金属学报(中文版), 2018, 54(11): 1553-1566
doi: 10.11900/0412.1961.2018.00372
高熵合金的变形行为及强韧化
Deformation Behavior and Toughening of High-Entropy Alloys
吕昭平, 雷智锋, 黄海龙, 刘少飞, 张凡, 段大波, 曹培培, 吴渊, 刘雄军, 王辉
北京科技大学新金属材料国家重点实验室 北京 100083
LU Zhaoping, LEI Zhifeng, HUANG Hailong, LIU Shaofei, ZHANG Fan, DUAN Dabo, CAO Peipei, WU Yuan, LIU Xiongjun, WANG Hui
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
 Cite this article:
LU Zhaoping, LEI Zhifeng, HUANG Hailong, LIU Shaofei, ZHANG Fan, DUAN Dabo, CAO Peipei, WU Yuan, LIU Xiongjun, WANG Hui. Deformation Behavior and Toughening of High-Entropy Alloys[J]. Acta Metallurgica Sinica, 2018, 54(11): 1553-1566 doi:10.11900/0412.1961.2018.00372

摘要:

高熵合金是近年来涌现出的一种新型金属材料。不同于传统合金设计以1种或2种元素为主添加其它合金元素为辅的方案,高熵合金由多种元素以等原子比或近等原子比的成分组成,具有独特的原子结构特征,因而呈现出诸多不同于传统合金的独特性能。自高熵合金被首次报道以来,目前已经研发出了一系列的高熵合金体系,在物理、化学、热力学性能方面显示出独有的优势,尤其在力学行为方面显示出高强、高硬、耐磨、耐蚀、抗高温软化等优异的性能,在国际学术界引起了广泛的关注和研究兴趣,已经成为新的研究热点。本文从高熵合金变形机理研究存在的挑战出发,主要综述了高熵合金的力学性能和变形行为特点,已经提出的强韧化方案及相关机理,并对未来高熵合金变形行为的研究进行了简单展望。

关键词: 高熵合金 ; 强韧化 ; 变形行为与机理

Abstract:

A new alloy design concept, high-entropy alloys (HEAs), has attracted increasing attentions and becomes a new research highlight recently. Different from traditional alloy design strategy which usually blends with one or two elements as the principal constituent and other minor elements for the further optimization of properties, HEAs are multicomponent alloys containing several principle elements (usually ≥5) in equiatomic or near equiatomic ratio. Due to their unique atomic structure, HEAs possess a lot of distinguished properties. Since the discovery of HEAs, a variety of HEA systems have been developed and shown unique physical, chemical and thermodynamic properties, especially the promising mechanical properties such as high strength and hardness, abrasion resistance, corrosion resistance and softening resistance. Here in this short review manuscript, starting from the research challenges for understanding the deformation mechanism of HEAs, this work briefly summarized the mechanical properties and deformation behavior of HEAs, reviewed the proposed strengthening-toughening strategies and their corresponding deformation mechanism in HEAs. A brief perspective on the research directions of mechanical behavior of HEAs was also proposed.

Key words: high-entropy alloy ; toughening ; deformation behavior and mechanism

金属材料在人类社会的发展中一直起着举足轻重的关键作用,我国的科技发展也对高性能的新型金属材料提出了更高要求。传统合金的设计理念以1种或2种元素为主,添加少量其它元素为辅来改变或优化性能,目前已经开发了大量的实用合金。但经过多年的开发,传统合金的性能已经趋于瓶颈,亟需颠覆性的新型合金设计理念。高熵合金就是近年涌现的一种具有广阔应用潜力的新型高性能金属材料。高熵合金于2004年首次被报道[1],它打破了传统合金以混合焓为主的单主元成分设计理念,以构型熵为主设计的一类新型多主元金属材料。

高熵合金又称多主元合金,其研究最早开始于18世纪后期。1981年,Cantor教授和他的学生Vincent制备研究了多种等原子比合金(如无说明,文中成分表达式均是原子比),其中Fe20Cr20Ni20Mn20Co20形成了单相fcc结构。之后,Cantor教授的学生Chang重复Vincent的实验,并于2004年将结果公开发表[2]。同一年,台湾清华大学Yeh等[1]也独立公开发表了多主元合金的研究结果,通过实验结果和相关的理论研究,首次提出高熵合金的概念。此后,高熵合金引起了学术界的关注,它的研究进入了一个快速发展的阶段。

Yeh等[1]最早将高熵合金定义为包含5种及以上组成元素,且每个组元原子分数在5%到35%之间的合金。传统合金的研究认为,合金组元多会形成金属间化合物,从而使合金结构变得复杂。但对高熵合金的研究[1]发现,其高混合熵增强了固溶体的相稳定性,促使合金形成简单固溶体。在统计热力学中,熵是表征系统混乱度的参数。系统的熵值越大,说明系统混乱度越大。根据Boltzmann热力学统计原理,体系的混合熵∆Sconf可以表示为[1]

Δ S conf = k ln ω (1)

其中, k 是Boltzmann常数; ω 是热力学概率,代表宏观态中包含的微观态总数。对于多组元合金,n种元素等原子比混合形成固溶体时[1]

Δ S conf = R ln n (2)

其中,R=8.314 J/(K·mol),为气体常数。由公式可知,当合金的组元数达到5种或者5种以上时,混合熵已大于1.609R,这就是高熵合金称谓的来源。而当组元数超过13时,混合熵的增加趋于平缓,所以一般情况下,高熵合金的组元数会控制在5~13之间。随着进一步的研究发展,高熵合金的定义有所拓展,目前三元[3]和四元[4]的近等原子比合金也被认为是高熵合金。除了高熵合金这个称谓,此类多组元合金也常被称为成分复杂合金(compositionally complex alloy)[5]、等原子比多组元合金(equiatomic multicomponent alloy)[2]、多主元合金(multi-principal element alloy)[6]等。

高熵合金的研究在早期阶段主要集中在合金成分设计上,多组元的设计理念决定了高熵合金的种类繁多,不同的组元元素种类和含量都会对合金的微观结构和性能产生一定的影响,研究者们通过对元素种类和含量的控制,改善合金的组织,致力于将合金的性能达到最优化。已开发研究的合金体系大体可以分为2大类:一类是以Al及第IV周期元素Fe、Co、Ni、Cr、Cu、Mn、Ti为主的合金系;一类是以难熔金属元素Mo、Ti、V、Nb、Hf、Ta、Cr、W等为主的难熔高熵合金系。目前研究最广泛的高熵合金是FeCoNiCrMn,其具有单一的fcc单相组织,室温下的抗拉强度为563 MPa,延伸率达52%[7]。TixAlCoCrFeNi系列高熵合金中通过调节Ti含量,发现当Ti原子比为0.5时合金的压缩塑性最理想,高达23.3%[8]。AlxCoCrFeNi系列高熵合金随着Al含量的增加,合金从单相fcc变为fcc和bcc双相,最终完全变为bcc。当x=0.9时,合金的硬度达到最高[9]

随着对高熵合金研究的展开,除了调控合金成分,更多性能优化的有效方法被提出。比如在高熵合金中引入相变诱发塑性(transformation induced plasticity,TRIP)效应使合金的强塑性都得到改善,Li等[10]成功制备双相TRIP高熵合金Fe50Mn30Co10Cr10,其强度、塑性和加工硬化率都较等原子比FeCoNiCrMn有所提高。Huang等[11]在bcc结构的TaxHfZrTi中通过改变Ta的含量引入hcp相,TRIP效应使合金保持了高强度的同时提高了塑性,为解决bcc合金室温脆性的问题提供了新的思路。He等[12]通过在FeCoNiCr合金中添加微量元素Ti、Al,制备出具有弥散纳米析出相的高熵合金,室温屈服强度为645 MPa,抗拉强度甚至超过了1 GPa, 塑性延伸率为39%,加工硬化效果显著。另外,共晶高熵合金的成功制备也为高熵合金的优化性能提供了新的思路,现已成功制备出了在低温-196 ℃至高温700 ℃这样宽的温度范围内,都具有极高强度和塑性的AlCoCrFeNi2.1共晶高熵合金[13]。虽然高熵合金诞生只有14年时间,但国内外研究单位积极参与,期间涌现了大量的高质量研究成果。高熵合金的性能优化研究,从单一的合金调控发展到如今已有多样化的有效强化策略,大幅提高了高熵合金的应用前景,在高超声速飞行器发动机用超高温材料、高性能战斗部材料、抗辐照核能用材料、轻质装甲防护材料、极地破冰船用材料、低温服役装置用材料、航空航天轻质材料等重要工业领域的发展中,高熵合金已经可以提供作为关键材料的选择和支撑。

1 高熵合金的结构特征及变形机理研究的挑战

高熵合金的多主元特性,使得其具有异于传统单一主元合金的结构特征,进而对其变形机理产生影响。高熵合金高的组态熵效应在简化合金的显微组织上起到了重要的作用,使得高熵合金主要由简单的fcc、bcc或hcp相结构组成,但也造成了高熵合金的化学无序和点阵畸变的特征。高熵合金虽然具有拓扑长程有序,但其晶体单胞中每个点阵位置的金属原子并非唯一确定,即高熵合金具有长程化学无序效应[14]。同时由于高熵合金中各个原子的尺寸差异,各原子周围将会产生点阵应变[15]。除了尺寸差异外,各组元的键合能以及晶体结构差异也会引起大的晶格畸变[16]。而在传统合金中,大部分基体原子都具有相同的周围环境。因此高熵合金的点阵畸变被认为远大于传统合金。高熵合金的化学无序效应和点阵畸变效应能够直接影响高熵合金的力学、物理和化学性能。

1.1 结构的化学无序效应及其对变形机理研究的影响

传统的固溶强化理论,固溶体中的溶剂与溶质是严格区分的[17]。而高熵合金中,每种组元含量相当,并没有严格的溶质与溶剂之分。因而基于传统合金的固溶强化理论,如Fleischer模型[18]、Labusch模型[19]等,是否仍然适用于高熵合金目前尚不清晰。Senkov等[20]根据TaNbHfZrTi高熵合金中各组元之间的模量不匹配度和尺寸不匹配度的差异,将该高熵合金简化为二元合金,(Ta+Nb+Ti)为溶剂,40%(Zr+Hf) (原子分数)为溶质,估算得到的屈服强度比实际值大18%。这种将多主元合金简化为稀固溶体合金的方法尽管简单直接,但局限性大,普适性不强,能否准确预测所有高熵合金的强度需要进一步验证。Toda-Caraballo等[21]利用Mooren模型计算多组元合金的原子距离,进一步提出计算高熵合金弹性不匹配度的模型,从而将Labusch模型[19]修正后应用于高熵合金。这一计算高熵合金固溶强化的模型虽然对于部分高熵合金的强化效果预测得很好,但仍然是将其中某一组元看成溶质,并未摆脱溶质溶剂的束缚,因此其普适性也值得商榷。

另外,早期科研工作者普遍认为高熵合金为随机固溶体,即高熵合金的各组元是完全无序占位的[1,22]。然而,考虑到合金中各个金属组元本身的特性(如原子尺寸,电负性等),高熵合金各个金属组元之间的相互作用并非完全均匀,其很可能并非绝对的随机固溶体,而是存在复杂的化学短程有序。Zhang等[23]通过X射线散射、中子散射以及扩展X射线精细结构(EXAFS)系统研究了NiCoCr中熵合金的短程有序结构,他们发现Cr原子倾向于在固溶体中与Ni和Co原子形成键合。Santodonato等[24]发现Al1.3CoCrCuFeNi高熵合金即使是在液态下,其金属元素也并非是随机混合的,Al-Ni、Cr-Fe和Cu-Cu倾向于成键。Singh等[25]通过基于电子结构的热动力学理论证明了Al-Co-Cr-Fe-Ni系高熵合金中也存在复杂的短程有序结构。因此,化学长程无序的高熵合金很可能存在复杂的化学短程有序。深入理解高熵合金原子尺度的变形机理,不可避免地需要考虑到其化学短程有序结构。高熵合金复杂的化学短程有序,一方面能够影响合金的位错滑移阻力,从而影响位错的滑移方式;另一方面也会改变合金的层错能,从而影响合金的孪晶变形。此外,化学短程有序也会影响合金的相稳定性,从而对合金的应力诱导相变产生影响。然而,由于高熵合金的多主元特性,其化学短程有序结构极其复杂,对这一原子尺度的化学不均匀性的研究尚十分困难,这也为进一步研究其化学短程有序与其变形行为之间的关系带来了重大挑战。

1.2 点阵畸变效应及其对变形机理研究的影响

大量研究结果[26,27]表明,高熵合金大的点阵畸变对其性能起到了重要作用,特别是其高的强度。然而,如何定量表征和理解高熵合金的点阵畸变目前各执一词,仍无定论[22,28~30]。最初原子尺寸差被用来量化点阵畸变度[22],但这一参数不是实验所测得,并不能反映合金的真实点阵畸变度。Tong等[28]利用对分布函数(PDF)来分析合金局部结构与整体结构所得点阵常数的差异,以此来定量描述高熵合金的点阵畸变。Song等[30]利用第一性原理密度泛函理论来研究高熵合金的点阵畸变,认为难熔高熵合金的点阵畸变度远大于FeCoNiCrMn系高熵合金的点阵畸变度。正是由于目前对高熵合金的点阵畸变效应尚无清晰的理论认知,采用何种有效的手段来研究其点阵畸变以及如何量化其点阵畸变度尚需进一步的探索。

值得注意的是,合金的变形行为与其缺陷,如位错、孪晶等,关系密切。经典的固溶强化理论、析出强化理论以及位错强化理论等的计算公式里均涉及到合金的Burgers矢量模(b),如:

位错强化[31],

τ = τ 0 + αGb ρ dis (3)

其中, τ 为剪切流变应力, τ 0 为材料的本征强度, α 为常数, G 为剪切模量, ρ dis 为位错密度。

固溶强化[32],

σ s = A x a 0 2 b (4)

其中, σ s 为流变应力, A 为常数, x 为溶质原子含量, a 0 为溶剂点阵参数。由于高熵合金大的晶格畸变,其Burgers矢量很可能不是一个定值,而是一个分布[33]。也就是说高熵合金的Burgers矢量尚无法准确定义,这也为理论计算和理解高熵合金的各种强化机制带来了挑战。同时,位错的运动对合金的塑性变形也起到了至关重要的作用。稀固溶体中,位错线一般认为呈直线型。而在高熵合金这一存在大点阵畸变多主元合金中,位错线很可能并非直线。这就为实验上定量描述高熵合金的塑性变形带来了挑战。高熵合金中的位错周围应力场能否用弹性连续介质模型来计算呢?位错的应变能、线张力及位错间的相互作用力等都与位错的Burgers矢量密切相关,而高熵合金Burgers矢量的不确定性也为人们从理论上理解高熵合金中的位错带来了挑战。更为重要的是,高熵合金中位错增殖的临界分切应力是否也存在局部不均匀性呢?这也会直接影响高熵合金塑性变形机理的研究。

总之,高熵合金已经发展成一种极富应用前景的先进材料,由于其多主元的特性,大的晶格畸变以及复杂的化学短程有序,传统合金的变形机理对于高熵合金可能并不一定完全适用。这就需要从高熵合金的结构特征入手,重新审视传统变形机理在高熵合金中的适用性。

2 典型高熵合金的力学性能与变形行为
2.1 fcc结构高熵合金的力学行为

fcc结构的高熵合金主要是FeCoNiCrMn系高熵合金,以及在此基础上进行成分变化得到的三元、四元甚至六元及以上高熵合金。该系高熵合金是最早提出的一类五元高熵合金,由于其成分均匀、无明显宏观偏析且组织稳定,因此是目前研究最为广泛的fcc高熵合金。

在室温以及低温条件下,FeCoNiCrMn高熵合金均表现出优异的塑性[34]。293 K条件下,大晶粒尺寸的FeCoNiCrMn (约155 μm)合金屈服强度为125 MPa,抗拉强度约450 MPa,延伸率可达80%;当晶粒尺寸减小至4.4 μm时,其屈服强度与抗拉强度分别约提高至460与630 MPa,延伸率仍达60%。室温下,FeCoNiCrMn高熵合金的变形方式以位错滑移为主。在塑性变形的初始阶段,位错沿最密排面{111}开动,滑移方向为1/2<110>;随后全位错分解成1/6<112> Shockley分位错和大量层错,与传统fcc合金类似。大量扩展位错的形成抑制了交滑移,因此均匀的平面滑移是FeCoNiCrMn高熵合金室温变形的主要方式。与室温相比,低温下FeCoNiCrMn合金中存在大量纳米孪晶参与变形,孪晶界以及孪晶中存在的固定位错成为位错滑移的阻碍,随着变形的加剧其加工硬化率并未出现明显的降低,仍可保持稳定[35],因此强度、塑性出现明显的上升。液氮温度下(77 K),与室温相比,其屈服强度与抗拉强度大幅度提高了约85%和约70%,分别达到759和1280 MPa,且延伸率超过80%[7]

高温下FeCoNiCrMn高熵合金的力学性能下降明显,在1073 K温度条件下,FeCoNiCrMn合金软化严重,抗拉强度不足200 MPa。He等[36]研究FeCoNiCrMn的高温流变行为发现,高温以及大应变速率(>2×10-5)条件下,变形机制以位错攀移为主,且Cr、Mn元素发生定向扩散,富集形成第二相,引发应力集中,促进了裂纹的形核与长大,导致材料强度降低。

2.2 bcc结构高熵合金的力学行为

bcc高熵合金通常具有较高的强度,但是塑性较低,譬如(FeCoNiCrMn)89Al11合金[37],其抗拉强度超过1.2 GPa,而延伸率不足5%。但有些bcc体系的高熵合金也具有一定的拉伸塑性,譬如TaNbHfZrTi系高熵合金。TaNbHfZrTi难熔高熵合金最早由Senkov等[20]设计,退火态TaNbHfZrTi的室温屈服强度超过1.1 GPa,延伸率可达约10%[38];而铸态TaNbHfZrTi压缩塑性超过50%,屈服强度可达929 MPa。与室温强度相比,TaNbHfZrTi的高温强度不尽如人意[39]。随着温度的提高,其强度下降趋势明显。当温度较低时,TaNbHfZrTi变形方式以位错滑移为主,并辅以少量的形变孪晶,变形过程均匀连续,因此其强度保持在675 MPa以上;当温度超过1073 K时,高扩散速率促使晶界上再结晶的发生,不稳定的亚晶界成为裂纹形核点,同时也是裂纹扩展的通道,最终裂纹快速扩展,导致材料失稳。因此1073 K以上条件下,TaNbHfZrTi合金强度急剧降低,当温度达到1473 K时,屈服强度降至92 MPa。

另外一类bcc难熔高熵合金为NbMoTaW系高熵合金,该类高熵合金是由元素周期表中几种高熔点元素组成,具有高的熔点以及优异的高温稳定性[4,40],因而高温应用潜力很大。铸态NbMoTaW高熵合金组织以bcc固溶体为主,枝晶晶界处存在少量偏析但含量极少(<5%)。室温下NbMoTaW屈服强度可达1058 MPa,但塑性差,压缩最大变形量仅为1.5%。但随着温度的升高,其塑性逐渐提高。1000 ℃时延伸率可达16%,此时其屈服强度为548 MPa,表现出优异的抗软化能力。从压缩变形行为上看,室温下NbMoTaW裂纹沿着压缩方向迅速扩展,表明其失效模式为纵向裂纹而非剪切;高温下(大于韧脆转变温度),NbMoTaW开始由脆变韧,裂纹与压缩方向呈约40°角,材料的断裂通过剪切的方式完成,因而塑性明显提高。对于该系合金来说,室温脆性已经成为制约其加工、成形以及后续应用的关键因素之一,因此迫切需要提高其室温塑性,目前已经有相关研究正在开展。

2.3 hcp结构高熵合金的力学行为

广泛研究的高熵合金大多数是fcc或bcc结构,其成分构成以过渡族金属为主。近年来,以镧系稀土元素为主的高熵合金被大量设计出来[41,42],这类高熵合金往往具有hcp结构,YGdTbDyHo为其中的典型代表。

YGdTbDyHo的晶体结构近似于单质Mg,晶格参数a约为0.363 nm,c约为0.566 nm[43]。Soler等[44]发现YGdTbDyHo晶间会有少量富Y相存在,且hcp基体中存在弥散分布的氧化物析出相。为避免富Y相对力学测试产生影响,准确表征YGdTbDyHo基体的强度,利用聚焦离子束切出不同直径的微柱试样(2、5及10 μm)进行压缩实验。研究表明,随着微柱试样的直径增大,会导致氧化物析出相体积分数升高,因而使其强度也得到增强。

另一类hcp结构的高熵合金是TiZrHf系高熵合金,主要是三元的TiZrHf系中熵合金。该合金具有一定的强度和塑性。拉伸条件下屈服强度超过800 MPa,抗拉强度约1 GPa,塑性接近20%。Rogal等[45]也设计出具有hcp结构的TiZrHfSc高熵合金,在室温下,以基面{0001}以及柱面{1010}为滑移面的滑移系均可开动,滑移方向以<21 10>为主,均匀的变形行为使其屈服强度可达700 MPa左右,延伸率亦接近20%。

2.4 双相高熵合金的力学行为

随着高熵合金研究的进展,双相固溶体组织的合金也被认为是高熵合金,目前的双相高熵合金主要有FeCoNiCrAlx以及 (FeCoNiCrMn)100-xAlx系列高熵合金(fcc+bcc、双相bcc)[37,46]、Fe50Mn30Co10Cr10高熵合金(bcc+hcp)[10]、ScYLaTiZrHf高熵合金(双相hcp)[47]等。

不同合金元素的添加往往会导致高熵合金的晶体结构、微观组织以及力学性能发生较大的变化[48,49,50]。其中,Al元素添加会使高熵合金原有的单相固溶体组织、结构发生规律性的改变,因此引起了广泛关注。

He等[37]在FeCoNiCrMn高熵合金基体中添加了不同原子比的Al元素(0~20%,原子分数,下同),系统地研究了Al元素的添加对FeCoNiCrMn结构、组织以及力学性能的影响。研究发现,随着Al元素增加,(FeCoNiCrMn)100-xAlx合金从最初的单一fcc结构(Al<8%)转变为fcc+bcc双相结构(Al=8%~16%),最终当Al的含量大于16%时结构演化为单相bcc结构。组织结构的变化带来的是力学性能的变化。随着fcc逐渐向bcc转变,合金强度上升,但同时塑性降低[37],而当Al原子分数大于11%时,试样由于过脆已无法进行拉伸实验。当合金保持在fcc单相区时,Al元素的添加带来的是晶格畸变的增大,此时强度的提高由固溶强化所致;bcc结构与fcc结构相比,其强度更高而塑性较低,因此bcc相的增多使得合金更强更脆。当Al原子分数大于11%时,bcc相比例进一步增高,且在bcc相内部析出纳米级脆性相A2相,使合金变得更脆。

相似地,Wang等[9]设计了FeCoNiCrAlx系列高熵合金,在FeCoNiCr基体中添加不同原子比的Al元素,随着Al元素的提高,出现相同的晶体结构及组织转变(fcc→fcc+bcc→bcc+bcc)。与此同时,在梯度温度下的硬度测试表明,室温下bcc组织的硬度明显高于fcc组织,而在高温下,虽然二者皆有所软化,但bcc组织的硬度仍然略高于fcc组织。

2.5 共晶高熵合金

传统的fcc或bcc单一固溶体结构的高熵合金往往难以同时兼顾强度与塑性。如前所述,fcc高熵合金塑性好而强度低,而bcc高熵合金强度优异但塑性不足。鉴于此,Lu等[51]设计出具有类似珠光体层状结构的AlCoCrFeNi2.1 “共晶”高熵合金,同时具有良好的强度与塑性,为高熵合金的设计提供了一种新的思路。

铸态AlCoCrFeNi2.1高熵合金具有fcc与bcc双相结构[13,52]。fcc (L12)相富Co、Cr、Fe 3种元素,而bcc (B2)相以Ni、Al元素为主,二者以片层状结构交替排列,与珠光体中的铁素体与渗碳体相类似。除此之外,bcc相中存在一定量的纳米级富Cr析出相。室温下该合金的屈服强度约为545 MPa,抗拉强度约为1.1 GPa,延伸率可达18%左右,比较好地实现了强度与塑性的兼顾。在材料变形的初始阶段,应变主要集中在相对较软的fcc组织中,而较硬的bcc组织较少参与变形。随着变形加剧,fcc组织中增殖的位错开始在两相界面处聚集,造成应力集中。当局部应力超过bcc组织的临界应力时,微裂纹形核并快速扩展,最终材料失稳。从其宏观变形来看,材料断裂模式主要为解理断裂。尽管AlCoCrFeNi2.1合金的延伸率接近20%,但未出现明显的颈缩现象。

此外,近年来,CoCrFeNiNb0.45和CoCrFeNiTa0.4等一系列具有相似结构的共晶高熵合金被设计出来[53],共晶高熵合金的研究以及应用前景值得期待。

2.6 TRIP韧塑化高熵合金

高强高韧材料的开发一直是材料研究的热点,然而,大多数传统强化方式在提高强度的同时会造成塑性的降低。具有稳定单一固溶体组织的高熵合金也不能避免这种趋势。针对此挑战,近年来“亚稳工程”的概念也被尝试应用于高熵合金,通过调整高熵合金的成分来降低固溶体相的稳定性和堆垛层错能,在变形过程中发生应力诱导相转变,从而提高其宏观塑性变形能力,实现韧塑化的目的。

在传统FeCoNiCrMn高熵合金的基础上,Li等[10]利用这一“亚稳工程”概念设计相变诱导双相高熵合金。按照调整双相微观结构降低高温相热力学稳定性实现界面强化和降低室温相稳定性实现相变诱导强化的思路设计了Fe80-xMnxCo10Cr10 (原子分数)高熵合金。通过改变Mn含量,冷却过程中会产生fcc到hcp结构的马氏体相变,从而得到双相微观结构和具有较低堆垛层错能的单相合金。实验表明,相比于Mn含量为45%的单相hcp高熵合金和传统的FeCoNiCrMn高熵合金,Mn含量为30%的Fe50Mn30Co10Cr10双相高熵合金在强度和塑性上均有明显提升,在晶粒尺寸为4.5 μm时,其强度提升近100 MPa,延伸率提升约30%。与此同时,双相高熵合金的加工硬化率也有明显的升高。

除此之外,对于缺乏塑性的难熔高熵合金,“亚稳工程”的设计思路亦行之有效。Huang等[11]在脆性难熔高熵合金中通过调控相的热力学和机械稳定性,通过形变、相变的动态协同耦合,在保持高强度的同时,实现了塑性的大幅增加,获得了高韧塑性的难熔高熵合金。传统的TaHfZrTi具有单相bcc结构,其室温抗拉强度在1500 MPa,但是延伸率很低(约4%);随着Ta元素含量的减少,bcc相的稳定性降低,在凝固过程中就会发生高温bcc到hcp的无扩散相变,合金由单相bcc结构转变为bcc+hcp结构。当Ta含量分别为0.6和0.5时,其断裂强度依然接近1100 MPa,而延伸率分别增至20%和27%;Ta0.4HfZrTi合金出现双屈服现象,屈服强度分别约为400和800 MPa。这一结果为设计高强高韧的难熔高熵合金并推动实际工程应用打下基础,同时也为解决bcc高熵合金中强度-塑性矛盾提供了一个新思路。

3 高熵合金的强韧化
3.1 细晶强化

在材料变形过程中,晶界能有效地阻碍位错运动从而提高材料的屈服强度。材料的屈服强度与晶粒尺寸之间具有Hall-Petch关系 , 即屈服强度随着晶粒尺寸的减小而提高。在传统金属材料中已经证实,通过细化晶粒,可以达到非常高的强化效果,甚至可以同时提高强度和塑性。细晶强化在高熵合金中同样可以起到显著的强韧化效果,同时由于高熵合金独特的化学和结构特征,细晶强化也具有其独特的特征。

Liu等[50]系统研究了fcc结构FeCrNiCoMn高熵合金中的晶粒长大过程和Hall-Petch关系计算出其Hall-Petch系数为677 MPa/μm1/2。根据Wu等[54]的统计,传统fcc合金的Hall-Petch系数一般不超过600 MPa/μm1/2。由此可见,与传统材料相比,高熵合金具有严重的晶格畸变,使得位错运动时需要克服更大的晶格阻力,从而导致了高熵合金具有更好的细晶强化效应。

Otto等[34]研究了不同晶粒尺寸FeCrNiCoMn在不同温度下的力学性能, 发现在同等温度下,晶粒尺寸为4.4 μm样品的力学表现总是优于更大晶粒的样品。随着温度降低,细化晶粒带来的强化效果更加明显。Sun等[55]进一步细化了FeCrNiCoMn 的晶粒,晶粒尺寸降至500 nm左右时,材料的屈服强度接近900 MPa,抗拉强度达到1250 MPa左右,但塑性变形能力有较大减弱。Juan 等[56]通过控制退火温度和时间从而细化晶粒,在bcc结构难熔高熵合金TaNbHfZrTi中实现了强度和塑性的同时提高。

上述结果表明,在高熵合金中合理控制晶粒尺寸大小是十分有效的强韧化手段。大量研究都采用控制材料的退火温度和时间从而调控晶粒尺寸,Seol等[57]独辟蹊径,通过在FeCoNiCrMn和Fe40Mn40Cr10Co10中添加微量B元素有效地修饰了其晶界结构并减小晶粒尺寸,在保持材料优异塑性的情况下使材料的屈服强度提高了超过100%,拉伸强度也提高了40%左右。一方面,B元素倾向于在多晶材料的界面处(晶界与相界等)偏析, 增加了晶界的凝聚力,降低了界面能和晶界在受力情况下灾难性失效的概率;另一方面,B元素在晶界处的修饰能有效增强晶界拖曳效应并降低再结晶过程中Gibbs-Thomson力,与未添加B的高熵合金相比,显著细化了晶粒,同时提升了材料的塑性和强度。利用机械合金化制得高熵合金粉末,经压制烧结也可制得晶粒十分细小(达纳米级)的块体高熵合金,但在球磨和烧结过程中极易引入杂质或气孔,在此不做赘述。

3.2 固溶强韧化

固溶强化的原理是将不同于基体材料的金属或非金属原子,融入到基体材料点阵间隙或结点上,使基体的局部点阵发生变化产生晶格畸变,从而产生应力场,增大位错的阻力,阻碍其运动,进而使基体金属的变形抗力随之提高[58]。与传统金属相同,高熵合金的固溶强化有置换固溶强化和间隙固溶强化2种途径。

为了提高高熵合金的综合力学性能,在保证合金晶体结构不变的前提下,研究人员尝试了添加不同金属元素对高熵合金进行置换固溶强化,能够起到一定程度的强化效果。如前所述,He 等[37]在典型高熵合金FeCoNiCrMn中加入了不同含量的Al元素,当Al含量小于8% (原子分数)时,合金形成单相固溶体,随着Al含量的增加,合金硬度几乎没有变化,合金抗拉强度有略微提升,但塑性降低了10%左右。Stepanov 等[59]在FeCoNiCrMn中添加少量V元素,发现与初始合金相比,FeCoNiCrMnV0.5的强度和塑性并没有明显的变化。Liu 等[60]调控FeCoNiCrMn中的Mn含量,发现随着Mn含量的改变,(FeCoNiCr)100-xMnx在保持单相fcc结构及层错能没有明显变化的情况下,其拉伸强度和塑性相差不大。传统合金中的固溶强化大多是针对稀固溶体的,在稀固溶体中位错穿过溶剂晶格与离散的溶质原子交互作用。而在高熵合金中没有传统的溶质与溶剂之分,即没有溶剂晶格,它类似于一种具有固定原子比例的无序定比化合物[61]。因此,现有实验结果表明置换固溶强化对高熵合金的强化效果十分有限。

间隙固溶强化是指在基体中添加与基体原子半径差大于41%的元素,使其进入基体晶格间隙中从而达到强化效果。常见的间隙固溶强化元素有H、C、B、N、O 5种。由于间隙原子尺寸较小,在高熵合金中可以产生较大的晶格畸变,因而对高熵合金的强化效果会显著大于置换固溶强化。Wang等[62]在Fe40.4Ni11.3Mn34.8Al7.5Cr6高熵合金中添加了不同含量的C对合金进行间隙固溶强化,随着C含量的增加合金的强度显著提高,并且塑性也随之提升。C的加入降低了合金的层错能,提高了晶格摩擦力,使位错滑移方式从波浪滑移转变为平面滑移,并且C的加入细化了合金中的位错结构,形成Taylor点阵,显微变形带与晶界的交互作用松弛了塑性应变集中,使合金呈现出较高的强度以及塑性。Stepanov 等[63]在FeCoNiCrMn中加入0.1%C (原子分数)元素,在提高基体合金间隙固溶强化的同时降低了孪晶动力学,提高了位错的活跃性,促进了位错交滑移,与相同处理条件的FeCoNiCrMn相比,强度显著提升,并且保留了较高的塑性。Xie 等[64]利用真空热压烧结法在FeCoNiCrMn中加入0.1%N (原子分数)元素,屈服强度提高了200 MPa,并且塑性仅降低了3.4%,小原子半径的N元素很容易溶入固溶体晶格中,明显增加了晶格畸变能,促进了固溶强化。Chen等[65]将少量O元素加入ZrTiHfNb0.5Ta0.5中,提高了难熔高熵合金的高温与室温强度。因此,相比于置换固溶强化,间隙固溶强化为高熵合金提供了一条更有效的强化途径。并且相对置换固溶元素来说,小原子元素具有廉价的优势,因此间隙固溶强化受到了越来越多的高熵合金科研工作者的关注。

3.3 共晶组织强韧化

高熵合金具有优异的综合性能,但是常见的单相固溶体高熵合金很难实现强度和韧性的平衡。如前所述,单相fcc高熵合金具有良好的塑性,但其强度一般较低,比如最典型的fcc高熵合金FeCoNiCrMn断裂延伸率可达50%,而屈服强度只有约410 MPa[7];单相bcc高熵合金具有较高的强度,但其塑性较低,比如TaHfZrTi具有1.5 GPa的高拉伸强度,但其塑性只有大约4%[11]。另外,由于包含多种高浓度的元素,高熵合金具有较差的流动性和可铸性,从而带来较大的成分偏析,严重限制了其在工业上的应用。为了解决这些问题,Lu 等[51]提出了“共晶”高熵的合金设计理念,为简化高熵合金的工业化生产带来机遇。

Lu 等[51]设计出高强度bcc相和高韧性fcc相相结合的AlCoCrFeNi2.1“共晶”高熵合金,并成功制备出具有工业尺寸的AlCoCrFeNi2.1共晶高熵合金铸锭。与其它块体高熵合金不同,其铸造性能优异,铸锭中含有较少的铸造缺陷;另外,该共晶高熵合金具有细小薄片状fcc/B2的微观组织结构,合金呈现出良好的强度和塑性结合,其良好的机械性能可以维持到700 ℃。这种新的设计思路有望提高高熵合金较差的铸造性能,解决宏观成分偏析等技术问题。Gao等[13]详细研究了AlCoCrFeNi2.1共晶高熵合金的微观组织结构,发现其优异的综合力学性能归因于fcc相和B2相的协同变形作用,fcc (L12)相通过位错平面滑移和层错变形充当软性相,而B2相通过纳米析出强化充当强化相。在AlCoCrFeNi2.1共晶高熵合金中,fcc相具有较低的层错能,与B2相之间的相界面为半共格结构,这种界面可以承受较高的应力; B2相中分布着直径20 nm左右的富Cr纳米析出相,这些析出相通过Orowan机制阻止位错滑移进而提高B2相的强度。Jiang 等[66]研究了由fcc固溶相和Fe2Nb型Laves相组成的CoFeNi2V0.5Nb0.75共晶高熵合金经过不同温度退火后的组织与性能变化,发现当退火温度高于600 ℃时合金中产生NbNi4型金属间化合物,随着退火温度的升高,该金属间化合物的体积分数增加而共晶区域随之减少,当退火温度为800 ℃时,在Fe2Nb型Laves相中尤其是共晶胞界处产生纤维状组织结构,使得共晶组织中两相连接更为紧密,因此呈现出优异的压缩性能。He等[67]利用二元相图设计了共晶高熵合金CoCrFeNiNbx,大原子尺寸Nb元素的添加引起剧烈的晶格畸变,进一步增强了fcc基体的强度,随着Nb含量的增加,合金的硬度也随之升高,其中CoCrFeNiNb0.25具有“亚共晶”组织结构,压缩强度高达2 GPa并且具有接近40%的压缩塑性,具有良好的强韧平衡性。

3.4 TWIP效应强韧化

开发同时具备高强度和高塑性的金属材料一直以来都是一个挑战,很多强化方式如析出强化和固溶强化在提高材料强度的同时往往会牺牲一部分塑性。在传统金属材料中,孪晶诱导塑性变形(TWIP)和TRIP则较好地克服了这一问题。TWIP效应是指材料在外力作用下变形时诱发产生形变孪晶,导致材料能在保持较高强度的同时,仍能保持很高的延伸率。TWIP效应受材料的层错能影响,当层错能较低时,变形过程中合金的扩展位错的宽度较大,滑移过程中很难束集,因此阻碍了位错交滑移,在这种情况下,合金中诱发第二变形机制,即孪晶变形。因此,降低合金的层错能可以促使合金变形方式由位错滑移变形转变为孪晶变形,进而提高合金的力学性能。因此,近年来科研工作者们不断设计开发新型高熵合金,试图将TWIP效应引入到高熵合金当中,以此提高其强度及塑性。

典型fcc系高熵合金FeCoNiCrMn室温变形过程中的低强度严重影响了其工业应用,但其在低温变形时会诱发TWIP效应生成大量的纳米孪晶,产生动态Hall-Petch效应并阻碍位错滑移,因此呈现出优异的断裂韧性[7]。Huang 等[68]利用第一性原理从成分、磁性和应力3个方面综合分析了FeCoNiCrMn层错能随温度的变化,发现随着温度的降低,其层错能也随之降低,在室温下其层错能约为21 mJ/m2,0 K时其层错能低至3.4 mJ/m2,显著低于室温时的层错能;另外,随着温度的降低,其变形机制由位错滑移转变为TWIP与位错滑移协同变形,温度低至一定程度时更有可能会发生TRIP效应。Deng 等[69]去除了FeCoNiCrMn中具有高层错能的元素Ni,并且降低了Co和Cr的含量以避免富Cr金属间化合物的生成,设计出具有低层错能四元高熵合金Fe40Mn40Co10Cr10,该合金在室温变形过程中在<112>//TD和<111>//TD (TD为拉伸方向)取向附近晶粒产生大量纳米孪晶,因此合金呈现出良好的力学性能。

Gludovatz 等[70]报道了具有优异力学性能的中熵合金NiCoCr,该合金在室温下抗拉强度可达将近1 GPa,断裂延伸率约为70%,断裂韧性也高达275 MPa·m1/2,各项性能均明显优于五元高熵合金FeCoNiCrMn,这是因为NiCoCr变形方式以孪晶变形为主导,在变形过程中为合金提供了稳定的加工硬化率,因此克服了高强度与高韧性的竞争关系,推迟了颈缩,促进了合金的塑性,为合金提供了除位错塑性变形之外的变形模式来适应外加应力。Laplanche 等[71]通过透射电镜测量扩展位错宽度的方法,发现NiCoCr的层错能为(22±4) mJ/m2,比通过相同方法得到的FeCoNiCrMn的层错能[72](30±5) mJ/m2低将近25%,其相对较宽的扩展位错在塑性变形初期阶段会阻碍位错交滑移并促进平面滑移。另外,NiCoCr的孪晶临界剪切应力与FeCoNiCrMn接近,但其较高的屈服强度以及加工硬化率使得NiCoCr在较低应变水平下即可达到此应力状态,因此孪晶可以在较大的应变区域内通过动态Hall-Petch效应为合金提供稳定的加工硬化,进而获得优异的综合力学性能。

3.5 TRIP效应强韧化

在传统材料譬如钢铁材料和钛合金中, TRIP效应已被证明可以显著提高材料的韧塑性[73,74]。Wu 等[75]在非晶合金中引入TRIP效应,大幅度提高了非晶合金的韧塑性,获得了具有大的拉伸塑性和加工硬化能力的非晶复合材料。TRIP效应也被尝试用于设计具有优异综合力学性能的高熵合金。在fcc高熵合金体系中,随着层错能的降低,合金的塑性变形机制由位错滑移转变为孪晶变形,层错能继续降低则转变为马氏体相变变形。马氏体板条与孪晶相同,可以充当平面障碍减少位错滑移的平均自由通道,位错堆积在这些平面缺陷与基体的界面处会产生显著的背应力[76],进而阻碍其它位错的滑移,促进加工硬化率。

Li 等[10]设计了具有优异综合力学性能的TRIP双相高熵合金,在Fe80-xMnxCo10Cr10 (x=45、40、35、30,原子分数,%)高熵合金中,随着x的降低,fcc相稳定性逐渐降低,合金变形机制由位错主导塑性转变为孪晶诱导塑性再到相变诱导塑性变形,当x=30时,实现双相TRIP高熵合金Fe50Mn30Co10Cr10;变形前fcc组织中存在大量由于1/6<112> Schockley不全位错滑移所形成的层错,这些层错在后续加载过程中成为ε马氏体相的形核位点,而在变形过程中由于相变产生的高密度相界阻碍了位错滑移,因此促进了合金的加工硬化,推迟了颈缩。基于这种思路,Li 等[77]发现通过Ab initio计算模拟不同x时Co20Cr20Fe40-xMn20Nix (x=0~20,原子分数,%)高熵体系中hcp和fcc的相稳定性,可建立有效设计具有TRIP效应的双相高性能高熵合金准则,根据模拟结果筛选出的Co20Cr20Fe34Mn20Ni6合金表现出了优异的拉伸强度和加工硬化能力。

已开发的fcc结构高熵合金通常具有优异的塑性,TRIP效应更提高了其综合强度和加工硬化能力。bcc结构高熵合金通常表现出极高的强度和硬度,但塑性和加工硬化能力较差,这严重限制了其实际应用前景,因此利用TRIP效应提升bcc高熵合金的塑性更具实际意义。Huang 等[11]以脆性bcc高熵合金TaHfZrTi为模型材料,通过亚稳工程降低高温相热稳定性并降低室温相机械稳定性,在该合金中减少Ta的含量降低了bcc相的稳定性,得到bcc和hcp的双相组织,受到外力时bcc相发生马氏体相变转变成hcp相。如图1a所示,降低Ta含量,虽然合金的强度有所下降,但TRIP效应的引入使材料的塑性大幅度增大,同时产生了大的加工硬化效应。与其它先进材料在强度-塑性图上对比(图1b),TRIP效应使bcc高熵合金在保持较高强度的同时大幅度提高了塑性。


图1

不同Ta含量高熵合金的拉伸真应力-真应变曲线以及韧塑化高熵合金与其它合金材料的强度-塑性对比

Fig.1

True tensile stress-strain curves of high-entropy alloys (HEAs) with different Ta contents (a), ultimate tensile strength and ductility of the ductilized HEAs, in comparison with other advanced alloys (b) (IF—interstitial free, TWIP—twinning induced plasticity, TRIP—transformation induced plasticity)

3.6 第二相强韧化

在金属材料中,通过第二相粒子阻碍位错运动从而提高屈服强度,是最常用的强化手段之一,被广泛应用于钢铁、铝基、铜基及高温合金材料中,在常温及高温下都是十分有效的强化机制。在金属材料中产生强化作用的第二相粒子一般具有含量高、尺寸小(十几到几十纳米之间)、均匀分布等特点。从产生途径上看,金属材料中的第二相强化粒子可以分为2种:一种是弥散强化,通过机械混入或者一次性析出的第二相粒子产生的强化作用,这种第二相粒子在高温下不回溶;另一种是析出强化,从过饱和固溶体中析出的第二相粒子产生的强化作用,在加热至高温时,这种粒子可以溶解在合金中。下面分别讨论这2种强化机制在高熵合金中的应用。

3.6.1 弥散强化 Rogal 等[78]在球磨过的FeCoNiCrMn 高熵合金粉末中添加5% (质量分数)的球状β-SiC 纳米颗粒(20~50 nm),经过均匀混料后在Ar气保护条件下热等静压15 min,制得纳米SiC颗粒弥散分布的FeCoNiCrMn高熵合金。与未添加纳米SiC颗粒同等条件制备的FeCoNiCrMn相比,压缩屈服强度从1180 MPa提高至1480 MPa (100 r/min)和1940 MPa (200 r/min),但塑性均有所下降。在200 r/min转速下混料的样品几乎没有均匀塑性变形过程,这是由于在高能球磨的过程中SiC颗粒与富Cr晶粒反应生成了大量硬质脆性M7C3碳化物,无法协调材料的塑性变形。采用相同的方法,Rogal 等[79]还在FeCoNiCrMn中添加了α-Al2O3纳米颗粒。5~35 nm的Al2O3颗粒弥散分布于晶粒尺寸为30~150 nm的fcc基体中,使材料的屈服强度从1180 MPa增至1600 MPa,但是复合材料的压缩强度和塑性均有所下降。

弥散的富Y2O3纳米颗粒能有效阻碍高温时铁素体钢中的位错运动和再结晶过程,从而提升材料在高温时的力学表现[80],这种利用氧化物弥散强化(ODS)的钢称作ODS钢。Hadraba等[81]借鉴这一设计思路,利用机械合金化的方法,在球磨过程中混入适量Y和Ti的金属粉末和O2 (Y、Ti和O2添加比例控制在大约能产生0.3% (质量分数)氧化物的水平),再经过放电等离子烧结(SPS)制备出了ODS FeCoNiCrMn高熵合金。与同等条件制备的高熵合金相比,ODS高熵合金的平均晶粒尺寸减小了50% (0.8 μm减至0.4 μm),室温和800 ℃高温拉伸强度分别增加了30%和70%,但室温下ODS高熵合金的拉伸塑性较差(不足2%)。

上述结果表明,在高熵合金中利用机械合金化的方法加入或者原位生成第二相粒子的确可以有效提升材料的强度,但材料的塑性也随之大幅下降。一般而言,通过机械混入或原位反应产生的第二相粒子与基体之间没有特定的晶体学位向关系,无法被位错线切割,在塑性变形过程中只能被位错线绕过。这类第二相粒子与基体间的界面能往往较高,在塑性变形中极易发生局部应力集中而萌生微裂纹,使材料过早发生塑性失稳。

3.6.2 析出强化 Chen等[82]对AlxCrFe1.5MnNi0.5 (x=0.3、0.5)在600~800 ℃进行不同时间的时效处理后发现,2种高熵合金的硬度都得到了显著提升。对其时效组织的表征发现,经时效后bcc基体中析出了大量ρ相(Cr5Fe6Mn8),这种四方结构的ρ相,与基体界面不共格,能显著强化基体,但对材料塑性影响较大。Tsai等[83]在固溶炉冷的Al0.3CoCrCu0.5FeNi高熵合金中发现大量纳米级条状和球形析出物,这些L12结构的(Ni, Cu)3Al析出相与基体界面具有共格关系,使材料的拉伸屈服强度从150 MPa左右提高到了300 MPa。这些高熵合金中的析出强化效果远不及传统镍基高温合金,其根本原因在于析出相含量不够高,分布不够均匀。若能在高度固溶的高熵合金基体中形成大量弥散分布稳定的纳米析出相,其力学性能可以得到进一步提升。

He 等[12]在单一fcc结构的FeCoNiCr高熵合金中添加不同含量的Ti和Al,通过适当的变形和热处理工艺,有效控制了析出相的种类、尺寸及分布状态。他们成功在(FeCoNiCr)94Ti2Al4 (后文以TA24表示)的时效组织中得到了大量弥散纳米γ'相,同时有害Heusler相含量得到了有效控制。如图2所示,经过30%冷变形和800 ℃时效处理18 h的TA24样品,其室温屈服强度提高到650 MPa左右,抗拉强度超过了1 GPa,断裂延伸率接近40%,且具有显著加工硬化效应。高分辨透射电镜和三维原子探针结果验证了基体中弥散析出相为L12结构的Ni3(Ti, Al)型γ'相(图3),尺寸在15~30 nm之间,体积分数约为21%。计算表明,析出强化对屈服强度贡献超过300 MPa,效果显著。经过70%冷轧变形和650 ℃、4 h时效处理的TA24样品,室温拉伸屈服强度超过1 GPa,仍保持了17%的塑性(图2),这种高的强度来自以析出强化为主、其它多种强化机制为辅的综合贡献。为控制成本,Zhao等[84]设计了一系列无Co非等比例高熵合金 (FeNi)67Cr15Mn10Al8-xTix (x=3、4和5)。在800 ℃热处理后3种合金中均形成了大量弥散γ'相(10~20 nm),除此之外,合金(FeNi)67Cr15Mn10Al4Ti4 (A3)和(FeNi)67Cr15Mn10Al3Ti5(A4) 中分别析出了少量Heusler相和 η 相,与He等[12]的观察结果相似。


图2

添加Ti和Al的FeCoNiCr高熵合金室温拉伸力学性能对比

Fig.2

Room-temperature tensile properties of FeCoNiCr HEA with Ti and Al addition


图3

Ti和Al添加FeCoNiCr高熵合金中析出相形貌及结构表征

Fig.3

Characterization of the morphology and structure of precipitate in FeCoNiCr HEA with Ti and Al addition

由上可以看出,传统合金中的强韧化手段均可用于高熵合金中,但其作用机理和产生的效果明显不同。由于显著的化学无序和晶格畸变效应,细晶强化、间隙强化等方式在高熵合金中能起到更加显著的效果;TWIP、TRIP效应也能通过对高熵合金热稳定性和机械稳定性的调整而实现,解决一些高熵合金的脆性问题,甚至同时提高高熵合金的强度和塑性。而这些强韧化手段的实现均可以通过适当的合金化和热处理工艺来具体实现,甚至可以实现多种强韧化机制同时、协同作用,起到更优的效果。

4 高熵合金的变形行为及研究展望

高熵合金的出现和发展,一方面为高性能金属材料的开发提供了新的选择,能够填补传统材料发展的某些性能不足,为高强韧部件、高温结构部件、低温服役装置等重要工业领域发展提供关键材料的选择和支撑;另一方面为研究化学无序的固体材料基础问题提供了模型材料。但是,目前对高熵合金基本科学问题的理解和关键应用的验证方面都还存在着重大挑战,关于高熵合金变形行为研究方面还存在诸多未解决之处,下面根据作者的理解举例如下。

(1) 结构模型

变形行为和机制的揭示,以及定量化变形模型的建立,离不开原子结构的理解和模型的建立,由于高熵合金精细结构表征和模型的模糊,高熵合金定量化的结构-性能关联依然缺失。结构模型的建立关键在于高熵合金中长程化学无序、短程有序的揭示和模型化,晶格畸变的定量表征和模型化,只有将化学无序和晶格畸变这2个关键结构特征定量化的描述,才有望建立高熵合金可靠的结构模型,为其变形行为和机制的研究打下基础。

(2) 变形机制

虽然具有晶体结构,微观变形机制也多是位错滑移和孪晶,但高熵合金的位错滑移特点和孪晶变形特点都很不同于传统合金。已经有研究表明,高熵合金的Peierls-Nabarro力不能再像传统合金中那样被忽略,位错的Burgers矢量可能不再是一个定值而是处在一个分布区间内,这些都对传统晶态合金变形机制和模型在高熵合金中的应用提出了挑战,需要对传统的经典变形模型进行修正,在此基础上将高熵合金的特点引入,建立适用于高熵合金具有预测性的变形机制和模型。

(3) 强韧化指导原则

由于高熵合金的相结构具有过饱和固溶体的特征,可以更加充分地利用多种强韧化手段,譬如固溶强化、第二相强韧化、细晶强韧化、相变强韧化等,但进行强韧化的指导原则并没有完全建立。譬如在高熵合金中可以固溶大量的C、O、B等小原子,但小原子是处于代位还是间隙位置尚有争论,小原子对层错能起到升高还是降低的作用也还没有澄清。但初步的研究已经表明,高熵合金中可以固溶大量的小原子,而且起到十分明显的强韧化作用,后续可对其作用的详细机理和优化进行研究。此外,通常认为高熵合金具有较低的层错能,而且温度对于层错能有着十分显著的影响,因此,研究高熵合金层错能调控的方案,进而改变高熵合金的TWIP和TRIP效应,预期也能对高熵合金强韧性的进一步优化起到一定指导作用。

(4) 多场耦合环境下的服役行为

由于较高的合金化元素含量,高熵合金在成本上相对传统合金并无太大优势,其应用应当瞄准极端服役环境和急需的极限材料。已有研究表明,高熵合金在极低温、极高温、辐照、腐蚀等极端环境中具有优异的性能,譬如,随着温度的降低高熵合金不仅没有韧-脆转变,还会随着温度的降低强度和韧性同时提高;在极高的温度条件下,也具有极为优异的相结构稳定性。但是,高熵合金在多种极端条件耦合情况下的服役行为研究还极为不充分,而这种在多场耦合环境下的服役行为对于高熵合金的关键应用突破又极为重要,因此,需要加大对高熵合金在多场耦合环境下服役行为的研究。

5 结语

尽管关于高熵合金还存在一些争论,譬如高熵合金的熵是否处于最大化,高熵合金的迟滞扩散效应、晶格畸变效应是否成立,其程度如何,化学元素占位的无序度如何等问题。但不可否认的是,高熵合金概念的出现,极大地拓展了工程材料开发的空间,使合金的开发空间由传统合金位于相图一角扩展到了相图中心的更广阔区域,为材料工作者开发新型高性能先进金属材料提供了更多新的选择和思路。同时,由于高熵合金具有诸多独特的性能特点,能够在一些传统材料性能达到极限而很难突破瓶颈的领域提供关键的高性能材料选择。

高熵合金下一步的研究应从基础研究和应用2个方面开展。基础研究方面应从高熵合金的关键结构特征出发,建立高熵合金的多尺度原子结构模型,研究高熵合金的变形、相变和韧塑化机理,建立高熵合金结构-性能关联。应用研究方面应当针对国防装备应用、航空航天等极端环境条件下服役的特种高性能高熵合金,制备出可用于宽温域工况条件且综合性能优异的新型装备高熵结构材料,并突破相关的制备加工工艺。

The authors have declared that no competing interests exist.

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An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4–160μm. Quasi-static tensile tests at an engineering strain rate of 10613s611 were then performed at temperatures between 77 and 1073K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to 652% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {111}〈110〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈110〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈112〉 Shockley partials. At later stages (6520% strain), nanoscale deformation twins were observed after interrupted tests at 77K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing (“dynamic Hall–Petch”), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study.
[本文引用: 2]
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Laplanche G, Kostka A, Horst O M, et al.Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy[J]. Acta Mater., 2016, 118: 152
DOI:10.1016/j.actamat.2016.07.038 URL
At low homologous temperatures (down to cryogenic temperatures), the CrMnFeCoNi high-entropy alloy possesses good combination of strength, work hardening rate (WHR), ductility, and fracture toughness. To improve understanding of the deformation mechanisms responsible for its mechanical properties, tensile tests were performed at liquid nitrogen and room temperature (7702K and 29302K) and interrupted at different strains to quantify the evolution of microstructure by transmission electron microscopy. Dislocation densities, and twin widths, their spacings, and volume fractions were determined. Nanotwins were first observed after true strains of 657.4% at 7702K and 6525% at 29302K; at lower strains, deformation occurs by dislocation plasticity. The tensile stress at which twinning occurs is 72002±023002MPa, roughly independent of temperature, from which we deduce a critical resolved shear stress for twinning of 23502±021002MPa. In the regime where deformation occurs by dislocation plasticity, the shear modulus normalized WHR decreases with increasing strain at both 7702K and 29302K. Beyond 657.4% true strain, the WHR at 7702K remains constant at a high value ofG/30 because twinning is activated, which progressively introduces new interfaces in the microstructure. In contrast, the WHR at room temperature continues to decrease with increasing strain because twinning is not activated until much later (close to fracture). Thus, the enhanced strength-ductility combination at 7702K compared to 29302K is primarily due to twinning starting earlier in the deformation process and providing additional work hardening. Consistent with this, when tensile specimens were pre-strained at 7702K to introduce nanotwins, and subsequently tested at 29302K, flow stress and ductility both increased compared to specimens that were not pre-strained.
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He J Y, Zhu C, Zhou D Q, et al.Steady state flow of the FeCoNiCrMn high entropy alloy at elevated temperatures[J]. Intermetallics, 2014, 55: 9
DOI:10.1016/j.intermet.2014.06.015 URL
61The high-temperature deformation mechanism of the FeCoNiCrMn HEA is characterized.61Two stages of stress exponent (n) depending on the strain rates are obtained.61Activation volume was calculated and verified through stress relaxation tests.61Dislocation climb mechanism controls the high strain rate region.61Viscous glide of dislocations mechanism controls the low strain rate region.
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He J Y, Liu W H, Wang H, et al.Effects of Al addition on structural evolution and tensile properties of the FeCoNiCrMn high-entropy alloy system[J]. Acta Mater., 2014, 62: 105
DOI:10.1016/j.actamat.2013.09.037 URL
A series of six-component (FeCoNiCrMn)10061xAlx (x=0–20at.%) high-entropy alloys (HEAs) was synthesized to investigate the alloying effect of Al on the structure and tensile properties. The microstructures of these alloys were examined using transmission electron microscopy, and crystalline phase evolution was characterized and compared with existing models. It was found that the crystalline structure changed from the initial single face-centered cubic (fcc) structure to a duplex fcc plus body-centered cubic (bcc) structure and then a single bcc structure as the Al concentration was increased. Resulting from the structural changes there were also corresponding variations in tensile properties. In the single fcc region, alloys behaved like a solid solution with relatively low strength but extended ductility. In the mixed structure region, alloys behaved like a composite with a sharp increase in strength but reduced ductility. In the single bcc region, alloys became extremely brittle. In this study, close correlation between the microstructure and mechanical properties was also discussed and presented.
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DOI:10.1016/j.jallcom.2015.07.209 URL
61Successful cold rolling of a BCC HfNbTaTiZr high entropy alloy into a thin sheet.61True tensile stress and ductility of the sheet are 129502MPa and 4.7%.61True tensile stress and ductility of the sheet annealed at 100002°C are 126202MPa and 10%.
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Senkov O N, Wilks G B, Scott J M, et al.Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys[J]. Intermetallics, 2011, 19: 698
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Takeuchi A, Amiya K, Wada T, et al.High-entropy alloys with a hexagonal close-packed structure designed by equi-atomic alloy strategy and binary phase diagrams[J]. JOM, 2014, 66: 1984
DOI:10.1007/s11837-014-1085-x URL
High-entropy alloys (HEAs) with an atomic arrangement of a hexagonal close-packed (hcp) structure were found in YGdTbDyLu and GdTbDyTmLu alloys as a nearly single hcp phase. The equi-atomic alloy design for HEAs assisted by binary phase diagrams started with selecting constituent elements with the hcp structure at room temperature by permitting allotropic transformation at a high temperature. The binary phase diagrams comprising the elements thus selected were carefully examined for the characteristics of miscibility in both liquid and solid phases as well as in both solids due to allotropic transformation. The miscibility in interest was considerably narrow enough to prevent segregation from taking place during casting around the equi-atomic composition. The alloy design eventually gave candidates of quinary equi-atomic alloys comprising heavy lanthanides principally. The XRD analysis revealed that YGdTbDyLu and GdTbDyTmLu alloys thus designed are formed into the hcp structure in a nearly single phase. It was found that these YGdTbDyLu and GdTbDyTmLu HEAs with an hcp structure have delta parameter ( δ ) values of 1.4 and 1.6, respectively, and mixing enthalpy (Δ H mix )02=02002kJ/mol for both alloys. These alloys were consistently plotted in zone S for disordered HEAs in a δ -Δ H mix diagram reported by Zhang et al. (Adv Eng Mater 10:534, 2008 ). The value of valence electron concentration of the alloys was evaluated to be 3 as the first report for HEAs with an hcp structure. The finding of HEAs with the hcp structure is significant in that HEAs have been extended to covering all three simple metallic crystalline structures ultimately followed by the body- and face-centered cubic (bcc and fcc) phases and to all four simple solid solutions that contain the glassy phase from high-entropy bulk metallic glasses.
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Gao M C, Alman D E.Searching for next single-phase high-entropy alloy compositions[J]. Entropy, 2013, 15: 4504
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Feuerbacher M, Heidelmann M, Thomas C.Hexagonal high-entropy alloys[J]. Mater. Res. Lett., 2015, 3: 1
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Soler R, Evirgen A, Yao M, et al.Microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure[J]. Acta Mater., 2018, 156: 86
DOI:10.1016/j.actamat.2018.06.010 URL
The microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure was performed. The phase state and chemical homogeneity of the solid solution were analysed with respect to crystal structure, phase stability, and oxide formation. It was found that Y-rich precipitates form at grain boundaries and that the alloy is prone to oxidation, leading to a homogeneous distribution of 651062nm-sized oxides in the grain interiors. The plastic response at the sub-grain level was studied in terms of the activated slip systems, critical resolved shear stresses (CRSS), and strain hardening using micropillar compression tests. We observe plastic slip on the basal system, with a CRSS of 196±14.762MPa. Particle strengthening and strength dependence on sample size are discussed on the basis of dislocation particle interaction and mechanical size effects.
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Rogal Ł, Czerwinski F, Jochym P T, et al.Microstructure and mechanical properties of the novel Hf25Sc25Ti25Zr25 equiatomic alloy with hexagonal solid solutions[J]. Mater. Des., 2016, 92: 8
DOI:10.1016/j.matdes.2015.11.104 URL
61The novel Hf25Sc25Ti25Zr25 high entropy alloy with nearly single hexagonal phase was design61Arrangements of laths typical for the Widmanst01tten structure have been identified.61Annealing at 1000°C/5h led to precipitation of the Sc plate-like cubic phase embedded in the hexagonal matrix.61Hexagonal solid solution decompose by the discontinuous solid-state reaction HPC1HT76HCP’1HT+α-Sc.61Successful modelling of the disordered alloy as an ensemble of periodic systems
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Wang W R, Wang W L, Yeh J W.Phases, microstructure and mechanical properties of AlxCoCrFeNi high-entropy alloys at elevated temperatures[J]. J. Alloys Compd., 2014, 589: 143
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Takeuchi A, Amiya K, Wada T, et al.Dual HCP structures formed in senary ScYLaTiZrHf multi-principal-element alloy[J]. Intermetallics, 2016, 69: 103
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Liu W H, Lu Z P, He J Y, et al.Ductile CoCrFeNiMox high entropy alloys strengthened by hard intermetallic phases[J]. Acta Mater., 2016, 116: 332
DOI:10.1016/j.actamat.2016.06.063 URL
Face-centered-cubic (fcc) type high entropy alloys (HEAs) exhibit outstanding ductility even at the liquid nitrogen temperature, but they are relatively weak in strength which is far from the requirements for practical structural applications. One of the general concepts employed previously in alloy design is the suppression of ‘brittle’ intermetallic compound formation which usually leads to a serious embrittlement. Surprisingly, we reveal in this study that the precipitation of hard σ and μ intermetallic compounds tremendously strengthened the CoCrFeNiMo0.3HEA but without causing a serious embrittlement. It exhibits a tensile strength as high as 1.202GPa and a good ductility of 6519%. A careful study of the deformation behavior reveals that the fcc matrix exhibits an extremely high work hardening exponent of 0.75, which suppresses the propagation of microcracks originated at these brittle particles. Our work presents a very successful demonstration of using complex hard intermetallic particles to manipulate the properties of fcc-type HEA systems. Furthermore, lattice distortion has been carefully measured in powder-metallurgy materials by line broadening from X-ray diffraction (XRD). It is interesting to discover that lattice planes are highly distorted in HEAs and this distortion also contributes to solid solution hardening.
[本文引用: 1]
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Lin C M, Tsai H L.Effect of annealing treatment on microstructure and properties of high-entropy FeCoNiCrCu0.5 alloy[J]. Mater. Chem. Phys., 2011, 128: 50
DOI:10.1016/j.matchemphys.2011.02.022 URL
This study examined the microstructure and electrochemical corrosion behaviour of high-entropy FeCoNiCrCu 0.5 alloys annealed at various temperatures. The alloy microstructures were characterized and analyzed chemically by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Further, the effects of annealing temperatures of 350 °C, 650 °C, 950 °C, and 1250 °C with a holding time of 24 h at each temperature on the alloy microstructure and properties were investigated. XRD spectra of the as-cast specimens and those heated to 1250 °C showed a face-centred cubic (FCC) solid-solution phase. All specimens contained a matrix, which included a Cu-depleted phase, a Cr-rich phase (second structure), and a Cu-rich phase (third structure). The Cr-rich phase precipitated in the matrix after annealing at 1250 °C. The microstructure of the Cu-rich phase showed spinodal decomposition with an increase in the annealing temperature from 350 °C to 1250 °C. Differential scanning calorimetry (DSC) analysis revealed that in the as-cast high-entropy FeCoNiCrCu 0.5 alloy specimen annealed below 1300 °C, the Cu-rich (Cu–(Ni, Co, Cr, Fe)) phases precipitated in the matrix by spinodal decomposition. The electrochemical corrosion behaviours of the as-cast and annealed specimens were evaluated by potentiodynamic polarization performed in immersion tests. The as-cast and annealed specimens were severely corroded in 3.5% NaCl solution; the main corrosion mechanism was the precipitation of the Cu-rich phase in the matrix. The Cu-rich phase was susceptible to corrosion, and its potential differed considerably from that of the matrix. The Cl 61 ions preferentially attacked this susceptible area (Cu-rich phase). This preferential attack was attributed to the fact that the presence of copper in the alloy degraded the corrosion resistance, thereby leading to corrosion by pitting.
[本文引用: 1]
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Liu W H, Wu Y, He J Y, et al.Grain growth and the Hall-Petch relationship in a high-entropy FeCrNiCoMn alloy[J]. Scr. Mater., 2013, 68: 526
DOI:10.1016/j.scriptamat.2012.12.002 URL
A high-entropy FeCoNiCrMn alloy with a single face-centered cubic phase was synthesized and subsequently annealed at different temperatures to systematically investigate the grain growth behavior. It was observed that the growth kinetics could be described by a power law of 3 and the activation energy for growth was about 321.7kJmol611. The hardness of the alloys was measured as a function of grain size, and the result was found to follow the classical Hall–Petch strengthening, though with a relatively high hardening coefficient.
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Lu Y P, Dong Y, Guo S, et al.A promising new class of high-temperature alloys: eutectic high-entropy alloys[J]. Sci. Rep., 2014, 4: 6200
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Lu Y P, Gao X Z, Jiang L, et al.Directly cast bulk eutectic and near-eutectic high entropy alloys with balanced strength and ductility in a wide temperature range[J]. Acta Mater., 2017, 124: 143
DOI:10.1016/j.actamat.2016.11.016 URL
High entropy alloys (HEAs) usually possess weak liquidity and castability, and considerable compositional inhomogeneity, mainly because they contain multiple elements with high concentrations. As a result, large-scale production of HEAs by casting is limited. To address the issue, the concept of eutectic high entropy alloys (EHEAs) was proposed, which has led to some promise in achieving good quality industrial scale HEAs ingots, and more importantly also good mechanical properties. In the practical large-scale casting, the actual composition of designed EHEAs could potentially deviate from the eutectic composition. The influence of such deviation on mechanical properties of EHEAs is important for industrial production, which constitutes the topic of the current work. Here we prepared industrial-scale HEAs ingots near the eutectic composition: hypoeutectic alloy, eutectic alloy and hypereutectic alloy. Our results showed that the deviation from eutectic composition does not significantly affect the mechanical properties, castability and the good mechanical properties of EHEAs can be achieved in a wide compositional range, and at both room and cryogenic temperatures. Our results suggested that EHEAs with simultaneous high strength and high ductility, and good liquidity and castability can be readily adapted to large-scale industrial production. The deformation behavior and microstructure evolution of the eutectic and near-eutectic HEAs were thoroughly studied using a combination of techniques, including strain measurement by digital image correlation, in-situ synchrotron X-ray diffraction, and transmission electron microscopy. The wavy strain distribution and the therefore resulted delay of necking in EHEAs were reported for the first time.
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Jiang H, Han K M, Gao X X, et al.A new strategy to design eutectic high-entropy alloys using simple mixture method[J]. Mater. Des., 2018, 142: 101
DOI:10.1016/j.matdes.2018.01.025 URL
Eutectic high entropy alloys (EHEAs) hold promising industrial application potential, but how to design EHEA compositions remains challenging. In the present work, a simple and effective strategy by combining mixing enthalpy and constituent binary eutectic compositions was proposed to design EHEA compositions. This strategy was then applied to a series of (CoCrFeNi)M x (M62=62Nb, Ta, Zr, Hf) HEAs, leading to the discovery of new EHEAs, namely, CoCrFeNiNb 0.45 , CoCrFeNiTa 0.4 , CoCrFeNiZr 0.55 and CoCrFeNiHf 0.4 . The microstructure of these new EHEAs comprised of FCC and Laves phases in the as-cast state. The experimental result shows that this new alloy design strategy can be used to locate new EHEAs effectively.
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Wu D, Zhang J Y, Huang J C, et al.Grain-boundary strengthening in nanocrystalline chromium and the Hall-Petch coefficient of body-centered cubic metals[J]. Scr. Mater., 2013, 68: 118
DOI:10.1016/j.scriptamat.2012.09.025 URL
Nanocrystalline Cr (nc-Cr) was synthesized by electrodeposition. Samples with various grain sizes (19–57nm) were prepared by annealing the as-deposited sample. Microstructures were examined using X-ray and electron microscopy, and the mechanical properties were evaluated using nanoindentation. The strength of nc-Cr samples apparently obeyed the classical Hall–Petch relationship. It was found that hardening potency caused by grain refinement was generally higher in body-centered cubic metals than that in face-centered cubic and hexagonal close-packed metals. A possible explanation was offered.
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Sun S J, Tian Y Z, Lin H R, et al.Transition of twinning behavior in CoCrFeMnNi high entropy alloy with grain refinement[J]. Mater. Sci. Eng., 2018, A712: 603
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Juan C C, Tsai M H, Tsai C W, et al.Simultaneously increasing the strength and ductility of a refractory high-entropy alloy via grain refining[J]. Mater. Lett., 2016, 184: 200
DOI:10.1016/j.matlet.2016.08.060 URL
61The HfNbTaTiZr solid solution refractory alloy exhibits low rate and high activation energy of grain growth at 1200~1350°C.61The slow grain boundary migration is proposed to be a result of the solute-drag mechanism.61Five componential atoms drag grain boundary and the slowest two components, Nb and Ta, explain the high activation energy and low rate of grain growth.61Grain refinement simultaneously increases tensile strength and ductility.61The alloy with a grain size of 38 μm has an excellent combination of tensile yield strength, 958MPa, and ductility, 20%.
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Seol J B, Bae J W, Li Z M, et al.Boron doped ultrastrong and ductile high-entropy alloys[J]. Acta Mater., 2018, 151: 366
DOI:10.1016/j.actamat.2018.04.004 URL
A new class of materials called high-entropy alloys (HEAs) constitutes multiple principal elements in similar compositional fractions. The equiatomic FeMnCrCoNi(at%) HEA shows attractive mechanical properties, particularly under cryogenic conditions. Yet, it lacks sufficient yield and ultimate tensile strengths at room temperature. To strengthen these materials, various strategies have been proposed mainly by tuning the composition of the bulk material while no efforts have been made to decorate and strengthen the grain boundaries. Here, we introduce a new HEA design approach that is based on compositionally conditioning the grain boundaries instead of the bulk. We found that as little as 30 痯pm of boron doping in single-phase HEAs, more specific in an equiatomic FeMnCrCoNi and in a non-equiatomic FeMnCrCo(at%), improves dramatically their mechanical properties, increasing their yield strength by more than 100% and ultimate tensile strength by &sim;40% at comparable or even better ductility. Boron decorates the grain boundaries and acts twofold, through interface strengthening and grain size reduction. These effects enhance grain boundary cohesion and retard capillary driven grain coarsening, thereby qualifying boron-induced grain boundary engineering as an ideal strategy for the development of advanced HEAs.
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Stepanov N D, Shaysultanov D G, Salishchev G A, et al.Effect of V content on microstructure and mechanical properties of the CoCrFeMnNiVx high entropy alloys[J]. J. Alloys Compd., 2015, 628: 170
DOI:10.1016/j.jallcom.2014.12.157 URL
Crystal structure, microstructure, microhardness and compression properties of CoCrFeMnNiVx (x=0, 0.25, 0.5, 0.75, 1) high entropy alloys were examined. The alloys were produced by vacuum arc melting and studied in as-solidified and homogenized (annealing at 1000°C for 24h) conditions. The CoCrFeMnNi alloy was a single-phase fcc solid solution in both conditions. The CoCrFeMnNiV0.25 alloy had a single-phase fcc structure in as-solidified condition, but 652vol.% fine particles of a sigma phase precipitated after annealing. The alloys with x=0.5, 0.75 and 1.0 contained the sigma phase already in as-solidified condition. The sigma-phase volume fraction increased with an increase in the V content, and in CoCrFeMnNiV the sigma phase became the matrix phase. After homogenization treatment, the volume fraction of the sigma phase increased in all three alloys by 658% due to additional precipitation of fine particles inside the fcc phase. Phase composition and microstructure of the alloys was analyzed employing criteria for solid solution/intermetallic phase formation. The effect of alloys’ chemical composition on the volume fraction of constitutive phases was discussed. A modified valence electron concentration (VEC) criterion, which takes into account localized lattice distortions around V atoms, was suggested to correctly predict sigma phase formation in the CoCrFeNiMnVx alloys. It was demonstrated that the volume fraction of sigma phase was proportional to the cumulative Cr and V concentration. Mechanical properties of the alloys were greatly affected by the sigma phase. The CoCrFeMnNi and CoCrFeMnNiV0.25 alloys were soft and ductile, but an increase in the sigma-phase volume fraction resulted in continuous strengthening and loss of ductility.
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Liu S F, Wu Y, Wang H T, et al.Stacking fault energy of face-centered-cubic high entropy alloys[J]. Intermetallics, 2018, 93: 269
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Wu Z, Bei H, Pharr G M, et al.Temperature dependence of the mechanical properties of equiatomic solid solution alloys with face-centered cubic crystal structures[J]. Acta Mater., 2014, 81: 428
DOI:10.1016/j.actamat.2014.08.026 URL
Compared to decades-old theories of strengthening in dilute solid solutions, the mechanical behavior of concentrated solid solutions is relatively poorly understood. A special subset of these materials includes alloys in which the constituent elements are present in equal atomic proportions, including the high-entropy alloys of recent interest. A unique characteristic of equiatomic alloys is the absence of “solvent” and “solute” atoms, resulting in a breakdown of the textbook picture of dislocations moving through a solvent lattice and encountering discrete solute obstacles. To clarify the mechanical behavior of this interesting new class of materials, we investigate here a family of equiatomic binary, ternary and quaternary alloys based on the elements Fe, Ni, Co, Cr and Mn that were previously shown to be single-phase face-centered cubic (fcc) solid solutions. The alloys were arc-melted, drop-cast, homogenized, cold-rolled and recrystallized to produce equiaxed microstructures with comparable grain sizes. Tensile tests were performed at an engineering strain rate of 10613s611 at temperatures in the range 77–673K. Unalloyed fcc Ni was processed similarly and tested for comparison. The flow stresses depend to varying degrees on temperature, with some (e.g. NiCoCr, NiCoCrMn and FeNiCoCr) exhibiting yield and ultimate strengths that increase strongly with decreasing temperature, while others (e.g. NiCo and Ni) exhibit very weak temperature dependencies. To better understand this behavior, the temperature dependencies of the yield strength and strain hardening were analyzed separately. Lattice friction appears to be the predominant component of the temperature-dependent yield stress, possibly because the Peierls barrier height decreases with increasing temperature due to a thermally induced increase of dislocation width. In the early stages of plastic flow (5–13% strain, depending on material), the temperature dependence of strain hardening is due mainly to the temperature dependence of the shear modulus. In all the equiatomic alloys, ductility and strength increase with decreasing temperature down to 77K.
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Wang Z W, Baker I, Cai Z H, et al.The effect of interstitial carbon on the mechanical properties and dislocation substructure evolution in Fe40.4Ni11.3Mn34.8Al7.5Cr6 high entropy alloys[J]. Acta Mater., 2016, 120: 228
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Stepanov N D, Shaysultanov D G, Chernichenko R S, et al.Effect of thermomechanical processing on microstructure and mechanical properties of the carbon-containing CoCrFeNiMn high entropy alloy[J]. J. Alloys Compd., 2017, 693: 394
DOI:10.1016/j.jallcom.2016.09.208 URL
61CoCrFeNiMn high entropy alloy doped with 102at.% C was produced by vacuum arc melting.61After homogenization annealing at 100002°C the alloy has single fcc phase structure.61The alloy was cold rolled to 80% thickness strain and annealed at 600–110002°C.61After annealing at 80002°C the alloy combines high strength with good ductility.61High strength of the annealed alloy mainly is due to strong Hell-Petch strengthening.
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Xie Y C, Cheng H, Tang Q H, et al.Effects of N addition on microstructure and mechanical properties of CoCrFeNiMn high entropy alloy produced by mechanical alloying and vacuum hot pressing sintering[J]. Intermetallics, 2018, 93: 228
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Chen Y W, Li Y K, Cheng X W, et al.Interstitial strengthening of refractory ZrTiHfNb0.5Ta0.5Ox (x= 0.05, 0.1, 0.2) high-entropy alloys[J]. Mater. Lett., 2018, 228: 145
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Jiang L, Lu Y P, Wu W, et al.Microstructure and mechanical properties of a CoFeNi2V0.5Nb0.75 eutectic high entropy alloy in as-cast and heat-treated conditions[J]. J. Mater. Sci. Technol., 2016, 32: 245
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He F, Wang Z J, Cheng P, et al.Designing eutectic high entropy alloys of CoCrFeNiNbx[J]. J. Alloys Compd., 2016, 656: 284
DOI:10.1016/j.jallcom.2015.09.153 URL
Based on surveying the existing binary phase diagrams with eutectic points, a strategy of designing eutectic high entropy alloys (EHEAs) with desired high strength and ductility is proposed. Based on the computer-aided thermodynamic calculations, the pseudo eutectic binary alloy system of CoCrFeNiNb x (x=0.1, 0.25, 0.5 and 0.8) was designed. The experimental results show that the eutectic alloys are composed of a ductile face centered cubic (FCC) phase and a hard Laves phase with fine laminar structures. The designed alloys show excellent integrated mechanical properties of ductility and strength. For the CoCrFeNiNb 0.5 alloy, the compressive fracture strength and strain can reach above 2300MPa and 23.6%, respectively.
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Huang S, Li W, Lu S, et al.Temperature dependent stacking fault energy of FeCrCoNiMn high entropy alloy[J]. Scr. Mater., 2015, 108: 44
DOI:10.1016/j.scriptamat.2015.05.041 URL
The stacking fault energy (SFE) of paramagnetic FeCrCoNiMn high entropy alloy is investigated as a function of temperature viaab initiocalculations. We divide the SFE into three major contributions: chemical, magnetic and strain parts. Structural energies, local magnetic moments and elastic moduli are used to estimate the effect of temperature on each term. The present results explain the recently reported twinning observed below room-temperature and predict the occurrence of the hexagonal phase at cryogenic conditions.
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Deng Y, Tasan C C, Pradeep K G, et al.Design of a twinning-induced plasticity high entropy alloy[J]. Acta Mater., 2015, 94: 124
DOI:10.1016/j.actamat.2015.04.014 URL
We introduce a liquid metallurgy synthesized, non-equiatomic Fe40Mn40Co10Cr10 high entropy alloy that is designed to undergo mechanically-induced twinning upon deformation at room temperature. Microstructure characterization, carried out using SEM, TEM and APT shows a homogeneous fcc structured single phase solid solution in the as-cast, hot-rolled and homogenized states. Investigations of the deformation substructures at specific strain levels with electron channeling contrast imaging (ECCI) combined with EBSD reveal a clear change in the deformation mechanisms of the designed alloy starting from dislocation slip to twinning as a function of strain. Such twinning induced plasticity has only been observed under cryogenic conditions in the equiatomic FeMnNiCoCr high entropy alloy. Thus, despite the decreased contribution of solid solution strengthening, the tensile properties of the introduced lean alloy at room temperature are found to be comparable to that of the well-studied five component FeMnNiCoCr system.
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Gludovatz B, Hohenwarter A, Thurston K V S, et al. Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures[J]. Nat. Commun., 2016, 7: 10602
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Laplanche G, Kostka A, Reinhart C, et al.Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi[J]. Acta Mater., 2017, 128: 292
DOI:10.1016/j.actamat.2017.02.036 URL
The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77K) and room temperature (293K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2 dislocations dissociated into 1/6 Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22±4mJm 612 , which is 6525% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16μm grain size is 260±30MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi.
[本文引用: 1]
[72]
Okamoto N L, Fujimoto S, Kambara Y, et al.Size effect, critical resolved shear stress, stacking fault energy, and solid solution strengthening in the CrMnFeCoNi high-entropy alloy[J]. Sci. Rep., 2016, 6: 35863
DOI:10.1038/srep35863 PMID:27775026 URL
High-entropy alloys (HEAs) comprise a novel class of scientifically and technologically interesting materials. Among these, equatomic CrMnFeCoNi with the face-centered cubic (FCC) structure is noteworthy because its ductility and strength increase with decreasing temperature while maintaining outstanding fracture toughness at cryogenic temperatures. Here we report for the first time by single-crystal micropillar compression that its bulk room temperature critical resolved shear stress (CRSS) is ~33–4365MPa, ~10 times higher than that of pure nickel. CRSS depends on pillar size with an inverse power-law scaling exponent of –0.63 independent of orientation. Planar 0565<6511065>65{111} dislocations dissociate into Shockley partials whose separations range from ~3.5–4.565nm near the screw orientation to ~5–865nm near the edge, yielding a stacking fault energy of 3065±65565mJ/m2. Dislocations are smoothly curved without any preferred line orientation indicating no significant anisotropy in mobilities of edge and screw segments. The shear-modulus-normalized CRSS of the HEA is not exceptionally high compared to those of certain concentrated binary FCC solid solutions. Its rough magnitude calculated using the Fleischer/Labusch models corresponds to that of a hypothetical binary with the elastic constants of our HEA, solute concentrations of 20–50 at.%, and atomic size misfit of ~4%.
[本文引用: 1]
[73]
Herrera C, Ponge D, Raabe D.Design of a novel Mn-based 1 GPa duplex stainless TRIP steel with 60% ductility by a reduction of austenite stability[J]. Acta Mater., 2011, 59: 4653
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Sun F, Zhang J Y, Marteleur M, et al.Investigation of early stage deformation mechanisms in a metastable β titanium alloy showing combined twinning-induced plasticity and transformation-induced plasticity effects[J]. Acta Mater., 2013, 61: 6406
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[75]
Wu Y, Xiao Y H, Chen G L, et al.Bulk metallic glass composites with transformation-mediated work-hardening and ductility[J]. Adv. Mater., 2010, 22: 2770
DOI:10.1002/adma.201000482 PMID:20422654 URL
A bulk metallic glass (BMG) composite with large tensile ductility and work-hardening capability (see figure) was developed by applying the 090008transformation-induced plasticity090009 concept to amorphous alloys. The current approach is not believed to be limited to the current BMG composite but could promote ductility in other BMG systems, offering a new paradigm for developing BMGs with improved ductility as practical engineering materials.
[本文引用: 1]
[76]
Bouaziz O, Allain S, Scott C P, et al.High manganese austenitic twinning induced plasticity steels: A review of the microstructure properties relationships[J]. Curr. Opin. Solid State Mater. Sci., 2011, 15: 141
DOI:10.1016/j.cossms.2011.04.002 URL
A significant increase in the research activity dedicated to high manganese TWIP steels has occurred during the past five years, motivated by the breakthrough combination of strength and ductility possessed by these alloys. Here a review of the relations between microstructure and mechanical properties is presented focusing on plasticity mechanisms, strain-hardening, yield stress, texture, fracture and fatigue. This summarized knowledge explains why TWIP steel metallurgy is currently a topic of great practical interest and fundamental importance. Finally, this publication indicates some of the main avenues for future investigations required in order to sustain the quality and the dynamism in this field.Highlights? TWIP steels exhibit a breakthrough combination of strength/ductility. ? A significant increase in TWIP steel development activity has occurred during the past five years. ? A review of the relations between microstructure and mechanical properties is presented. ? The main avenues for future investigations are indicated.
[本文引用: 1]
[77]
Li Z M, Körmann F, Grabowski B, et al.Ab initio assisted design of quinary dual-phase high-entropy alloys with transformation-induced plasticity[J]. Acta Mater., 2017, 136: 262
DOI:10.1016/j.actamat.2017.07.023 URL
We introduce a new class of high-entropy alloys (HEAs), i.e., quinary (five-component) dual-phase (DP) HEAs revealing transformation-induced plasticity (TRIP), designed by using a quantum mechanically based and experimentally validated approach. Ab initio simulations of thermodynamic phase stabilities of Co 20 Cr 20 Fe 40-x Mn 20 Ni x (x=0 20at. %) HEAs were performed to screen for promising compositions showing the TRIP-DP effect. The theoretical predictions reveal several promising alloys, which have been cast and systematically characterized with respect to their room temperature phase constituents, microstructures, element distributions and compositional homogeneity, tensile properties and deformation mechanisms. The study demonstrates the strength of ab initio calculations to predict the behavior of multi-component HEAs on the macroscopic scale from the atomistic level. As a prototype example a non-equiatomic Co 20 Cr 20 Fe 34 Mn 20 Ni 6 HEA, selected based on our ab initio simulations, reveals the TRIP-DP effect and hence exhibits higher tensile strength and strain-hardening ability compared to the corresponding equiatomic CoCrFeMnNi alloy.
[本文引用: 1]
[78]
Rogal Ł, Kalita D, Tarasek A, et al.Effect of SiC nano-particles on microstructure and mechanical properties of the CoCrFeMnNi high entropy alloy[J]. J. Alloys Compd., 2017, 708: 344
DOI:10.1016/j.jallcom.2017.02.274 URL
Novel metal matrix nanocomposites were designed exploring the CoCrFeMnNi high entropy alloy as a matrix and SiC spherical nanoparticles with a diameter of 20–50nm as a reinforcement phase, and manufactured by mechanical alloying followed by hot isostatic sintering. After reaction at 1000°C for 15min the microstructure consisted of a matrix with a face centered solid solution of all elemental ingredients with traces of M 23 C 6 /M 7 C 3 carbides (where M=Cr, Fe, Co), σ-phase and SiC nano-particles distributed along grain boundaries of the matrix. Additions of 5wt.% of SiC nano-particles increased the room temperature compressive yield strength of the CoCrFeMnNi base from 1180MPa to 1480MPa, accompanied by a decrease in compressive strength and plasticity. The results are discussed in terms of mechanisms controlling the strengthening process in nano-composites with high entropy alloy matrix.
[本文引用: 1]
[79]
Rogal Ł, Kalita D, Litynska-Dobrzynska L.CoCrFeMnNi high entropy alloy matrix nanocomposite with addition of Al2O3[J]. Intermetallics, 2017, 86: 104
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[80]
Zinkle S J, Ghoniem N M. Operating temperature windows for fusion reactor structural materials [J]. Fusion Eng. Des., 2000, 51-52: 55
DOI:10.1016/S0920-3796(00)00320-3 URL
A critical analysis is presented of the operating temperature windows for nine candidate fusion reactor structural materials: four reduced-activation structural materials (oxide-dispersion-strengthened and ferritic/martensitic steels containing 8–12%Cr, V–4Cr–4Ti, and SiC/SiC composites), copper-base alloys (CuNiBe), tantalum-base alloys (e.g. Ta–8W–2Hf), niobium alloys (Nb–1Zr), and molybdenum and tungsten alloys. The results are compared with the operating temperature limits for Type 316 austenitic stainless steel. Several factors define the allowable operating temperature window for structural alloys in a fusion reactor. The lower operating temperature limit in all body-centered cubic (BCC) and most face-centered cubic (FCC) alloys is determined by radiation embrittlement (decrease in fracture toughness), which is generally most pronounced for irradiation temperatures below 650.3 T M where T M is the melting temperature. The lower operating temperature limit for SiC/SiC composites will likely be determined by radiation-induced thermal conductivity degradation, which becomes more pronounced in ceramics with decreasing temperature. The upper operating temperature limit of structural materials is determined by one of four factors, all of which become more pronounced with increasing exposure time: (1) thermal creep (grain boundary sliding or matrix diffusional creep); (2) high temperature He embrittlement of grain boundaries; (3) cavity swelling (particularly important for SiC and Cu alloys); or (4) coolant compatibility/corrosion issues. In many cases, the upper temperature limit will be determined by coolant corrosion/compatibility rather than by thermal creep or radiation effects. The compatibility of the structural materials with Li, Pb–Li, Sn–Li, He and Flibe (Li 2BeF 4) coolants is summarized.
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[81]
Hadraba H, Chlup Z, Dlouhy A, et al.Oxide dispersion strengthened CoCrFeNiMn high-entropy alloy[J]. Mater. Sci. Eng., 2017, A689: 252
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Chen S T, Tang W Y, Kuo Y F, et al.Microstructure and properties of age-hardenable AlxCrFe1.5MnNi0.5 alloys[J]. Mater. Sci. Eng., 2010, A527: 5818
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Tsai M H, Yuan H, Cheng G M, et al.Morphology, structure and composition of precipitates in Al0.3CoCrCu0.5FeNi high-entropy alloy[J]. Intermetallics, 2013, 32: 329
DOI:10.1016/j.intermet.2012.07.036 URL
High-entropy alloy is a new class of metallic materials with great potential for many applications. However, their microstructural characteristics, particularly those of precipitates, remain poorly understood. This has hindered the establishment of structure-property relationship in these alloys. Here, we report the morphology, crystal structure and composition of the precipitates in the Al0.3CoCrCu0.5FeNi high-entropy alloy. Two types of precipitates were identified, namely the plate-like and the spherical precipitates. Their formation sequence and mechanism during the cooling process are discussed based on thermodynamics.
[本文引用: 1]
[84]
Zhao Y L, Yang T, Zhu J H, et al.Development of high-strength Co-free high-entropy alloys hardened by nanosized precipitates[J]. Scr. Mater., 2018, 148: 51
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