四川大学材料科学与工程学院 成都 610065
文献标识码: 分类号 TG146 文章编号 1005-3093(2016)08-0609-05
收稿日期: 2015-08-17
网络出版日期: 2016-09-28
版权声明: 2016 《材料研究学报》编辑部 《材料研究学报》编辑部
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摘要
通过XRD, SEM, EDS分析和显微硬度测试, 研究了Fe含量对CrMoVNbFex高熵合金铸态组织的相结构变化、微观组织和力学性能的影响。结果表明, 随Fe含量的增加, 合金相结构由单一的bcc结构固溶体逐步转化为bcc和σ两相结构。合金的铸态组织为典型树枝晶, Mo主要分布在枝晶内, Fe和Cr主要分布在枝晶间, 随Fe含量的增加, Nb在枝晶间的含量增加。随Fe含量的增加, 合金的组织显著细化, 而且显著提高合金的硬度, 最高硬度达到HV950。
关键词：
Abstract
The effect of Fe content on the phase constituent, microstructure and mechanical properties of CrMoVNbFex high-entropy alloys were investigated by using of XRD, SEM, EDS and microhardness tester. The results reveal that CrMoVNbFex alloys exhibited as a solid solution of single bccphase without Fe, whereas bcc solid solution+ intermetallic σ phase was observed with the increase of Fe content. The as-cast alloys show a microstructure with typical casting dendrites. It was found that Mo mainly exist in the dendrite, Fe and Cr concentrated mainly in the interdendriticspace and the Nb content in the interdendriticspace was slightly higher than that in the dendrite. With the increasing Fe content, the microstructure of alloys is refined and the hardness is enhanced significantly and the maximum hardness value of the CrMoVNbFex high-entropy alloys reaches HV950.
Keywords：
2004年叶均蔚提出了高熵合金的概念, 他指出高熵合金的组成元素n≥5, 其中每种元素的原子百分数至少大于5%, 但不得超过35%。由于元素种类较多并以等摩尔比存在于合金中, 从而使合金的混合熵(混乱度)很高, 结果获得fcc或bcc单相固溶体结构[1-3]。独特的结构使高熵合金拥有很多传统合金所不具有的优异特性, 如高硬度、高加工硬化、耐高温软化、耐高温氧化、耐腐蚀、高电阻率等特性[4-7], 因此在要求高硬度且耐磨耐温耐蚀的工具、模具、化学领域、船舰的耐蚀高强度材料、涡轮叶片及耐热材料等领域具有很大的应用潜力[8]。近年来, 高熵合金的研究取得了较大的进展, 新的合金系相继提出, 如单相fcc结构的CoCrFeNiCu, CoCrFeNiMnCu, CoCrFeNiCuV等, 单相bcc结构的CoCrFeNiAl, MnCrFeNiCuAl以及具有fcc+bcc复相结构的CoCrFeNiCuAl0.8, CoCrFeNiCuAlSi等[9]。研究发现, fcc结构的高熵合金强度较低, 硬度不超过300HV, 但塑性高, bcc结构合金的硬度可达到600HV以上, 但塑性较低, 脆性大[10-12]。对AlxCoCrFeNi合金的研究发现, 随Al含量的增加, 合金相由单相的fcc结构固溶体转变为fcc+bcc复相结构和单相bcc结构。合金的硬度由fcc结构的116HV提高到bcc结构的509HV[11]。O.N.Senkov采用高熔点元素合成了Ta25Nb25W25Mo25和Ta20Nb20W20Mo20V20 bcc结构的合金, 具有超高的硬度和良好的高温强度[12]。研究发现, 通过不同合金含量的变化来控制组织结构, 并利用单相固溶体基体上析出元素偏聚, 少量的金属间化合物来调控性能[13-15]。CoCrCuFeNiTix合金在Ti含量为0 mol时, CoCrCuFeNiTix合金表现出了良好的塑性性能; Ti含量增加时, 晶界析出金属间化合物, 使合金的强度提高, 但塑性明显下降[13]。研究发现, 随着Fe含量的增加, AlCoCrFexMo0.5Ni合金中的bcc相逐渐增多, 而σ相(CoCr相)逐渐减少, 合金的硬度值降低, 耐磨性能也随之下降[14]。对CrxCuFe2Mo0.5Nb0.5Ni2合金研究发现[15], Cr含量的增加, 有利于Fe、Cr和Ni元素的分布相对均匀, 合金硬度随Cr含量的增加而逐渐增加。这些结果表明, 不同合金体系中, 合金化元素和其他组元之间的相互作用不同, 进而对其组织和性能造成不同的影响。
从热力学角度来看, 由于各种合金元素混合后的熵, 焓的改变以及各原子尺寸的差异, 导致合金相结构形成规律的复杂性, 这也给相应的性能控制带来一定的不确定性。一些文献也给出了相应高熵合金相形成的判据, 用来指导合金设计和性能研究[3, 16, 17]。本文在具有bcc结构的CrMoVNb合金基础上探讨Fe含量的变化对CrMoVNbFex (x=0, 0.2, 0.4, 0.6, 0.8, 1.0)五元高熵合金的相结构, 微观组织及力学性能的影响, 为具有高硬度的bcc结构的高熵合金的相组成及力学性能有进一步的了解。
合金采用高纯(纯度大于99.9%)Cr, Mo, V, Nb和Fe作为原料, 在氩气保护下, 利用WS-4型非自耗真空熔炼炉熔炼不同Fe含量的CrMoVNbFex(x=0, 0.2, 0.4, 0.6, 0.8, 1.0)合金钮扣锭, 每个合金锭重复翻转熔炼6次以保证成分均匀。用DX-2000型X射线衍射仪对合金的相结构进行分析, 靶材选用Cu靶(λ=0.1542 nm), 工作电压为40 kV和25 mA, 扫描速率为0.06°/s。试样进行研磨、抛光, 再用王水腐蚀, 采用JSM-7500F型扫描电镜、能谱分析仪对合金的微区进行成分分析, 工作电压为20 KV。试样经抛光后进行显微硬度测试, 设备为DHV-1000Z型显微硬度计, 负荷选用1 kg, 加载时间15 s, 测量8个点求平均值。
图1为不同Fe含量的CrMoVNbFex合金的XRD图谱。由图所示结果可知, 当x=0时, 该合金的X射线衍射谱在2θ=40.58°、59.24°、73.82°、88.76°的位置存在4个清晰的衍射峰, 通过分析, 认为4个衍射峰对应于单相bcc结构, 这表明CrMoVNb四元合金为具有单相bcc结构的固溶体。当合金中Fe含量为0.2 mol时, 即x=0.2, 合金仍呈现单相bcc结构固溶体, 而当Fe的含量继续增加, 在XRD图谱上出现了新的衍射峰, 对衍射峰进行分析认为, 该新的衍射峰为Fe0.55Nb0.67相的衍射峰, 说明Fe的含量超过x=0.2后, CrMoVNbFex合金中出现了Fe0.55Nb0.67的σ相, 且随Fe含量的增加, σ相的衍射峰逐渐增多增强, 说明生成的σ相逐渐增多。由此可知, CrMoVNb合金中, 随Fe的加入, 使合金由bcc结构的单相固溶体转变为bcc固溶体和σ两相组成。
图1 铸态CrMoVNbFex(x=0, 0.2, 0.4, 0.6, 0.8, 1.0)高熵合金的X射线衍射谱
Fig.1 X-ray diffraction spectra of CrMoNbVFex high-entropy alloys
根据之前相关文献的报道, 热力学参数△Smix、δ、△Hmix和VEC(空位电子浓度)用于预判合金是否形成固溶体以及固溶体类型 [17]。定义:
式中,
表1 CrMoVNbFex高熵合金的△Smix、δ、△Hmix和VEC值
Table 1 The calculated parameters△Smix、δ、△Hmix and VEC of CrMoVNbFex alloys
| X | △Smix | δ | △Hmix | VEC |
|---|---|---|---|---|
| 0 | 11.53 | 4.85 | -4.00 | 5.50 |
| 0.2 | 12.58 | 5.06 | -4.53 | 5.62 |
| 0.4 | 13.01 | 5.17 | -4.96 | 5.73 |
| 0.6 | 13.24 | 5.22 | -5.29 | 5.83 |
| 0.8 | 13.35 | 5.32 | -5.56 | 5.92 |
| 1.0 | 13.38 | 5.42 | -6.72 | 6.00 |
根据Sheng Guo等人的研究表明[3, 17], 当合金的δ, △Hmix和△Smix满足0<δ<8.5, -22≤△Hmix≤7 kJ/mol, 11≤△Smix≤19.5 J/(Kmol)则形成单相固溶体; 而当VEC<6.78时, 合金形成单一BCC结构, 当VEC≥8.0时, 合金为单一fcc结构, 介于6.78和8.0之间则为bcc+fcc结构。本实验中, 当合金不含Fe时△Smix=11.53 J/(Kmol), δ=4.85, VEC=5.50, △Hmix=-4.00 kJ/mol, 合金满足形成固溶体的条件, 如图1的XRD图谱所示。但当Fe含量增加时, 合金的参数δ, △Hmix和△Smix也随之增加, 虽然也满足文献3和17提出的形成固溶体的条件, 但在δ≥5和△Hmix≤-5.00 kJ/mol的情况下, 将从固溶体中析出相应的金属间化合物, 形成固溶体与金属间化合物相的混合组织。另外, 根据张勇[9]提出的高熵合金的Ω-δ相形成规律, 在Fe含量增加后也会形成部分金属间化合物。也由表1中的δ和△Hmix的结果可知, 本实验中在Fe的含量增加到0.4 mol时, 除了bcc固溶体相以外, 还会有部分金属间化合物形成。根据表2, 由于Fe与Nb有最负的结合焓(-16 kJ/mol), 所以Fe与Nb容易结合, 而形成Fe0.55Nb0.67的金属间化合物。
表2 各元素之间的结合焓
Table 2 The values of
| Element | Cr | Mo | V | Nb | Fe |
|---|---|---|---|---|---|
| Cr | 0 | -2 | -7 | -1 | |
| Mo | 0 | -6 | -2 | ||
| V | -1 | -7 | |||
| Nb | -16 |
图2为铸态CrMoVNbFex高熵合金显微组织的SEM-BSE像。由图可见, 合金铸态组织为典型的树枝晶结构, 白亮区域为枝晶部分(A), 灰暗区域为枝晶间(B)。而且随Fe含量的增加, 铸态的组织逐渐细化, 枝晶间隙也明显细化, 表明Fe在CrMoVNbFex合金中可起到细化铸态组织的作用。
图2 铸态CrMoVNbFex高熵合金的背散射电子像
Fig.2 SEM backscattered electron images of the as-cast CrMoVNbFex high-entropy alloys (a) x=0; (b) x=0.2; (c) x=0.4; (d) x=0.6; (e) x=0.8; (f) x=1.0
表3为CrMoVNbFex高熵合金不同区域(如图2所标注的A, B)的能谱测试结果。结果表明, 不含Fe的CrMoVNb四元合金中, 高熔点元素Mo在枝晶间的含量较低, 它主要分布在枝晶内, 而低熔点的Cr在枝晶间的含量明显高于枝晶内, 表明Cr在凝固过程中向枝晶间富集。Nb和V在枝晶内和枝晶间的含量基本一致, 表明在枝晶内和枝晶间呈均匀分布, 在合金凝固过程中没有发生富集。随着Fe含量的增加, Mo元素的分布没有明显变化, Fe与Cr一样在枝晶间的含量明显高于枝晶内, 因此合金凝固后Fe主要分布在枝晶间。实验结果还表明, Fe的加入使V在枝晶间的含量稍低于枝晶内, 而Nb在枝晶间的含量则高于枝晶内。造成各元素分布不均匀的现象是因为, 合金从高温液态开始凝固过程中, 高熔点的Mo和Nb以固溶体形式率先形核并以树枝晶形式长大(即亮白色区域A), 与此同时, 低熔点的Cr和Fe则大部分被排挤到剩余液相中去, 随着温度的继续降低, 富含Cr和Fe的枝晶间区域开始凝固(即灰色区域)。由于Fe在枝晶间富集, 根据表2可知, Fe-Nb的结合焓为-16 kJ/mol, 所以Fe倾向于同枝晶间的Nb结合, 析出Fe0.55Nb0.67的σ相, 这也使得枝晶内的Nb含量较高。随着合金中Fe含量的增加, σ相也增多。
表3 铸态CrMoVNbFex (x=0, 0.2, 0.4, 0.6, 0.8, 1)合金系中不同显微组织区域的的化学成分
Table 3 Compositions of different microstructure areas in as-cast CrMoVNbFex (x=0, 0.2, 0.4, 0.6, 0.8, 1) high-entropy alloys
| x | Region In fig .2 | Atom fraction/% | ||||
|---|---|---|---|---|---|---|
| Fe | Cr | Mo | V | Nb | ||
| Normal | 0 | 25.00 | 25.00 | 25.00 | 25.00 | |
| 0 | A | 0 | 23.68 | 26.49 | 23.42 | 26.40 |
| B | 0 | 40.71 | 9.37 | 23.41 | 26.51 | |
| Normal | 4.80 | 23.80 | 23.80 | 23.80 | 23.80 | |
| 0.2 | A | 1.74 | 20.67 | 29.82 | 21.63 | 26.13 |
| B | 14.29 | 35.78 | 3.17 | 16.70 | 30.05 | |
| Normal | 9.10 | 22.80 | 22.70 | 22.70 | 22.70 | |
| 0.4 | A | 3.46 | 23.40 | 24.71 | 22.34 | 26.09 |
| B | 15.34 | 34.82 | 3.90 | 16.02 | 29.84 | |
| Normal | 13.04 | 21.74 | 21.74 | 21.74 | 21.74 | |
| 0.6 | A | 4.75 | 26.70 | 19.75 | 23.67 | 25.13 |
| B | 16.12 | 34.31 | 4.37 | 15.65 | 29.55 | |
| Normal | 16.67 | 20.83 | 20.83 | 20.83 | 20.83 | |
| 0.8 | A | 7.47 | 23.27 | 23.49 | 23.73 | 22.04 |
| B | 21.48 | 28.03 | 5.93 | 15.74 | 28.81 | |
| Normal | 20.00 | 20.00 | 20.00 | 20.00 | 20.00 | |
| 1.0 | A | 10.10 | 23.98 | 22.83 | 23.22 | 19.88 |
| B | 25.87 | 25.38 | 6.39 | 15.05 | 27.31 | |
为了研究不同Fe含量对合金显微硬度的影响, 对合金的硬度进行了测试, 结果如图3所示。由图可以看出, 合金中未加入Fe时, 合金的硬度值为HV641; 随Fe含量增加, 合金硬度值逐渐升高, 最高达到HV950, 因此Fe的添加使合金的硬度得到显著提高。合金硬度的提高可能与枝晶内固溶的Fe含量增加产生的固溶强化, σ相的析出以及组织细化造成的强化效应增加有关。
图3 不同Fe含量的CrMoVNbFex合金的硬度
Fig.3 Microhardness of CrMoVNbFex high-entropy alloys at different Fe contents
由以上实验结果可知, 在bcc结构的CrMoVNb四元高熵合金中加入Fe, 可以显著细化合金的铸态组织。随着Fe含量的增加, 合金仍能保持bcc固溶体相结构, 同时有金属间化合物σ相在枝晶间析出, 结果使合金的硬度有显著提高, 最高可达到HV950。
1. 在四元bcc晶体结构的CrMoVNb合金中, 随着Fe含量的增加, CrMoVNbFex高熵合金仍能保持bcc固溶体相结构, 但随Fe含量的增加, 在bcc固溶体中析出金属间化合物σ相。
2. 合金的铸态组织为典型的树枝晶状, Cr和Fe主要富集在枝晶间, 随Fe含量的增加, V在枝晶间的含量稍低于枝晶内, 而Nb在枝晶间的富集程度较高, Mo的分布在枝晶内和枝晶间基本均匀。而且Fe的加入使合金的铸态组织显著细化。
3. Fe可显著提高合金的硬度。硬度随Fe含量的增加而增加, 最高值达到HV950。
The authors have declared that no competing interests exist.
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Tensile properties of the Cr-Fe-Ni-Mn non-equiatomicmulticomponent alloys with different Cr contents ,
A series of Fe 40 Mn 28 Ni 32026102x Cr x (x02=024, 12, 18, 24 (at.%)) multicomponent alloys was prepared by vacuum arc melting. The Fe 40 Mn 28 Ni 28 Cr 4 , Fe 40 Mn 28 Ni 20 Cr 12 and Fe 40 Mn 28 Ni 14 Cr 18 alloys were ductile single phase fcc solid solutions. The Fe 40 Mn 28 Ni 8 Cr 24 alloy had intermetallic sigma phase matrix and was extremely brittle after homogenization. The tensile properties of the Fe 40 Mn 28 Ni 28 Cr 4 , Fe 40 Mn 28 Ni 20 Cr 12 and Fe 40 Mn 28 Ni 14 Cr 18 solid solution alloys were examined in recrystallized condition with average grain size of ~021002μm. The yield strength increased from 21002MPa of the Fe 40 Mn 28 Ni 28 Cr 4 alloy to 31002MPa of the Fe 40 Mn 28 Ni 14 Cr 18 alloy. The elongation to fracture of the alloys decreased from 71% to 54%, respectively. Solid solution strengthening by the constitutive elements of the alloys was calculated using Labush approach. Strong solid solution strengthening by Cr was predicted. Gypen and Deruyttere approach was used to estimate solid solution strengthening of the Fe 40 Mn 28 Ni 32026102x Cr x alloys. Good correlation between predicted solid solution strengthening and the experimental yield strength values was found.
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| [11] |
To have a further investigation of their physical properties such as electrical, magnetic, and thermal properties, the microstructure and mechanical property of Al x CoCrFeNi alloys have to have a detailed and systematic study. We present the effects of Al content on microstructure and mechanical property of as-cast, -homogenized, and -deformed Al x CoCrFeNi alloys in this study. It shows that single FCC and single BCC solutions, and duplex FCC–BCC are principal phases in these alloys. The x intervals for duplex FCC–BCC of different states of these alloys are investigated. The spinodal decomposition of less Al–Ni and more Al–Ni phases is the major reaction during homogenization of the alloys. The morphology of the less and more Al–Ni phases is also discussed. There is no stress-induced phase transformation during 50%-rolling deformation, and the main strengthening mechanism is work hardening. The hardening ability of FCC was about twice that of BCC in this alloy system.
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| [12] |
Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20refractory high entropy alloys,
Two refractory high entropy alloys with compositions near Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20, were produced by vacuum arc-melting. Despite containing many constituents, both alloys had a single-phase body-centered cubic (BCC) structure that remained not only stable after exposure to 1400 degrees C, but also disordered, as confirmed by the absence of superlattice reflections in neutron diffraction data. Compressive flow properties and microstructure development of these alloys were determined from room temperature up to 1600 degrees C. Limited compressive plasticity and quasi-cleavage fracture at room temperature suggest that the ductile-to-brittle transition for these alloys occurs above room temperature. At 600 degrees C and above, both alloys showed extensive compressive plastic strain. The yield stress of both alloys dropped by 30-40% between room temperature and 600 degrees C, but was relatively insensitive to temperature above 600 degrees C, comparing favorably with conventional superalloys. (C) 2011 Elsevier Ltd. All rights reserved.
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| [13] |
QiaoY, Effect of Ti on the microstructure and properties of CoCrCuFeNiTix high-entropy alloys , |
| [14] |
Effect of iron content on wear behavior of AlCoCrFexMo0.5Ni high-entropy alloys,
A new high-entropy alloy system, AlCoCrFeMoNi, was designed based on the AlCoCrCuFeNi, by replacing Cu with Mo to improve strength and thermal stability. The effect of iron content on hardness and wear behavior of AlCoCrFe x Mo 0.5 Ni alloys is related to the microstructural change. As the iron content is increased, the microstructure changes from “dendritic” for Fe0.6 and Fe1.0 alloys to “polygrain” for Fe1.5 and Fe2.0 alloys. The two constituent solution phases are BCC and σ, but the volume fraction of the BCC phase increases with the iron content. The hardness declines correspondingly because the σ phase is harder than the BCC phase. The wear resistance also declines as the Fe content increases, but the wear resistance of Fe2.0 is much lower than that of Fe1.5, which result is inconsistent with their similar hardness levels. Composition analysis of the worn surface and worn debris indicates that, the major wear mechanism of all these HE alloys is abrasion. The oxidation test at the pin/disk interface flash temperature, 500°C, indicates that the oxidation rate of Fe2.0 markedly exceeds that of Fe1.5, indicating more oxides abrade the surface resulting lower wear resistance.
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| [15] |
Microstructure and Mechanical Performance ofCrxCuFe2Mo0.5Nb0.5Ni2 high-entropy alloys, CrxCuFe2-Mo0.5Nb0.5Ni2高熵合金的微观组织与力学性能,
采用电弧熔炼工艺制备了CrxCuFe2Mo0.5Nb0.5Ni2（x=0,0.5,1.0,2.0,摩尔比）高熵合金,采用X射线衍射分析（XRD）、扫描电镜（SEM）和显微硬度计对合金的物相结构、微观组织形貌、元素分布和硬度进行了分析。结果表明,合金物相主要由面心立方固溶体相（FCC）、体心立方固溶体相（BCC）和密排六方固溶体相（HCP）组成。Cr含量的增加,有利于BCC相的形成。合金组织主要呈树枝晶和枝晶间结构组成。合金中Nb、Mo和Cu元素分别偏聚于枝晶和枝晶间区域,Fe、Cr和Ni元素的分布相对均匀。合金硬度随Cr含量的增加而逐渐增加,但增幅较小。
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| [16] |
Ting TingZuo, Zhi Tang, Michael C.Gao, Karin. ADahmen, Peter K. liaw, Zhao ping Lu, Microstructures and properties of high-entropy alloys , |
| [17] |
C.T.Liu, Phase stability in high entropy alloys: ‘Formation of solid-solution phase or amorphous phase,
The alloy design for equiatomic multi-component alloys was rationalized by statistically analyzing the atomic size difference,mixing enthalpy,mixing entropy,electronegativity,valence electron concentration among constituent elements in solid solutions forming high entropy alloys and amorphous alloys.Solid solution phases form and only form when the requirements of the atomic size difference,mixing enthalpy and mixing entropy are all met.The most significant difference between the solid solution forming high entropy alloys and bulk metallic glasses lies in the atomic size difference.These rules provide valuable guidance for the future development of high entropy alloys and bulk metallic glasses.
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