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.     

Strain-dependent constitutive analysis of hot deformation and hot workability of T4-treated ZK60 magnesium alloy

The hot deformation behavior of T4-treated ZK60 magnesium alloy was investigated in a compression test conducted with a thermo-mechanical simulator at a temperature range of 523 K to 673 K and a strain rate of 0.001 s^?1 to 1 s^?1. The results show that the flow stress increases as the deformation temperature decreases and the strain rate increases. Strain-dependent constitutive relationships were developed using regression method and artificial neural network, and good agreements between the experimentally measured values and the predicted ones were achieved. The work hardening analysis and onset of dynamic recrystallization (DRX) were investigated. The processing map reveals a domain of DRX at the temperature range of 620–673 K and strain rate range of 0.001–0.01 s^?1, with its peak efficiency of 32% at 623 K and 0.001 s^?1, which are the optimum values of the parameters for hot working of the T4-treated ZK60 alloy. The strain level has a great effect on the processing maps and lower temperatures and higher strain rates should be avoided during hot working processes. DRX model indicates that DRX of ZK60 alloy is controlled by the rate of nucleation, which is slower than the rate of migration.     

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.     

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).     

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。     

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.     

Effect of Deformation Conditions on the Dynamic Recrystallization of GH690 Alloy

采用Gleeble-3500热模拟试验机进行高温等温压缩实验,研究了变形条件对GH690合金高温变形动态再结晶的影响。结果表明:GH690合金动态再结晶过程是一个受变形温度和应变速率控制的过程,在应变速率为0.001～1s-1的实验条件下,GH690合金获得完全动态再结晶组织所需的温度随变形速率的增大而升高;动态再结晶晶粒尺寸随变形温度升高而增大。采用力学方法直接从流变曲线确定了GH690合金发生动态再结晶的临界应变量,并回归出临界应变量与Z参数的关系式:εc=1.135×10-3Z0.14233。GH690合金的主要动态再结晶机制是原始晶界凸起形核的不连续动态再结晶机制(DDRX),而新晶粒通过亚晶逐渐转动而形成的连续动态再结晶机制(CDRX)则起辅助作用。     

Effect of strain rate on hot deformation characteristics of GH690 superalloy

In order to clarify the effect of strain rate on hot deformation characteristics of GH690 superalloy, the hot deformation behavior of this superalloy was investigated by isothermal compression in the temperature range of 1000–1200 °C and strain rate range of 0.001–10 s^?1 on a Gleeble–3800 thermo-mechanical simulator. The results reveal that the flow stress is sensitive to the strain rate, and the dynamic recrystallization (DRX) is the principal softening mechanism. The strain rate of 0.1 s^?1 is considered to be the critical point during the hot deformation at 1000 °C. The DRX process is closely related to the strain rate due to the adiabatic temperature rise. The strain rate has an important influence on DDRX and CDRX during hot deformation. The nucleation of DRX can be activated by twin boundaries, and there is a lower fraction of ∑ 3ⁿ (n=1, 2, 3) boundaries at the intermediate strain rate of 0.1 s^?1.     

Nucleation mechanisms of dynamic recrystallization in Inconel 625 superalloy deformed with different strain rates

The effects of strain rates on the hot working characteristics and nucleation mechanisms of dynamic recrystallization (DRX) were studied by optical microscopy and electron backscatter diffraction (EBSD) technique. Hot compression tests were conducted using a Gleeble-1500 simulator at a true strain of 0.7 in the temperature range of 1000 to 1150 °C and strain rate range of 0.01 to 10.00 s-1. It is found that the size and volume fraction of the DRX grains in hot-deformed Inconel 625 superalloy firstly decrease and then increase with increasing strain rate. Meanwhile, the nucleation mechanism of DRX is closely related to the deformation strain rate due to the deformation thermal effect. The discontinuous DRX (DDRX) with bulging of original grain boundaries is the primary nucleation mechanism of DRX, while the continuous DRX (CDRX) with progressive subgrain rotation acts as a secondary nucleation mechanism. The twinning formation can activate the nucleation of DRX. The effects of bulging of original grain boundaries and twinning formation are firstly gradually weakened and then strengthened with the increasing strain rate due to the deformation thermal effect. On the contrary, the effect of subgrain rotation is firstly gradually strengthened and then weakened with the increasing strain rate.     

Deformation Mechanism and Hot Workability of Extruded Magnesium Alloy AZ31

Using the flow stress curves obtained by Gleeble thermo-mechanical testing, the processing map of extruded magnesium alloy AZ31 was established to analyze the hot workability. Stress exponent and activation energy were calculated to characterize the deformation mechanism. Then, the effects of hot deformation parameters on deformation mechanism,microstructure evolution and hot workability of AZ31 alloy were discussed. With increasing deformation temperature, the operation of non-basal slip systems and full development of dynamic recrystallization(DRX) contribute to effective improvement in hot workability of AZ31 alloy. The influences of strain rate and strain are complex. When temperature exceeds 350 °C, the deformation mechanism is slightly dependent of the strain rate or strain. The dominant mechanism is dislocation cross-slip, which favors DRX nucleation and grain growth and thus leads to good plasticity. At low temperature(below 350 °C), the deformation mechanism is sensitive to strain and strain rate. Both the dominant deformation mechanism and inadequate development of DRX deteriorate the ductility of AZ31 alloy. The flow instability mainly occurs in the vicinity of 250 °C and 1 s-1.     

THE HOT DEFORMATION BEHAVIOR AND DYNAMIC RECRYSTALLIZATION MODEL OF 35CrMo STEEL

The isothermal single-stage compression of 35CrMo structural steel has been carried out by using Gleeble 1500 simulator at the temperature range of 950℃ to 1150℃ and strain rate range of 0.01s-1 to 10s-1. The effect of hot deformation parameters, such as strain rate, deformed temperature and initial grain size on the flow stress behavior was investigated. The activation energy of tested alloy was calculated, which is 378.16kJ/mol; The relationships between the peak stress (σp), the peak stain (εp), the critical strain (εc) and Z parameter were established. The micro structure evolution shows the pre-existing austenite grain boundaries constitute the principal nucleation sites for dynamic recrystallization (DRX), and the initial austenite grain size affects the grain size of DRX slightly. The kinetic mathematical model of DRX of 35CrMo is: XDRX=1-exp(-3.23-2.28) and Ddyn = 2.252× 10Z-0.22.     

Dynamic Recrystallization Behavior of Bimodal Size SiCpReinforced Mg Matrix Composite during Hot Deformation

The(submicron+micron) bimodal size Si Cp-reinforced Mg matrix composite was compressed at the temperature of 270–420 °C and strain rate of 0.001–1 s~(-1). Then, dynamic recrystallization(DRX) behavior of the composite was investigated by thermodynamic method and verified by microstructure analysis. Results illustrated that the composite possess the lower critical strain and higher DRX ratio as compared to monolithic Mg alloys during hot deformation process. The predicted DRX ratio increased with the proceeding of compression, which was well consistent with the experimental value. Results from thermodynamic calculation suggested that the occurrence of DRX could be promoted by Si Cp, which would be further proved by microstructure analysis. Formation of particle deformation zone around micron Si Cp played a significant role in promoting DRX nucleation. Nevertheless, the distribution of submicron Si Cp was increasingly uniform with the proceeding of compression, which could fully restrain grain growth. Therefore, the corporate effects of micron and submicron Si Cp on DRX contributed to the improvement of DRXed ratio and the refinement of grain size for the composite during compression process.     

基于位错密度理论的核级316LN不锈钢高温本构模型

研究了316LN奥氏体不锈钢在1050~1200 ℃、应变速率0.1,1和50 s^-1下的压缩变形行为,分析了变形温度和应变速率对热流曲线的影响。基于位错密度理论,建立了316LN钢的热变形本构模型,并揭示了316LN钢的软化机理。结果表明,在高温低应变速率（小于0.1 s^-1）条件下,动态再结晶（DRX）为主导软化机理;在高温高应变速率（大于1 s^-1）条件下,动态回复（DRV）为主导软化机理;在高温及应变速率为0.1和1 s^-1条件下,DRV和DRX共同作用。构建的模型可以很好地预测316LN钢的热变形行为,其Pearson相关系数为0.9956,平均相对误差绝对值为3.07%,为一个精确的本构模型。     

热挤压态FGH95合金的动态再结晶机制研究

针对热挤压态FGH95合金进行变形温度为1050~1120 ℃、变形量为50%和70%、应变速率为10?4~1 s?1的热压缩试验,研究该合金动态再结晶（DRX）的组织演变和形核机制。结果表明:提高变形温度和降低应变速率可以促进小角度晶界向大角度晶界迁移,有利于动态再结晶晶粒的长大;变形温度和变形量对热挤压态FGH95合金的动态再结晶机理的影响不明显,而应变速率对动态再结晶机制影响较大;随着应变速率的增加,热挤压态FGH95合金由不连续动态再结晶机制逐渐转变为连续动态再结晶机制;热挤压态FGH95合金的动态再结晶以不连续动态再结晶形核机制为主,以连续动态再结晶形核机制为辅;在1050 ℃、1 s?1变形条件下,热挤压态FGH95合金发生连续动态再结晶形核。     

Hot Deformation Behavior and Processing Maps of a Medium Manganese TRIP Steel

The hot deformation behavior of a medium-Mn steel was studied in terms of hot compression flow curves in the temperature range of 850–1050 ℃ and strain rates of 0.05–10 s~(-1).The thermo-mechanical analysis was carried out and suggested that the microstructure during deformation was completely austenite which had high tendency for dynamic recrystallization(DRX).The flow behavior was characterized by significant flow softening at deformation temperatures of 950–1050 ℃ and lower strain rates of 0.05–5 s~(-1), which was attributed to heating during deformation, DRX and flow instability.A step-by-step calculating procedure for constitutive equations is proposed.The verification of the modified equations indicated that the developed constitutive models could accurately describe the flow softening behavior of studied steel.Additionally, according to the processing maps and microstructure analysis, it suggested that hot working of medium Mn steel should be carried out at 1050 ℃, and the strain rate of 0.05–10 s~(-1) resulted in significantly recrystallized microstructures in the in steel.The flow localization is mainly flow instability mechanism for experimental steel.     

铸态TB6钛合金β热变形中的动态再结晶行为

利用Thermecmaster-Z型热模拟试验机在β相区对铸态TB6钛合金进行了热压缩试验,并对其动态再结晶行为进行了研究。结果表明,合金在β热变形过程中主要存在两类形核位置:原始β晶界附近及β晶粒内部,相应地存在两类动态再结晶机制:不连续动态再结晶和连续动态再结晶。在较高应变速率(≥0.01s-1)时,以不连续动态再结晶机制为主,但动态再结晶发生的程度较低,不能通过此机制使组织获得明显细化;在低应变速率(≤0.001s-1)和高变形温度(≥950℃)时,以连续动态再结晶机制为主。此时,合金动态再结晶晶粒直接由亚晶转变而成,组织均匀、细小。     

Flow Behavior and Microstructures of Inconel 625 Superalloy at the Elevated Temperature

The deformation behavior of Inconel 625 superalloy was investigated by means of hot compression tests. The flow stress curves were obtained in the temperature and strain rate ranges of 950-1200 ℃ and 0.01-10 s-1, respectively. Optical microscopy was used to evaluate the microstructural evolution of the alloy under different conditions examined. The results show that the flow stress decreases with decreasing strain rate and increasing temperature, and the activation energy is about 654.502 kJ/mol. Microstructure observations show that with increasing temperature, the sizes and volume fraction of dynamic recrystallization (DRX) grains increase. The strain has no remarkably effect on the sizes of DRX grains, but with increasing strain the volume fraction of DRX grains increases. During hot compression of Inconel 625 superalloy at elevated temperature, the occurrence of DRX was the main softening mechanism. The DRX mechanism of Inconel 625 superalloy can be mainly attributed to the discontinuous dynamic recrystallization (DDRX).     

AerMet100超高强钢在热加工过程中的动态再结晶模型研究

为了准确预测AerMet100超高强钢在热加工过程中的微观组织演变,通过系列等温热压缩试验分析了合金在温度为800~1040℃、应变速率为0.01~10s-1、变形量为15~60%的热变形行为,并建立了动态再结晶(DRX)体积分数和晶粒尺寸的DRX模型。通过计算获得了AerMet100钢本构模型中的Zener-Hollomon参数,用于建立DRX模型。通过建立的DRX模型定量预测了热变形参数对合金微观组织演变的影响,结合微观组织观察发现,高温低应变速率和较大的变形程度有利于DRX充分发生,使组织细化和均匀化。模型预测结果与实验结果吻合较好,验证了所建立的DRX模型的准确性。结果表明,建立的DRX模型可以定量预测AerMet100钢在不同变形参数下进行热加工时的微观组织演变规律。     

Mg-3Y合金大应变量轧制过程中的孪生辅助动态再结晶及组织演变

采用预挤压加单道次大应变量热轧制的方法制备了Mg-3Y（质量分数,%）合金板材。并研究了大应变量轧制过程中不同孪晶类型对合金动态再结晶（DRX）及组织演变的影响。结果表明,在挤压比为8:1的预挤压过程中,合金内部发生了几乎完全的动态再结晶。而在接下来的大应变量热轧制过程中,孪生变形尤其是■压缩孪晶及■双孪晶在协调合金的塑性应变中发挥了重要作用。此外,大量动态再结晶在压缩孪晶及双孪晶内部发生,并扩展到非孪晶区域,有效缓解了轧制过程中的内应力集中。上述2个过程对提高合金在大应变量轧制中的成形性均起到了促进作用。     

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.