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金属学报, 2022, 58(11): 1399-1415 DOI: 10.11900/0412.1961.2022.00370

综述

异构金属材料的设计与制造

张显程,, 张勇, 李晓, 王梓萌, 贺琛贇, 陆体文, 王晓坤, 贾云飞, 涂善东

华东理工大学 承压系统与安全教育部重点实验室 上海 200237

Design and Manufacture of Heterostructured Metallic Materials

ZHANG Xiancheng,, ZHANG Yong, LI Xiao, WANG Zimeng, HE Chenyun, LU Tiwen, WANG Xiaokun, JIA Yunfei, TU Shantung

Key Laboratory of Pressure Systems and Safety, Ministry of Education, East China University of Science and Technology, Shanghai 200237, China

通讯作者: 张显程,xczhang@ecust.edu.cn,主要从事极端环境装备安全保障研究

责任编辑: 肖素红

收稿日期: 2022-08-04   修回日期: 2022-09-06  

基金资助: 国家自然科学基金项目(51725503)

Corresponding authors: ZHANG Xiancheng, professor, Tel:(021)64253149, E-mail:xczhang@ecust.edu.cn

Received: 2022-08-04   Revised: 2022-09-06  

Fund supported: National Natural Science Foundation of China(51725503)

作者简介 About authors

张显程,男,1979年生,教授,博士

摘要

通过构筑金属异构材料以实现强韧均衡的材料设计和制造方法,已成为机械工程和材料科学等领域的前沿方向与研究热点。近年来,对异构金属材料内部多种强韧化机制的理解已逐渐深入,而建立强韧化增益效果与微观结构特征参量的定量关联,进而指导强韧化工艺研发,对异构金属材料的设计理论、制造成形及性能表征具有重要意义。本文主要综述了近年来异构金属材料的微观结构调控理论基础与常用制造工艺的进展。首先按照微观调控手段对异构金属材料进行分类,随后综述了异构金属材料微观结构调控的若干理论基础,最后以“自上而下”和“自下而上”对强韧化工艺进行分类,介绍了常见的异构金属材料制备工艺方法。在此基础上,对异构金属材料设计与制造面临的挑战和发展方向进行了讨论与展望。

关键词： 异构金属材料; 强韧化; 背应力; 微观结构设计; 制备工艺

Abstract

The design and manufacture of heterostructured metallic materials for the balanced improvement of strength and ductility by interior microstructure construction have been the research frontiers and focus in mechanical engineering and materials science. Recently, the understanding of multiple hardening mechanisms in heterostructured metallic materials has progressively advanced. Although establishing quantitative relationships between hardening effects and microstructural parameters and further instructing the research and development of manufacturing for a superior combination between strength and ductility will be of significant value to the design theory, the manufacturing processes and property characterization of heterostructured metallic materials are crucial. In this article, the research progress on the theoretical foundations of designing microstructures and manufacturing processes for heterostructured metallic materials was reviewed. First, the heterostructured metallic materials from the perspective of their microstructural regulation method were categorized. Second, the theoretical foundations for the microstructural regulation of heterostructured materials were reviewed. Third, the manufacturing process for heterostructured materials was classified in terms of the up-bottom and bottom-up approaches as well as reviewed. Finally, the challenges and future development of the design and manufacture of heterostructured metallic materials were addressed.

Keywords： heterostructured metallic material; strength and ductility; back stress; microstructural design; manufacturing process

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本文引用格式

张显程, 张勇, 李晓, 王梓萌, 贺琛贇, 陆体文, 王晓坤, 贾云飞, 涂善东. 异构金属材料的设计与制造[J]. 金属学报, 2022, 58(11): 1399-1415 DOI:10.11900/0412.1961.2022.00370

ZHANG Xiancheng, ZHANG Yong, LI Xiao, WANG Zimeng, HE Chenyun, LU Tiwen, WANG Xiaokun, JIA Yunfei, TU Shantung. Design and Manufacture of Heterostructured Metallic Materials[J]. Acta Metallurgica Sinica, 2022, 58(11): 1399-1415 DOI:10.11900/0412.1961.2022.00370

随着我国航空航天、交通运输和海洋装备等重大工程领域的快速发展,其装备对于结构材料提出了高性能、轻量化和高可靠性的迫切需求。正所谓“一代材料,一代装备”,金属及其合金作为工业领域中不可或缺的结构材料,其综合服役性能的不断提高推动着相关工业技术的进步。强度和韧性作为金属材料最核心的2个力学性能指标,决定着金属材料在工程应用中的使役表现,因此强韧均衡是工程结构材料设计的永恒主题[1]。过去30年的广泛研究表明,传统的形变强化、固溶强化、析出强化和相变强化等强化手段[2~5],在一定程度上提升了其强度而牺牲了韧性,呈现出明显的强度-韧性倒置关系,且材料的强度愈高倒置关系就愈突出[1]。基于仿生学的材料设计正成为一个快速增长并具有巨大发展空间的领域。具有优异综合力学性能和强韧性匹配的天然生物材料往往具有比较复杂的结构要素特征,如非均匀几何形态及空间分布、多尺度、多相与多层次结构等[6,7]。由此出发,科学家们“师法自然”,从自然界中获取灵感,在金属材料中构筑出一些新颖的微观结构,如梯度结构[8]、层片结构[9]和多级孪晶结构[10]等。相关设计范式已被证实能够有效地提高材料的强韧综合性能[11]。

与传统均匀结构材料相比,上述材料内部空间结构、强度乃至成分上呈现出非均匀分布的特性,此类存在“软区”和“硬区”结构单元的金属材料也被称为“异构金属材料”[12]。从21世纪初,Wang等[13]首次提出双峰结构协同提升金属材料强韧性伊始,20年来已有上万篇关于异构金属材料力学性能与变形机理的文章。在ScienceDirect数据库中,异构金属材料强韧性相关文章已20000余篇。在此基础上,已有若干综述总结了异质结构的力学性能及其存在的变形强化机制。例如,Sun等[10]详细综述了多级纳米孪晶金属的强化效应与制造工艺。Li等[14]系统描述了梯度结构中的力学性能与变形机制。Misra等[15]综述了近年来多相及多级材料在提升金属强韧性方面的作用。Sathiyamoorthi和Kim[16]概述了异构金属材料在新材料方面的应用。Wu和Fan[17]详述了异质结构变形过程中应变去局部化机制。Li等[18]阐述了异质结构内几何必需位错堆积诱导的强化效应。Zhu等[12]系统总结了异构金属材料的设计理念与内部异质区非均匀变形引起的背应力强化。目前,对异构金属材料的变形与强化机制效应的研究已比较全面。随着相关理论、模型和制备工艺水平的发展,人们在纳观-微观-宏观等不同尺度上对异构金属材料的微观结构设计与精准调控逐步成为可能,有望进一步推动异构金属材料领域的发展。本文综述了异构金属材料微观结构优化设计的理论基础与常见调控强韧化工艺的进展,并对该领域亟待解决的问题以及未来发展做出展望。

1 异构金属材料分类与倒置曲线

根据微观调控方式,大致可以将异构金属材料划分为以下3类：(1) 调控材料晶粒尺寸或结构,制备具有跨尺度特征晶粒分布的微观结构,如梯度结构[19]、核壳结构[20]和层片结构[21]等；(2) 在金属材料内引入大量缺陷与特殊界面,进而诱发额外的应变硬化等机制,如梯度位错胞[22]、纳米孪晶结构[23]和多级结构[24]等；(3) 调控材料在微纳尺度上的元素与相组成,激发多尺度强韧化机制,如异质双相结构[25]和复合层压板结构[26]等(如图1[19~23,25,26]所示)。

图1

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图1   异构金属材料的分类[19~23,25,26]

Fig.1   Microstructures of various heterogeneous structured metals and alloys[19-23,25,26]


异构金属材料主要通过上述3种调控方式诱发额外的韧化机制,提高金属材料强度的同时保持或提升其韧性,寻求强度与韧性在Ashby图中的最佳匹配[27~30],如图2[11,19,20,22,25~28,30]所示。精准的调控方式依托于先进制备技术,中国在此领域做出了一系列原创性研究工作,引领了异构金属材料领域的发展,实现了金属结构材料强韧性的不断拓维。为加速异构金属材料的迭代优化与工业应用,还需持续推进“材料成分-制造工艺-微观结构-服役性能”多层次的系统研究,首先要厘清强韧化工艺对材料内部微观结构的影响规律和内部微观结构对材料强度与韧性协同提升的内在机制等科学问题,其次,利用成熟的强化机制理论反过来对材料微观结构进行优化设计,并选择合适的强韧化工艺进行制造,最终形成基于材料内部微观结构调控的强韧均衡材料精准设计和制造方法。

图2

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图2   均匀结构与异构金属材料强韧性示意图[11,19,20,22,25~28,30]

Fig.2   Schematic plot of yield strength and elongation of heterogeneous microstructure versus homogeneous microstructure (The sequence of various heterogeneous microstructure is arbitrary)[11,19,20,22,25-28,30]


2 异构金属材料设计理论基础

如上文所述,近年来异构金属材料设计理念已在传统金属材料的倒置曲线上表现出了明显的强韧均衡优势,涵盖广阔的强韧性包络面。已有大量综述[10,12,14,15,17,31,32]证实异构金属材料内部存在多重强韧化机制及其重要作用,而如何定量地建立材料微观结构特征参量与强韧化增益效果之间的关系,从而确定主控材料力学性能的微观参量,将使金属材料微观结构设计更加有的放矢,强韧均衡的按需设计也将成为可能。本节简要介绍异构金属材料强韧化机制并重点概述其主要影响因素。

2.1 背应力强化(异质变形强化)

在异构金属材料的塑性变形过程中,软硬区的力学不相容性会导致明显的非均匀变形,表现出显著的包申格(Bauschinger)效应[11,33]。单轴拉伸卸载再加载曲线(图3a[33])可以定量地描述此效应,也就是背应力(σb)[33]:

σb=σr+σu2

式中,σu和σr分别对应卸载与再加载时的屈服应力。实际上,基于此测得的背应力应由2个部分组成[34]。一部分为加卸载过程中异质区非均匀变形导致的残余应力,即晶间背应力[34]；此外,如图3b[35]所示,软硬区之间的非均匀变形在塑性变形过程中还会诱发巨大的应变梯度,将在异构界面处累积大量的几何必需位错,进而产生晶内背应力,即随动或背应力强化[34],在图中还包含了晶内背应力的反作用力——硬相中的前应力[35]。因此,无论晶间背应力还是晶内背应力,物理起源都是由于材料非均匀微观结构的非均匀变形,例如取向错配、晶粒尺寸差异和夹杂等因素,如图3c所示。

图3

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图3   背应力计算方法与机理示意图[33,35]

Fig.3   Schematics of the calculation and mechanism of back stress

(a) calculation of back stress (σb) in unloading-reloading loop[33] (σu—unload yield stress, σr—reload yield stress)

(b) schematic of the mechanism of back stress and forward stress[35] (τa—an applied shear stress to pile up a dislocation against the boundary, n—the number of pile-up dislocations)

(c) components of back stress (HDI—heterodeformation-induced)

(d) back stress of gradient and homogeneous structures[33]


从前述可知,晶间背应力是异构金属材料加卸载过程中残余应力导致的必然产物,而背应力强化(晶内背应力)则由异构金属材料内复杂位错活动诱导[14],因此后者直接控制着异构金属材料额外的加工硬化行为,也吸引着大量学者对此进行深入的研究[11,36~41]。背应力强化随着材料屈服逐渐增加,从梯度结构和均匀结构的卸载再加载曲线可知[33],梯度结构具有更大的背应力斜率与增量,这表明以梯度结构为代表的异构金属材料能比均匀结构积累更多的几何必需位错,诱导更显著的背应力强化(图3d[33]),使得异构金属材料获得优异的强韧性匹配。因此,如何调控背应力强化成为进一步优化金属材料强韧均衡中的关键科学问题。

2.1.1 晶粒尺寸分布调控

基于应变梯度塑性理论,σb与应变梯度存在如下关系[42]：

σb∝μbR2∂2γ∂x2

式中,μ为Young's模量,R为几何必需位错贡献于背应力的作用半径,b为Burgers矢量模,γ为位错滑移引起的剪切应变,x为加载方向。 式(2)表明,背应力强化本质上为应变的二阶梯度效应,因此,可以增加异构金属材料内异质区的应变梯度来提升强化效果。Li等[42]在J2流动准则[43]的框架下,考虑几何必需位错累积产生的背应力强化效应,建立了基于位错密度的梯度结构本构模型,预测结果与实验结果符合良好。仿真对比发现通过减小表面晶粒尺寸,梯度结构的强韧性协同将得到进一步改善。然而此强化规律受到梯度纳米结构表层机械驱动的晶粒长大机制影响[41],因此还需进一步研究。Lin等[44]通过幂指数经验公式定义了梯度指数(n'),其描述了梯度结构中不同晶粒尺寸分布对应的梯度程度,即

d=dmax-(dmax-dmin)(1-t)n'

式中,d为归一化深度t对应的晶粒尺寸,dmax和dmin分别为沿着梯度结构最大和最小的晶粒尺寸,研究[44]发现调控梯度结构中n'可以进一步提升强韧性匹配,在梯度结构纯Ni中,n'为3时,其延性甚至超过了粗晶纯Ni,最佳的粗细晶匹配有效抑制了试样中部粗晶区域在变形过程中的表面粗糙与细晶的塑性失稳现象。He等[45]通过分子动力学模拟发现增大n'可以增加材料强度。进一步地,Cao[46]基于原子尺度模拟发现梯度结构能够减小引起反Hall-Petch效应的临界尺寸,且此临界尺寸与n'成反比,引起此现象的本质原因在于梯度结构中晶界迁移的削弱与晶间塑性变形的增强。

另外,Li等[40]通过多组异质结构Cu的对比模拟发现,减小硬相尺寸和增大软相尺寸能够增加异质区应变梯度,开发出强韧性匹配更好的微观结构。Zhang等[38]通过考虑软硬相强度差主导的背应力强化机制,建立了描述双峰异构金属材料力学性能的晶体塑性本构模型,该模型不仅能够同时预测多种尺寸分布的双峰结构拉伸行为,还能定量地描述实验加卸载曲线求得的背应力演化,克服了未有效拟合实验背应力数据造成的背应力强化高估问题。并在此基础上对比了微观结构特征参量对力学行为的影响,结果表明,细化双峰结构中的硬相尺寸比调控软相尺寸能更有效地提升双峰结构的背应力强化效应,进一步提升材料的应变硬化能力(图4a和b[38])。

图4

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图4   微观结构特征参量对异构金属材料力学性能的影响[26,38,50]

Fig.4   Effects of the microstructural parameters on the mechanical properties of heterogeneous structures

(a, b) effects of size of coarse and fine grains on the back stress hardening[38]

(c) schematic of interface affected zone in the laminate structure[26]

(d) effect of thickness of interface affected zone on the back stress[26]

(e) volume fraction of fine grains on the stress-strain curves[50] (CR—cold rolling, HS—heterogeneous structured, CG—coarse grain, YS—yield strength, UE—uniform elongation, UTS—ultimate tensile strength, EFT—elongation to fracture)

(f) back stress[50] (σh—HDI stress, σeff—effective stress obtained from the loading-unloading-reloading test)


然而,细化晶粒尺寸的强化方法并非始终有效。Ma等[47]和Huang等[26]通过调控层压板中的层片厚度,先后确认了界面影响区的存在,界面影响区定义为具有应变梯度的区域,该区域能够有效地积累位错,产生背应力强化,随着层片厚度的减小,直到层片内相邻的界面影响区开始重叠,强韧性都会同时提升,表明层压板存在最佳层片厚度(lIAZ),大概为[26]:

lIAZ≈μσy2b

式中,σy为材料屈服应力。因此,该研究结果表明异构金属材料内严重位错累积区的重叠反而会削弱强韧化效果,晶粒尺寸的调控也需考虑有效位错堆叠区,应最大程度地发挥异构金属材料内可容纳位错区,诱导更显著的背应力强化(图4c和d[26])。Zhao等[48,49]建立异构金属材料位错密度与堆积理论,模拟了层压板结构拉伸变形,预测了不同层压板厚度的力学性能,发现随着界面间距的减小,界面影响区的体积分数增大能增加背应力强化效应,提升金属材料强韧性。此外,Shin等[50]通过在钛合金内调控异质层片结构的软硬相体积分数,发现增大硬相区体积分数能够提升异构金属材料的背应力强化,进一步提升强度与韧性(图4e和f[50])。

2.1.2 相成分调控

精准调控材料在微纳尺度上的相含量和分布,激发多尺度的强化机制亦是调控金属强韧性的有效策略之一。双相结构中的软硬相塑性变形时诱发的应变梯度也会诱发显著的背应力强化效应[51,52]。例如,典型的双相钢结构中的硬相马氏体与软相铁素体能够在提高强度的同时,表现出一定的加工硬化和良好的塑性,这种良好的力学性能归因于软硬相异质区的协同强化作用[51,53]。针对复合结构材料,Fan等[9]和Wu等[54]基于软硬相协同变形设计思路制备出Ti-Al复合材料,发现双相复合板结构能有效抑制硬相中的应变局部化并约束微裂纹的扩展[55],展示出良好的强韧性匹配。相成分调控策略的有效性在先进高强钢和高熵合金设计领域也已被证实,相关工作已在许多综述中展示[16,56,57],在此不再赘述。

2.2 特殊界面调控

材料的力学行为很大程度上也受到微观界面的影响。已有大量的工作[12,47]表明,引入特殊界面(例如相界面、异构界面、孪晶界面等)能够同时提升材料的强韧性匹配,此节讨论如何通过调控特殊界面优化异构材料强韧性。

2.2.1 异构界面调控

异构金属材料中,多种形式的金属相界面与软硬相界面排布,即异构界面分布也会对异构金属材料的力学行为产生影响。Sun等[58]和Ma等[59]通过调控中锰钢的双相异构界面,发现引入C元素偏析可提高位错发散与形核的阻力,进而显著提高材料的屈服强度。Liu等[60]对比了粗晶层片排布与弥散排布异构金属材料的力学性能,发现后者具有更优异的强韧性匹配,在弥散排布异质结构的变形过程中,更加均匀分布的软相能够有效抑制失稳剪切带的形成,形成大量微剪切带,提升均匀变形能力,同时提升背应力强化。Flipon等[61]利用晶体塑性有限元方法对比了不同粗晶排布的双峰结构力学性能,结果表明3种粗晶排布对双峰结构的宏观力学行为影响较小,但是粗晶聚集分布易于局部剪切带的形成与扩展,同时促进局部剪切带宽化,对材料微观变形均匀性有劣化作用。Zhang等[62]在晶体塑性框架下对比3种典型的双峰晶粒尺寸分布异质结构,即核壳结构、层片排布结构和粗晶弥散结构(图5[62]),在粗晶弥散排布结构中印证了大量微剪切带的形成,并定量研究了3种不同粗晶排布异质结构中界面附近应变梯度主导的背应力强化,发现粗晶弥散结构拥有最大的异构界面密度,其能够有效引起更大的平均应变梯度,进而诱导更高的背应力强化(图5d[62]),揭示了异构界面密度与应变硬化率的线性关系。

图5

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图5   软相排布对力学性能的影响[62]

Fig.5   Effects of the distribution of coarse grains on the mechanical properties of heterogeneous structures[62]

(a-c) constructions of representative volume element (RVE) of harmonic (a), lamellar (b), and dispersed (c) structures

(d) back stress hardening of three heterogeneous structures


2.2.2 孪晶界调控

孪晶界作为一种特殊低能态共格晶界,因其优异的强韧化效应而受到广泛关注[63]。不同于传统晶界,孪晶界不仅能有效阻碍位错运动,还能作为位错的滑移面容纳大量位错,使材料保持良好的强度和韧性[64,65]。相较于微米尺度和亚微米尺度的孪晶,纳米尺度的孪晶对材料的强化效应显著提高[66,67]。纳米孪晶的变形机理和微观结构演化已在前人的综述[10,68]中得到详细介绍,本节侧重于阐述纳米孪晶金属的微观结构特征参数对力学性能的影响。

在纳米孪晶材料中,不少学者针对孪晶厚度对材料强韧性的影响进行了研究。Lu等[63]采用脉冲电沉积技术制备出具有不同孪晶厚度的纯Cu样品,发现纳米孪晶Cu材料的强度随孪晶厚度的减小而增大,当孪晶厚度为15 nm时,材料强度达到最大值,如图6a[63]所示。极值强度的出现是由于随孪晶厚度的减小,纳米孪晶Cu的主导塑性变形机制从位错与孪晶的相互作用转变为孪晶结构中预存的位错运动。此外,Li等[69]通过分子动力学模拟提出该临界孪晶厚度以及极值强度与晶粒尺寸密切相关,晶粒尺寸越小,临界孪晶厚度也越小,对应材料的极值强度越高。应变速率敏感指数(m)是衡量材料塑性变形的基本参量之一[70]。Lu等[71]总结了孪晶厚度对纳米孪晶Cu应变速率敏感指数影响的实验结果。如图6b[71]所示,随孪晶厚度从微米尺度减小到纳米尺度,m迅速增加,当厚度为15 nm左右时,m比微米尺度样品高近乎一个数量级,其归因于位错和大量孪晶界的交互作用。材料的层错能也会对纳米孪晶金属的强韧性有显著影响,在加工过程中,位错和孪晶会大量出现在层错能较低的金属材料中。Zhao等[72]对拥有不同层错能的Cu-Zn合金进行高压扭转处理和拉伸测试发现,在Cu-Zn合金中存在一个最佳层错能,该层错能对应的材料在严重塑性变形后具有较高的强度和最佳的韧性(图6c[72])。提高孪晶材料韧性的关键是在塑性变形时材料中要有充足的位错和孪晶累积,对于层错能过低的金属,经过严重塑性变形后,材料中的层错处于饱和状态,在后续的塑性变形中就难以累积层错和孪晶,可以通过热处理来减少材料严重塑性变形过程中产生的位错和孪晶来相应地提高韧性。You等[73]通过直流电沉积工艺制备纳米孪晶Cu,并结合晶体塑性和分子动力学模拟发现,改变孪晶平面和加载方向之间的角度会引起纳米孪晶Cu的主导变形机制发生改变,共存在3种主导变形机理,即位错在孪晶间滑移、位错穿过孪晶界和不全位错运动导致的孪晶界迁移。Zhang等[74]通过建立理论模型表明,可以通过合理设置孪晶平面和加载方向之间的角度来提高纳米孪晶材料的力学性能。如图6d[74]所示,当2者角度为60°时,发射位错所需要的临界应力最大,可以有效抑制剪切带的启动。

图6

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图6   微观结构参数对纳米孪晶金属力学性能的影响[63,71,72,74]

Fig.6   Effects of microstructure parameters on mechanical properties for nanotwinned metals

(a) effect of twin thickness on stress-strain curves[63] (nt—nanotwin, ufg—ultrafine grain, cg—coarse grain)

(b) effect of twin thickness (λ) or grain size (D) for nanocrystalline Cu on strain rate sensitivity index (m)[71]

(c) effect of stacking fault energy on stress-strain curves[72]

(d) effect of the angle between loading axis and twin boundary (θ) on the ratio of critical stress for dislocation emission and shear modulus[74] (σcr—critical stress, G—shear modulus)


近年来,在金属中构建多级纳米结构也被证实为一种有效提高强韧性的方法,例如在易形成纳米孪晶的金属中构建多级纳米孪晶结构可以进一步改善材料的力学性能[10]。目前学者们对多级纳米孪晶材料力学性能影响因素的研究十分有限,主要集中在多级纳米孪晶厚度对强韧性的影响。

Sun等[75]通过分子动力学模拟发现两级纳米孪晶材料存在2个屈服软化现象。随着一级孪晶厚度的减小,材料的塑性变形机制由全位错主导转变为不全位错主导,材料逐渐发生软化。随后材料又因主导变形机制转变为位错阻塞而得到强化。随着一级孪晶厚度的进一步减小,主导的塑性变形机制又转变为平行于孪晶界的不全位错诱发的孪晶界迁移和退孪生,材料再次发生软化。Zhu等[76]使用基于位错密度的理论模型来描述多级孪晶的变形行为,随着一级孪晶厚度的增加,流动应力先增加后减小再增加,其趋势转变时对应的孪晶厚度分别称为第一次软化临界厚度和第二次软化临界厚度。如图7a[76]所示,第一次软化对应的临界一级孪晶厚度与二级孪晶厚度无关,而第二次软化对应的临界一级孪晶厚度随着二级孪晶厚度的增加先减小后增大,此外2个屈服软化现象对应的临界一级孪晶厚度都会随着晶粒尺寸的增大而增大。Yuan和Wu[77]通过分子动力学模拟发现,在一级孪晶厚度和晶粒尺寸相同的情况下,随着二级孪晶厚度的减小,材料的流动应力先升高后减小,存在一个临界二级孪晶厚度使材料的强度达到最大,并且该临界二级孪晶厚度随着一级孪晶厚度的减小而减小(图7b[77])。随着二级孪晶厚度的减小,塑性变形机制从位错穿过晶界和孪晶界转变为二级孪晶的退孪生和一级孪晶的迁移。Zhu等[78]建立了基于位错的理论模型,进一步验证了在一级和二级孪晶中存在最佳孪晶厚度,并表明临界孪晶厚度与晶粒尺寸满足比例关系。

图7

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图7   孪晶厚度对多级孪晶金属力学性能的影响[76,77]

Fig.7   Effects of twin thickness on mechanical properties for hierarchical nanotwinned metals

(a) effect of secondary twin thickness (L2) with D on transition twin thickness (L1Tr)[76]

(b) effect of L2 on average flow stress[77] (L1—primary twin thickness)


3 异构金属材料制造工艺

异构金属材料的制备工艺,可划分为“自上而下”法和“自下而上”法2种技术手段,如图8所示。其中,“自上而下”法是指通过机械变形的方式细化晶粒,形成多尺度的晶粒分布,从而在材料内部获得异质微观结构。常见的自上而下法有表面机械碾磨处理[29]、超声表面滚压[79]以及异步轧制[80]等。“自下而上”法的原理则是通过物理或者化学的方法,将原子团簇或不同成分粉末混合形成微观组织具有异构特征的金属材料[32,81~83]。本节将综述近年来发展的异构金属材料强韧化工艺及其核心调控参数。

图8

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图8   异质金属材料制造技术手段的分类原理图

Fig.8   Classification schematic of precise preparation techniques of heterogeneous metal materials


3.1 “自上而下”工艺

3.1.1 表面强化工艺

喷丸(shot peening,SP)工艺是常见的传统机械表面处理技术之一,其工艺原理如图9a[84]所示。大量硬质弹丸在压缩空气的驱动下形成喷射流,反复地碰撞样品表面,导致材料表面发生严重塑性变形,表面晶粒持续的破碎与细化,从而制备出由表及里的梯度结构。喷丸工艺的主要优点是可以适应不同形状的样品,并且操作方便,成本相对低廉。但是,在长时间的处理过程中,弹丸的冲击能量较难精确控制,且存在表面粗糙度较差的问题[84,85],难以精准地调控材料表层的微观结构。

图9

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图9   表面强化工艺示意图以及其主要的工艺参数[29,84,87,89,91]

Fig.9   Schematics of the surface strengthening treatments and their main process parameters

(a) shot peening (SP)[84]

(b) surface mechanical attrition treatment (SMAT)[87]

(c) surface mechanical grinding treatment (SMGT)[29] (TD—transverse direction, ND—normal direction, SD—shear direction, ν1—rotation velocity, ν2—sliding velocity)

(d) fast multiple rotation rolling (FMRR)[89] (P—pressure, ν— horizontal velocity, ω— rotational speed)

(e) ultrasonic surface rolling process (USRP)[91] (ap—preset depth)


表面机械研磨处理(surface mechanical attrition treatment,SMAT)工艺是较早用于制备异构金属材料的主要技术手段之一。早在1999年,Lu等[86]研发了表面机械磨损处理工艺并采用此工艺分别在Fe和不锈钢中制备出了梯度纳米结构,表面晶粒极限尺寸约为几十纳米。如图9b[87]所示,SMAT的工作原理是通过高功率激振器驱动弹丸高速撞击材料表面,在材料表层持续产生高应变的塑性变形,应变量及变形速率随着深度增加而减小,最终在材料表面形成梯度纳米结构。相较于SP工艺,SMAT工艺可以较为精准地调控弹丸的冲击能量、冲击角度以及冲击频率等工艺参数,从而有利于理想微观结构的精准制备,但仍存在弹丸在使用过程中表面状态不易于控制,材料表面粗糙度不理想等问题。

2008年,Li等[88]在SMAT工艺基础上进行了改良,首创了表面机械碾压处理(surface mechanical grinding treatment,SMGT)工艺并且在纯Cu材料上制备出梯度纳米结构。此外,为了有效地细化表层晶粒,消除变形过程中的瞬时温升,可以通过液氮环境加工,将表层晶粒极限尺寸降低至几个纳米。在SMGT过程中,硬质材料制成的半球形压头浸入冷却介质中并压入高速旋转的圆柱形样品 (图9c[29]),然后沿样品的轴向以相对较低的速率滑动。不同于SMAT中材料表层受不易于控制的冲击载荷而发生塑性变形,SMGT工艺通过调控试样转速ν1、压头的滑动速率ν2与路径以及施加载荷等工艺参数,从而使材料表层受到范围可调的剪切应变和应变速率,制备出较为理想的梯度纳米结构。

2012年,Chui等[89]提出一种金属材料的表面强化制备工艺,即多重旋转碾压(fast multiple rotation rolling,FMRR)工艺。通过在材料的最表层设置多个滚珠尖端滚动,从而提高表面纳米化的效率,在低碳钢中制备出了极限尺寸约8 nm的等轴晶粒。如图9d[89]所示,FMRR工艺允许多个自由滚动的滚珠同时工作,可以减小表面摩擦力并提高晶粒细化的效率,从而使材料在更短的加工时间内获得表面完整性较好的梯度纳米结构。与SMAT 相比,SMGT和FMRR产生的梯度结构层更厚更平滑,并且结构规律性更好,可重复性更高。

Wang等[90]研发的表面超声滚压处理(ultrasonic surface rolling process,USRP)技术将传统滚压技术与超声技术结合,沿工件表面法线方向对硬质合金工作头施加一定幅度的超声频机械振动,工作头将静压力和超声冲击振动传递到旋转的金属材料表面,使材料表面产生大幅度的塑性变形(图9e[91])。与此同时,在超声波冲击和静压力滚压联合作用下,保证了滚珠和变化曲面的连续接触,同时金属材料表面产生剧烈而均匀的塑性变形[92]。此外,由于实现了工作滚珠与材料表面的近“无摩擦”冲击滚压效果,减少了对材料表面的划伤,获得了理想的表面质量。相较于SMGT工艺,USRP工艺提供了更多可供调控的工艺参数用于构筑理想微观结构,例如超声振动频率和振幅等。Cao等[93]选取振动频率为20 kHz且振动幅度为30 μm超声滚压工艺,在S45C钢上制备了极限尺寸为50 nm的梯度纳米层片结构并显著地改善了材料的表面完整性,表面硬度提升一倍,粗糙度降低一倍,极大提高了材料的抗疲劳性能。另外,超声表面滚压工艺对于复杂曲面零部件的加工具有独特的优势,Zhang等[94]设计了一种适用于复杂曲面,微观结构可调控的USRP系统,此系统可以沿着多曲度零件表面产生顺应性的变形,从而在加工过程中曲面受到稳定而均匀的静载荷加载并获得较高的表面质量和较理想的异构金属材料。

综上,表面强化工艺正朝着以下3个方向发展。(1) 可控参数丰富化：与超低温加工环境箱和超声波发射器等辅助设备协同加工,激发更加多元的工艺参数,允许梯度金属材料微观结构实现更加精准的调控；(2) 微观结构与宏观表面完整性调控一体化：在微观结构优化的基础上,需同时兼顾加工材料的表面完整性,提高强化工艺效率；(3) 复杂条件下工业应用：为使表面强化工艺在工业环境下得到更广泛的应用,还需依托于传统加工系统与运维监测系统,使其可服役于严苛环境,应用于硬质难强化、复杂表面形态的工件。

3.1.2 剧烈塑性块体强化工艺

冷轧(cold rolling,CR)工艺结合不同热处理条件对于大规模制备高强韧性的材料具有良好的适用性以及经济性[95]。如图10a[96]所示,轧制时2个旋转轧辊的转速相同,通过冷轧量和轧制次数的增加实现塑性变形量的不断累积。Sabooni等[97]通过冷轧和退火工艺在AISI 304L不锈钢中制备出双峰结构,有效地改善了材料的力学性能。

图10

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图10   块体剧烈塑性变形工艺以及其主要的工艺参数[96,99,100,103]

Fig.10   Schematics of severe deformed treatments for bulk and their main process parameters

(a) cold rolling (CR)[96] (ν1', ν2'— rotation velocities of top and bottom roller, respectively)

(b) asymmetric rolling (ASR)[99]

(c) equal channel angular pressing (ECAP)[100] (Φ—angle between channels, Ψ—outer curvature angle)

(d) accumulative roll bonding (ARB)[103] (HPT—high pressure torsion)


相较于传统的冷轧,异步轧制(asymmetric rolling,ASR)的特点是轧辊速度ν1'和ν2'不对等(图10b[98]),因此,可以通过选择合适的ν1'和ν2'的组合,显著降低轧制压力与轧制扭矩,减少轧制道次,提高轧制效率[98,99]。Ren等[99]的研究表明,当选取ν1'和ν2'比值为1.8时,在AA6016薄片上制备出了极限尺寸约200 nm的异质结构。此外,研究发现材料的强韧匹配随着v1'和v2'的比值增大而显著提升。

等通道转角挤压(equal channel angular pressing,ECAP)工作原理如图10c[100]所示,材料在外加载荷的作用下被压入两通道的交界处时,试样内部发生近似理想的纯剪切变形。由于挤压前后试样的截面形状和面积不发生改变,故多道次挤压可以获得相当大的累积应变量。等通道转角挤压工艺由于变形过程中不改变材料的横截面面积和截面形状,故只需较低的工作压力即可实现材料反复定向的剪切变形。并且,此工艺适用于制备三维大尺寸的块体材料,具有较大的工业应用潜力。Sun等[101]对Mg-Al-Ca-Mn合金采用32道次的等通道转角挤压工艺,制备出了力学性能和耐腐蚀性能协同增强的双峰异质结构。

累积叠轧(accumulative roll bonding,ARB)工艺的原理[102]是将两块形状、尺寸相同的薄板材料叠合在一起,在一定温度下进行轧制,使其轧合成一个整体；然后,将轧制成整体的板料从中截断,再经叠合、轧制等重复操作,实现塑性变形的累积。由于ARB工艺中涉及到多道工序,因此在制备过程中大量工艺参数,例如薄板的高径比,叠轧载荷以及叠轧扭转速率等,皆可用于提高异质结构制备的调控精准性。Ma等[103]将工业铜和青铜圆盘表面进行了机械抛光和超声波清洗,随后将两层或多层材料牢固结合,堆积起来并结合高压扭转(high pressure torsion,HPT)工艺,获得沿半径方向更均匀的变形,预热处理后立即用2个旋转的轧辊对板材进行研磨和压缩,这便是一个ARB循环, 如图10d[103]所示。多次循环后试样厚度减小,晶粒得到细化,梯度层变厚,随后在240℃下退火2 h,使部分晶粒再结晶,最终在界面附近形成异质纳米层片结构。

3.2 “自下而上”工艺

“自下而上”工艺可以实现从纳观、微观和宏观3种不同尺度来进行异质金属结构材料的设计和制造。纳观尺度设计法从原子层设计出发制备异构材料,如电沉积法[104~106]；微观尺寸设计法通常以微米级金属粉末作为原料,通过成型技术将粉末组装成宏观样品,如粉末冶金法[107]；宏观尺度设计法从毫米级区域形成多尺度晶粒或梯度成分,从而形成宏观异构金属材料[108]。

电沉积(electrodeposition,ED)工艺通过两电极之间的电流,将电解液中的离子还原沉积到电极上,形成所需沉积物。由于电沉积工艺的高度均匀与可控性,被广泛应用于异构金属材料的制备[109~112]。在直流电沉积过程中,可以通过调控电解液温度、浓度、沉积时间等来调控沉积物的形态、成分、织构等微观参量[105]。Cheng等[104]通过沉积纯Cu制备高度可调的梯度纳米孪晶结构(图11a[104]),由于晶界附近超高密度的几何必需位错,梯度纳米孪晶结构的整体强度甚至超过了结构中的最强组分,大幅度地改善了结构强韧性匹配。Zhang等[113]通过调节电流密度和添加剂成功调控出双峰Ni,其内部核壳组织和大量共格孪晶大幅提升了材料的应变硬化,扩展了均匀材料的强韧性极限。脉冲电流的引入则提供了更多可调参数,例如超高的电流密度、沉积速率等,可以有效地调控晶粒度和微观形貌[105]。Daryadel等[106]通过脉冲电沉积技术直接打印三维纳米孪晶Cu,沉积物完全致密且无杂质与微观缺陷,显示出超高的强度。Cui等[114]通过超声沉积技术发现多层沉积Cu能进一步降低单层沉积Cu的晶粒尺寸。

图11

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图11   自下而上设计法及其主要工艺参数[104,107,108,119]

Fig.11   Schematics of the bottom-up design method and their main process parameters

(a) electrodeposition (ED)[104] (b) powder metallurgy (PM)[107,119] (c) additive manufacturing (AM)[108] (h2—hatch spacing)


粉末冶金(powder metallurgy,PM)工艺通常包括2个步骤,首先控制金属粉末的塑性变形,得到所需尺寸的金属粉末,进而烧结获得块体材料。可以筛选粉末实现微观结构的调控。例如可在气磨设备中调控初始粉末的粒径、研磨压力、研磨遍数等得到不同尺寸的金属粉末[20,107,115,116]。另外,还可通过优化烧结温度和时间来进一步调控双元或多元粉末合成后的微观结构,实现异构金属材料的调控[117,118]。Ota等[115]利用气磨工艺制备出异质核壳结构,有效抑制微观结构变形局部化,同时提升强韧性(图11b[107,119])。将金属粉末与多种复合材料粉末混合,可对异构金属材料进一步开发[120]。Vajpai等[121]和Wang等[122]利用合金金属粉末烧结过程中的相变等特性,调控出多种形态的核壳结构,促进多强化机制的协同作用。Fu等[123]烧结3种尺寸金属粉末合成多峰结构复合材料,其对应变局部化的抑制效果超过单峰及双峰结构,实现强韧性的大幅提升。

增材制造(additive manufacturing,AM)技术被誉为有望产生“第三次工业革命”的代表性技术,是个性化制造模式发展的引领技术,其原理是利用计算机辅助设计逐点把材料累积形成面,逐面累积成为体,这一成形原理给材料设计从传统的纳/微观设计向宏观设计发展提供了新契机。增材制造因其独特的冷热循环效应,在微观尺度上易形成异构金属材料(图11c[108])。Lu等[124]和Yao等[125]利用该特性开发出多款低温高强韧性的增材制造多组元合金材料。通过调节增材制造工艺(如增材制造类型、扫描策略、激光功率等)、异种粉末类型和体积分数等参数可以在构件内形成多尺度晶粒或成分梯度结构。Tan等[108]利用直接能量沉积技术(direct energy deposition,DED)制造出具有可控体积分数和空间周期分布的异构金属材料。与大多数报道的线性格式多材料不同,这项工作利用DED空间设计和制造的独特灵活性,通过将2种类型钢(即高强的C300马氏体时效钢和高韧性的316L不锈钢)配置在一个部件空间,从而将2种材料的优点合并,有助于设计出强韧均衡的合金或部件。此外,利用DED技术或选区激光熔化技术制备出双层梯度组织材料已成为材料宏观尺度设计的重要方式,如双梯度SS316L/In718合金等[126],实现了强塑均衡材料的可控制造。

4 总结与展望

异质微观结构的构筑可作为独立方法或协同合金化策略来进一步突破传统金属材料的强韧性倒置关系。经过20年的发展,异构金属材料领域已逐渐成熟,研究方向已远不止局限于单一异构形式,科学家们持续从大自然中汲取灵感,已逐渐探索出多级、多尺度、多元的异构形式,确保金属材料变形过程中充足的强韧化机制。然而大自然物竞天择的筛选效率显然已不能满足日益增长的性能需求,“取法乎上,仅得其中”时刻提醒我们要取自自然,更要优于自然,亟需从已有的异构金属材料体系中量化微观结构参量与强化机制之间的关联,进而通过高通量测试技术、数据科学等手段进一步对异构金属材料进行优化设计,逐步逼近金属材料强韧性极限。其中,许多困难和挑战需要进一步克服,主要包括：

(1) 大多数精密巧妙的异构金属材料微观形态设计不具备经济可行性。由于当前的众多关于异构材料的研究制备工艺过程繁琐,成本高昂,对环境资源负担较重,目前仍集中在基于实验室的研究设施中。深化产学研合作,对接企业需求,以期发展适合工业化应用的大尺寸、低成本的可靠性制造方法,是异构金属材料领域实际应用将会面临且必须跨越的鸿沟。

(2) 面向性能优化的异构金属材料设计从“试错”到“精准”的转变。对现有多种异构金属材料的微观结构-力学性能数据库进行统计分析,运用机器学习等数据科学方法对多级异构金属材料微观结构的未来演化进化做出预测,弥补传统筛选优化的长耗时缺陷,推动异构金属材料强韧优化设计。

(3) 材料微观结构的异质性并不总能带来强韧性的提升,因此需在加工过程中保持必要异质性,避免非必要异质性,如缺陷、孔隙等。未来应澄清“制造工艺-微观结构-服役性能”的内在关联机制,进而在传统强化方法的基础上,对现有制备工艺方法进行精益优化设计,同时结合数据科学方法,发展数字制造、增材制造、纳米制造等新的制造方法,形成更精细、更先进、高可靠性的强韧化工艺库,以实现材料微观结构特征的精准调控。

(4) 目前异构金属材料领域的研究重点集中于强韧匹配,理解其塑性变形机制。亟待开展多轴、疲劳、蠕变、腐蚀等复杂服役条件下失效机理与破坏形式的研究,揭示微观损伤演化规律。此外,异构金属材料特殊的微观结构将引起基于传统损伤动力学的寿命设计和评定方法在一定程度上的失效。亟需考虑复杂微观结构-时空载荷-环境耦合下跨尺度破坏物理机制的科学描述,揭示多尺度、多维度、多场耦合的失效动力学与损伤演化规律,发展关键微观结构参量敏感的寿命设计理论与方法,以满足异构材料优异强韧匹配、高可靠性和长寿命等目标的服役需求。

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The strength-ductility trade-off has been a long-standing dilemma in materials science. This has limited the potential of many structural materials, steels in particular. Here we report a way of enhancing the strength of twinning-induced plasticity steel at no ductility trade-off. After applying torsion to cylindrical twinning-induced plasticity steel samples to generate a gradient nanotwinned structure along the radial direction, we find that the yielding strength of the material can be doubled at no reduction in ductility. It is shown that this evasion of strength-ductility trade-off is due to the formation of a gradient hierarchical nanotwinned structure during pre-torsion and subsequent tensile deformation. A series of finite element simulations based on crystal plasticity are performed to understand why the gradient twin structure can cause strengthening and ductility retention, and how sequential torsion and tension lead to the observed hierarchical nanotwinned structure through activation of different twinning systems.

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Conventional polycrystalline metals become stronger with decreasing grain size, yet softening starts to take over at the nanometer regime, giving rise to the strongest size at which the predominate strengthening mechanism switches to softening. We show that this critical size for the onset of softening can be tuned by tailoring grain size gradient, and raising in the gradient shifts the size toward the smaller value. The decrease in the strongest size is prompted by mitigation of grain boundary-mediated softening processes accompanying by enhanced intragranular plastic deformations. We found that the nanograins smaller than 6 nm, mainly involving intergranular sliding in homogeneous structures, reveal anomalous plastic deformation in gradient systems, which is mediated by partial dislocation nucleation, faulting and twinning activated in a gradient stress field. The results on extended dislocation slip and gradient plasticity, stemming from the structure heterogeneity, shed light on an emerging class of heterogeneous nanostructured materials of improved strength-ductility synergy.

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The strength of polycrystalline materials increases with decreasing grain size. Below a critical size, smaller grains might lead to softening, as suggested by atomistic simulations. The strongest size should arise at a transition in deformation mechanism from lattice dislocation activities to grain boundary-related processes. We investigated the maximum strength of nanotwinned copper samples with different twin thicknesses. We found that the strength increases with decreasing twin thickness, reaching a maximum at 15 nanometers, followed by a softening at smaller values that is accompanied by enhanced strain hardening and tensile ductility. The strongest twin thickness originates from a transition in the yielding mechanism from the slip transfer across twin boundaries to the activity of preexisting easy dislocation sources.

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Twin-thickness-controlled plastic deformation mechanisms are well understood for submicron-sized twin-structural polycrystalline metals. However, for twin-structural nanocrystalline metals where both the grain size and twin thickness reach the nanometre scale, how these metals accommodate plastic deformation remains unclear. Here, we report an integrated grain size and twin thickness effect on the deformation mode of twin-structural nanocrystalline platinum. Above a ∼10 nm grain size, there is a critical value of twin thickness at which the full dislocation intersecting with the twin plane switches to a deformation mode that results in a partial dislocation parallel to the twin planes. This critical twin thickness value varies from ∼6 to 10 nm and is grain size-dependent. For grain sizes between ∼10 to 6 nm, only partial dislocation parallel to twin planes is observed. When the grain size falls below 6 nm, the plasticity switches to grain boundary-mediated plasticity, in contrast with previous studies, suggesting that the plasticity in twin-structural nanocrystalline metals is governed by partial dislocation activities.

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