一种新型镍基高温合金的动态再结晶行为

采用等温热压缩实验研究了一种新型镍基高温合金在不同热变形条件下(变形温度1040～1120℃、应变量0.35～1.2、应变速率0.1 s^-1)的动态再结晶行为。通过光学显微镜(OM)、扫描电子显微镜(SEM)和电子背散射衍射仪(EBSD)研究变形温度和应变量对合金热变形过程中组织演变和动态再结晶(DRX)形核机制的影响。结果表明,根据加工硬化率曲线能够准确确定DRX出现的临界应力和临界应变。合金的DRX晶粒体积分数随变形温度和应变量的增加而增加。在高温低应变速率下,不连续动态再结晶(DDRX)和连续动态再结晶(CDRX)形核机制同时发生。随着变形温度的升高,CDRX形核机制减弱,而CDRX机制在高温条件下占据主导。随着应变量的增加,合金中DDRX机制逐渐变强。热变形后期,CDRX仅作为辅助形核机制发挥作用。另外,Σ3孪晶界的形成有助于DRX晶粒的形核。     

AM355不锈钢的热变形行为

使用Gleeble-3800热模拟试验机对锻造态AM355不锈钢进行等温热压缩试验,应变速率选择0.01~10 s^-1,变形温度选择1173~1423 K。热变形后的组织通过光学显微镜、电子背散射衍射、透射电镜进行观察。基于Arrhenius模型采用峰值应力构建了本构方程,并对其改进得到了准确度更高的本构方程。采用动态材料模型构建了热加工图。由热加工图与变形后的组织得到了真应变为0.9时的热加工窗口。结果表明,适用于AM355钢的最优热加工区域为变形温度1250~1300 K、应变速率0.01~0.03 s^-1与变形温度1300~1400 K、应变速率0.01~10 s^-1及变形温度1400~1423 K、应变速率0.5~10 s^-1,该区域下能量耗散率均小于0.36,且发生了完全的动态再结晶。此外,还确立了完全动态再结晶时奥氏体晶粒尺寸d_drx与Z参数的关系。     

23Cr-2.2Ni-6.3Mn-0.26N节Ni型双相不锈钢动态再结晶行为研究

利用热模拟试验机在变形温度为1073～1423 K,应变速率为0.01～10 s~(-1)的条件下对23Cr-2.2Ni-6.3Mn-0.26N节Ni型双相不锈钢动态再结晶行为进行研究。结果表明,试样在低温高应变速率变形时以两相动态回复(DRV)为主,而在高温低应变速率变形时以奥氏体动态再结晶(DRX)为主,且在0.01和0.1 s~(-1)较低应变速率下,奥氏体相再结晶晶粒尺寸随变形温度升高而增大。试样的软化机制与Z参数有关,在低Z值条件下,热变形软化以奥氏体相DRX为主。基于热变形方程得到试样的表观应力指数为5.18,热变形表观激活能为391.16 kJ/mol,并利用Sellars双曲正弦模型建立了峰值流变应力与Z参数关系本构方程。DRX临界应力随应变速率增加和变形温度减小而增大,DRX临界应变随变形温度减小而增加,且随应变速率增加(0.1～10 s~(-1))在较低变形温度下先增大后减小。确定了DRX临界应力(应变)和峰值应力(应变)的关系,DRX特征参数和Z参数相关模型,以及奥氏体相DRX体积分数模型。利用所建模型对DRX行为进行预测,表明应变速率增加和变形温度下降会推迟DRX发生。     

节Ni型2101双相不锈钢的高温热加工行为研究

采用Gleeble-3800热力模拟试验机在温度为1123~1423 K、应变速率为0.001~10 s~(-1)的条件下对2101双相不锈钢进行了热压缩实验,以研究热变形参数对其热加工行为的影响规律。结果表明,相同应变速率下,随温度升高,流变曲线由动态再结晶向动态回复转变。变形速率由0.001 s~(-1)增至0.01和0.1 s~(-1)提高了动态再结晶温度范围,而1和10 s~(-1)的较高应变速率不利于动态再结晶。在应变速率为0.001~0.1s~(-1)、变形温度为1253~1323 K时,峰值应力所对应的应变越小,奥氏体动态再结晶越容易发生,有利于等轴状再结晶组织形成。低应变速率下,变形温度升高使奥氏体再结晶晶粒长大,且Zener-Hollomon参数较大时,动态再结晶效果变差与Mn稳定奥氏体能力较Ni弱有关。基于热变形方程计算得到该不锈钢热变形激活能Q=464.49 k J/mol,略高于2205双相不锈钢,并建立了峰值流变应力本构方程。结合不同变形条件下的应变曲线和显微组织,根据热加工图确定了最佳热加工区域为应变速率在0.001~0.1 s~(-1)、变形温度为1220~1350 K,该区域功率耗散系数处于0.40~0.47的较高值,发生了明显奥氏体动态再结晶。     

Mg-6Gd-1.2Y-0.53Zr合金的热变形行为及热加工图

采用热模拟试验法研究了变形温度(340～500℃)和应变速率(0.01～25 s^-1)对均匀化态Mg-6Gd-1.2Y-0.53Zr合金动态再结晶(DRX)临界应变及体积分数的影响,通过构建热加工图优化了其热加工工艺参数范围。结果表明,在0.01～1 s^-1的低应变速率下,该合金的动态再结晶(DRX)临界应变量随变形温度的升高而升高,而在10～25 s^-1高应变速率下,DRX临界应变量随变形温度的升高而略微下降。应变速率及变形温度的升高都使DRX体积分数增大,在500℃、25 s^-1条件下,合金的动态再结晶体积分数最高,达90.0%。根据构建的热加工图,当变形量在30%～80%之间时,较佳的热加工工艺区间为400～500℃、0.01～1 s^-1以及420～500℃、10～25 s^-1。在10～25 s^-1应变速率下,当变形量为10%～80%时,合金最适宜的变形温度为460～500℃。     

Fe-Mn-Al-Ni-C低密度钢铸锭的再结晶特征与热加工图

在变形温度为925～1150℃和应变速率为0.01～10 s^-1的条件下，采用THERMECMASTOR 100 kN热模拟试验机研究了Fe-15Mn-15Al-5Ni-1C低密度钢铸锭的热变形行为，分析了其动态再结晶(DRX)特征，并绘制了其在不同应变量下的热加工图。结果表明：该铸锭变形后的组织主要由高温铁素体(δ-F)、奥氏体(A)、α-铁素体(α-F)和κ-碳化物组成。δ-F和κ-碳化物的存在使得铸锭的热加工性能变差，只有在变形温度升高到1125℃或者应变速率下降到0.02 s^-1时，铸锭才能获得再结晶组织，实现软化。Fe-15Mn-15Al-5Ni-1C低密度钢存在两个适宜的热加工区域，区域1:变形温度为1125～1150℃,应变速率为0.01～0.5 s^-1;区域2:变形温度为925～1080℃,应变速率为0.01～0.02 s^-1。     

新型Co-Ni基高温合金的热变形行为及微观组织演变

利用Gleeble-3800热模拟试验机对新型Co-Ni基高温合金进行热压缩试验,研究其在变形温度为950～1100℃、应变速率为0.01～10 s^-1、真应变为0.693时的热变形行为和微观组织演变。结果表明,合金流动应力随变形温度的升高或应变速率的降低而减小。合金平均晶粒尺寸随变形温度的升高而增加,降低变形温度和提高应变速率可细化动态再结晶晶粒。基于EBSD和TEM分析表明,合金热变形过程中非连续动态再结晶(DDRX)作为主要动态再结晶(DRX)机制,孪晶形核作为辅助形核机制。     

13Cr超级马氏体不锈钢热压缩变形行为与组织演变

通过Gleeble-3500热模拟试验机对13Cr超级马氏体不锈钢进行单道次压缩变形试验,系统研究变形温度在950~1150 ℃、应变速率为0.001~10 s^-1条件下的热变形行为。利用双曲正弦模型建立了13Cr超级马氏体不锈钢的流变应力本构方程,求得试验钢的热变形激活能为412 kJ/mol,并基于动态材料模型(DMM)理论绘制了材料的热加工图,得出材料的最佳热变形工艺参数窗口为:变形温度1032~1072 ℃,应变速率0.039~0.087 s^-1。组织演变结果表明,试验钢在高变形温度和低应变速率的条件下,容易发生动态再结晶。当应变速率一定时(0.01 s^-1),变形温度从950 ℃升到1050 ℃,动态再结晶的体积分数从18.7%升高到60.1%,组织的再结晶程度提高,晶粒均匀细小;当变形温度一定时(1050 ℃),随着应变速率的降低,动态再结晶的晶粒长大粗化。     

一种中碳高硅弹簧钢的热变形行为及流变应力模型

利用MMS-200热模拟试验机对一种中碳高硅弹簧钢进行了单道次热压缩试验,研究了该钢在变形温度为900~1 100 ℃及应变速率为0.1~10 s^-1条件下的热变形行为,建立了应变补偿的Arrhenius流变应力预测模型。结果表明,应变速率和变形温度对该弹簧钢的奥氏体动态再结晶过程有显著影响。当变形速率为0.1、5、10 s^-1时,在所有变形温度下均发生奥氏体动态再结晶;当变形速率为1 s^-1且变形温度超过950 ℃时,奥氏体发生动态再结晶,其热变形激活能为445.5 kJ/mol。通过对真应力的预测值与试验值的对比,得出应变补偿Arrhenius模型具有准确性和预测性,其相关系数为0.976,平均相对误差为4.73%。     

Nb-Ti连铸坯热芯大压下轧制动态再结晶行为研究

采用热模拟单道次压缩实验,研究了Nb-Ti连铸坯热芯大压下轧制中动态再结晶行为及奥氏体晶粒转变规律。结果表明,变形温度越高,应变速率越低,发生动态再结晶的临界应变值越小,动态再结晶越充分。在变形温度1 350 ℃,继续增加应变至0.8和增加应变速率至10 s^-1,奥氏体晶粒尺寸并未得到进一步细化,反而较应变0.5和应变速率5 s^-1下的奥氏体晶粒更加粗大。这是因为高温粘塑性区的金属晶间粘性流动增加,位错增殖速度增大,在动态再结晶过程中会重新形成新的无畸变再结晶晶粒,这些新的无畸变晶粒的亚动态再结晶动力学极快,在较大驱动力下使奥氏体晶界快速迁移,从而使奥氏体发生一定程度的粗化。     

Research on Hot Working Behavior of Low-Nickel Duplex Stainless Steel 2101EI北大核心CSCD

The thermal deformation difference of two phases for duplex stainless steel (DSS) makes hot working difficult, 2101 DSS substitute Mn, N for Ni to stabilize austenite phase, which will significantly affect hot deformation behavior. Hot compression tests in the temperature ranging from 1123 to 1423 K and strain rate ranging from 0.001 to 10 s(-1) were carried out on a Geeble-3800 thermal simulator for 2101 DSS. At the same strain rate, the flow curve characteristics of 2101 DSS changed from dynamic re-crystallzation (DRX) to dynamic recovery with increasing deformation temperature. Increasing defomation stain rate from 0.001 s(-1) to 0,01 and 0.1 s(-1) increased DRX temperature range, but higher strain rate of 1 and 10 s(-1) is not beneficial to DRX occurrence. In the deformation temperature region of 1253 similar to 1323 K and low strain rate of 0.01 similar to 0.1 s(-1), the smaller strain value corresponding to the peak stress, the austenite DRX is more likely to occur, which is beneficial to the equiaxed recrystallized grains formation. At low strain rate, the recrystallization grain grows up with the increase of deformation temperature, the worse effect of austenite DRX is related to weakened austenite stabilized ability of Mn substitution for Ni at high Zener-Hollomon parameter values. Based on the thermal deformation equation, the apparent activation energy Q was calculated as 464.49 kJ/mol, which is slightly higher than that of 2205 DSS, and the constitutive equation of the peak flow stress was established. By combining with flow curve and microstructure analysis, the processing map exhibits the optimum processing conditions are in deformation temperature ranging from 1220 to 1350 K and strain rate ranging from 0.001 to 0.1 s(-1) with high power dissipation co-efficient of 0.40 similar to 0.47, under which the austenite DRX obviously occurred.     

Hot Deformation and Dynamic Recrystallization Behavior of Austenite-Based Low-Density Fe–Mn–Al–C Steel

The hot deformation and dynamic recrystallization(DRX) behavior of austenite-based Fe–27Mn–11.5Al–0.95 C steel with a density of 6.55 g cm-3were investigated by compressive deformation at the temperature range of900–1150 °C and strain rate of 0.01–10 s-1. Typical DRX behavior was observed under chosen deformation conditions and yield-point-elongation-like effect caused by DRX of d-ferrite. The flow stress characteristics were determined by DRX of the d-ferrite at early stage and the austenite at later stage, respectively. On the basis of hyperbolic sine function and linear fitting, the calculated thermal activation energy for the experimental steel was 294.204 k J mol-1. The occurrence of DRX for both the austenite and the d-ferrite was estimated and plotted by related Zener–Hollomon equations. A DRX kinetic model of the steel was established by flow stress and peak strain without considering dynamic recovery and d-ferrite DRX. The effects of deformation temperature and strain rate on DRX volume fraction were discussed in detail. Increasing deformation temperature or strain rate contributes to DRX of both the austenite and the d-ferrite, whereas a lower strain rate leads to the austenite grains growth and the d-ferrite evolution, from banded to island-like structure.     

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。     

21Cr节镍型双相不锈钢高温形变过程的静态软化分析

采用热模拟研究了21Cr双相不锈钢在高温变形道次间隔时间内的静态软化行为,讨论了变形温度、应变速率和变形程度对静态再结晶行为及微观组织的影响。结果表明,变形条件通过影响两相内部应变分配进一步影响双相不锈钢静态软化行为。随着变形温度和变形程度增加,铁素体相内承担的应变增加,铁素体内部再结晶程度增加,促进双相不锈钢的静态软化程度增加;而随着应变速率的增加,试验钢静态软化率的变化规律与奥氏体相承担的应变变化规律相同,都呈现出先降低后升高的变化趋势,奥氏体相在应变速率为1 s^-1时的内部再结晶程度最低。21Cr双相不锈钢静态再结晶激活能约为301 kJ/mol。     

Hot Deformation Behavior and Processing Map of a Cu-Bearing 2205 Duplex Stainless Steel

The hot deformation behavior and processing map of Cu-bearing 2205 duplex stainless steel(2205-Cu DSS) were investigated at temperatures of 950-1150℃and strain rates of 0.01-10 s~(-1).The effects of Cu addition and different deformation parameters on deformation behavior were,respectively,characterized by analyzing flow curves,constitutive equations and microstructures.The results indicated that the shapes of flow curves strongly depended on the volume fraction of two phases.When deformed at low strain rate,DRV in ferrite was prompted with increase in the temperature and was further developed to continuous DRX.At high strain rate,flow localization preferentially occurred in ferrite at low deformation temperature due to the strain partitioning and relatively less fraction of ferrite.The activation energy for 2205-Cu DSS was 452 kJ/mol and was found to connect with the variation of strain,strain rate and deformation temperature.The optimum hot deformation parameters for 2205-Cu DSS were obtained in the temperature range of 1100-1150℃and strain rate range of 0.1-1 s~(-1)with a peak power dissipation efficiency of 41%.Flow localization was the main way to lead to flow instability.Meanwhile,the Cu-rich precipitates were generated within a few ferrite grains when deformed at temperature lower than 1000℃.The interaction between dislocations and Cu-rich precipitates at high strain rate,as well as the limited DRV in ferrite and DRX in austenite,contributed to the complex microstructure and flow behavior.     

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.     

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。     

Microstructure Evolution and Strain-Dependent Constitutive Modeling to Predict the Flow Behavior of 20Cr–24Ni–6Mo Super-Austenitic Stainless Steel During Hot Deformation

Hot compression tests were carried out with specimens of 20 Cr–24 Ni–6 Mo super-austenitic stainless steel at strain rate from 0.01 to 10 s~(-1) in the temperature range from 950 to 1150 °C, and flow behavior was analyzed. Microstructure analysis indicated that dynamic recrystallization(DRX) behavior was more sensitive to the temperature than strain rate, and full DRX was obtained when the specimen deformed at 1150 °C. When the temperature reduced to 1050 °C, full DRX was completed at the highest strain rate 10 s~(-1) rather than at the lowest strain rate 0.01 s~(-1) because the adiabatic heating was pronounced at higher strain rate. In addition, flow behavior reflected in flow curves was inconsistent with the actual microstructural evolution during hot deformation, especially at higher strain rates and lower temperatures. Therefore, flow curves were revised in consideration of the effects of adiabatic heating and friction during hot deformation. The results showed that adiabatic heating became greater with the increase of strain level, strain rate and the decrease of temperature, while the frictional effect cannot be neglected at high strain level. Moreover, based on the revised flow curves, strain-dependent constitutive modeling was developed and verified by comparing the predicted data with the experimental data and the modified data. The result suggested that the developed constitutive modeling can more adequately predict the flow behavior reflected by corrected flow curves than that reflected by experimental flow curves, even though some difference existed at 950 °C and0.01 s~(-1). The main reason was that plenty of precipitates generated at this deformation condition and affected the DRX behavior and deformation behavior, eventually resulted in dramatic increase of deformation resistance.