金属学报, 2025, 61(6): 857-865 DOI: 10.11900/0412.1961.2023.00158

研究论文

Ti30Ni50Hf20 高温形状记忆合金的热变形行为

姜沐池1,2, 宫继双3, 杨兴远2, 任德春,2, 蔡雨升2, 李秉洋4,5, 吉海宾,2, 雷家峰1,2

1 中国科学技术大学 材料科学与工程学院 沈阳 110016

2 中国科学院金属研究所 师昌绪先进材料创新中心 沈阳 110016

3 中山大学 航空航天学院 广州 510275

4 中国航天科技创新研究院 先进材料与能源中心 北京 100163

5 北京大学 工学院 北京 100871

Hot Deformation Behavior of Ti30Ni50Hf20 High-Temperature Shape Memory Alloy

JIANG Muchi1,2, GONG Jishuang3, YANG Xingyuan2, REN Dechun,2, CAI Yusheng2, LI Bingyang4,5, JI Haibin,2, LEI Jiafeng1,2

1 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China

2 Shi -changxu Advanced Materials Innovation Center, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

3 School of Aeronautics and Astronautics, Sun Yat-Sen University, Guangzhou 510275, China

4 Advanced Materials and Energy Center, China Academy of Aerospace Science and Innovation, Beijing 100163, China

5 College of Engineering, Peking University, Beijing 100871, China

通讯作者: 任德春,dcren14s@imr.ac.cn,主要从事钛合金及形状记忆合金材料研究;吉海宾,hbji@imr.ac.cn,主要从事结构钛合金研发

责任编辑: 肖素红

收稿日期: 2023-04-10   修回日期: 2023-09-12  

基金资助: 国家自然科学基金项目(52205431)

Corresponding authors: REN Dechun, associate professer, Tel:(024)83970131, E-mail:dcren14s@imr.ac.cn;JI Haibin, professor, Tel: (024)83970131, E-mail:hbji@imr.ac.cn

Received: 2023-04-10   Revised: 2023-09-12  

Fund supported: National Natural Science Foundation of China(52205431)

作者简介 About authors

姜沐池,男,1995年生,博士生

摘要

Ti-Ni二元形状记忆合金的相变温度较低,限制了其在高温领域的应用,添加Hf元素可有效提升相变温度,但也会导致合金可变形性降低。因此,需要研究三元Ti-Ni-Hf高温形状合金热变形行为,确定热加工窗口。本工作采用真空感应熔炼制备Ti30Ni50Hf20高温形状记忆合金,利用OM和Gleeble-3800热模拟试验机等,对Ti30Ni50Hf20高温形状记忆合金的热变形行为进行了研究,变形温度为700、800和900 ℃,应变速率为0.01、0.1、1和10 s-1。结果表明,Ti30Ni50Hf20高温形状记忆合金在高温热变形过程中具有负温度、正应变敏感性,其流变应力随应变速率增加而增大,随变形温度升高而减小,且随变形温度升高再结晶现象增强;利用Arrhenius热变形表达式建立了Ti30Ni50Hf20高温形状记忆合金热加工的本构方程,其应变激活能为527.447 kJ/mol,根据本构方程计算得到的峰值应力理论值与实验值相吻合;根据Ti30Ni50Hf20高温形状记忆合金的动态模型建立其热加工图,确定其最佳热变形加工参数为变形温度880~900 ℃,应变速率0.01~0.04 s-1

关键词: 高温形状记忆合金; 热变形行为; 本构方程; 热加工图

Abstract

The application of Ti-Ni binary alloy at high temperatures is hindered by its low phase transition temperature. To enhance this transition temperature, researchers have explored the addition of metal Hf to Ti-Ni alloy. However, Ti-Ni-Hf high-temperature shape memory alloy exhibits brittleness and lacks favorable thermal deformation characteristics. Thus, a comprehensive investigation of its thermal deformation behavior is essential. During the hot working process, the material undergoes shape and microstructural changes, which are influenced by various processing factors. Consequently, optimizing processing parameters, including temperature, strain, and strain rate, is crucial for producing defect-free components with the desired microstructure. To optimize the hot working technology, single-pass compression tests on a Gleeble-3800 thermo-simulation machine were conducted and the hot deformation behavior and workability of the high-temperature shape memory alloy Ti30Ni50Hf20 were explored. These tests covered a temperature range of 700-900 oC and a strain rate range of 0.01-10 s-1. Flow stress-strain curves for Ti30Ni50Hf20 under different deformation conditions were generated and the evolution of the alloy's microstructure at varying deformation temperatures under a strain rate of 0.01 s-1, as well as at a deformation temperature of 900 oC with different deformation rates were examined. Utilizing a dynamic material model, a processing diagram was constructed and the impact of process parameters on the alloy's processing performance was analyzed. The results indicate that the recrystallization of Ti30Ni50Hf20 high-temperature shape memory alloy increases with the deformation temperature. This alloy exhibits negative temperature sensitivity and positive strain sensitivity, with flow stress increasing as the strain rate rises and decreasing with higher deformation temperatures. A constitutive equation for Ti30Ni50Hf20 high-temperature shape memory alloy during hot working is established, employing the Arrhenius hot deformation equation. The calculated strain activation energy was determined to be 527.447 kJ/mol. It revealed a consistent match between the theoretical and actual peak stress values. Through the assessment of the hot working diagram, the optimal processing parameters are identified as a deformation temperature in the range of 880-900 oC and a strain rate of 0.01-0.04 s-1.

Keywords: high-temperature shape memory alloy; hot deformation behavior; constitutive equation; thermal processing map

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姜沐池, 宫继双, 杨兴远, 任德春, 蔡雨升, 李秉洋, 吉海宾, 雷家峰. Ti30Ni50Hf20 高温形状记忆合金的热变形行为[J]. 金属学报, 2025, 61(6): 857-865 DOI:10.11900/0412.1961.2023.00158

JIANG Muchi, GONG Jishuang, YANG Xingyuan, REN Dechun, CAI Yusheng, LI Bingyang, JI Haibin, LEI Jiafeng. Hot Deformation Behavior of Ti30Ni50Hf20 High-Temperature Shape Memory Alloy[J]. Acta Metallurgica Sinica, 2025, 61(6): 857-865 DOI:10.11900/0412.1961.2023.00158

Ti-Ni形状记忆合金(shape memory alloy,SMA)是通过固态相变实现形状记忆效应的一种金属间化合物,具有良好的生物相容性和综合力学性能,但Ti-Ni二元合金的相变温度一般低于100 ℃,这严重制约了Ti-Ni形状记忆合金的应用发展[1~5]。Ti-Ni基高温形状记忆合金具有较高的马氏体和奥氏体相变温度,可广泛用于制备航空发动机、核电站以及其他高温报警装置中的驱动部件,成为近年来形状记忆合金领域研究的热点方向[6,7]。研究[8~11]表明,添加Hf、Pt和Pd等元素可以有效提升Ti-Ni形状记忆合金的相变温度,其中Ti-Ni-Hf系形状记忆合金具有价格相对低、性能优异等优点,是目前最具有工程化应用前景的高温形状记忆合金之一。

热加工变形行为是考量金属材料实现加工成形的重要因素,Ti-Ni-Hf系金属间化合物的热变形除了受本身属性影响以外,主要与变形温度、应变速率等变形参数有关。研究[12]表明,材料的本构方程能够直接反映热加工工艺参数之间的关系,在此基础上建立的热加工图可以预测材料的加工性能,避开加工失稳区,确保材料成形及性能提升。马昕等[13]研究了Ni60Ti40形状记忆合金的热变形特性,结果表明该合金的热变形激活能为213 kJ/mol,具有3个稳定变形区、1个失稳区。朱乐宗等[14]的研究结果表明,多元线性回归法能预测轧制态Ni-40Ti合金的热变形行为。赵亚楠等[15]研究了Ni50.9Ti49.1形状记忆合金的热变形行为,结果表明该合金具有负温度、正应变敏感性。目前围绕Ti-Ni形状记忆合金的热变形行为展开了较多研究,但针对Ti-Ni-Hf系高温形状记忆合金的研究较少,主要集中在研究时效温度、时间等对铸造Ti-Ni-Hf高温形状记忆合金组织和性能的影响,而对于热变形后的Ti-Ni-Hf高温形状记忆合金的研究较少。Karelin等[16]在制备Ti-Ni-Hf高温形状记忆合金棒材时发现,只有Hf含量为1.5% (原子分数)的合金成功制备成棒材,其余成分均未成功。究其原因是高Hf含量Ti-Ni-Hf合金具有较高的硬度和脆性,极难进行热变形[10,17,18]。而低Hf含量无法满足航空航天领域对高温形状记忆合金的需求。因此,有必要开展高Hf含量Ti-Ni-Hf系高温形状记忆合金的热变形行为研究,全面理解其热变形能力。

本工作针对航空航天领域对高温形状记忆合金的需求,以Ti30Ni50Hf20高温形状记忆合金为研究对象,通过热模拟实验研究该合金在不同温度和变形速率下的热变形行为,建立该合金的本构方程及热加工图,研究热变形工艺对合金流变应力、组织演化的影响,为Ti-Ni-Hf高温形状记忆合金的热加工工艺提供数据支持和理论依据。

1 实验方法

按Ti∶Ni∶Hf = 3∶5∶2的原子配比将高纯海绵Ti、电解Ni以及海绵Hf进行配料,采用真空感应炉进行合金熔炼,打磨后的试样尺寸为直径45 mm、长80 mm,成形后的合金按照GB/T 14265—2017和GB/T 23942—2009测得其化学成分(质量分数,%)为:Ni 36.8,Hf 45.2,Cr 0.004,Fe 0.028,Nb 0.006,C 0.0053,N 0.0024,O 0.043,H 0.0011,Ti余量。利用线切割在合金铸锭上切取直径8 mm、长12 mm的热模拟压缩试样,机加至表面光亮。采用Gleeble-3800热模拟试验机进行热模拟压缩实验,热变形参数为:应变速率0.01、0.1、1和10 s-1,变形温度700、800和900 ℃,升温速率10 ℃/s。试样升到实验温度后保温5 min,总压缩变形量为80%,将样品立刻取出水淬,以保留其在高温变形下的组织状态。将原始样品及热压缩后样品线切割后进行机械磨抛,采用体积比为HNO3∶HF∶H2O = 1∶4∶5的腐蚀液对样品进行腐蚀,利用Axiovert 200MAT金相显微镜(OM)进行组织观察。利用Q1000差示扫描量热仪(DSC)进行相变行为测试。

2 实验结果与讨论

2.1 显微组织与相变温度

铸态Ti30Ni50Hf20高温形状记忆合金的显微组织OM像和DSC曲线分别如图1a和b所示。可以看出,铸态Ti30Ni50Hf20高温形状记忆合金的显微组织由枝晶和胞晶构成,马氏体相变开始温度为271 ℃。图1c为Ti50Ni50形状记忆合金的DSC曲线,马氏体相变开始温度为76 ℃。可见,添加Hf元素后的Ti30Ni50Hf20高温形状记忆合金,其马氏体相变开始温度较传统等原子比Ti-Ni合金提高了195 ℃。

图1

图1   铸态Ti30Ni50Hf20高温形状记忆合金的显微组织OM像和DSC曲线及Ti50Ni50形状记忆合金的DSC曲线

Fig.1   OM image (a) and DSC curve (b) for as-cast Ti30Ni50Hf20 high-temperature shape memory alloy, and DSC curve for as-cast Ti50Ni50 shape memory alloy (c)


2.2 流变应力-应变曲线

不同变形温度和应变速率下Ti30Ni50Hf20高温形状记忆合金的应力-应变曲线如图2所示。可以看出,不同变形条件下Ti30Ni50Hf20高温形状记忆合金的热变形流变应力变化趋势可分为3个阶段[19~21]。第一阶段:在变形初期,随着变形量增加,流变应力快速增大直至达到最大值,该阶段样品内部位错随变形量增加快速增殖,导致合金的变形抗力增大,产生加工硬化。在恒定温度下随着应变速率增加,加工硬化现象更加明显,曲线斜率也更大[18];第二阶段:随变形量增加,当流变应力达到峰值后,金属内部畸变能不断升高,当其达到一定程度时产生动态回复或动态再结晶,应力随应变增加逐渐降低,出现流动软化现象[20]。第三阶段:随变形量继续增大,加工硬化与动态软化逐渐达到动态平衡,流变应力趋于稳定,在图2中表现为应力-应变曲线最终趋于平缓[21]

图2

图2   不同温度和应变速率下Ti30Ni50Hf20高温形状记忆合金应力-应变曲线

Fig.2   Stress-strain curves of Ti30Ni50Hf20 high-temperature shape memory alloy at different temperatures and strain rates (ε˙)

(a) 700 oC (b) 800 oC (c) 900 oC


图3是不同应变速率下峰值应力与变形温度的关系曲线。可以看出,Ti30Ni50Hf20合金热变形的峰值应力与变形温度、应变速率密切相关。在恒定的应变速率下,随变形温度升高合金的峰值应力降低;在恒定的变形温度下,随应变速率升高合金的峰值应力增大。基于图3曲线的变化规律,Ti30Ni50Hf20合金的热变形峰值应力随变形温度降低或应变速率升高而升高,为负温度及正应变敏感材料[21]

图3

图3   Ti30Ni50Hf20高温形状记忆合金峰值应力随温度和应变速率变化曲线

Fig.3   Variation curves of peak stress (σp) of Ti30Ni50Hf20 high-temperature shape memory alloy with temperature and ε˙


变形温度对Ti30Ni50Hf20合金流变应力的影响规律如图4所示。可以看出,随着变形温度升高流变应力逐渐减小,达到峰值的应变逐渐减小。这是因为在高温变形过程中,随着变形温度升高,材料的热激活能作用增强,原子间结合力降低,导致晶体在产生滑移时的临界分切应力呈现下降趋势,因此合金的流变应力随温度升高而降低;当流变应力达到峰值后,变形过程中合金内部产生大量位错导致合金发生动态再结晶、动态回复等,致使合金发生软化效应,应力-应变曲线趋于平缓[22,23]

图4

图4   不同变形温度下Ti30Ni50Hf20高温形状记忆合金的应力-应变曲线

Fig.4   Stress-strain curves of Ti30Ni50Hf20 high-temperature shape memory alloy under different deformation temperatures at strain rates of 0.01 s-1 (a), 0.1 s-1 (b), 1 s-1 (c), and 10 s-1 (d)


2.3 本构方程

由Ti30Ni50Hf20高温形状记忆合金在不同温度和应变速率下的应力-应变曲线(图2)可知,当变形温度一定时,流变应力随应变速率的升高而增大,为表征该高温记忆合金的热变形特性,采用式(1)~(3)对其热变形过程中的变形行为进行分析[22,24,25]

ε˙=A1σpn1exp-QRT       (ασp<0.8)
ε˙=A2expβσp-QRT        (ασp>1.2)
ε˙=Asinhασpnexp-QRT

式中,ε˙为应变速率;σp为峰值应力(MPa);AA1A2为结构因子(s-1);nn1为加工硬化指数;R为气体常数(8.3145 J/(mol·K));Q为变形激活能(J/mol);T为变形温度(K);β为与材料有关的参数;α为应力水平参数。当合金在高温变形过程中满足以上3种关系,即式(1)~(3)时,则变形过程中变形激活能与温度无关,对上述公式分别取对数,可以得到:

lnε˙=lnA1+n1lnσp-QRT        (ασp<0.8)
lnε˙=lnA2+βσp-QRT        (ασp>1.2)
lnε˙=lnA+nln[sinh(ασp)]-QRT

将不同变形温度下Ti30Ni50Hf20高温形状记忆合金的峰值应力(图4)与应变速率分别代入 式(5)和(6)中,得到的峰值应力与应变速率之间关系图,如图5所示。其中图5a的斜率为1 / β图5b的斜率为1 / n1,根据α = β / n1,计算得出n1 = 12.04,β = 0.018,α = 0.0015。将α和应变速率代入到 式(6)中建立ln[sinh(ασp)]与lnε˙的关系(图5c),结果表明ln[sinh(ασp)]与lnε˙成线性关系。

图5

图5   Ti30Ni50Hf20高温形状记忆合金峰值应力与应变速率的关系

Fig.5   Relationships between σp and ε˙ of Ti30Ni50Hf20 high-temperature shape memory alloy

(a) σp-lnε˙ (b) lnσp-lnε˙

(c) ln[sinh(ασp)]-lnε˙ (α—stress level parameter)


采用Zener-Holloman参数(Z)分析合金塑性变形过程中温度与应变速率之间的关系,Ti30Ni50Hf20合金本构方程可表示为[22,24,25]

Z=A[sinh(ασp)]n=ε˙expQRT

当应变速率一定时,假设在一定范围内Q保持不变,对 式(7)进行对数转换可以得到:

lnZ=lnA+nln[sinh(ασp)]=lnε˙+QRT
ln[sinh(ασp)]=A'+B'1000T

式中,A'B'分别为图6拟合后曲线的截距和斜率

图6

图6   Ti30Ni50Hf20高温形状记忆合金ln[sinh(ασp)]与1000 / T的关系

Fig.6   Relationship between ln[sinh(ασp)] and 1000 / T of Ti30Ni50Hf20 high-temperature shape memory alloy


将ln[sinh(ασp)]与T代入上 式(9),根据关系式绘制Ti30Ni50Hf20合金ln[sinh(ασp)]与1000 / T的关系图,如图6所示。可见,ln[sinh(ασp)]与1000 / T之间符合线性关系。

然后运用Zener-Holloman参数法求解Ti30Ni50Hf20合金高温塑性变形的变形激活能等材料常数。根据 式(6),可以得到[24]

Q=Rln[sinh(ασp)]1Tε˙lnε˙ln[sinhασp]T

式中,ln[sinh(ασp)]1T为ln[sinh(ασp)]-1000 / T函数关系斜率(图6),ε˙lnε˙ln[sinh(ασp)]为ln[sinh(ασp)]-lnε˙的函数关系斜率(图5c),将R和上述2个斜率值代到 式(10)中可求出Q,将Q代入 式(7)中可以计算出Z和lnZ,绘制出lnZ-ln[sinh(ασp)]之间的函数关系式,如图7所示。图7表明,lnZ-ln[sinh(ασp)]在应变速率为0.01~10 s-1和变形温度为700~900 ℃的条件下,符合线性关系,表明本工作实验拟合结果正确。

图7

图7   Ti30Ni50Hf20高温形状记忆合金lnZ与ln[sinh(ασp]的关系

Fig.7   Relationship between lnZ and ln[sinh(ασp)] of Ti30Ni50Hf20 high-temperature shape memory alloy (Z—Zener-Hollmon parameter)


lnZ-ln[sinh(ασp)]函数关系的斜率和截距分别为n和lnA。计算得出Q = 527.447 kJ/mol,n = 9.59,lnA = 56.2。得到Ti30Ni50Hf20高温形状记忆合金的本构方程为:

ε˙=exp56.2-527.447RT[sinh(0.0015σp)]9.59

采用Z参数的 式(7)表示为:

Z=2.56×1024[sinh(0.0015σp)]9.59=ε˙exp527.4478.3145T                        
σp=10.0015lnZ56.219.59+Z56.229.59+112

将变形温度和应变速率代入 式(7)求得Z,再将Z代入 式(13)中进行Ti30Ni50Hf20高温形状记忆合金本构方程验证,计算出峰值应力,计算值与实验值的对比如图8所示。可以看出,理论计算值与实验值相吻合,采用上述分析建立的本构方程计算的结果能较好地反映实验结果,因此本工作通过计算建立的本构方程可以很好地表征Ti30Ni50Hf20合金的热变形特性。

图8

图8   Ti30Ni50Hf20高温形状记忆合金峰值应力理论计算值与实验值对比图

Fig.8   Comparisons of measured and calculated peak stresses of Ti30Ni50Hf20 high-temperature shape memory alloy


2.4 热变形过程中合金组织变化

图9为Ti30Ni50Hf20高温形状记忆合金在应变速率为0.01 s-1时不同变形温度下显微组织的OM像。可以看出,与原始组织(图1a)相比,枝晶和胞晶消失,变形后合金组织呈现一定的流线特征(图9ab)。随着变形温度升高,再结晶现象逐渐显著。当变形温度为800 ℃时(图9b),在变形带附近出现尺寸较小的再结晶晶粒;当变形温度为900 ℃时(图9c),大部分晶粒都发生了再结晶,而再结晶的发生也是图4a中当流变应力达到峰值后迅速降低的主要原因。在再结晶的作用下,合金产生的软化与加工硬化相互抵消,最终导致流变应力达到动态稳定(图4a),应力-应变曲线则表现为流变应力随变形温度升高趋于平缓[24,25]

图9

图9   应变速率为0.01 s-1时不同变形温度下Ti30Ni50Hf20高温形状记忆合金显微组织的OM像

Fig.9   OM images for Ti30Ni50Hf20 high-temperature shape memory alloy at strain rate of 0.01 s-1 at deformation temperatures of 700 oC (a), 800 oC (b), and 900 oC (c)


图10为变形温度为900 ℃时,Ti30Ni50Hf20高温形状记忆合金在不同应变速率下变形后显微组织的OM像。可以看到,随着应变速率的增加,合金组织内部变形带更加密集(图10bc),晶粒受挤压变形,在晶粒内部有滑移带产生(图10b),在变形过程中大量位错缠绕堆积,在应力-应变曲线中表现为流变应力随应变速率的增加而增大(图2)。

图10

图10   变形温度为900 ℃时Ti30Ni50Hf20高温形状记忆合金在不同应变速率下显微组织的OM像

Fig.10   OM images for Ti30Ni50Hf20 high-temperature shape memory alloy at deformation temperature of 900 oC under strain rates of 0.1 s-1 (a), 1 s-1 (b), and 10 s-1 (c) (Inset in Fig.10b shows the slip band)


2.5 热加工图

根据动态材料模型理论,合金热加工过程中单位体积内所吸收的能量(P)与耗散量(G)和耗散系数(J)之间的关系为[24]

P=σε=G+J=0ε˙σdε˙+0σε˙dσ

式中,σ为应力,ε为应变。GJP中的占比由应变速率敏感指数(m)决定。本工作中Ti30Ni50Hf20合金m的计算方法如下式[23]

m=JG=ε˙σσ˙ε˙=lnσlnσε, T

式中,σ˙为应力率。理想线性耗散时,m = 1,J具有最大值Jmax = P / 2。

功率耗散系数(η)是材料成形过程中显微组织演变所耗散能量与线性耗散能量的比值,它与JJmaxm的关系如下[24]

η=JJmax=2m1+m

根据Prasad材料流变失稳准则[25]

ξ(ε˙)=lnmm+1lnε˙+m<0

式中,ξ为室温因子。对等温热压缩模拟实验相关数据进行分析,分别对应力和应变速率取对数,将lnσlnε˙的函数关系进行三次样条插值拟合,得到:

lnσ=a+blnε˙+c(lnε˙)2+d(lnε˙)3

式中,abcd为三次拟合后的系数。

根据 式(14)和(17)求出不同温度下的m,即:

m=b+2clnε˙+3d(lnε˙)2

根据式(16)、(18)得到:

ξ(ε˙)=lnmm+1lnε˙+m=2c+6dlnε˙mm+1+m

根据 式(16)计算出不同温度、不同应变速率下的η,在T-lnε˙平面内绘制η等高线图,即为功率耗散图(图11)。由图可得,Ti30Ni50Hf20高温形状记忆合金耗散值随变形温度的升高而升高。再将计算得到的不同温度、不同应变速率所对应的ξ,在T-lnε˙平面内绘制出流变失稳图。将流变失稳图与功率耗散图相叠加,可构建出基于动态材料模型(dynamic material model,DMM)理论的Ti30Ni50Hf20合金热加工图[26~31]

图11

图11   Ti30Ni50Hf20高温形状记忆合金功率耗散图

Fig.11   Power dissipation diagram of Ti30Ni50Hf20 high-temperature shape memory alloy


图12即为Ti30Ni50Hf20高温形状记忆合金的热加工图,等高线代表功率耗散系数,灰色区域代表失稳区域,绿色区域代表可加工区域。可以看出,Ti30Ni50Hf20高温形状记忆合金的可加工区域由3个区域构成:区域I,温度为700~780 ℃,应变速率为0.01~0.69 s-1的低温、低应变速率区域;区域II,温度为755~900 ℃,应变速率为0.79~9.77 s-1的高温、高应变速率区域;区域III,温度为880~900 ℃,应变速率为0.01~0.04 s-1的高温、低应变速率区域。

图12

图12   Ti30Ni50Hf20高温形状记忆合金热加工图

Fig.12   Thermal processing diagram of Ti30Ni50Hf20 high-temperature shape memory alloy


区域I和区域II的η均小于0.14,通常这种情况会发生绝热剪切变形或局部塑性流动。区域III的η在0.43~0.49之间,是整个功率耗散图中功率耗散系数最大的部分,研究[29~31]表明,η越大,材料的热加工性能越好。依据Ti30Ni50Hf20高温形状记忆合金的热加工图,Ti30Ni50Hf20合金加工性能较差,可加工区域较小,结合实际加工效率等因素,该合金的最优加工参数为:温度880~900 ℃,应变速率0.01~0.04 s-1

3 结论

(1) Ti30Ni50Hf20高温形状记忆合金的流变应力随变形温度升高而减小,随应变速率的增加而增大。合金为负温度、正应变敏感材料,计算得到的本构方程为:ε˙=exp56.2-527.447RT[sinh0.0015σ]9.59

(2) 随变形温度升高合金再结晶现象明显增强,峰值应力降低,随应变速率增加合金晶界相互缠绕曲折程度加剧,导致变形过程中流变应力急剧增加,峰值应力增大。

(3) 建立了Ti30Ni50Hf20高温形状记忆合金的热加工图,其最优加工参数为:温度880~900 ℃,应变速率0.01~0.04 s-1

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为研究铸态Al19.3Co15Cr15Ni50.7高熵合金的热变形行为,利用Gleeble-3500热模拟试验机对试样进行热压缩实验,获得了在应变速率为0.001~0.1 s-1和变形温度为973~1273 K条件下的真应力-真应变曲线。根据 Arrhenius模型构建该合金应变为0.1~0.7的流变应力下的本构方程,得出不同应变条件下的变形激活能和材料参数,以应变ε为自变量将材料常数进行6次多项式拟合,并对本构方程进行验证。基于动态材料模型的功率耗散理论和失稳判据构建功率耗散图和失稳图,并将2者叠加,得到应变为0.3~0.7的热加工图。结果表明,1273 K时的流变应力曲线呈现明显的动态回复的特征,其他温度下的流变应力曲线呈现动态再结晶的特征,并且流变应力随变形温度的下降或应变速率的提升而提升。建立本构方程并进行验证,判定系数R2 = 0.956,较高的判定系数表明建立的流变应力本构模型能够比较精确地预测合金的流变应力。高温压缩后的SEM结果显示:相比于铸态微观组织,合金经历热变形之后,组织中出现一些空隙与局部塑性流动,长条状的B2相发生了弯折、断裂,且断裂处的长条状B2相被撕裂成连续的片层状。最后基于动态材料模型(DMM)理论计算出热加工图,从而得到优异的热加工区间。

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