金属学报, 2022, 58(11): 1427-1440 DOI: 10.11900/0412.1961.2022.00317

综述

从局域应力/应变视角理解异构金属材料的强韧化行为

范国华1, 缪克松,1, 李丹阳2, 夏夷平3, 吴昊1

1.南京工业大学 先进轻质高性能材料研究中心 南京 211816

2.哈尔滨工业大学 空间环境与物质科学研究院 哈尔滨 150001

3.哈尔滨工业大学 材料科学与工程学院 哈尔滨 150001

Unraveling the Strength-Ductility Synergy of Heterostructured Metallic Materials from the Perspective of Local Stress/Strain

FAN Guohua1, MIAO Kesong,1, LI Danyang2, XIA Yiping3, WU Hao1

1.Key Laboratory for Light-weight Materials, ‎Nanjing Tech University, Nanjing 211816, China

2.Laboratory for Space Environment and Physical Sciences, Harbin Institute of Technology, Harbin 150001, China

3.School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

通讯作者: 缪克松,miaokesong@njtech.edu.cn,主要从事异构金属材料形变机制与先进表征技术研究

责任编辑: 肖素红

收稿日期: 2022-06-27   修回日期: 2022-07-17  

基金资助: 国家重点研发计划项目(2020YFA0405900)
国家自然科学基金项目(51927801)
国家自然科学基金项目(52171117)
江苏省自然科学基金项目(BK20202010)
江苏省高等学校基础科学研究项目(22KJB430027)

Corresponding authors: MIAO Kesong, Tel:(025)83589102, E-mail:miaokesong@njtech.edu.cn

Received: 2022-06-27   Revised: 2022-07-17  

Fund supported: National Key Research and Development Program of China(2020YFA0405900)
National Natural Science Foundation of China(51927801)
National Natural Science Foundation of China(52171117)
Natural Science Foundation of Jiangsu Province(BK20202010)
Basic Science Research Project for Higher Education Institutions of Jiangsu Province(22KJB430027)

作者简介 About authors

范国华,男,1981年生,教授,博士

摘要

同步提升强度与塑性是金属材料研究的不懈追求之一。近年来,异构设计通过调控力学性质存在显著差异的组元相的空间分布,突破了金属材料强度与塑性难兼得的瓶颈。异构变形诱导强化、应变分配、延迟颈缩、界面影响区等主流理论为异构金属材料设计提供了有力指导,上述理论均指出,在受载过程中,异构金属材料组元相的局域应力与局域应变存在独特特征,并伴随偏离经典理论预测的变形和断裂行为。本文综述了异构金属材料在早期变形阶段、塑性变形阶段和断裂阶段中局域应力和局域应变演化,归纳了异构金属材料中变形行为、断裂行为与局域应力、局域应变的交互关系及对力学性能的影响,为高性能异构金属材料的设计和研发提供新的思路。

关键词: 异构设计; 局域应力; 局域应变; 变形行为; 断裂行为

Abstract

The concurrent enhancement of strength and ductility is an unremitting pursuit in metallic material research. Recently, by deliberately controlling the spatial distribution of domains with substantially different mechanical properties, heterostructured architecture has overcome the limitation of strength-ductility synergy in metallic materials. Mainstream theories, such as hetero-deformation-induced hardening, strain partition, premature local necking delay, and interface affected zone, have provided crucial guidance for the designing of preferable heterostructured metallic materials. These theories suggest that the domains of heterostructured metallic materials present unique local stress and strain characteristics upon loading, accompanying deformation and fracture behaviors that deviate from the predictions of classical theories. In this study, the evolutions of local stress and strain during the early deformation, plastic deformation, and fracture stages of heterostructured metallic materials were reviewed. Moreover, interactions between deformation or fracture behaviors and local stress or strain as well as their effects on mechanical properties are summarized, presenting a new perspective for designing and developing high-performance heterostructured metallic materials.

Keywords: heterostructured architecture; local stress; local strain; deformation behavior; fracture behavior

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范国华, 缪克松, 李丹阳, 夏夷平, 吴昊. 从局域应力/应变视角理解异构金属材料的强韧化行为[J]. 金属学报, 2022, 58(11): 1427-1440 DOI:10.11900/0412.1961.2022.00317

FAN Guohua, MIAO Kesong, LI Danyang, XIA Yiping, WU Hao. Unraveling the Strength-Ductility Synergy of Heterostructured Metallic Materials from the Perspective of Local Stress/Strain[J]. Acta Metallurgica Sinica, 2022, 58(11): 1427-1440 DOI:10.11900/0412.1961.2022.00317

金属材料是人类文明的支柱之一,青铜时代、铁器时代等均以冶炼和制造金属工具为显著标志。现代金属材料研究领域中,材料科研工作者们追求使金属同时具有轻质、高强度、高塑韧性等优异综合性能。轻质高强可实现构件轻量化,提高交通、航空航天等领域的运输效率,节能减排;高塑韧性则有利于材料的加工成型,实现复杂形状部件成型,同时保障部件的服役安全性,避免造成重大经济损失和人员伤亡[1,2]。在金属材料学发展的近百年时间里,人们发现并基于经典的4大强化机制(固溶强化、加工硬化、细晶强化和第二相强化)改善金属材料的力学性能。然而,这些手段在优化材料强度的同时往往伴随着塑韧性的降低,材料的力学性能遵循“香蕉曲线”[3,4]

自然界中存在大量具备优异综合性能的材料,它们成为指导人造金属材料综合性能提升的良好范例[5,6]。例如,生物贝壳中特殊叠层结构组合的无机质比简单结构无机质的断裂韧性高出3个数量级。近年来,在充分挖掘金属材料微观组织设计潜力的基础上,异构设计(heterostructured architecture)的理念被提出并广受关注[7,8]。异构金属材料中包含力学性能差异显著的组元,并且组元的分布遵循特定的空间结构[9],诸如,梯度结构[10~12]、层状结构[13~15]、谐波结构[16,17]、双联通结构[18]等。图1a[19]所示为通过表面塑性变形技术制备晶粒尺寸呈梯度分布的纯Cu,强度提升时大幅缓解了塑韧性的降低,梯度异构Cu的力学性能超越了均匀金属材料的“香蕉曲线”,在强度-塑性坐标空间内向右上角迁移(图1b[19])。广义上,可以将这种优异强度和塑性的组合视为实现了金属材料的“强韧化”。如图1c和d[15]所示,通过热压和轧制工艺,将晶粒尺寸存在较大差异的纯Al进行层状结构复合后,其力学性能同样超越了均匀结构纯Al。利用气流研磨机实现对粉末颗粒的可控剧烈塑性变形后,结合热压烧结可以制备如图1e[16]所示的谐波结构:空间上超细晶三维网壳结构包覆着粗大晶粒。谐波结构设计已经在钢、钛合金、铜合金、高熵合金等多种材料体系中实现应用,同步提升了金属材料的强度和韧性(图1f[16])。

图1

图1   几种典型的异构设计策略及其强韧化效果[15,16,19]

Fig.1   Typical heterostructured architectures and their effects on strength and ductility synergy

(a, b) gradient heterostructure (FG and CG are referred to fine grain and coarse grain, respectively)[19] (c, d) layered heterostructure (TD—transverse direction)[15] (e, f) harmonic heterostructure[16]


异构金属材料中实现了多元微观组织(晶粒尺寸差异、物相差异、成分差异等)的空间分布控制,这一分布控制将对材料产生2个方面的影响:① 在不承受载荷时,由于多元微观组织的差异性(屈服强度、弹性模量、膨胀系数、晶体取向等)在材料内部产生局域残余应力,这些局域应力将会直接影响材料的力学特性[20,21];② 当承受外载荷时,同样由于微观组织和性能的差异性,不同组元的弹性变形、塑性变形及断裂行为不同。作为一个整体,在材料的不同宏观变形阶段,材料内不同的位置可能同时存在弹性变形、塑性变形及微裂纹等,异构设计事实上调控了不同组元在不同宏观变形阶段的局域应变的分布与演化,进而影响材料的宏观力学性能[22,23]。如图2a[9]所示,在早期变形阶段,异构金属材料中软相组元和硬相组元先保持弹性变形,随着外加载荷的逐步增加,异构金属材料中较软的组元将优先屈服,而硬相组元仍然保持弹性变形;直至外加载荷使硬相组元也屈服,进入塑性变形阶段。图2b[24]为通过中子衍射原位实验得到的Ti/Al层状异构材料中不同物相的晶格应变,可清晰地观察到与多阶段变形理论吻合的晶格应变演化规律。

图2

图2   异构金属材料的多个变形阶段[9,24]

Fig.2   The multi-stages of deformation in heterostructured metallic materials

(a) three deformation stages of heterostructured metallic materials[9]

(b) lattice strain evolution of Ti/Al layered heterostructured materials measured by neutron diffraction[24]


阐明异构设计实现金属材料强韧化的主流理论包括:异构变形诱导强化理论(hetero-deformation induced hardening)[25,26]、应变分配理论(strain partition)[17,27]、延迟颈缩理论(delaying premature local necking)[28,29]、界面影响区理论(interface affected zone)[30]等,上述理论在众多异构设计范例中很好地关联了“微观组织-变形行为-强韧化”,它们的共性之一在于都强调了局域应力与局域应变对于异构金属材料力学性能提升的重要作用。例如,异构变形诱导强化理论中认为软相组元和硬相组元的变形不匹配将引起局域应变梯度,需要几何必需位错(geometrically necessary dislocation,GND)的累积以实现协调,而位错的累积将诱发局域应力状态的改变,从而影响软相和硬相的变形行为。本文聚焦异构金属材料的局域应力和局域应变演化,归纳了异构金属中变形行为、断裂行为与局域应力、局域应变的交互关系及对力学性能的影响,为高性能异构金属材料的设计和研发提供新的思路。

1 早期变形阶段

早期变形阶段主要涉及异构金属材料的弹性变形以及早期的塑性变形,局域应力/应变的分布与演化直接影响了屈服强度及早期形变微结构。均匀金属材料中微观尺度上塑性变形机制(位错滑移、孪生等)的大量激活对应宏观尺度的屈服行为,因而屈服强度一方面取决于塑性变形机制的临界分切应力(critical resolved shear stress,CRSS)的大小,另一方面取决于分切应力是否达到CRSS。CRSS受晶体结构、纯度、温度、应力状态等因素的影响。以应力状态为例,Spitzig[31]发现单晶Fe和多晶钢在三向静水压应力状态下的CRSS提高,使材料得以强化;Zhou等[32]研究指出,高水平的三向静水压应力可以提升纳米晶纯Ni中晶界迁移与滑动的CRSS,有效抑制由位错主导到晶界迁移与滑动主导的形变机制转变,使纳米晶纯Ni的强度提升至5 GPa。尽管异构金属材料的组元往往处于复杂应力状态,其CRSS可能与均匀金属材料存在差异,但目前在异构金属材料中通过直接设计和调控CRSS实现材料强化的案例较少。

异构金属材料的分切应力受到多个因素影响:首先,组元性质各异,由于热膨胀、塑性变形能力等不匹配,在制备过程中易形成高水平局域残余应力[33]。Long等[21]研究发现,表层具有局域残余压应力的梯度纳米晶Cu在循环加载过程中,拉伸强度和压缩强度不对称,而退火后拉压不对称性减弱。Yang等[20]通过同步辐射实验发现,在外加拉应力过程中,具有最高水平局域残余压应力的区域弹性阶段更长。换言之,局域残余压应力抵消了一部分作用于该区域的外加应力。其次,异构金属材料中组元模量差异在弹性变形阶段导致了局域应力的分区[24]。Huang等[24]在Ti/Al层状异构材料、Huang等[30]和Ma等[34]在铜/青铜层状异构材料中均发现,在软相层已经进入塑性变形阶段时,硬相层仍处于弹性阶段,此时材料中主要由硬层承担应力。尽管软相组元已发生塑性变形机制的激活,异构金属材料的宏观屈服往往发生于硬相组元屈服后,硬-软相组元的弹-塑性变形不匹配也将影响分切应力。对铜/黄铜[35,36]、非均质片层钛[37]等异构金属材料形变机制的研究发现,软相组元的早期塑性变形会在异构界面附近产生GND以协调局域应变梯度,这些GND在界面附近软相组元一侧塞积,从而改变了界面附近区域的局域应力状态,并形成了长程应力场,实现材料屈服强度的提升[8,9,25,26,35~38] (图3[9])。Pan等[39]通过循环扭转的方法制备了纳米位错胞结构呈非均匀梯度分布的稳定单相高熵合金Al0.1CoCrFeNi,纳米尺度的低角晶界位错胞提供了与高角晶界相当的强化效果,其屈服强度可达细晶/粗晶组元的2~3倍。在循环加载实验中测得样品在宏观应变量为0.6% (接近屈服)时,背应力高达260 MPa,接近屈服强度的50%。Li等[40]通过热压的方法在纯Ti中实现了粗晶和细晶呈层状分布的结构,其屈服强度与细晶一致。通过对粗/细晶层状异构纯Ti板晶格偏应变的分布与演化(图4[40])及早期形变后位错组态的分析发现,由于层状结构与晶粒取向之间的几何关系以及具有hcp结构的纯Ti的变形机制,层界面附近将发生大量的位错塞积,从而产生背应力,抑制已激活的滑移系继续发射位错。随外加应力增加,层界面附近将持续累积局域应力,直至分切应力使高CRSS的<c + a>位错或相邻细晶中的位错开始运动。此外,由于不同的塑性变形机制贡献的应变张量不同[41],改变组元变形机制也将直接影响局域应变的分布和演化。

图3

图3   异构金属材料的强化机制[9]

Fig.3   The strengthening mechanism of heterostructured metallic materials[9]

(a) pile-ups of geometrically necessary dislocations (GNDs)

(b) strain and strain gradient versus distance from the domain interface

(c) local stress versus distance from the domain interface


图4

图4   粗/细晶层状异构纯Ti板的晶格应变演化[40]

Fig.4   The evolution of lattice strain for grain A in layered heterostructured pure titanium with alternating coarse- and fine-grain layers[40] (φ1—Euler angle)

(a) εxx, the strain component along normal direction (ND, x axis)

(b) εyy, the strain component along rolling direction (RD, y axis)

(c) εzz, the strain component perpendicular to ND and RD (z axis)

(d) εxy, the shear strain component in the x-y plane

(e) εyz, the shear strain component in the y-z plane

(f) Euler angle map of grain A

(g) orientation schematic diagram of grain A


值得一提的是,异构界面的结构也可以影响分切应力,进而显著改变材料的屈服强度。Chen等[42]通过改变物理气相沉积的速率,调节界面结构和化学成分的梯度,实现了组元之间的错配界面的尺度从几纳米到几十纳米的调控。相较于具有2D界面的Cu/Nb层状异构材料,10 nm厚的3D界面使异构金属材料的屈服强度增加了50% (图5[42,43])。通过原子尺度的模拟与分析,位错源可能出现在界面扭曲最强的位置,而不仅取决于Schmid因子。可动位错的形核主要发生在2D界面,2D界面为局域应力集中源[43]。相比之下,3D界面的局域应力集中位置更为分散,需要更大的外加应力使位错从3D界面发射,从而使得材料的屈服强度得以提升。

图5

图5   具有3D界面的Cu/Nb层状异构材料的组织表征与力学性能[42,43]

Fig.5   The microstructures and properties of Cu/Nb layered heterostructured material with 3D interface[42,43]

(a) a high density of dislocations at 3D interface[42]

(b) stress-strain plots for micropillar compression[42] (ε˙—strain rate) (c, d) microstructures[42] (e, f) TEM images (PVD—physical vapor deposition, ARB—accumulative roll bonding)[42] (g, h) HRTEM images[43] (i, j) schematic diagrams of dislocation behaviors[43]


异构金属材料局域应力/应变在早期变形阶段的特点可以归纳如下:加载前局域应力分布不均匀(局域残余应力),加载过程中局域应力演化不均匀(组元模量、弹-塑性阶段等差异导致局域应力分区);在弹性变形阶段,各组元的局域应变均线性增加,当软相组元屈服后在异构界面附近形成局域应变梯度。局域应力与局域应变是交互影响的,例如局域应力通过改变变形机制影响局域应变演化,与此同时,局域应变梯度需要GND协调,反过来影响了局域应力状态等。因此,理解异构金属材料屈服强度的提升及早期形变微结构的形成需要综合考虑局域应力和局域应变的影响。

2 塑性变形阶段

前已述及异构设计影响了组元的局域应力状态[44,45],因而异构金属材料在塑性变形阶段的变形机制往往显著区别于均匀金属材料。Wu等[46,47]发现梯度异构材料在宏观单向拉伸过程中,微观上纳米晶表层和粗晶芯部均处于多向局域应力状态。这是由于塑性变形阶段纳米晶表层优先发生塑性失稳而产生侧向收缩,与粗晶芯部的均匀变形不协调,诱发侧向的局域应力分量。多向局域应力状态激活了更多滑移系的启动,促进了位错间的交互和缠结。Cheng等[48]基于微观结构分析与分子动力学计算模拟,发现梯度纳米孪晶Cu在变形初期将形成独特的位错结构:大量GND富集束。位错富集束中超高的位错密度在变形过程中显著提升了梯度纳米孪晶Cu的加工硬化能力。这一设计策略同样适用于高熵合金[39],通过在晶粒内引入尺寸呈梯度分布的位错胞稳定结构,在塑性变形阶段将激活大量细小的层错和孪晶生成,贡献优异的塑性和加工硬化能力。Xia等[15]利用对位错类型敏感的同步辐射X射线白光Laue微衍射技术对层状异构材料中的变形机制进行了细致研究。图6a[15]为Al/Al层状异构材料中粗晶层部分区域的取向示意图,选择其中2点进行Laue微衍射得到衍射花样图6b和c[15]。Laue微衍射的衍射花样与观测区域的位错结构密切相关,当观测区域存在GND时,衍射斑点形状拉长,拉长方向与GND对应的滑移系相关[49]。基于刃型位错开动模拟fcc金属开动不同滑移系时的衍射花样(图6d[15]),比对标定层状异构材料中的Al启动了[1¯01](11¯1)和[101](111¯)滑移系。然而,实验标定激活的滑移系Schmid因子均明显低于该取向晶粒的最大Schmid因子,证明了局域应力状态对异构金属材料塑性变形机制的调控能力。相似的结果在Ti/Ti、Ti/Al层状异构材料中均有报道。Li等[40]通过透射电镜研究发现,仅引入层状结构时,Ti的早期塑性变形机制仍以低CRSS的柱面<a>位错滑移为主,而当层界面两侧的Ti存在晶粒尺寸差异时,层界面处将累积高水平的局域应力,激活高CRSS的锥面<c + a>位错滑移。Miao等[50]发现当Ti与Al以层状结构轧制复合并沿轧制方向进行单向拉伸时,区别于轧制纯Ti通过{ 112¯2}压缩孪晶的激活协调变形,Ti/Al层状异构材料中Ti的{ 101¯2}拉伸孪晶发生了退孪生,并一定程度上抑制{ 112¯2}压缩孪晶的激活。从局域应变角度分析,拉伸孪晶退孪生和压缩孪晶孪生均提供了宏观单向拉伸变形所需的应变张量,与此同时,退孪生引入的切应变分量更小,有利于协调变形。

图6

图6   利用同步辐射X射线白光Laue微衍射技术分析Al/Al层状异构材料中的变形机制[15]

Fig.6   Analyses of deformation mechanism in Al/Al layered heterostructured materials by synchrotron radiation polychromatic X-ray Laue microdiffraction[15]

(a) inverse pole figure map (b, c) Laue patterns taken from the red and blue points marked in Fig.6a (d) simulations of Laue peak streaking directions corresponding to all the 12 possible slip systems (a'—lattice constant, SF—Schmid factor)


在塑性变形阶段,异构金属材料中独特的局域应力分布和演化促进了多种变形机制的协同激活,有利于位错-位错、位错-孪晶等交互过程,因而提升了异构金属材料的加工硬化能力,即变形过程中单位应变增量下应力的提升(dσ / dε,其中σ为应力,ε为应变)。依据Hart和Considére准则[51,52],高加工硬化能力对于稳定材料的塑性变形能力有积极意义,已有众多研究通过数据图像相关技术(digital image correlation,DIC)验证了异构设计对局域应变的优化作用[53~57]。例如,均匀纳米晶材料由于缺乏加工硬化能力,往往形成细长的剪切带迅速贯穿样品导致失效。Yuan等[58]在梯度结构的无间隙原子钢中发现,梯度结构使纳米晶层表面的早期剪切带沿着加载方向发生扩展,演化成局域应变集中区域(图7a[58])。当变形集中于剪切带时,金属材料实际参与变形的区域远小于样品的标距。通过剪切带的非局域化,拓展参与变形的区域,可为位错、孪晶等的形成、储存和交互提供充分的空间。Wang等[59]对比了纳米晶层单独承载时(图7b[59])和有梯度结构支持时(图7c[59])的局域应变演化,指出异构设计对剪切带有稳定作用,促进了大量分散分布的剪切带形成。这一现象在众多材料体系中均得到验证[60~62],表明该设计策略具备普适性。值得一提的是,剪切带中的应变集中将诱发晶粒粗化和位错等缺陷的湮灭,为位错、孪晶等的后续形成和存储提供空间,进而进一步协助稳定塑性变形[63];另一方面,剪切带区域和非剪切带区域间相互约束的切应变分量也有促进非剪切带区域塑性变形的作用[59]。通过异构设计稳定或者非局域化应变集中,有助于提升材料的综合力学性能。

图7

图7   梯度异构实现剪切带的非局域化[58,59]

Fig.7   Delocalization of shear bands achieved by gradient heterostructure[58,59]

(a) contour maps of axial strain on nanostructured surface (X, Y, and Z are referred to transverse direction, loading direction, and normal direction, respectively)[58]

(b) contour maps of axial strain on freestanding nanostructured surface (εy —strain component along loading direction)[59]

(c) contour maps of axial strain on nanostructured surface supported by gradient heterostructured substrate[59]


综上所述,在塑性变形阶段,异构金属的局域应力/应变的分布和演化通常是不均匀的,贡献异构金属材料优异加工硬化能力的因素主要包括:① 复杂局域应力状态激活了均匀金属材料中难启动的变形机制;② 通过改善局域应变分布,抑制局域应变集中及其诱发的过早失效。上述2个因素共同促进了位错、层错、孪晶等的增殖和交互,一方面提高了异构金属材料的加工硬化能力,另一方面延长了异构金属材料的加工硬化阶段。

3 断裂行为特点

前文已讨论了异质结构金属在早期变形阶段和塑性变形阶段中局域应力/应变和特征变形机制之间的关系,并指出局域应力/应变是解释异构金属材料强度-塑性优异组合的关键。强度-塑性的改善使异构金属材料具备更优异的韧性和抵抗失效的能力。因此,探究异构金属材料的断裂失效行为及其与力学性能的关系十分重要。

在断裂力学中,材料的增韧主要包括2个方面:本征韧化(intrinsic toughening)和非本征韧化(extrinsic toughening)[64]。本征韧化作用于裂纹的尖端,与材料本征的塑性有关,是材料固有的抵抗断裂的能力,主要通过裂纹尖端产生大量缺陷强化,使裂纹形核、扩展过程更为困难;非本征韧化则主要作用于裂纹的后端,通过减小局域应力集中从而抑制裂纹扩展,常见于复合材料中纤维变形耗散能量[65]、陶瓷中的沿晶断裂[66]等情况。而异构金属材料的设计主要通过调控软相-硬相的空间分布以实现本征和非本征增韧,提升材料的塑韧性。

异构设计提升韧性的案例在大自然中随处可见,例如海边的贝壳[67,68] (图8a[68])和人的牙齿[69]、骨骼[70,71] (图8b[71])等。这些自然界中高断裂韧性的材料具备一个共同特征:微-纳米尺度的硬相和软相交替堆叠而形成“砖瓦结构”(brick-and-mortar structure)[72]。贝壳是由较硬的矿物质和较软的有机蛋白质交替形成[67];骨骼的基本单元则是由羟基磷灰石和胶原蛋白复合而成[73]。基于自然界的启示和对异构材料断裂机制的深入认识,近年来,科研工作者们在大量传统材料体系中实现了异构设计的应用和发展,通过硬相和软相在介-微观多级尺度的构型设计实现了材料整体强韧性的大幅度提高。Liu等[74]通过简单的热变形-淬火配分方法制备了双相超强钢,该材料在抗拉强度超过2 GPa的同时保持了20%的拉伸延伸率(图8c[74])。该双相钢是由片层状分布的马氏体基体和亚稳的奥氏体组成,与贝壳的微观结构相似,研究发现亚稳奥氏体的相变诱发塑性机制(transformation-induced plasticity,TRIP)和界面开裂效应(delamination)共同作用,有效地提高了金属的强韧性。Koyama等[73]受到骨骼结构的启发,开发了由硬的马氏体片层和软的亚稳奥氏体组成的多级纳米片层的双相钢,指出多级纳米片层结构对裂纹的偏转引起了粗糙度诱导裂纹终止(roughness-induced crack termination,RICT)机制,而亚稳奥氏体转变为马氏体的过程中产生的体积变化将引入局域压应力从而限制裂纹的形核和扩展,即相变诱导裂纹终止(transformation-induced crack termination,TICT)机制。RICT和TICT 2种机制的共同作用可最大程度抑制疲劳裂纹的扩展过程,因此该异构金属材料的疲劳韧性明显优于传统的双相钢(图8d[73])。高熵合金因具有区别于传统合金的优异综合性能受到广泛关注[75],而通过多级异构设计的强韧化方案同样在共晶高熵合金中获得成功的应用和实践。Shi等[76]采用定向凝固方法设计并制备了一种仿生鱼骨结构的共晶高熵合金,在不牺牲强度的情况下,获得了超高的断裂韧性。研究发现超高断裂韧性是由于共晶片层中B2相容纳了大量微裂纹,这些微裂纹在片层界面处发生明显钝化不进入L1软相,多级层状结构的缓冲机制进一步抑制了微裂纹的扩展,使材料表现出极强的塑性变形能力和断裂韧性。

图8

图8   自然界中异构案例及应用[68,71,73,74]

Fig.8   Cases of heterostructures in nature and applications

(a) shell[68] (b) bone[71]

(c) microstructure and mechanical properties of ultrastrong dual-phase steel (σy—yield strength, σyu—upper yield strength, σyl—lower yield strength, σuts—ultimate tensile strength, εu—uniform elongation, εf—fracture elongation, G—gauge length, σ0—effective yield strength, KJIC—crack-initiation fracture toughness, Kss—crack-growth toughness, E'—effective modulus, JIC—size-independent fracture toughness, Δamax—maximum length for stable crack extension, B—specimen thickness, W—specimen width, a—crack length)[74]

(d) microstructure and fatigue properties of bone-like heterostructured steel (R—stress ratio)[73]


众多研究案例指出,异构设计大幅阻碍了异构金属材料中微裂纹的扩展,提高了对微裂纹的容纳能力[53,54,56]。微裂纹萌生后并不会迅速扩展贯穿材料导致完全失效,因此,微裂纹也成为了异构金属材料中协调变形的“变形机制”[23,57]。异构金属材料的断裂行为同样可以归纳为异构设计对局域应力/应变的影响。Wu等[14]通过粉末烧结-热压复合工艺制备了TiBw/Ti-Ti(Al)层状异构材料。如图9a和b[14]所示为层状异构材料的微观组织及力学性能,该层状异构材料由TiB晶须增强Ti层(TiBw/Ti)和固溶Al元素的Ti层(Ti(Al))交替堆叠而成。微裂纹的形核和扩展与局域应力密切相关,图9c和d[14]中考虑了层状结构对裂纹尖端的驱动力(Jhet)和对裂纹扩展的抵抗力(Rhet)的影响,相比于均匀金属材料的裂纹驱动力(Jhom),层状结构使Ti(Al)层中形核的微裂纹在TiBw/Ti层中的裂纹驱动力(Jhet)明显降低。图9e[14]所示为微裂纹扩展过程示意图。当Ti(Al)层中微裂纹尖端前的塑性区到达TiBw/Ti层时,虽然裂纹还没进入相邻层,但较软的TiBw/Ti层具有更低的裂纹驱动力(图9c[14])和更高的裂纹抵抗力,抑制了微裂纹的进一步扩展(图9d[14])。此外,裂纹尖端区域往往存在更加复杂的多向局域应力状态[77],有利于促进裂纹尖端的塑性区内开动不常见的位错机制。位错的交互作用使得塑性区快速硬化,起到钝化裂纹的作用[78]。与此同时,异构金属材料内部大量容纳的微裂纹同样贡献了局域拉伸应变,使得异质结构金属表现出极高的断裂延伸率[24,54]

图9

图9   TiBw/Ti-Ti(Al)层状异构材料的断裂行为研究[14]

Fig.9   Fracture analyses of TiBw/Ti-Ti(Al) layered heterostructured materials[14]

(a) microstructure (b) comparisons of tensile properties (c, d) effect of layered heterostructure on crack driving force and crack resistance (Jhom—crack-tip driving force for homogeneous structure, Jhet—crack-tip driving force for heterogeneous structure, Rhom—resistance to crack propagation for homogeneous structure, Rhet—resistance to crack propagation for heterogeneous structure, ahom—half of critical crack length for homogeneous structure, ahet—half of critical crack length for heterogeneous structure, r—affected zone size of heterogeneous interface) (e) schematic diagram of the effect of layered heterostructure on the plastic zone at the microcrack tip (rp—plastic zone size at the microcrack tip)


异构金属材料断裂行为与局域应力/应变的关系可以归纳为:异构设计调控的局域应力通过影响裂纹扩展的驱动力和抵抗力、激活裂纹尖端塑性区域多种变形机制等途径,实现对微裂纹扩展的抑制,有效稳定材料内部的微裂纹。不易扩展的微裂纹起到了提供局域应变、协调局域应变分布的作用。值得一提的是,结合不同材料体系中可能存在的特殊机制,如相变诱导塑性、孪生诱导塑性、相变诱导残余应力等,可进一步提高异构金属材料的强度-韧性匹配,该类过程中局域应力/应变的分布和演化行为也更为复杂。然而,该方面研究目前主要集中于适用材料体系的扩展和微观结构参数实验试错等方向。大量研究发现,异构金属材料的微观组织特征,如硬相-软相的尺寸[79]、硬相-软相的比例[80]、硬相-软相的分布方式[81]、位向[82]等,均会影响异构金属材料的断裂失效行为,如何优化异构设计方案以实现最优的强度-塑/韧性结合亟需系统研究,并发展具有普适性的研发策略,这可能需要在实验的基础上进一步结合断裂力学[83,84]、塑性力学[85~87]等数值模拟方法进行预测和完善。此外,由于异构设计本征的空间特征,异构金属材料往往存在较为明显的各向异性,但该方面的研究尚不全面。例如梯度异构材料的力学性能大多仅从垂直于梯度方向的拉伸实验获得、层状异构材料的力学性能研究主要基于平行于层方向的拉伸实验或垂直于层方向的弯曲实验展开等。系统探明异构金属材料的各向异性,并结合材料的实际服役力学条件进行异构设计优化,也是亟待研究和发展的重要方向。

4 结论与展望

异构金属材料中异质组元对力学的响应不同,导致受载过程中相互约束,有效调控了组元的局域应力、局域应变。异构金属材料中局域应力和局域应变的分布和演化贯穿了早期变形阶段、塑性变形阶段和断裂阶段,异构设计对局域应力的有效调控,是异构金属材料表现出区别于均匀金属材料的塑性变形机制、断裂机制等的关键因素,变形机制和断裂机制的改变一方面表现为微观组织的演化过程,另一方面影响了变形、断裂机制提供的局域应变,表现为调控、优化了异构金属材料中的局域应变分布和演化。值得关注的是,异构金属材料中往往同时存在多种变形机制、断裂机制的交互作用,协同实现了对异构金属材料加工硬化能力、损伤容限、断裂延伸率的改善,从而贡献了异构金属材料优异的强度、塑性和韧性。未来的研究方向主要包括2个方面。

(1) 微观组织设计优化与可控制备:气相沉积、增材制造、精密铸轧、智能制造等技术和领域的蓬勃发展,为金属材料中复杂构型的设计制备、跨尺度的异构构筑等提供了更多可能,有望进一步提升异构金属材料性能极限。与此同时,如何将异构设计与工业设备耦合,发展、实现工业规模生产的制备技术也是亟待拓展的方向之一。

(2) 局域应力与局域应变表征和解析:目前局域应变可以通过DIC、电子背散射衍射技术等方法实验表征,局域应力的实验量化研究较为有限。通过发展局域应力/应变表征技术,耦合有限元模拟、晶体塑性理论等模拟方法,解析局域应力/应变与异构金属材料变形/断裂行为的相关性,有望丰富和拓宽金属材料异构设计理论,指导高性能异构金属材料研发。

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DOI      PMID      [本文引用: 1]

In human-made malleable materials, microdamage such as cracking usually limits material lifetime. Some biological composites, such as bone, have hierarchical microstructures that tolerate cracks but cannot withstand high elongation. We demonstrate a directionally solidified eutectic high-entropy alloy (EHEA) that successfully reconciles crack tolerance and high elongation. The solidified alloy has a hierarchically organized herringbone structure that enables bionic-inspired hierarchical crack buffering. This effect guides stable, persistent crystallographic nucleation and growth of multiple microcracks in abundant poor-deformability microstructures. Hierarchical buffering by adjacent dynamic strain-hardened features helps the cracks to avoid catastrophic growth and percolation. Our self-buffering herringbone material yields an ultrahigh uniform tensile elongation (~50%), three times that of conventional nonbuffering EHEAs, without sacrificing strength.Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

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