Mechanical Behaviors and Microstructural Characteristics of TC11 Alloy during Hot Deformation

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Hot Deformation Behavior of TB8 Alloy near the β-Transus

通过热模拟压缩试验,对TB8钛合金β相变点附近的高温变形行为进行了研究.热模拟压缩试验的变形温度为650～900℃,应变速率为0.01～10 s-1.通过试验分别得到了TB8钛合金双相区(α+β)和单相区(β)的流变应力曲线,并分别研究了流变应力与变形温度、应变速率和微观组织演化的关系.在10 s-1的高应变速率下,真应力-真应变曲线在850和900℃出现了双峰,这一现象未见报道.通过本构关系推导,得到了TB8钛合金双相区(α+β)和单相区(β)的表观激活能分别为233.0151和197.8987 kJ/mol.另外,建立了TB8钛合金双相区(α+β)和单相区(β)相应的流变应力本构方程.     

Characterization of Hot Deformation Behavior of a Novel Al–Cu–Li Alloy Using Processing Maps

The isothermally compression deformation behavior of an elevated Cu/Li weight ratio Al–Cu–Li alloy was investigated under various deformation conditions.The isothermal compression tests were carried out in a temperature range from 300 to 500 °C and at a strain rate range from 0.001 to 10 s-1.The results show that the peak stress level decreases with temperature increasing and strain rate decreasing,which is represented by the Zener–Hollomon parameter Z in the hyperbolic sine equation with the hot deformation activation energy of 218.5 k J/mol.At low Z value,the dynamic recrystallized grain is well formed with clean high-angle boundaries.At high Z value,a high dislocation density with poorly developed cellularity and considerable fine dynamic precipitates are observed.Based on the experimental data and dynamic material model,the processing maps at strain of 0.3,0.5 and 0.7 were developed to demonstrate the hot workability of the alloy.The results show that the main softening mechanism at high Z value is precipitate coarsening and dynamic recovery;the dynamic recrystallization of the alloy can be easily observed as ln Z B 29.44,with peak efficiency of power dissipation of around 70%.At strains of 0.3,0.5 and 0.7,the flow instability domains are found at higher strain rates,which mainly locate at the upper part of processing maps.In addition,when the strain rate is 0.001 or 0.02 s-1,there is a particular instability domain at 300–350 °C.     

Influence of Microstructural Evolution on the Hot Deformation Behavior of an Fe–Mn–Al Duplex Lightweight Steel

The hot deformation behavior of Fe–26 Mn–6.2 Al–0.05 C steel was studied by experimental hot compression tests in the temperature range of 800–1050 °C and strain rate range of 0.01–30 s21 on a Gleeble-3500 thermal simulation machine. The microstructural evolution during the corresponding thermal process was observed in situ by confocal laser scanning microscopy. Electron backscattered diffraction and transmission electron microscopy analyses were carried out to observe the microstructural morphology before and after the hot deformation. Furthermore, interrupted compression tests were conducted to correlate the microstructural characteristics and softening mechanisms at different deformation stages.The results showed that hot compression tests of this steel were all carried out on a duplex matrix composed of austenite and d-ferrite. As the deformation temperature increased from 800 to 1050 °C, the volume fraction of austenite decreased from 70.9% to 44.0%, while that of d-ferrite increased from 29.1% to 56.0%. Due to the different stress exponents(n) and apparent activation energies(Q), the generated strain was mostly accommodated by d-ferrite at the commencement of deformation, and then both dynamic recovery and dynamic recrystallization occurred earlier in d-ferrite than in austenite.This interaction of strain partitioning and unsynchronized softening behavior caused an abnormal hot deformation behavior profile in the Fe–Mn–Al duplex steel, such as yield-like behavior, peculiar work-hardening behavior, and dynamic softening behavior, which are influenced by not only temperature and strain rate but also by microstructural evolution.     

Characterization of the Hot Deformation Behavior of Cu–Cr–Zr Alloy by Processing Maps

Hot deformation behavior of the Cu–Cr–Zr alloy was investigated using hot compressive tests in the temperature range of 650–850 °C and strain rate range of 0.001–10 s-1. The constitutive equation of the alloy based on the hyperbolic-sine equation was established to characterize the flow stress as a function of strain rate and deformation temperature. The critical conditions for the occurrence of dynamic recrystallization were determined based on the alloy strain hardening rate curves. Based on the dynamic material model, the processing maps at the strains of 0.3, 0.4 and 0.5were obtained. When the true strain was 0.5, greater power dissipation efficiency was observed at 800–850 °C and under0.001–0.1 s-1, with the peak efficiency of 47%. The evolution of DRX microstructure strongly depends on the deformation temperature and the strain rate. Based on the processing maps and microstructure evolution, the optimal hot working conditions for the Cu–Cr–Zr alloy are in the temperature range of 800–850 °C and the strain rate range of 0.001–0.1 s-1.     

High-Temperature Oxidation Kinetics and Behavior of Ti–6Al–4V Alloy

Owing to the high-temperature reactivity of titanium, the oxidation and alloying of titanium during hot working processes is an important variable. The oxidation behavior of Ti–6Al–4V alloy in air was investigated at various temperatures between 850 and 1100 °C for different times. The oxidation kinetics were determined by isothermal oxidation weight gain experiments. The results showed that the oxidation kinetics approximately obeyed a parabolic law. The activation energy of oxidation was estimated to be 199 and 281 kJ mol^?1 when temperature was above and below the beta transformation temperature (T _β), respectively. A model to predict oxidation extent was established based on experimental observations. The oxide scales mainly consisted of TiO₂ with a small amount of Al₂O₃ and TiVO₄. The alpha case was defined as solid solution formed because of oxygen diffusion into the substrate. The difference in the morphology and the formation mechanism of the alpha case at different temperature ranges was mainly owing to the participation of the grain boundary and grain orientation of the nucleation site.     

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.     

The Influence of the Alloy Microstructure on the Oxidation Behavior of Ti–46Al–1Cr–0.2Si Alloy

The influence of microstructure of the two-phase alloyTi–46Al–1Cr–0.2Si on the oxidation behavior in air between600 and 900°C was studied. The oxidation rate, type of scale, and scalespallation resistance were strongly affected by the type of microstructure,i.e., lamellar in as-cast material and duplex after extrusion at1300°C. The oxidation rate was affected by the size and distribution ofthe ₂-Ti₃Al phase, being faster for the extrudedmaterial with coarse ₂-Ti₃Al. The type of oxide scaledetermines the spalling resistance. Cast material developed a uniform scalethat spalled off after short exposure times at 800 and 900°C when a criticalthickness was reached. The extruded material presented a heterogeneous scalewith predominant thick regions formed on -TiAl-₂-Ti₃Algrains and thin scale regions formed on -TiAl grains. Thistype of scale could permit an easier relaxation in the matrix of stressesgenerated by both thermal-expansion mismatch between scale and alloy andoxide growth, resulting in a higher spallation resistance.     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

Deformation Behavior in the Isothermal Compression of Hydrogenated Ti–5.6Al–4.8Sn–2.0Zr–1.0Mo Alloy

The isothermal compression of hydrogenated Ti–5.6Al–4.8Sn–2.0Zr–1.0Mo alloy has been carried out. The experimental result shows that the additional hydrogen significantly decreases the flow stress of Ti–5.6Al–4.8Sn–2.0Zr–1.0Mo alloy. The minimum peak stress at deformation temperature of 830–900 °C corresponds to the hydrogen content of 0.4 wt.%, alternatively the appropriate hydrogen content raises the true strain rate up to one order of magnitude in comparison with the received Ti–5.6Al–4.8Sn–2.0Zr–1.0Mo alloy. X-ray diffraction examination shows the appropriate hydrogen content accelerates the phase transformation so as to improve the workability of this alloy. However, the hydride phase appears when the hydrogen content is about 0.734 wt.%, which increases of the flow stress in comparison to the flow stress of this alloy with hydrogen content of 0.4 wt.%.     

Ti-22Al-25Nb合金热变形行为研究

在温度940～1000℃、应变速率10-2～50s-1、最大变形程度50%条件下利用Gleeble-1500型热模拟试验机对Ti-22Al-25Nb合金的高温流动应力变化规律进行了研究,分析了热变形参数对流动应力的影响规律,并利用Zener-Hollomon参数建立了该合金的本构关系。试验结果表明,应变速率的降低或温度的升高都会使合金的流动应力降低;变形过程中产生的流动软化现象与温升效应和组织变化有关;高应变速率(≥10s-1)条件下发生的应力不连续屈服现象与晶界突然增殖大量可动位错有关,与固溶原子的钉扎无关。     

Ti-6Al-7Nb合金高温塑性变形行为及热加工图研究

本文以Ti-6Al-7Nb合金为研究对象,采用Gleeble-3500热模拟压缩试验机进行不同温度和应变速率压缩试验。分析了Ti-6Al-7Nb合金在变形温度1023 K、1073 K、1123 K、1173 K,应变速率为0.005 s-1、0.05 s-1、0.5 s-1、5 s-1和10 s-1,最大变形量为60%下的高温变形行为及热加工特性。结果表明：变形温度与应变速率对Ti-6Al-7Nb合金的流动应力影响较大,其中应变速率是影响加工硬化过程的主要因素。Ti-6Al-7Nb合金在发生热塑性变形时后的物相主要有：初生α相、片层状α相、次生α相、片层状β相以及发生球化的初生α相等。Arrhenius本构方程模型适用于低温低应变速率和高温高应变速率形变条件的Ti-6Al-7Nb合金高温变形。利用MATLAB构建计算确定了合金最佳塑性变形区间为：应变速率0.0067 s-1-0.1353 s-1和温度1100-1173 K,在该区间有可能获取Ti-6Al-7Nb合金最佳的塑性变形工艺参数。     

Ti-22Al-26Nb合金热变形本构方程建立及软化行为研究

作为最具潜力的航空航天高温结构材料,Ti2AlNb基合金具有高的比强度和良好的高温蠕变性能。本文对热轧态Ti-22Al-26Nb合金高温变形中的力学行为和再结晶行为进行研究,建立其高温本构关系模型,对其中呈现出的动态再结晶多应力峰值曲线特征（以1000℃,0.1s-1为例）进行拟合分析。结果表明：基于双曲正弦函数建立Ti-22Al-26Nb合金的高温本构关系模型的精度较高,最大误差为2.6%,可以很好地描述合金在高温变形时各热力学参数之间高度非线性的复杂关系,由修正的Avrami方程预测得知再结晶体积分数与应变呈现典型的再结晶动力学增长趋势,揭示了该合金高温变形过程中复杂的软化行为。     

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.     

Ti-1300合金的热变形行为研究

采用Gleeble-1500型热模拟试验机对Ti-1300近β钛合金进行了等温恒应变速率压缩试验.变形温度范围为:920～1010℃,应变速率范围为:0.01～10 s-1,最大变形量为80%.根据试验数据建立了Ti-1300合金高温热变形行为的流变应力模型,得出该合金的变形激活能为177.59 kJ/mol.结合样品的显微组织分析可知,该合金在低应变速率下发生了动态再结晶,且随着温度的升高,再结晶晶粒呈现长大的趋势:在高应变速率下以动态回复为主.结果表明,为获得细小的再结晶组织,Ti-1300钛合金宜在相变点以上50～150℃的温度范围内采用较低的变形速率进行锻造.     

应用加工图理论研究Ti2AlNb基合金的高温变形特性

基于动态材料模型（DMM），建立了Ti2AlNb基合金（Ti-22Al-25Nb）在温度94012-1060℃，应变速率0．001s^-1-10s^-1范围内的加工图，并利用该图分析了合金的高温变形特性。结果发现：在温度94012～97012，应变速率0．4s^-1～10s^-1和温度970℃—1060℃，应变速率1s^-1～10s^-1范围为流动失稳区，前者范围内主要发生绝热剪切变形和45°角剪切开裂，功率耗散率达到最小值；后者区域内以局部塑性流动和纵向开裂为主，功率耗散率小于33％。热加工图的其余部分为塑性加工的“安全区”，主要发生再结晶。在温度94012～970℃，应变速率0．001s^-1-0．4s^-1范围，以α2／O相板条球化为主；在温度970℃～1030℃，应变速率0．001s^-1～1S^-1范围，功率耗散率为35％-45％，呈现连续再结晶特征。在温度1030℃～1060℃。麻蛮谏率0．001s^-1-0.1s^-1范围。功率耗散率为45％～66％。达最大值,发生连续再结晶晶粒长大。     

Ti-6Al-4V合金超塑性变形及微观组织演变

通过高温拉伸试验研究了Ti-6Al-4V合金的高温变形力学行为和超塑性,并对试样断口附近的组织进行了观察。结果表明,随着变形温度的升高或初始应变速率的降低,Ti-6Al-4V合金的流动应力明显减小;Ti-6Al-4V合金的最佳超塑性变形工艺参数为880℃/0.001s-1,最大延伸率为689%,峰值应力仅为30.03MPa;在超塑性拉伸过程中,试样变形区发生明显的动态再结晶,使片层状的α相晶粒破碎、细化和等轴化,促进超塑性的增加;随着变形温度的提高、变形量增大和变形时间的加长,再结晶α相发生了聚集长大,从而使显微组织明显粗化。对于双态组织的两相钛合金,最佳超塑性变形温度应低于或等于片层状α→β转变的终了温度。     

Hot Deformation Behavior and Dynamic Recrystallization Characteristics in a Low-Alloy High-Strength Ni–Cr–Mo–V Steel

This study presented a quantitative investigation of deformation behavior and dynamic recrystallization of low-alloy highstrength Ni–Cr–Mo–V steels during hot deformation.A series of isothermal compression experiments were performed at temperatures ranging from 800 to 1200°C and strain rates from 0.01 to 10 s~(-1)with a height reduction of 60%.A complete Arrhenius constitutive model and processing maps were developed.The results showed that the constitutive model had the ability to predict the flow stress with an average absolute relative error of\5.7%.The processing maps constructed at strains of 0.2,0.4,and 0.8 showed that flow instability was prone to occur at higher strain.Dynamic recrystallization tended to take place at higher temperatures(900–1200°C)and lower strain rates(0.01–1 s~(-1)).The critical strain for the onset of dynamic recrystallization was determined,and a kinetics model was developed.The predicted results for recrystallization volume fraction and flow stress were compared with the experimental data,which indicated that the model was accurate and reliable.     

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。     

Effect of Nb Content on the Hot Deformation Behavior of S460ML Steel

The hot deformation behavior of S460 ML steel with different Nb contents was investigated by single-pass compression experiment on the Gleeble-1500 thermo-machine simulator at deformation temperatures ranging from 900 to 1200 ℃ and strain rates from 0.1 to 5.0 s~(-1). The critical strain of dynamic recrystallization(DRC) under different conditions was determined by using working hardening rate-strain curves. The relationship between critical strain and peak strain of DRC was discussed. By means of regression analysis method, the hot deformation activation energy of steels and the mathematical model for predicting DRC critical strain were calculated and established, respectively. The results showed that DRC occurred during hot deformation. As the deformation temperature increased and strain rate decreased, the critical strain of DRC decreased. The critical strain(εc) showed linear relationship with peak strain(εp). As Nb content increased,the needed deformation temperature for occurring DRC increased, while the needed strain rate for it decreased. The activation energy for hot deformation of S460 ML steel increased with increasing Nb content.