中国腐蚀与防护学报, 2025, 45(2): 271-282 DOI: 10.11902/1005.4537.2024.184

临氢关键材料服役行为研究专刊

氢促进局部塑性变形理论的发展趋势

张倩茹, 孙擎擎,

中山大学材料学院 深圳 518107

Hydrogen Enhanced Localized Plasticity: A Critical Review

ZHANG Qianru, SUN Qingqing,

School of Materials, Sun Yat-sen University, Shenzhen 518107, China

通讯作者: 孙擎擎,E-mail:sunqq7@mail.sysu.edu.cn,研究方向为金属使役行为与延寿技术

收稿日期: 2024-06-14   修回日期: 2024-08-14  

基金资助: 国家自然科学基金.  52101115

Corresponding authors: SUN Qingqing, E-mail:sunqq7@mail.sysu.edu.cn

Received: 2024-06-14   Revised: 2024-08-14  

Fund supported: National Natural Science Foundation of China.  52101115

作者简介 About authors

张倩茹,女,1999年生,硕士生

摘要

认识金属中氢与位错之间的交互作用是理解氢脆微观机制的关键。本文介绍了氢促进局部塑性变形理论(HELP)的提出、内涵与研究进展,并进行述评。针对HELP机制的主要现存问题,给出了一种氢与位错交互作用研究的新范式,并作出展望。

关键词: 氢脆 ; 机理 ; 氢促进局部塑性变形

Abstract

The key of understanding hydrogen embrittlement mechanism of metals is to fully elucidate the interaction between hydrogen and dislocation. This paper introduces the history, content and development of the theory of hydrogen enhanced localized plasticity (HELP) and reviews it critically. The unsettling questions regard HELP mechanism are emphasized and addressed. In order to answer the unsettling questions, a new research methodology to reveal the interaction between hydrogen and dislocation is presented and prospected.

Keywords: hydrogen embrittlement ; mechanism ; hydrogen enhanced localized plasticity

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本文引用格式

张倩茹, 孙擎擎. 氢促进局部塑性变形理论的发展趋势. 中国腐蚀与防护学报[J], 2025, 45(2): 271-282 DOI:10.11902/1005.4537.2024.184

ZHANG Qianru, SUN Qingqing. Hydrogen Enhanced Localized Plasticity: A Critical Review. Journal of Chinese Society for Corrosion and Protection[J], 2025, 45(2): 271-282 DOI:10.11902/1005.4537.2024.184

氢能是一种来源丰富、绿色低碳、应用广泛的二次能源,正逐步成为全球能源转型发展的重要载体之一。作为未来国家能源体系的重要组成部分,氢能开发利用技术取得突破为实现零排放的能源利用提供重要解决方案。

氢脆失效是氢能产业安全发展的重要瓶颈性问题[1~4]。当材料中的氢浓度超过临界值,会产生各种不可逆氢损伤,如钢中白点、焊接冷裂纹、氢化物以及氢致马氏体等。氢能使材料塑性下降,引起滞后断裂。如果充氢时不产生不可逆氢损伤,则试样在除氢后再在空气中慢拉伸,塑性能恢复。人们把氢致塑性损失和氢致滞后断裂统称为氢脆。氢脆瞬时发生,事先毫无征兆,往往酿成灾难性的恶性事故,严重地威胁着人们生命财产安全。因此,氢脆被认为是对金属结构件最有危害性的因素之一[5~10]

认清金属氢脆的机理,对防治氢脆危害和抗氢脆合金设计至关重要。1875年Johnson即认识到金属中的氢脆现象,然而,至今对氢致滞后断裂机理仍存在巨大争议[11]。通常氢致滞后断裂的机理分为脆性断裂理论和塑性断裂理论。前者认为氢致开裂过程不以塑性变形为先决条件,后者认为任何金属的开裂过程均以局部塑性变形为先决条件。脆性断裂理论包括氢压理论[12]、氢降低原子键合力理论(亦称弱键理论,HEDE)[13]以及氢化物理论[14,15]等。塑性断裂理论主要包括氢促进塑性变形(HELP)理论[16]以及氢/应力诱发空位机制[17]等。此外,随着研究的不断深入,国内外研究人员提出了更多的氢脆机理与模型。例如,单智伟研究团队[18]通过环境透射电子显微镜内的原位弯曲实验并结合原子尺度模拟计算,提出在金属Al中由于局部塑性变形引发小角晶界的动态形成从而促进材料氢脆断裂的新机制。上述几种理论都有存在的理由、适用的范围和对象。具体如下:

(1) 对于各种金属和合金来说,氢压理论并不适用。氢压理论是指材料中的H聚集成含H2的气泡,当氢气泡中的内压引起的应力场强度因子大于或等于材料断裂韧性时,气泡壁将开裂。氢压理论不适用于金属的原因在于:(a) 氢压裂纹出现的临界可扩散氢浓度远远大于慢拉伸时引起的塑性损失或恒温载荷氢致滞后断裂开裂的临界氢浓度;(b) 氢压理论无法解释氢致滞后断裂的可逆性。

(2) 对于容易形成脆性氢化物的金属(Ti、Nb、V、Zr)及其合金,氢致脆性断裂可归因于氢化物理论。然而,对于大部分金属(Fe、Al、Ni、Cu、Mg等不能形成稳定氢化物的金属)及其合金而言,氢化物理论并不适用。

(3) 产生氢脆的合金拉伸速率较低、拉伸温度往往为常温,一般很难产生空位聚集并长大为微米级空洞,故氢/应力诱发空位机制依旧存疑。

(4) 对于不容易形成脆性氢化物的金属与合金,弱键理论是氢致滞后断裂的主流理论之一。弱键理论虽有第一性原理计算支撑,但尚缺乏直接实验证据。此外,氢仅仅降低键合力而不降低塑性功无法解释滞后断裂门槛值KIH远低于KIC的事实。

(5) HELP理论认为H能促进位错发射和运动,当扩散/富集的H浓度大于临界值时,在比空拉时更低的外应力下,即会发生氢致塑性变形[16]。近年来,随着研究的不断深入,HELP机制逐渐成为氢致滞后断裂的主流机理之一。

对于大部分金属(Fe、Al、Ni、Cu、Mg等不能形成稳定氢化物的金属)及其合金来说,目前HEDE与HELP理论被广泛接受。当然,在很多情况下可能是两种或两种以上理论同时起作用。然而,主流氢致断裂理论自身仍然存在一些尚待解决的基础科学问题,譬如:(1) HELP机制尚无法解释局部塑性增强和宏观脆性断裂之间的矛盾;(2) 基于HELP机制的位错组织演化与材料脆性断裂之间的关联性尚未建立。本文首先介绍HELP理论的提出、发展与主要问题,随后重点介绍氢对位错集群行为的影响研究进展,以及一种氢与位错交互研究的新范式,并对HELP机制的发展趋势做出展望。研究HELP机制尚未解决的基础科学问题,有助于提升对金属材料氢致滞后断裂机理的认知,对基于氢脆的物理机制开发抗氢脆合金、促进氢能产业的安全发展具有重要意义。

1 HELP理论的提出、发展与问题

HELP在氢致滞后断裂中扮演重要角色的认知,最早可以追溯到1972年Beachem的工作[19]。他发现氢致断裂的断口表面有撕裂脊和韧窝等塑性特征,并且指出断口表面特征与金属组织状态密切相关。然而,Beachem的这一论断并非基于对变形组织演化的观察或者氢影响塑性的直接观测[19]。对氢促进位错运动的直接观测,是20世纪8,90年代由伊利诺伊大学的研究团队(代表人物有Birnbaum、Robertson以及Sofronis)完成的[20~26]。该研究团队在带环境室的高压电镜中原位观察表明,拉伸试样如通入Ar气,则原来静止的位错保持不动;但如通入相同压力的H2(压力很小95 Torr,否则H2散射电子,成像不清楚),则位错运动,而且其速率随氢压升高而升高(详见图1a~g中310S不锈钢在不同氢压下的位错塞积图像[22]);若把H2抽出,则位错停止或反向运动。对bcc金属α-Fe,fcc金属Al、Ni和不锈钢以及hcp金属α-Ti均获得相同的结论[20~26]。氢促进位错运动的原因被归结为氢氛围的弹性屏蔽效应。相距为r的两个共面平行刃位错的相互作用力τD[16]

τD=μb/[2π(1-v)r]

图1

图1   氢促进位错运动的原位电镜观察[22]与氢气团弹性屏蔽效应[16]

Fig.1   In-situ electron microscopy observation of hydrogen-enhanced dislocation motion[22] and elastic shielding of hydrogen atmosphere[16]: (a-f) images showing the dislocation pile-up at different hydrogen pressure (0、35、50、70、90、95 Torr) in 310S stainless steel, (g) after introducing H2, dislocation pile-up of 310S stainless steel moves from the black lines to the white lines, (h) the elastic force between dislocations of Nb varies with the dislocation spacing in the presence and absence of hydrogen


式中,μ为剪切模量,b为Burgers矢量的模,v是Poisson比。对于金属Nb,当不存在氢氛围时,τD/μ (剪切应力与剪切模量之比)的值随r/b (沿滑移面归一化距离)的变化见图1h的黄色实线。如果位错周围存在氢氛围,则氛围中所有氢原子作用在该位错上的合应力τH[16]

τH=-[μVH/2π(1-v)NA]02πC(r, ϕ)sin2ϕrdrdϕ

其中,rϕ是氢原子在球坐标系中的坐标,VH为氢的偏摩尔体积,NA为Avogadro常数,C(r, ϕ)为氢的浓度分布。对于金属Nb,τH/μ的值随r/b的变化见图1h中的最下方小于0的黄色虚线(氢浓度C0 = 0.01)和蓝色点线(氢浓度C0 = 0.1)。由于τH < 0,故作用在氢氛围中位错上的合力τH + τD < τD,即氢氛围的弹性屏蔽作用使位错的运动阻力下降。

单智伟团队[27]通过基于原位环境透射电镜的新型定量实验方法,对比研究了α-Fe中单根螺位错在含氢/不含氢条件下的运动行为,证明了氢促进螺位错的运动。图2结果表明,当向真空样品室通入2 Pa的氢气并在电子束辐照的帮助下对样品进行充氢后,试样中螺位错的启动应力σc从104.8 MPa降低至75.6 MPa,下降幅度为27%以上;同时,相应的位错最大滑动位移δmax也大幅增加,从10.8 nm增加到17.8 nm。该结果在单个位错尺度上验证了HELP机制。

图2

图2   α-Fe中单根螺位错在有氢和无氢环境下的运动情况[27]

Fig.2   Motion of a single screw dislocation of α-Fe in the presence and absence of hydrogen[27]: (a) bright-field transmission electron microscope image showing the pillar after a series of cyclic compression loading and unloading sessions. A mobile dislocation tagged as 1 in the boxed region is magnified and observed in Fig.2b, the white spots indicate the pinning points, (b) configurations of dislocation 1 at σmax in vacuum (N = 1) and in 2 Pa H2 (N = 2), (c) loading engineering stress σ and the digitally tracked projected glide distance δ of dislocation 1 in a typical load cycle are shown as a function of time. The critical stress for activating the dislocation (σc) and the maximum glide distance (δmax) are also indicated, (d, e) measured δmax and σc of dislocation 1 as a function of loading cycle number in vacuum (d) and in 2 Pa H2 (e). Errors for measurements of δmax and σc are ±1.4 nm and ±9.5 MPa, respectively. Error bars represent standard deviation. The tests in 2 Pa H2 were started after the pillar had been exposed to the 2 Pa H2 atmosphere for ~2 h. Scale bars, 100 nm


以伊利诺伊大学研究团队为代表的科研人员基于环境电镜技术和理论计算手段,给出了HELP机制的直接物理图像,提出了氢弹性屏蔽理论,这奠定了HELP理论的基础。进一步地,通过应力弛豫实验发现,氢可以促进Ni[16]、Ni-C[16]、bcc-Fe[28]、310S不锈钢[29]的应力弛豫,降低位错热激活运动的面积及热激活焓。氢降低位错热激活面积就是使应变速率升高,从而使位错运动速率升高。此外,研究还表明,氢可以导致应变集中和滑移共面性,促使滑移带变得更粗更直,滑移台阶更高,滑移间距更大[26]。利用分子动力学、位错动力学等计算模拟手段,人们发现氢能够促进位错的发射和增殖,能够促进裂纹的形核与扩展[30,31]。上述研究结果丰富了HELP理论的内涵,给出了有价值的见解,但仍然无法建立HELP机制与最终断裂之间的关系,原因如下:

(1) 氢弹性屏蔽理论是基于金属Nb的计算得出,而Nb的氢致断裂机制为氢化物理论,而非HELP机制;氢弹性屏蔽理论只考虑了氢对位错间弹性应力场的影响,对氢与位错芯的交互作用关注不够;只有当氢富集浓度较大时(超过10%,见图1h)才会有明显的弹性屏蔽效应,而实际上金属中的氢浓度很难富集到如此程度,Chen等[32]的研究表明氘在晶界和位错等缺陷处的富集浓度不过0.1%。

(2) 环境电镜的观测、弹性屏蔽理论以及位错动力学模拟处理的都是离散位错,而离散位错对应的只是金属材料变形的早期阶段,远远未到断裂失效的阶段;此外,环境电镜采用的样品为薄膜样品,比表面积远大于真实应用场景下的工程材料,位错与表面的交互作用不能排除。

(3) 应力弛豫结果是基于宏观力学行为得到的,难以关联最终断裂,特别是无法揭示氢致断裂的微观机制。

(4) 滑移带是位错在样品表面的凸起,而样品表面的位错演化与芯部是不同步的,故基于滑移带的观测结果无法为建立HELP机制与最终断裂之间的关系提供有说服力的证据;此外,在比较氢对滑移带特征的影响时,未考虑晶体取向的影响,降低了“氢促滑移共面性”这一结论的科学严谨性。

为了建立HELP机制与氢致最终断裂之间的关系,需要系统研究氢与位错的交互作用,关键是准确揭示氢对位错集群行为(collective behaviour)的影响规律。根据低能位错理论,随着应变的提高,退火态金属中的位错会从离散位错形态逐步发生自组织化(self-organization),即会产生位错的集群行为,表现为不同类型的位错组态/组织。例如,材料断裂前位错的演化路径一般为:离散位错→位错缠结/位错墙→位错胞→更加细小的位错胞。相比于离散位错行为,位错在中高应变区的集群行为与材料最终断裂之间的关联性更为显著,因此有必要系统研究氢对位错集群行为的影响规律。

2 氢对位错集群行为的影响研究

目前,关于氢对金属材料在中高应变区变形组织演化影响的研究不多,且已有文献结果存在争议[33~47]:氢促进位错组态的演化[33,34,37,39,45]、氢阻碍位错组态的演化[40,41]、以及氢对位错组态演化没有影响[34~36,38]的结论均有报道。例如,Wang等[33]研究了原位电化学充氢对bcc纯铁位错组态的影响,结果表明与未充氢试样相比,相同应变下充氢试样存在更多的位错缠结(图3a)。Wilcox和Smith[34]观察应变约0.1的纯镍发现,充氢试样的位错密度是未充氢试样的两倍,但是在更高的应变下位错密度在充氢前后的试样中没有区别。McInteer等[35]与Robertson和Birnbaum[36]却发现氢未改变纯镍拉伸试样中的位错组态和密度,与Wilcox和Smith[34]的结果相矛盾。2017年,Wang和Robertson等[37]采用高压扭转处理充氢和未充氢镍,并比较了不同应变下的位错组织特征,见图3b。研究表明,当扭转应变为0.23时,充氢与未充氢试样均呈位错胞结构,且充氢样的位错胞尺寸更小;当扭转应变增大到0.45时,未充氢样品仍旧呈位错胞结构,而充氢试样已发展成为条带状组织。然而,Harris等[38]研究表明,无论在室温还是液氮温度,充氢与未充氢纯镍拉伸试样在相同应变下的位错胞大小、位错与晶界的交互作用均未表现出差异性。人们对高熵合金中氢与位错交互作用亦进行了一定的探索。Pu等[43]比较了CrMnFeCoNi高熵合金与316L不锈钢的氢脆敏感性,并把高熵合金氢脆敏感性的提升归因于氢促位错平面滑移效应在高熵合金中的减弱,即在充氢高熵合金中只发现了代表着位错交滑移的位错胞,而在316L不锈钢充氢样中出现了代表着位错平面滑移的平直状位错组织结构(HDDWs)。Nygren等[44]比较了充氢前后等原子比FeNiCoCr高熵合金的位错组织演化,表明位错缠结在充氢前后的样品中均会出现,而位错胞只存在充氢后的样品中。Bertsch等[45]指出氢促进了FeNiCoCrMn高熵合金中位错胞的演化,并促进了变形带密度的增加。Nygren等[44]和Bertsch等[45]均观察到高熵合金充氢后变形会出现孪晶,而Pu等[43]未报道变形孪晶的生成。文献[47]指出C-N共掺杂促进了FeMnCoCr高熵合金位错的平面滑移,导致更多的氢富集在晶界或孪晶界,从而致使氢脆现象的发生。Cheng等[48]的研究表明,氢可以促进(FeCoNi)86Al7Ti7高熵合金在拉伸过程第一阶段变形带的形成。虽然上述工作存在矛盾甚至相反的研究结果,但均加深了人们对高熵合金变形组织演化以及氢脆机理的理解。

图3

图3   氢对典型金属位错组态的影响[33,37,39,42]

Fig.3   Hydrogen effects on the dislocation configurations of typical metals[33,37,39,42]: (a) bcc pure iron[33], in-situ electrochemical hydrogen charging with uniaxial tension, (b) fcc pure nickel[37], high pressure torsion after high-pressure hydrogen charging, (c) the ferritic-pearlitic low carbon steel (SS400)[39], in-situ fatigue loading under high pressure, (d) 316L austenitic stainless steel[42], fatigue loading after high-pressure hydrogen charging


此外,氢对金属材料疲劳过程中的位错组态的影响研究也备受关注[39~42]。Wang等[39]对空气和高压氢气环境(40 MPa)相同应力强度因子范围的铁素体-珠光体SS400低碳钢疲劳断口附近的位错形态进行分析(图3c),可见在空气中疲劳裂纹尖端位错组织以拉长的位错胞状组织和迷宫状组织的形式存在,位错组织化行为在距离裂纹~56 µm处开始消失;而氢气环境中位错胞状组织更小并多为等轴形状,位错组织的存在范围延伸~104 µm。氢可以细化SS400低碳钢疲劳裂纹尖端位错胞尺寸,说明氢促进了低碳钢疲劳过程中位错自组织的演化。然而,Ogawa等[40,41]在纯铁中却观察到,氢反而阻碍了纯铁疲劳裂纹尖端附近位错组织的演化进程。上述相互矛盾的报道表明,氢对中高应变区位错集群行为及其他变形组织演化的影响研究仍待进一步进行,针对该科学问题的物理图像仍然是模糊的。

造成该物理图像模糊的因素有很多,譬如合金纯度、晶粒尺寸、晶体取向、氢浓度、应变量与应变速率的差异,而其中最重要的影响因素是晶体取向。必须明确指出,文献[33~47]均未考虑到位错组织与晶体取向之间的关联性。研究表明,金属在塑性变形过程中的位错组织演化特征严重依赖于晶体取向[49~57]。例如,Hansen和黄晓旭等[49~57]基于大量实验结果,提出了fcc金属位错组织的3种类别(图4a):(1) 接近[110]的区域为类型1(图4e中Type 1),位错组织呈板条状(cell block),由几何必须位错边界(GNBs)和附生位错边界(IDBs)构成。几何必须位错边界与位错滑移面(111)基本平行,角度偏差小于10°;(2) 接近[100]的区域为类型2 (图4e中Type 2),位错组织呈胞状(cell)。位错胞之间的取向差较小;(3) 接近[111]的区域为类型3 (图4e中Type 3),类型3与类型1类似,区别在于类型3的几何必须位错边界不与位错滑移面(111)平行,角度偏差可高达35°。此外,bcc金属的位错组织同样呈现明确的晶体取向关联性,且跟fcc金属位错组织的晶体取向关联性呈倒易关系,如图4b所示[58]。变形孪晶组织的演化也存在严重的晶体取向关联性,见图4c (课题组未发表结果)。变形孪晶与不全位错的性质密切相关,故氢与位错交互作用必将影响变形孪晶组织的演化。研究表明,金属在疲劳加载过程中的位错组织演化也遵循晶体取向关联性,见图4d[59]。而图3d所示的316L不锈钢在高压充氢后疲劳加载导致的变形组织并未考虑晶体取向关联性[42]。研究[33~47]在揭示金属中可扩散氢与塑性载体位错之间的相互作用时做出了巨大努力,给出了一些初步的物理图像,这非常难能可贵。但遗憾的是,它们均未考虑变形组织的晶体取向关联性。对晶体取向的忽视,将严重降低研究工作的严谨性,甚至得出错误的氢致滞后断裂机理。因此,若要准确揭示金属材料中高应变区的氢与位错交互作用机理,需要首先考虑变形组织的晶体取向关联性。

图4

图4   金属变形组织的晶体取向关联性[54,58,59]

Fig.4   Orientation dependence of deformed microstructure in metals: (a) orientation dependence of dislocation structure of fcc metals with medium to high stacking fault energy during uniaxial tension[54], (b) orientation dependence of dislocation structure in bcc metals[58], (c) orientation dependence of deformation twins in fcc metals, black dots indicate orientations that can produce deformation twins (unpublished results from our group, 316L stainless steel was used in this study), (d) orientation dependence of dislocation structure during fatigue loading of fcc metals[59], (e) types of dislocation structure with different orientations, types 1, 2, and 3 corresponding to I, II, and III in (a) and (b), respectively


直到2019年,Bertsch等[45]在研究氢对纯镍单向拉伸过程中位错组织演化的影响时,才考虑了位错组织的晶体取向依赖性。研究表明,充氢样品在[011]取向晶粒为条带状和胞状,而在[001]和[111]取向晶粒则只有胞状结构。此外,纯镍充氢前后胞状结构的大小也不同。定量统计结果表明,无论对0.08或0.095应变,氢均促进了位错胞尺寸的细化,即氢促进了纯镍中位错的自组织化。Bertsch等[45]虽然考虑了位错组织的晶体取向关联性,但他们采用聚焦离子束(FIB)提取的是表面晶粒,即所揭示的是氢对表面晶粒位错组织演化的影响规律。研究表明[57,60],对于金属多晶体拉伸而言,表面晶粒与芯部晶粒所受的应力应变状态不同,二者存在不同的位错组织演化规律。一般来说,表面晶粒优先发生塑性变形,产生表层硬化现象;随着应变的进一步提升,表层的位错组织演化程度又会被芯部反超。由于常规金属工程结构件足够厚,故与表层晶粒相比,芯部晶粒的氢与位错交互作用将在构件的氢脆断裂中扮演更重要角色。因此,Bertsch等[45]揭示的氢对位错集群行为的影响仍然不是一个准确的物理图像。综上所述,在氢与位错交互作用的研究中除了要考虑变形组织的晶体取向关联性,还须考虑表层与芯部晶粒中变形组织演化的非同步性,即位错组织演化的表面效应。

此外,若要获取氢对位错集群行为影响的准确物理图像,还需要考虑TEM样品的取样方向,即注意FIB取样的规范性。这是因为,即便对同一位置进行TEM位错表征,当取样方向不同时,所得的位错组织特征亦不同[54]。对于fcc合金[001]晶面,当TEM试样平行于拉伸方向时,位错胞呈拉长状;当TEM试样垂直于拉伸方向时,位错胞为等轴状结构。因此,除了需要考虑位错组织演化的取向效应以及表面效应,还需要规范FIB取样操作,统一取样方向。

综合文献分析可以看出,若要揭示金属中氢与位错交互作用的准确物理图像,则需要考虑如下3个关键因素:

(1) 需要考虑变形组织的晶体取向关联性。位错组态以及变形孪晶等变形组织存在严重的晶体取向关联性,但目前绝大多数氢脆领域的变形组织表征都缺乏晶体取向信息,这严重降低了研究工作的严谨性,对氢致滞后断裂机理的解析造成不利影响。

(2) 需要考虑变形组织演化在表层与芯部的非同步性。目前,个别文献虽考虑了位错组织的晶体取向依赖性,但获得的是金属充氢前后表面晶粒的位错组织演化图像,结果具有误导性。

(3) 需要考虑TEM样品取样方向的规范性。当取样方向不同时,所得的位错组织特征亦不同。

3 氢对位错集群行为影响研究的新范式

对于多晶体而言,获取变形组织对应的晶体学取向信息有两种办法:一种是TEM样品制备后采用菊池花样确定取向信息,另一种是在TEM样品制备前通过EBSD等技术确定目标区域的取向信息。前者一般有电解双喷技术或离子减薄技术,需要采用事先标记的办法确定拉伸方向,而且目标区域所处的具体位置如是否靠近晶界不可知;后者容易获取样品的拉伸方向、目标取样区域的具体位置及其所处晶粒的形状等信息,随后采用FIB即可定点制备TEM薄膜试样。

本课题组在前期工作的基础上,通过大量摸索和失败,创新地应用EBSD-FIB-S/TEM等先进电镜表征手段的联用技术,充分考虑位错组织的晶体取向关联性、位错组织演化在表层和芯部的非同步性以及TEM样品取样方向的规范性,建立了氢对位错集群行为影响研究的新范式。即,如图5所示:塑性变形→晶体取向分析→FIB定点取样→Zone axis-STEM位错表征。对于图5所示的范式,需要注意的是:(1) FIB取样是基于EBSD获取的晶体取向信息,也就是说,本方法考虑了位错组织的晶体取向关联性;(2) 取样区域是变形金属的芯部,而非表层,如此避免了位错组织演化的表面效应,考虑到了位错组织演化在表层与芯部晶粒的非同步性;(3) FIB加工过程中的取样方向统一为平行于拉伸的方向,故考虑到了TEM样品取样方向的规范性。剥离了上述3大干扰因素,方能准确揭示氢对位错组织演化/集群行为的影响规律。

图5

图5   本课题组及其合作者发展的氢对位错集群行为影响研究的新范式

Fig.5   A new investigation methodology developed by the this group to reveal the effect of hydrogen on dislocation collective behavior of metals


基于该新范式,课题组获得了金属中氢与位错交互作用的准确物理图像,揭示了氢对纯镍/纯铜/NiCr等合金位错组织演化的影响规律,为金属变形组织演化理论提供了新见解[61~65]。基于中高层错能金属纯镍的研究结果表明[62]:拉伸至相同应变(8.3%),相比于未充氢样,0.0012% (原子分数)充氢样品[100]取向晶粒的位错密度增长了350%、[110]取向晶粒的位错密度增长了104%、[111]取向晶粒的位错密度增长了175%。纯镍不同取向晶粒中氢促位错密度增加幅度按如下顺序:[100] > [111] > [110]。图6展示了未充氢纯镍以及0.0004% (原子分数)充氢纯镍在同一应变(16.0%)的位错集群行为,并统计了不同取向晶粒中位错胞/IDB的尺寸(图6c)。对于[100]取向附近的晶粒,充氢前后样品均为位错胞结构;对于[110]和[111]取向附近的晶粒,充氢前后样品均为位错街区(Cell block)组织。0.0004% (原子分数)的氢显著加快了位错自组织化的进程,充氢后[100]取向的位错胞尺寸降低了74%、[110]取向的IDB尺寸降低了38%、[111]取向的IDB尺寸降低了51%。也就是说,在纯镍不同取向晶粒中,氢对位错集群行为的促进幅度按如下顺序:[100] > [111] > [110]。该顺序与0.0012% (原子分数)样品中氢对位错密度促进程度的顺序相同。上述结果表明:(1) 氢没有改变纯镍位错组织演化的晶体取向关联性;(2) 氢显著改变了纯镍位错的集群行为,促进了纯镍位错的增殖与自组织化;(3) 氢促位错组织演化这一现象具备显著的晶体取向关联性。进一步地,计算了局部应力和晶界的非协调性,结果表明:氢的存在显著提升了晶粒间局部应力的非协调性,如充氢0.0004% (原子分数)后Δσ[100]-[110]从16 MPa提升到218 MPa,充氢后Δσ[100]-[111]从104 MPa提升到249 MPa。结合拉伸曲线可知,由氢导致的晶界非协调性在氢致断裂中扮演了重要作用。此外,还半定量地探索了HELP与HEDE机制在不同氢浓度纯镍断裂中所扮演的相对权重,发现高浓度充氢合金中HEDE扮演更关键的角色,而低浓度充氢合金中HELP将发挥更重要的角色。这是能够区分HELP和HEDE机制相对权重的为数不多的工作。通过这项工作,本课题组发现了氢促进位错组织演化的晶体取向关联性,为氢脆机理提供了新视角与新见解。

图6

图6   纯镍中氢对位错集群行为的影响研究(应变为16.0%)[57]

Fig.6   Dislocation structures of differently orientated bulk grains in pure Ni without and with 400 ppm hydrogen after deformed to a strain of 16.0%. The 400 ppm H-charged Ni fractured at 16.0%[57]: (a1) [100], (a2) [110] and (a3) [111] corner grains of uncharged sample, (b1) [100], (b2) [110] and (b3) [111] corner grains of 0.0004% (atomic fraction) H-charged sample. The black circles in the IPFs correspond to the grain orientation along the tensile direction of the site of interest. Note that the diffraction pattern corresponds to the largest cellular area in the TEM foil, (c) Cell/IDB size statistics of the uncharged Ni and 0.0004% (atomic fraction) H-charged Ni at different strain levels


基于相同范式,课题组还研究了中低层错能金属纯铜中的氢与位错交互作用[63]。由于纯铜中较高的断后延伸率以及由此带来的剧烈晶格转动现象,纯铜断裂样品中难以找到[110]取向,故只比较了充氢前后纯铜[100]以及[111]取向中的位错组织。结果表明,氢对纯铜中位错集群行为没有可以察觉的促进作用,这也跟纯铜充氢后没有显著氢脆相吻合。相比于纯镍,纯铜中的氢与位错交互作用非常弱。进一步的计算模拟表明,相较于纯镍,氢在纯铜位错核心及其弹性应力场附近的富集程度极低。

另外,以金属单晶体为研究对象,也能够研究氢与位错交互作用晶体取向关联性。譬如,文献[66,67]利用TEM表征了纯镍块体单晶以及Ni-Cr合金块体单晶在充氢前后的位错组态。这是一种有效的方法,能够弥补上述新范式中多晶体在变形过程中的晶格转动以及初始晶体取向未知的缺点,能够为揭示氢与位错交互作用的晶体取向关联性提供更多的信息。然而,比较遗憾的是,尚未见到不同晶体取向的块体单晶中氢与位错交互作用的系统研究。除了块体单晶外,基于微纳加工技术制备的微柱单晶、微悬臂梁单晶可以为氢与位错的交互作用提供有价值的信息。例如挪威科技大学的团队[68~72]基于EBSD-FIB联用技术制备微悬臂梁单晶样品,然后在扫描电镜原位力学台上施加应力,原位观察充氢前后的微悬臂梁单晶样品的断裂行为,随后离子束减薄制备TEM薄膜样品,观察氢对位错组织的影响。他们的工作非常精湛,这种方法需要非常娴熟的微纳操作技巧。此外,微悬臂梁样品比TEM薄膜样品的比表面积小得多,位错镜像力或者说位错的表面效应可以被弱化许多。但是,微悬臂梁方法与图5所示的新范式相比,仍有一些局限或不足之处:(1) 微悬臂梁样品的尺寸为微米级别,氢溢出能力大大增强,因此在扫描电镜真空腔中进行原位加载时,样品中的氢浓度是否还能达到氢脆所需的氢浓度门槛值是存疑的;(2) 微悬臂梁样品的加载方向相对于样品坐标系一直在变化,加载速度是否足够慢(< 10-5 s-1)也未知,故难以揭示氢对位错组织演化影响的晶体取向关联性。此外,由于缺乏晶界,单晶体(无论块体还是微纳尺度)中的变形组织演化表征无法揭示变形过程中晶粒间的相互作用,这对多晶体工程合金的氢脆机理研究、特别是氢致沿晶开裂研究非常不利。总而言之,与图5所示的多晶体中氢与位错交互作用研究范式相比,基于单晶的研究范式虽具有一定的局限性,但仍然可以作为一个有力的补充。

4 结论与展望

认清金属氢脆的机理对防治氢脆危害和抗氢脆合金设计至关重要,其中最关键的问题之一是准确认识可扩散氢与塑性载体位错之间的交互作用。本文综述了50年来氢促进局部塑性变形(HELP)理论的发展趋势,梳理了该理论的提出、内涵、重要进展与现存问题,并基于本课题组的最新研究结果给出了部分问题的解决方案。具体如下:

(1) 伊利诺伊大学学派基于环境电镜表征和理论计算手段,奠定了HELP理论的基础。然而环境电镜的观测、弹性屏蔽理论以及位错动力学模拟处理的都是离散位错,而离散位错对应的只是金属材料变形的早期阶段,远远未到断裂失效的阶段。因此HELP理论的早期探索虽然给出了有价值的见解,但仍然无法建立HELP机制与最终断裂之间的关系。

(2) 相比于离散位错行为,位错在中高应变区的集群行为与材料最终断裂之间的关联性更为显著。为了建立HELP机制与最终断裂之间的关系,需要系统研究氢对位错集群行为的影响规律。然而,氢对金属在中高应变区位错组织演化的影响研究存在广泛争议,针对该科学问题的物理图像仍然是模糊的。

(3) 为了准确揭示金属中氢对位错集群行为的影响规律,则需要考虑如下3个关键因素:位错组织演化的晶体取向关联性、位错组织演化在表层与芯部的非同步性以及TEM样品取样方向的规范性。基于该重要认知,本课题组在前期大量摸索的基础上,创新地应用EBSD-FIB-S/TEM等先进电镜表征手段的联用技术,建立了氢对位错集群行为影响研究的新范式。利用该新范式,获得了金属中氢与位错交互作用的准确物理图像,发现了HELP机制的晶体取向关联性,即氢促进位错演化的程度在不同取向晶粒中遵循如下规律:[100] > [111] > [110]。另外,氢的存在显著提升了晶粒间局部应力的非协调性,且该非协调性在氢致断裂中扮演了重要作用。该发现为氢脆机理的进一步揭示提供了新视角与新见解。

(4) HELP机制的晶体取向关联性这一发现的普适性如何尚有待进一步研究。此外,塑性变形机制存在位错滑移、孪生以及相变3种类型。例如不锈钢、高熵合金等合金体系,由于其较低的层错能,塑性变形过程中也会产生变形孪晶、层错等其他变形组织,故未来也要系统研究氢对变形孪晶等其他形式变形组织演化的影响规律。该新范式虽然充分考虑了位错组织的晶体取向关联性、位错组织演化在表层和芯部的非同步性以及TEM样品取样方向的规范性,但忽略了多晶体在变形过程中的晶格转动效应,即所考虑的晶体取向为变形后的最终取向,而对初始取向的信息无法掌握。譬如,fcc金属在单向拉伸变形过程中,[110]附近取向的晶粒会向[111]取向附近转动。为了克服晶格转动效应带来的可能影响,后续有必要以单晶为研究对象,揭示不同晶体取向的单晶在中高应变区的氢与位错交互作用。结合单晶体和多晶体中的氢与位错交互作用的系统研究结果,为建立HELP机制与最终断裂之间的关系奠定科学依据。

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