IMECH-IR  > 非线性力学国家重点实验室
面心立方中熵合金的变形机制与强韧化机理
Alternative TitleDeformation and strengthening and toughening mechanism of face centered cubic medium entropy alloys
陈雪飞
Thesis Advisor武晓雷
2021-11-25
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline固体力学
Keyword中熵合金 化学短程序 L12 滑移带 异构
Abstract

在传统金属中,强度与塑性是一对力学性能中难以兼顾的矛盾。面心立方 (Face-Centered Cubic,FCC)晶体结构合金通常拥有较好的塑性和变形能力,但 是其强度普遍偏低。目前公认的强化方法:固溶强化、细晶强化、相变强化和沉 淀强化等,均难以使材料突破强度与塑性的倒置。近些年来,在 FCC 中/高熵合 金中发现了短程有序结构不但对位错有显著的阻碍作用,并且还可以降低局部的 层错能,引入诸如相变和孪生等更加丰富的塑性变形机制,对材料强韧化有着非 常重要的作用。此外,长程有序结构,例如沉淀相或者第二相,也具有不可忽视 的强化作用,在此类结构的作用下,材料的屈服强度可强化至 2GPa 以上。为了 改善强度与塑性的倒置,本文将沉淀相、纳米孪晶等亚结构引入异质结构设计。 拉伸变形过程中,不同的异质结构之间形成背应力,或者称异变诱导(HeteroDeformation Induced,HDI)应力。HDI 应变硬化大幅提升了材料整体的应变硬 化率,延缓颈缩的形成,可以有效地协同提高材料的强度与塑性。在 FCC 中/高 熵合金中,短程有序,长程有序,以及异构等因素对强度与塑性的具体作用机制, 目前有待深入解决。 本研究以 VCoNi 和 CoCrNi 两种典型的 FCC 中熵合金为研究对象,进行了 以下 3 个方面的研究:(1)短程有序结构对材料强化的影响机制;(2)长程有 序结构的强化机制;(3)多级滑移带体系联动对韧化的影响。 第一,在众多局部化学有序(Local Chemical Order,LCO)的研究中,化学 短程序(Chemical Short-Range Order,CSRO)是最难破译的难题。目前为止, CSRO 存在形式的具体证据依然缺失。本论文选择一个代表性 VCoNi 中熵合金, 该合金是一种 FCC 结构超级固溶体,并且材料处于亚稳态,具有形成化学有序 的倾向。第一性原理模拟也表明,VCoNi 合金中等原子比的 V、Co 和 Ni 三种元 素之间的结合能存在以下规律:该合金的固溶体更倾向于形成 V-Co 和 V-Ni 原 子对,但回避 V-V 原子对。短程有序结构的尺寸往往处于亚纳米尺度,目前的 实验技术难以实现直接对其结构的直接观察与分析。为此,基于原子尺度高分辨 透射结果,我们设计了独特的结构分析方法,成功规避数据拟合和多种可能性的 解释;在特定的晶带轴下,利用选区衍射、微纳束衍射,以及原子级高分辨,化 学能谱的透射电子显微镜照片,获得了 VCoNi 中熵合金的面心立方晶体结构 FCC 相中的 CSRO 的直接证据。其空间三维结构是单斜晶体,晶格常 数为 a=0.541 nm、b=0.264 nm、c=0.445 nm,基于对 CSRO 结构的深入理解,进一步研究了塑性变形过 程中,短程有序结构与位错的交互作用机制,阐明了短程有序结构的强化机理。 第二,在中/高熵合金的长程有序结构目前主要集中于沉淀强化方面的研究, 通过时效处理引入纳米级长程有序结构,可提高各类合金体系的抗拉强度至 GPa 级水平。其中,最具代表性的是L1ଶ型(A3B 型,面心正方晶体结构,八个顶角 均为 B 原子)的长程有序金属间化合物(Ordered Intermetallic Compounds,OICs), 其具有高强度的优势,但塑性是主要瓶颈。本文另辟蹊径的提出了一种双相异构 VCoNi 中熵合金(FCC 相+L1ଶ相)微观组织调控思路,主要的目的是形成由纳 米片层夹杂堆垛层错的特殊结构,实现双相 VCoNi 中熵合金屈服强度提高至 2.1 GPa 的强化效果,并保持了 16%的均匀延伸率。利用化学能谱的透射电子显微 镜、三维原子探针、XRD 配合第一性原理计算,深入探索这一新结构的强韧化 潜在机制。其中,双相 L12 长程有序显著提高了 VCoNi 中熵合金的强度,但是 在 FCC 和 L12 双相异构材料中的 HDI 应变硬化显著提高了材料整体的加工硬化 能力,延迟了颈缩和变形失稳的发生。为新型高强高韧中熵合金的制备提供了新 的强韧化思路。 第三,FCC 中/高熵合金的特点是高塑性,低强度。对于中/高熵合金韧化机 制的深入理解,是进一步实现强化的重要前提。本文以单相 CrCoNi 中熵为研究 对象,研究不同晶粒尺度对强度与塑性的关系,发现粗晶态样品不但屈服强度没 有下降,其均匀延伸率显著高于细晶材料,可实现高达 110%的均匀塑性。通过 原位透射电子显微镜,深入解析了超高塑性变形的韧化机制,提出了以滑移带诱 导塑性新机制:具体是塑性变形初期晶界作为位错源发生位错,并且塑性变形集 中于特定的滑移带内,滑移带中的位错包含全位错和不全位错两种类型;随着应 变的增加,滑移带之间发生交互作用,当不全位错在交割处相遇,则会形成 Lomer-Cottrell(LC)位错锁,钉扎住路过的位错,形成高密度位错塞积;为实现 持续稳定的塑性变形,以交滑移方式激活了更多新的侧向滑移带;以此方式,持 续促发多次滑移带,初始的毫米级晶粒逐步划分为微米级小区域;随着结构单元 的尺寸进一步降低,不全位错比例增加,诱使高密度纳米孪晶形成,促使晶粒进 一步被细化至纳米级。这种动态的晶粒细化,是超大晶粒 CoCrNi 依然能保持较 高的屈服强度的原因之一,晶内逐级形成的高密度纳米孪晶也是实现超高塑性的 潜在机理。 综上,本文在传统金属材料的强韧化研究基础上,针对新型中熵合金材料, 系统研究了其特有的短程、长程有序结构的强化机制;在增塑方面,一方面,本 文发现了中熵合金的双相结构中也存在异构金属材料的 HDI 应变硬化,可显著 提高材料的均匀塑性;另一方面,基于原位透射电镜技术,本文发现由于短程有 序结构存在诱导形成了以滑移带为主要形式的新型变形机制,能够在粗大晶粒材料中实现基于多级滑移带体系诱导的超强韧化效应。结合本文三部分内容,最终提出了面心立方中熵合金的强韧化新思路。

Other Abstract

Strength and ductility are a pair of perpetual paradox in conventional metals. The face-centered cubic (FCC) alloys exhibit hallmark ductility and fracture toughness, but have a glaring shortcoming of low strength. Up to now the mechanisms to strengthen FCC alloys are still traditional ways, e.g. solid solution, grain refinement, phase transformation, and precipitation, etc. But, it still can’t break through the trade-off of strength and plasticity. Recently, the short-range order (SRO) will hinder the dislocation and reduce the local stacking fault energy, which induces more deformation mechanisms, such as transformation or twinning. So, SROs play an important rple in strengthening and toughening. Another cooresponding structure, long range order, such as precipitates or second phase, has been proved its assignable strengthening effect in large number of researches. Even, the yield strength can reach ~2GPa. In order to increase strength and plasticity synergistically, sub-structures, such as precipitates or nano twins, can be designed as hetrostructures. During tensile deformation, in contrast to a homogeneous structure, the heterostructure may induce back stress, later corrected as hetero-deformation-induced (HDI) stress. This HDI stress will produce extra HDI hardening, which combined with forest hardening, can improve ductility especially at elevated strengths as widely reported. In FCC alloys, the specific effect and mechanism of short-range order, long-range order and hetero-structure on strength and plasticity needs to be further solved. In this work, two typical FCC alloys, VCoNi and CoCrNi alloys, were studied. And it includes three parts. ⅰ) the study on the short-range order and its influence on strengthening; ⅱ) long-range ordered L1ଶ and its effect on strengthening; ⅲ) the multiple slip bands and its effect on toughening. First, Among the LCOs that can develop to different extents, chemical short-range order (CSRO) is arguably the most difficult to decipher. Concrete evidence of CSRO has been sorely missing thus far. We begin our presentation by first selecting a representative alloy system. The VCoNi MEA, a face-centered-cubic (FCC) random solid solution, is a metastable phase with a high likelihood of partial chemical order. Specifically, we hypothesize that the single-phase FCC has a propensity for V-Co and V-Ni preference accompanied by V-V avoidance. Such CSROs, however, are notoriously difficult to observe in a direct manner. Howerver, here we discover that under an appropriate zone axis, micro/nano beam diffraction, as well as atomicresolution imaging and chemical mapping in transmission electron microscope, can observe CSRO in a face-centered-cubic VCoNi concentrated solution. Our complementary suite of tools unequivocally nails down the CSRO, including its spatial extent, atomic packing configuration and preferential lattice occupancy by the chemical species. The configuration of CSRO structure is monoclinic crystal,  a=0.541 nm, b=0.264 nm, c=0.445 nm. The stoichiometry is V2CoXNi(2-x). . Based on the in-depth research on CSRO, its interaction with dislocation and its strengthening and toughening mechanism is further studied. Second, In terms of precipitation-hardening in FCC-based HEAs, tensile strengths over 1 GPa have been achieved in several alloy systems strengthened. Traditionally, L1ଶ-ordered (A3B, Face centered square crystal structure, eight vertex angles are B atoms) intermetallic compounds (OICs) results in high strength and poor ductility. Dual phase VCoNi MEA (including FCC and L1ଶ phase) was studied here. The L1ଶ-OICs studied in this paper has a unique microstructure, which is composed of two layers of L1ଶ-OICs with a layer of stacking faults. This special structure is one of the important reasons why the yield strength is as high as ~2.1 GPa, and the uniform elongation is ~16%. Chemical mapping in transmission electron microscope, transmission electron microscope (TEM), three-dimensional atomic probe (3D-APT), XRD and first principles calculation was used to study he mechanism of strengthening and toughening. This L1ଶ -OICs phase provides a new idea for the strengthening and toughening of medium and high entropy alloys in the future. L1ଶ results in high strength in dual phase VCoNi MEA. However, the HDI strain hardening obviously increases the total work hardening in the dual phase MEA, which can effectively delay the necking and deformation instability. It provides a new strategy of strengthening and toughening of H/MEA. Third, the uniform elongation is high with a limited yield strength for FCC medium-entropy. To gain high strength, it is important to do deeply research on toughening mechanism. In this work a UCG CrCoNi was studied by in-situ TEM to explain the superior ductility. Compared with other smaller grain sizes, the uniform elongation of UCG can be as high as 110% without decreasing the yield strength. By in-situ TEM analysis, slip band induced plastic deformation mechanism, a new toughening mechanism to realize this ultra-high plastic deformation, is found: firstly, at the initial stage, dislocations occur at the grain boundary as the dislocation source, and the plastic deformation is concentrated in a specific slip band. The dislocations in the slip band include full dislocations and partials; With the increase of strain, the interaction between slip bands occurs. When partials meet at the intersection, LomerCottrell (LC) dislocation locks will be formed. And it will pin dislocations. Then here will form high-density dislocation tangle. To achieve continuous and stable plastic deformation, more new slip bands are activated by cross slip. In this way, multiple slip bands are continuously promoted, and the initial millimeter grain is gradually divided into micron small regions. With the further reduction of the size of structural units, the proportion of partials increases, which induces the formation of high-density nano twins and makes the grains further refined to nano scale. This dynamic grain refinement is one of the reasons why the super large grain cocrni can still maintain high yield strength. The step-by-step formation of high-density nano twins in the crystal is also a potential mechanism to realize ultra-high plasticity. In conclusion, based on the strengthening and toughening of conventional metals, in MEA alloys, its unique short-range and long-range order is systematically studied. For toughening, HDI strain hardening can effectively increase the uniform elongation in dual pahse MEA alloys as in heterostructure alloys. On the other hand, by in-situ TEM analysis, slip band is induced by SROs. Multiple slip bands result in ultra strengthening and toughening in ultra coase grain MEA alloys. Combined with the three parts, it proposes a new strategy of strengthening and toughening in FCC MEA alloys.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/87823
Collection非线性力学国家重点实验室
Recommended Citation
GB/T 7714
陈雪飞. 面心立方中熵合金的变形机制与强韧化机理[D]. 北京. 中国科学院大学,2021.
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