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。     

Zr-1.0Sn-1.0Nb-0.1Fe合金热变形行为及动态再结晶模型建立

热轧过程中的动态再结晶影响锆合金板材制备过程中的组织和织构遗传性,以及最终力学性能。在本文中,利用Gleeble 3800热模拟实验研究了Zr-1.0Sn-1.0Nb-0.1Fe合金在应变速率为0.01~10s_^-1范围下,变形温度在550℃~700℃的动态再结晶行为。通过对实测应力-应变结果的加工硬化速率分析,确定了动态再结晶开始发生的临界应变和峰值应变。动态再结晶是通过塑性变形过程中流变应力的软化来判断的,并量化为计算的动态回复曲线和实测的应力应变曲线之间的差异。采用计算的临界应变、峰值应变、Zener-Hollomon参数和动态再结晶体积分数对动态再结晶过程进行建模,最终利用EBSD统计所得的再结晶体积分数验证了Zr-1.0Sn-1.0Nb-0.1Fe合金的动态再结晶模型。     

铸态粗晶Ti-5553钛合金高温变形行为与机理研究

本文借助Gleeble-3800热模拟试验机系统地研究了铸态粗晶Ti-5553合金在温度700 ℃~1100 ℃、应变速率为0.001 s_^-1~10 s_^-1条件下的高温变形行为。研究结果表明合金的流变应力对变形温度和速率都有强敏感性,流变软化过程也随变形参数的改变呈现出不同的模式。通过经典的动力学模型,建立了合金高温变形的本构关系和激活能分布图,进一步基于动态材料模型构建了合金的热加工图并实现了对不同加工区间变形机制的识别。合金在低温区(700 ℃)和高速率区( 1 s_^-1)均展现出失稳变形的特征,包括外部开裂、绝热剪切带、局部流变等机制,在实际加工中应对这些加工区域进行规避。合金在800 ℃及中低速率( 0.1 s_^-1)变形下的主导机制为α相的动态析出,在中高温(900 ℃-1100 ℃)及中低速率变形下的主导机制为动态回复与动态再结晶的结合。此外,合金在高温较低应变速率(1100 ℃/0.01 s_^-1)条件的变形中表现出大范围动态再结晶的行为特点并伴随稳定的流变软化,因此此条件附近的参数区间被认定为该合金的最优加工窗口,应在实际加工中给予优先考虑。     

Ti600合金在(α+β)相区的热变形行为和本构模型

对具有片层状初始组织的Ti600合金的热变形行为进行了研究。变形温度范围为800~960 ℃,应变速率范围为10^-3~1 s^-1。随后提出了应变硬化指数（n）来表征流动软化和加工硬化之间的竞争。并且通过分析流变曲线和观察显微组织研究了该合金的软化行为。结果表明,变形参数对Ti600合金的流变行为有显著影响。当变形超过峰值应变之后,n值逐渐降低,动态软化过程开始占主导地位。微观组织分析表明：热变形过程中,α相的弯曲、破碎、动态回复和动态再结晶行为是造成Ti600合金软化的主要原因。最后基于实验数据,建立了3种本构模型,分别是应变补偿Arrhenius模型、Hensel-Spittel模型和修正的Arrhenius模型,来表征Ti600合金的流变行为。将3种模型预测的流变应力与实验结果进行比较,并计算其相关系数值和平均相对误差值来评估模型的准确性。3种模型的相关系数值分别为0.965、0.989和0.997,平均相对误差值分别为12.86%,9.74%和3.26%。这些结果表明,这3种模型都可以描述Ti600合金的流变行为,而修正的Arrhenius模型具有最高的预测精度。     

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。     

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。     

Pd-20W合金的热压缩变形行为和显微组织

采用Gleeble-3500热模拟试验机对Pd-20W合金进行热压缩试验,研究了合金在变形温度1000~1200 ℃、应变速率0.001~1.000 s^-1条件下的流变应力以及变形过程中的显微组织。结果表明,合金的流变应力在变形初期随着真应变的增大快速上升,出现峰值应力后逐渐下降并达到稳态或略有下降。该合金热压缩变形的流变应力行为可用Zener-hollomon参数来描述,拟合计算得到了该材料的形变激活能等参数,获得流变应力的本构方程。热压缩变形后合金组织呈现一定程度的协同变形特征,晶界动态再结晶趋势增强,合金的主要软化机制为动态再结晶,表现出典型的应变诱发晶界形核机制特点。     

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。     

新型钛合金Ti-555热变形行为及热加工图研究

本文通过高温热压缩试验研究Ti-555钛合金热变形过程中变形温度、应变速率对流变应力的影响,采用Arrhenius双曲正弦函数模型推导出Ti-555本构方程,并依据动态材料模型建立了ε=0.6时的热加工图。结果表明,Ti-555钛合金流变应力对应变速率和变形温度较为敏感,热变形时随变形温度升高或应变速率降低,流变应力下降。根据热加工图确定了两个热加工安全区参数为(1)变形温度为850～950 ℃、应变速率为0.6～10 s_^-1;(2)变形温度为950～1150 ℃、应变速率为0.36～0.9 s_^-1。     

316L不锈钢热加工硬化行为及机制

在Gleeble-1500热模拟试验机上, 通过高温压缩实验对316L不锈钢的热加工硬化特点和机制进行了研究. 根据Ludwik幂函数模型对实验数据进行了非线性拟合, 并用 Crussard-Jaoul分析法计算了Ludwik幂函数模型的n值. 实验结果表明: 316L不锈钢在热变形过程中易发生加工硬化, 真应力-应变曲线上未出现应力峰值; 热变形过程中发生了部分动态再结晶, 这一不完全的软化机制无法抵消热加工硬化的作用, 另外在热变形过程中发生了孪生行为, 这是热加工硬化的主要机制之一.     

High temperature deformation behavior and processing map for a nickel-titanium shape memory alloy

The hot deformation behavior of 49.2Ti-50.8Ni shape memory alloy was studied using hot compressive deformation testing in the temperature range of 1023-1323 K and at strain rates of 0.01-10 s^-1. The work-hardening rate was induced to analyze the stress-strain curves, and the critical stress σc and the dynamic recovery saturation stress σ_sat were measured which can be specified approximately by the expressions: σ_sat-1.12σ_p and σ_c-0.86σ_p. An Arrhenius model was calculated to describe the relationship between peak stress and the Z parameter. The relationship between deformation activation energy, the deformation conditions and the effect of Ni component in a binary TiNi alloy on the activation energy were discussed in this work. With the help of electron backscattering diffraction, a connected mode dynamic recrystallization microstructure was confirmed in peak efficiency regimes (850 °C & 0.01 s^-1 and 1050 °C & 10 s^-1) of the processing map.     

ZK31-1.5Y镁合金热变形行为及加工图

研究了ZK31-1.5Y镁合金在变形温度为250~450℃、应变速率为0.001~1 s-1条件下的热压缩变形特性,基于动态材料模型建立了热加工图,并结合真应力-真应变曲线确定了该合金在实验条件下的热变形机制及最佳工艺参数。结果表明:ZK31-1.5Y合金的真应力-真应变曲线主要以动态再结晶和动态回复软化机制为特征,峰值应力和稳态应力随变形温度的降低或应变速率的升高显著增加。合金功率耗散图和失稳图中分别包含了3个效率峰值区和1个马鞍形流变失稳区,峰区效率范围为38%~65%,叠加后形成的加工图给出了实验参数范围内热变形时的最优工艺参数,其热变形温度为350~450℃、应变速率为0.1~1 s-1。当应变量由0.1~0.6逐渐增大时对加工图分布规律影响不大。     

Incoloy825合金热变形过程中的本构模型和特殊晶界演变

采用原始JC模型、修正JC模型和应变补偿Arrhenius方程，描述了Incoloy825合金在不同温度（950～1150 °C）和应变速率（1~10 s^-1）下经摩擦和温升修正后的应力-应变曲线。结果表明，修正后曲线具有明显的动态再结晶特征。与原始JC模型和修正的JC模型相比，Arrhenius应变补偿模型更适合于描述Incoloy825合金热变形过程中的应力应变行为。温度和应变速率对特殊晶界的演变有显著影响。特殊晶界长度分数与动态再结晶分数呈正相关。与冷轧后退火处理工艺相比，热变形工艺调控的特殊晶界长度分数较低，热变形工艺不适合用来调整特殊晶界分数，其原因是在热变形过程中动态再结晶的大量形核造成较小的晶粒团簇。     

Hot deformation behaviors and dynamic recrystallization mechanism of Ti-35421 alloy in β single field

Hot deformation behaviors and microstructure evolution of Ti?3Al?5Mo?4Cr?2Zr?1Fe (Ti-35421) alloy in the β single field are investigated by isothermal compression tests on a Gleeble?3500 simulator at temperatures of 820?900 °C and strain rates of 0.001?1 s^?1. The research results show that discontinuous yield phenomenon and rheological softening are affected by the strain rates and deformation temperatures. The critical conditions for dynamic recrystallization and kinetic model of Ti-35421 alloy are determined, and the Arrhenius constitutive model is constructed. The rheological behaviors of Ti-35421 alloys above β phase transformation temperature are predicted by the constitutive model accurately. The EBSD analysis proves that the deformation softening is controlled by dynamic recovery and dynamic recrystallization. In addition, continuous dynamic recrystallization is determined during hot deformation, and the calculation model for recrystallization grain sizes is established. Good linear dependency between the experimental and simulated values of recrystallized grain sizes indicates that the present model can be used for the prediction of recrystallized grain size with high accuracy.     

Hot Deformation Behavior of NiTiHf Shape Memory Alloy Under Hot Compression Test

In this study, the hot deformation behavior of Ni₄₉Ti₃₆Hf₁₅ alloy was investigated. Compression tests were carried out at temperatures ranging from 800 to 1100?°C and at the strain rates of 0.001?C1/s. The peak stress decreases with increasing deformation temperature and decreasing strain rate, a behavior which can be described by plotting the Zener-Hollomon parameter as a function of stress. It was realized that dynamic recrystallization (DRX) was responsible for flow softening. Most of the samples exhibited typical DRX stress-strain curves with a single peak stress followed by a gradual fall down stress. Microstructure evolution showed that new recrystallized grains formed in the vicinity of grain boundaries. The hyperbolic-sine-type constitutive model of Ni₄₉Ti₃₆Hf₁₅ alloy was obtained to provide basic data for determining reasonable hot-forming process. The activation energy for hot deformation of the Ni₄₉Ti₃₆Hf₁₅ alloy was close to 410?kJ/mol.     

Flow characteristics and constitutive modeling for elevated temperature deformation of a high Nb containing TiAl alloy

The hot deformation behavior of a high Nb containing TiAl alloy with a nominal composition of Ti–42Al–8Nb–(W, B, Y) was investigated at temperatures ranging from 1000 °C to 1150 °C and strain rates from 10⁻³ s⁻¹ to 0.5 s⁻¹ on a Gleeble thermo-simulation machine. The work hardening regime and flow softening behavior of the alloy were analyzed in detail. The results revealed that the onset of dynamic recrystallization (DRX) was quite easy for the present alloy, whereas the dynamic recovery (DRV) was impeded during the hot deformation. The DRX kinetics was studied by Avrami-type equation. The low Avrami exponents of the proposed equation indicate a lower recrystallization rate compared to ordinary metals and alloys. Based on the classical hyperbolic-sine law and the kinematics of DRX, the constitutive equations of the work hardening-recovery period (i.e. flow stress before the peak) and flow softening process (i.e. flow curve after the peak stress) were established for the present alloy, respectively. Comparisons between the experimental and calculated results revealed that except the severely cracked specimens, the stress–strain curves predicted by the established model are in good agreement with experimental results.     

Ti2AlNb合金高应变率下流动应力特征与本构关系

利用电子万能试验机和分离式Hopkinson压杆得到Ti_2AlNb合金准静态拉伸曲线及高应变率下动态压缩应力-应变曲线,观察分析变形后试样的微观组织,研究其高应变率下的流动应力特征。结果表明:在应变率2500～7500 s-1范围内,Ti_2AlNb合金的流动应力对应变率有较强的敏感性,且具有应变强化、应变率增强及增塑效应;应变率为5500、6500、7500s-1的3组试样中观察到了与加载方向约成45°的绝热剪切带。改进Johnson-Cook本构模型,拟合实验数据得到Ti_2AlNb合金室温下的动态塑性本构关系,与实验对比,改进后的模型能够较好地描述Ti_2AlNb合金在高应变率下的流动应力。     

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.     

Hot deformation behavior of Ni–Fe–Ga-based ferromagnetic shape memory alloy – A study using processing map

Ni–Fe–Ga-based alloys form a new class of ferromagnetic shape memory alloys (FSMAs) that show considerable formability because of the presence of a disordered fcc γ-phase. The current study explores the deformation processing of this alloy using an off-stoichiometric Ni₅₅Fe₁₉Ga₂₆ alloy that contains the ductile γ-phase. The hot deformation behavior of this alloy has been characterized on the basis of its flow stress variation obtained by isothermal constant true strain rate compression tests in the 1123–1323 K temperature range and strain rate range of 10⁻³–10 s⁻¹ and using a combination of constitutive modeling and processing map. The dynamic recrystallization (DRX) regime for thermomechanical processing has been identified for this Heusler alloy on the basis of the processing maps and the deformed microstructures. This alloy also shows evidence of dynamic strain-aging (DSA) effect which has not been reported so far for any Heusler FSMAs. Similar effect is also noticed in a Ni–Mn–Ga-based Heusler alloy which is devoid of any γ-phase.