K and at strain rates ranging from 10⁻¹ to 10⁻³ s⁻¹. The experimental results show that the plasticity improves significantly with temperature and decreases obviously with increasing strain rate. When the temperature is below the recrystallization temperature, twinning and dislocation slip have been proven to be the dominant model of plastic deformation. With the temperature increasing, some DRXed grains can be observed at the twinned regions and grain boundaries, suggesting that both twinning-induced DRX and continuous DRX occurred in the deformation process.     

BT25钛合金在锻造过程中失稳变形和动态再结晶行为的数值模拟

分别利用失稳图和功率耗散图确定BT25钛合金失稳变形组织和动态再结晶变形组织的热力参数边界条件,并将其输入到Deform-3D有限元软件中,使加工图技术与有限元技术能够进行有效结合。利用二次开发后的软件对BT25钛合金在变形温度为950~1100 ℃和应变速率0.001~1 s^-1的条件下进行失稳变形组织和动态再结晶行为的模拟和预测,并通过对比金相组织,验证了该模拟结果的可靠性。结果表明,流动应力随变形温度的升高或应变速率的降低而降低;失稳变形组织集中在低温、高应变速率区域;高温和低应变速率均有利于动态再结晶（DRX）行为;微观组织的观察结果与模拟预测的结果吻合较好,说明本研究提出的加工图技术与有限元技术相结合的方法对模拟与预测金属锻造过程中的失稳变形组织和DRX行为是可行的。     

Characterization of hot deformation and microstructure evolution of a new metastable β titanium alloy

The hot deformation characteristics of as-forged Ti?3.5Al?5Mo?6V?3Cr?2Sn?0.5Fe?0.1B?0.1C alloy within a temperature range from 750 to 910 °C and a strain rate range from 0.001 to 1 s^?1 were investigated by hot compression tests. The stress?strain curves show that the flow stress decreases with the increase of temperature and the decrease of strain rate. The microstructure is sensitive to deformation parameters. The dynamic recrystallization (DRX) grains appear while the temperature reaches 790 °C at a constant strain rate of 0.001 s^?1 and strain rate is not higher than 0.1 s^?1 at a constant temperature of 910 °C. The work-hardening rate θ is calculated and it is found that DRX prefers to happen at high temperature and low strain rate. The constitutive equation and processing map were obtained. The average activation energy of the alloy is 242.78 kJ/mol and there are few unstable regions on the processing map, which indicates excellent hot workability. At the strain rate of 0.1 s^?1, the stress?strain curves show an abnormal shape where there are two stress peaks simultaneously. This can be attributed to the alternation of hardening effect, which results from the continuous dynamic recrystallization (CDRX) and the rotation of DRX grains, and dynamic softening mechanism.     

ZL270LF铝合金的热变形行为与组织演变

通过热压缩实验研究了ZL270LF铝合金在变形量为70%,温度为300~550 ℃,应变速率为 0.01~10 s^-1范围的热变形行为,建立了流变应力本构方程模型,绘制出了二维热加工图,确定了最佳热加工区域,采用电子背散射衍射（EBSD）和透射电子显微镜（TEM）技术研究了该合金的组织演变规律。结果表明：ZL270LF铝合金的流变应力随变形温度的升高和应变速率的降低而降低,热变形激活能为309.05 kJ/mol,最优热加工区为温度470~530 ℃、应变速率为0.01~1 s^-1。该合金在热变形过程中存在3种不同的DRX机制,即连续动态再结晶（CDRX）、不连续动态再结晶（DDRX）和几何动态再结晶（GDRX）,其中CDRX是ZL270LF铝合金动态再结晶的主要机制。     

FeCrNiMn双相高熵合金高温压缩过程中的组织演变

为了研究双相高熵合金（HEA）在高温变形过程中的微观组织演变,在900至1050 ℃的温度下进行了不同应变速率的压缩试验。选择了4种典型的流动曲线,并对相应的微观组织进行了分析,以研究双相HEA的动态再结晶（DRX）和织构演变。结果表明,在应变速率为0.1和0.01 s^-1时,变形试样的流动曲线完全不同。力学流动曲线的差异与DRX和织构演化过程有关。在1050 ℃和0.1 s^-1下压缩后,获得了结合<110>和<100>的双组分组织结构,这是因为高温下扩散控制的溶质阻力占主导地位。此外,bcc相的影响依赖于界面边界和颗粒周围的应变不均匀性,因为没有发生相变,大部分应变由fcc相容纳。     

Nucleation mechanisms of dynamic recrystallization for G3 alloy during hot compression

Dynamic recrystallization(DRX) mechanisms of a nickel-based corrosion-resistant alloy, G3, were investigated by hot compression tests with temperatures from 1050 to 1200 ℃ and strain rates from 0.1 to 5.0 s~(-1).Deformation microstructure was observed at the strain from 0.05 to 0.75 by electron backscatter diffraction(EBSD) and transmission electron microscope(TEM).Work hardening rate curves were calculated to analyze the effect of deformation parameters on the nucleation process.Results indicate that strain-induced grain boundary migration is the principal mechanism of DRX. Large annealing twins promote nucleation by accumulating dislocations and fragmenting into cell blocks. Continuous dynamic recrystallization is also detected to be an effective supplement mechanism, especially at low temperature and high strain rate.     

Optimization of process parameters for the manufacturing of rocket casings: A study using processing maps

Maraging steels possess ultrahigh strength combined with ductility and toughness and could be easily fabricated and heat-treated. Bulk metalworking of maraging steels is an important step in the component manufacture. To optimize the hot-working parameters (temperature and strain rate) for the ring rolling process of maraging steel used for the manufacture of rocket casings, a systematic study was conducted to characterize the hot working behavior by developing processing maps for γ-iron and an indigenous 250 grade maraging steel. The hot deformation behavior of binary alloys of iron with Ni, Co, and Mo, which are major constituents of maraging steel, is also studied. Results from the investigation suggest that all the materials tested exhibit a domain of dynamic recrystallization (DRX). From the instability maps, it was revealed that strain rates above 10 s⁻¹ are not suitable for hot working of these materials. An important result from the stress-strain behavior is that while Co strengthens γ-iron, Ni and Mo cause flow softening. Temperatures around 1125 °C and strain rate range between 0.001 and 0.1 s⁻¹ are suitable for the hot working of maraging steel in the DRX domain. Also, higher strain rates may be used in the meta-dynamic recrystallization domain above 1075 °C for high strain rate applications such as ring rolling. The microstructural mechanisms identified from the processing maps along with grain size analyses and hot ductility measurements could be used to design hot-working schedules for maraging steel.