GH2907合金热变形行为及热加工图

通过热模拟压缩实验研究了GH2907合金在变形温度为950~1100℃、应变速率为0.01~10s_^-1、变形量为60%条件下的热变形行为,流变应力随着变形温度的升高或应变速率的降低而显著降低;根据Arrhenius方程和Zener-Hollomon参数,计算了热变形激活能Q,建立了GH2907合金的热变形本构方程;根据动态材料模型,确定了GH2907合金在不同应变下的功率耗散图,功率耗散效率η较高的区域位于温度为1050~1100℃,应变速率为0.01~0.03s_^-1范围,在该变形区域内组织发生了明显的动态再结晶现象;基于Preased失稳判据,绘制了GH2907合金在不同应变下的热加工图,流变失稳区位于高温高应变速率区域,即温度为970~1100℃,应变速率为0.6~10s_^-1范围,在该变形区域内动态再结晶晶粒沿着绝热剪切带和局部流动分布。根据GH2907合金热加工图及微观组织分析得到适宜的加工区域是温度为1050~1100℃,应变速率为0.01~0.03s_^-1范围。     

低成本Ti-Al-V-Fe-O钛合金热变形行为及热加工图

通过真空非自耗熔炼炉制备了低成本Ti-6Al-2.5V-1.5Fe-0.15O合金,利用Gleeble-1500D热模拟机,研究了其热加工参数为：变形温度875℃-1100℃、应变速率0.001s_-1-1s_-1,变形量为70%时的热变形行为,建立了Ti-6Al-2.5V-1.5Fe-0.15O合金考虑应变量的Arrhenius本构方程,基于动态材料模型建立热加工图。研究结果表明：变形温度升高,应变速率降低,流变应力降低。通过本构方程计算可得两相区平均热激活能为398.824KJ/mol,远大于纯钛自激活能,表明热变形软化机制与动态再结晶有关。单相区热激活能为210.93KJ/mol,略大于纯钛自激活能,以动态回复为主。通过热加工图确定两个失稳区,中等变形温度(950℃-1070℃)高应变速率(0.31-0.1s_-1)易发生绝热剪切,结合热加工图确定适合的加工区间：应变速率为0.001-0.01s_-1,变形温度为875℃-925℃。     

一种新型Ni-Cr-Co基合金的热变形行为及其组织演变研究

采用Gleeble-3800热压缩实验机研究了新型Ni-Cr-Co基合金在1050~1250 ℃、0.001~1 s_^-1条件下的热变形行为,并利用EBSD探讨了变形温度和应变速率对合金组织演变和动态再结晶形核机制的影响。结果表明,流变应力随变形温度的升高而降低,而随应变速率的增大而增加。基于流变应力曲线,建立合金的Arrhenius本构方程和热加工图,得到热变形激活能为520.03 kJ/mol,最佳热加工区间为1175~1250 ℃、0.006~1 s_^-1,该区域最大功率耗散效率为45％。动态再结晶分数随变形温度的升高和应变速率的降低而增加,且动态再结晶过程形成均匀细小的等轴晶粒以及∑3孪晶界。动态再结晶形核主要以晶界“弓出”为特征的不连续动态再结晶机制主导。低温高应变速率下,持续亚晶转动诱导的连续动态再结晶作为辅助形核机制发挥作用。     

反应堆耐压壳体用Ti-5Al-3V-3Zr-0.7Cr合金热变形行为研究

利用Gleeble-3800型热模拟试验机对经过三次真空自耗熔炼的Ti-5Al-3V-3Zr-0.7Cr(wt%)合金进行热模拟等温压缩试验,研究了在750~900℃及0.001~1s_-1应变速率下的高温流变行为及再结晶行为。结果表明,在合金的高温变形过程中,流变曲线呈现出明显的先硬化后软化的流变行为特征,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;造成该合金流变软化的主要原因是动态再结晶。动态再结晶的临界应变与峰值应变之间成线性关系,随着温度和真应变增加,再结晶体积分数呈“S”型增加;应变速率减小,再结晶体积分数也呈抛物线增长。合金的最佳高温塑形变形参数为：750℃/0.001-0.01s_-1和850-900℃/ 0.01-0.1s_-1。     

锻态GH4742合金的热变形行为及组织性能演变

利用单道次等温压缩实验获得了锻态GH4742合金在变形温度为 1020~1150 ℃、应变速率为0.001~1 s_^-1、真应变为0.65时的真应力-应变曲线,构建了GH4742合金的热变形本构方程和热加工图,并采用SEM、EBSD等研究了热变形过程中微观亚结构以及γ′相的演变规律,建立了变形工艺条件-组织形态差异-性能变化之间的关联性。结果表明：合金的组织性能演化机制与Z参数密切相关,1080 ℃低温变形时,应变速率由0.001 s_^-1增加至1 s_^-1后,lnZ值由75.6增加至82.6,热效应增强,小角度晶界比例降低,动态再结晶比例增加,组织发生细化,基体硬度增加;1110 ℃高温变形时,随着应变速率增加,lnZ值由74增加至78.5,位错滑移和晶界迁移减缓,小角度晶界比例增加,动态再结晶比例降低,加工硬化程度增加,基体硬度增加。GH4742合金不发生动态再结晶晶粒粗化的临界lnZ值为73。结合热加工图和变形组织分析得出锻态GH4742合金良好的加工区域为变形温度1110～1150 ℃、应变速率0.01～0.1s_^-1。     

Ti60合金的热变形行为及工艺参数优化

在变形温度600～950℃,应变速率0.001～10s_^-1条件下,采用Thermecmaster-Z型热加工模拟试验机对Ti60合金进行等温恒应变速率压缩实验。通过分析流动应力行为,计算应变速率敏感指数m和应变硬化指数n,并综合考虑加工图和变形微观组织来研究该合金的热变形行为,得到优化的工艺参数范围。研究结果表明,Ti60合金的流动应力-应变曲线在不同热力参数条件下分别呈现流动稳态型和流动软化型。应变速率敏感指数m随着变形温度升高和应变速率降低而增大。应变硬化指数n随着变形温度升高而减小;随着应变速率的增加在低应变速率(0.001～0.1s_^-1)区间增大,在高应变速率(1～10s_^-1)区间减小;随着应变的增加在高温段(800～950℃)的低应变速率(0.001～0.1s_^-1)区间较缓慢地减小,在高温段(800～950℃)的高应变速率(1～10s_^-1)区间以及低温段(600～750℃)的所有应变速率(0.001～10s_^-1)区间较明显地减小。Ti60合金存在两个功率耗散效率峰值区域,其对应的热力参数窗口分别为温度725～875℃,应变速率≤0.003s_^-1和温度875～938℃,应变速率≤0.04s_^-1。从流动应力行为、应变速率敏感指数m、应变硬化指数n以及加工图综合考虑,Ti60合金的最佳热加工工艺参数为：温度800～875℃,应变速率0.001～0.003s_^-1,或温度875～938℃,应变速率0.001～0.04s_^-1。     

粉末TC4钛合金热压缩动态软化行为分析

通过热模拟压缩实验获得的应力应变曲线表明粉末TC4钛合金在温度为850~950℃,应变速率为0.1~10s_^-1范围内变形时具有加工硬化和连续的动态软化特性,建立了材料本构方程,很好的描述了粉末TC4钛合金的流变行为。进一步对动态软化行为进行了分析,并计算了各种因素对软化的影响程度。结果表明：变形温度越低,应变速率越小,流动软化程度越大;在应变速率为1s_^-1和10s_^-1时,主要是变形热导致流动软化;当应变速率为0.1s_^-1,温度为850℃和900℃时,有变形热、动态相变和α相形态演化三种软化因素,且温度越低,α相形态演化导致的软化占比越大,温度增加,动态相变软化所占比例增加;当应变速率为0.1s_^-1,变形温度为950℃时,有变形热和动态相变两种软化因素,变形量增加,动态相变软化所占比例增大。     

Ti-6Al-3Nb-2Zr-1Mo钛合金热变形行为及加工图

利用Gleeble-3800热模拟试验机,在变形温度为820-1060℃及应变速率为0.001-1s_-1参数范围内对Ti-6Al-3Nb-2Zr-1Mo钛合金进行等温恒应变速率压缩试验。建立了该合金的高温变形本构方程,得到两相区和单相区的表面激活能分别为764.714 和126.936 kJ/mol。基于DMM和Prasad失稳准则建立了应变为0.4和0.7时的热加工图。分析加工图发现: Ti-6Al-3Nb-2Zr-1Mo钛合金在840–1060 ℃,应变速率为0.001–0.1 s_-1,之间主要发生DRV/DRX,此区间变形时耗散率峰值51%分别出现在940℃/0.001s_-1和880℃/1s_-1,其变形后微观组织演变机制与热加工图匹配较好,当变形发生在820℃,较高应变速率(≥1s_-1)下该合金加工时易发生流变失稳现象。     

超高W含量NiW中/重合金热加工行为研究

采用Gleeble-1500D研究了高密度Ni-42W-10Co-1Mo(wt.%)中/重合金(MHA)在1150~1300°C和应变速率0.001~1s_?1^{下流变行为和}微观组织演化规律。研究结果表明,该合金初始组织由面心立方基体和初生σ相构成。流变应力对变形温度和应变速率敏感,在1150℃变形,应力-应变曲线呈现出典型的动态回复软化特征,而在1200-1350℃变形呈现出典型动态再结晶软化特征。基于应力-应变曲线建立Arrhenius本构方程,计算得到Ni-42W-10Co-1Mo合金热变形激活能为446.2 kJ/mol。基于动态材料模型构建该合金热加工图,发现在1300℃高应变速率下存在失稳区。通过微观组织观察,揭示其动态再结晶机制主要为不连续动态再结晶,σ相促进动态再结晶优先在其周围形核。最终优化获得了Ni-42W-10Co-1Mo合金最佳的热加工窗口：1250–1300°C,应变速率0.1–0.01s_?1^。     

粉末冶金态与铸态Ti-5553合金高温不连续屈服行为和绝热温升效应对比研究

本文系统地研究了粉末冶金态与铸态Ti-5553合金在温度为700 ℃~1100 ℃、应变速率为0.001 s_^-1~10 s_^-1条件下的高温不连续屈服行为和绝热温升效应,并对这两种同名义成分不同制备工艺的钛合金进行了对比研究。结果表明：两种合金不连续屈服的幅度均与应变速率呈正相关关系,并与温度呈近似负相关关系, 两种合金中出现的不连续屈服现象符合动态理论。在相同变形条件下,铸态合金中不连续屈服的幅度更大,其原因是相对于粉末冶金态合金,铸态合金中的起始位错密度低,这更有利于晶界处可动位错的突然增殖与扩展。两种合金在热变形中绝热温升的大小均随应变速率的升高而逐渐增大,并随着变形温度的升高而逐渐降低。在相同变形条件下,粉末冶金态合金的绝热温升效应相比与铸态合金较弱,这是因为粉末冶金态合金具有较低的变形抗力和较高的协调变形能力。     

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。     

Flow curve correction and processing map of 2050 Al–Li alloy

Hot compression tests of 2050 Al–Li alloy were performed in the deformation temperature range of 340–500 °C and strain rate range of 0.001–10 s^–1 to investigate the hot deformation behavior of the alloy. The effects of friction and temperature difference on flow stress were analyzed and the flow curves were corrected. Based on the dynamic material model, processing map at a strain of 0.5 was established. The grain structure of the compressed samples was observed using optical microscopy. The results show that friction and temperature variation during the hot compression have significant influences on flow stress. The optimum processing domains are in the temperature range from 370 to 430 °C with the strain rate range from 0.01 to 0.001 s^–1, and in the temperature range from 440 to 500 °C with the strain rate range from 0.3 to 0.01 s^–1; the flow instable region is located at high strain rates (3–10 s^–1) in the entire temperature range. Dynamic recovery (DRV) and dynamic recrystallization (DRX) are the main deformation mechanisms of the 2050 alloy in the stable domains, whereas the alloy exhibits flow localization in the instable region.     

High temperature deformation and processing map of a NiTi intermetallic alloy

The deformation behavior of a 49.8 Ni-50.2 Ti (at pct) alloy was investigated using the hot compression test in the temperature range of 700 °C–1100 °C, and strain rate of 0.001 s^?1 to 1 s^?1. The hot tensile test of the alloy was also considered to assist explaining the related deformation mechanism within the same temperature range and the strain rate of 0.1 s^?1. The processing map of the alloy was developed to evaluate the efficiency of hot deformation and to identify the instability regions of the flow. The peak efficiency of 24–28% was achieved at temperature range of 900 °C–1000 °C, and strain rates higher than 0.01 s^?1 in the processing map. The hot ductility and the deformation efficiency of the alloy exhibit almost similar variation with temperature, showing maximum at temperature range of 900 °C–1000 °C and minimum at 700 °C and 1100 °C. Besides, the minimum hot ductility lies in the instability regions of the processing map. The peak efficiency of 28% and microstructural analysis suggests that dynamic recovery (DRV) can occur during hot working of the alloy. At strain rates higher than 0.1 s^?1, the peak efficiency domain shifts from the temperature range of 850 °C–1000 °C to lower temperature range of 800 °C–950 °C which is desirable for hot working of the NiTi alloy. The regions of flow instability have been observed at high Z values and at low temperature of 700 °C and low strain rate of 0.001 s^?1. Further instability region has been found at temperature of 1000 °C and strain rates higher than 1 s^?1 and at temperature of 1100 °C and all range of strain rates.     

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.     

Hot working behavior of near alpha titanium alloy analyzed by mechanical testing and processing map

The hot deformation characteristics of the Ti−5.7Al−2.1Sn−3.9Zr−2Mo−0.1Si (Ti-6242S) alloy with an acicular starting microstructure were analyzed using processing map. The uniaxial hot compression tests were performed at temperatures ranging from 850 to 1000 °C and at strain rates of 0.001−1 s⁻¹. The developed processing map was used to determine the safe and unsafe deformation conditions of the alloy in association with the microstructural evolution by SEM and OM. It was recognized that the flow stress revealed differences in flow softening behavior by deformation at 1000 °C compared to the lower deformation temperatures, which was attributed to microstructural changes. The processing map developed for typical strain of 0.7 in two-phase field exhibited high efficiency value of power dissipation of about 55% at 950 °C and 0.001 s⁻¹, basically due to extensive globularization. An increase in strain rate and a decrease in temperature resulted in a decrease in globularization of α lamellae, while α lamellar kinking increased. Eventually, the instability domain of flow behavior was identified in the temperature range of 850−900 °C and at the strain rate higher than 0.01 s⁻¹ reflecting the flow localization and adiabatic shear banding. By considering the power efficiency domains and the microstructural observations, the deformation in the temperature range of 950−1000 °C and strain rate range of 0.001−0.01 s⁻¹ was desirable leading to high efficiencies. It was realized that (950 °C, 0.001 s⁻¹) was the optimum deformation condition for the alloy.     

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.     

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。     

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。     

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

Hot Deformation Mechanisms of an As-Extruded TiAl Alloy with Large Amount of Remnant Lamellae

The hot deformation mechanisms of an as-extruded Ti-44Al-5V-1Cr alloy with a large amount of remnant lamellae were investigated by hot compression tests at temperatures of 900-1250 °C and strain rates of 0.001-1 s^?1. The hot processing map of the as-extruded Ti-44Al-5V-1Cr alloy was developed on the basis of dynamic materials modeling and the Prasad criteria. There were four different domains in the hot processing map, according to the efficiency of power dissipation, η. The flow soft and hot deformation mechanisms for different domains were illustrated in the context of microstructural evolution during the process of deformation. As a result, the dynamic recrystallization and superplastic deformation occurred at 1125-1150 °C near 0.001 s^?1, and this region is suitable for superplastic forming. The α phase dynamic recrystallization and dynamic recovery occurred at 1250 °C and 0.1 s^?1. The existence of small amount of the γ and β phases effectively inhibited the growth of α grains.