退火温度对Fe35Ni30Cr20Al10Nb5 高熵合金的组织结构和性能的影响
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Effect of Annealing Temperature on Microstructure and Properties of a High-entropy Alloy Fe35Ni30Cr20Al10Nb5
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通讯作者: 付华萌,研究员,hmfu@imr.ac.cn,研究方向为非晶合金制备技术
责任编辑: 吴岩
收稿日期: 2023-05-08 修回日期: 2023-06-05
基金资助: |
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Corresponding authors: FU Huameng, Tel:
Received: 2023-05-08 Revised: 2023-06-05
作者简介 About authors
李云飞,男,1998年生,硕士生
系统地研究了退火处理温度对低熔点高熵合金Fe35Ni30Cr20Al10Nb5(摩尔比)的组织结构和性能的影响。结果表明:随着退火温度的提高,铸态Fe35Ni30Cr20Al10Nb5合金中富Fe-Cr元素的fcc相的体积分数逐渐减少,Laves相和B2-NiAl相的体积分数逐渐增大。准静态压缩实验结果表明,铸态样品的压缩塑性变形能力良好。随着退火处理温度的提高合金的屈服强度先提高后降低,在700℃退火的样品其屈服强度最高(为1247.7 MPa),但是塑性变形量比铸态有所降低。压缩屈服强度随退火处理温度降低,可归因于基体fcc相在高温下的分解。电化学测试结果表明,这种合金的耐腐蚀能力随退火处理温度的提高而单调提高,在900℃退火的样品其腐蚀电位为-72.02 mV。
关键词:
The effect of annealing-temperature on the microstructure and properties of the low-melting point high-entropy alloy Fe35Ni30Cr20Al10Nb5 (molar ratio) were systematically investigated. The results show that with the increasing annealing-temperature, the volume fraction of Fe-Cr-rich fcc phase gradually decreases, while the volume fraction of Laves phase and B2-NiAl phase gradually increases for the as-cast alloy. The quasi-static compression test results show that the as-cast alloy has good compressive plastic deformation ability, and the yield strength of the alloy increases and then decreases as the annealing-temperature increases. The decrease in compression yield strength with the rising annealing-temperature is mainly attributed to the decomposition of the fcc phase at higher temperatures. The electrochemical test results show that the corrosion resistance of the alloy increases monotonically with the annealing-temperature, and the free-corrosion potential of the alloy annealed at 900oC is -72.02 mV.
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本文引用格式
李云飞, 王金贺, 张龙, 李正坤, 付华萌, 朱正旺, 李宏, 王爱民, 张海峰.
LI Yunfei, WANG Jinhe, ZHANG Long, LI Zhengkun, FU Huameng, ZHU Zhengwang, LI Hong, WANG Aimin, ZHANG Haifeng.
21世纪初Cantor等[1]和Ye等[2]分别提出了多组元合金和高熵合金(HEAs)概念,其主要组成元素多于或等于5种,后来又发展出三种或四种组元的中熵合金。高熵或中熵合金打破了传统合金只含有一种或两种组元的理念,提高了合金的强度、耐磨性和耐腐蚀性。Fe-Co-Ni-Cr-Al体系高熵合金具有优异的综合力学性能,是目前较为成熟的高熵合金体系[3~5]。Fe4CrNiAl0.5Ti0.5高熵合金[6]的屈服强度达到1930 MPa,同时塑性超过70%。Lu等[7]提出了共晶高熵合金的概念,这类合金的铸造流动性较好并且熔点较低,由于其特别的组织结构,合金在力学和耐腐蚀性能方面有着优异表现[8,9]。用电弧熔炼的AlCoCrFeNi2.1高熵合金其室温拉伸断裂强度超过1 GPa,拉伸塑性超过15%[10]。
这种高性能共晶高熵合金可应用在某些极端服役环境,但是所含的Co元素使其核电领域的抗辐照性能降低。研究者针对核电使役环境分析发现,Nb元素具有低中子吸收截面和优异的耐腐蚀性。Shi等[11]研究了Nb对暴露在含氧熔融Pb中的Fe-Ni-Cr-Al高熵合金的组织结构和腐蚀行为的影响,发现Nb元素的加入可大大提高Fe-Ni-Cr-Al高熵合金在熔融Pb中的耐腐蚀性。Hamdy等[8]在奥氏体不锈钢中加入Nb元素,发现其在3.5%NaCl(质量分数)溶液中的耐腐蚀性能随着Nb含量的提高而提高。这表明,无Co、兼具良好耐腐蚀性的Fe-Ni-Cr-Al-Nb高熵合金的应用前景良好。系统分析Fe/Ni比例对该体系合金结构和性能的影响发现,Fe35Ni30Cr20Al10Nb5高熵合金具有优异的综合性能,Nb元素的引入可提高其耐腐蚀性和固溶强化[12,13]。大量的研究结果表明,热处理对调控合金的组织结构和性能也有显著的影响。鉴于此,本文系统研究热处理温度对Fe35Ni30Cr20Al10Nb5高熵合金的组织结构的影响,探讨组织演变规律、评价准静态加载条件下高熵合金力学性能随热处理温度的变化规律,并揭示其在3.5%NaCl(质量分数)溶液中的耐腐蚀性能及其机理。
1 实验方法
实验用纯金属原料的纯度为99.9%(质量分数)。用真空电弧熔炼实验用高熵合金纽扣锭,其名义成分为Fe35Ni30Cr20Al10Nb5(摩尔比)。在惰性的保护气氛下进行真空电弧熔炼,纽扣锭反复熔炼5次以保证合金成分的均匀性。选择在SXL-1400C箱式电阻炉中进行退火处理,退火温度分别为700、800和900℃,保温24 h后空冷至室温。
用MTISYJ-400高精密划片切割机从退火后的高熵合金棒上切取直径为5 mm长度为7.5 mm的压缩试样。用KB30SRFA硬度仪测试样品的硬度,加载载荷为200 g,加载时间为10 s。用INSTRON 5582电子万能试验机测试样品准静态力学性能,应变速率为1 × 10-3 s-1。利用Gamry Interface 1000电化学工作站测试样品的耐腐蚀性能,参比电极为饱和甘汞电极(SCE),辅助电极为Pt片,扫描电压频率为0.333 mV/s。使用Rigaku Smartlab型X射线衍射仪(XRD)表征合金的晶体结构,扫描范围为10°~90°,扫描速率为10 (°)/min。使用附带能谱仪(EDS)的JSM-7001F场发射扫描电镜(FE-SEM)表征合金的微观形貌以及元素分布。
2 实验结果
2.1 微观组织结构
图1给出了铸态和退火后的Fe35Ni30Cr20Al10Nb5高熵合金的XRD谱。根据与标准PDF卡片的比对,铸态Fe35Ni30Cr20Al10Nb5高熵合金主要由fcc、B2-NiAl双相结构组成,XRD谱中还出现了Fe2Nb Laves相的衍射峰,PDF卡片编号为17-0908。随着退火温度的提高,B2-NiAl相衍射峰的强度逐渐提高而峰位向低角度偏移,Fe2Nb相衍射峰的数量明显增多。退火温度为800℃时fcc相的衍射峰向右偏移,表明第二相的析出使晶格常数改变,如图1b所示。随着退火温度的提高Fe2Nb相衍射峰增强,表明退火处理温度的提高促进了Fe2Nb相的生成。同时,fcc相的衍射峰随着退火处理温度的提高而减少,表明在高温下fcc相可能发生了分解。
图1
图1
不同状态Fe35Ni30Cr20Al10Nb5高熵合金的XRD谱
Fig.1
XRD spectra of Fe35Ni30Cr20Al10Nb5 high-entropy alloy in different states
图2a~2d给出了不同退火态Fe35Ni30Cr20Al10Nb5高熵合金的背散射像(BSE),图3给出了不同状态高熵合金的成分面分布(EDS)。铸态合金呈现树枝晶组织形貌,枝晶臂的宽度约为10 μm,在枝晶间形成了网状的共晶组织形貌。图中的深色枝晶基体为fcc富Fe-Cr相,枝晶间的白色区域为Fe2Nb化合物,黑色点状析出物为B2-NiAl相,各组成相在图2中分别用箭头标出。随着退火温度的提高,深色枝晶臂的尺寸逐渐增大,合金中富Fe-Cr相的溶解使其体积分数减少,而B2-NiAl相和Fe2Nb金属间化合物的体积分数呈现增大的趋势。在800℃退火后的合金其组织形貌发生粗化,在枝晶相中析出了大量的Fe2Nb化合物和B2-NiAl相。在900℃退火后枝晶间B2-NiAl相的含量大幅度提高,枝晶中仍然有大量Fe2Nb化合物和B2-NiAl相。表1列出了EDS检测到的Fe35Ni30Cr20Al10Nb5合金不同区域的化学成分。
图2
图2
不同状态Fe35Ni30Cr20Al10Nb5高熵合金的微观形貌
Fig.2
Microstructure of Fe35Ni30Cr20Al10Nb5 high-entropy alloy in different states (a) as-cast, (b) 700oC, (c) 800oC, (d) 900oC
图3
图3
不同状态Fe35Ni30Cr20Al10Nb5高熵合金的EDS面分布
Fig.3
EDS surface distribution of Fe35Ni30Cr20Al10Nb5 high entropy alloy in different states (a) as-cast, (b) 700oC, (c) 800oC, (d) 900oC
表1 不同状态Fe35Ni30Cr20Al10Nb5高熵合金的EDS结果
Table 1
Alloys | Chemical composition / %, atomic fraction | ||||
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Fe | Ni | Cr | Al | Nb | |
As-cast | 34.43 | 39.22 | 20.50 | 10.97 | 4.89 |
700oC | 34.87 | 29.35 | 20.66 | 10.34 | 4.78 |
800oC | 34.35 | 29.11 | 20.39 | 11.08 | 5.08 |
900oC | 35.29 | 28.98 | 20.62 | 10.74 | 4.37 |
2.2 力学性能
图4
图4
不同状态Fe35Ni30Cr20Al10Nb5高熵合金的准静态压缩性能
Fig.4
Quasi-static compression properties of Fe35Ni30-Cr20Al10Nb5 high-entropy alloy under different states
2.3 耐腐蚀性能
图5
图5
不同状态的Fe35Ni30Cr20Al10Nb5高熵合金的动电位极化曲线
Fig.5
Dynamic potential polarization curves of Fe35Ni30Cr20Al10Nb5 high-entropy alloy under different states
表2 不同状态Fe35Ni30Cr20Al10Nb5高熵合金在3.5%NaCl(质量分数)溶液中的腐蚀参数
Table 2
Temperature / oC | Ecorr vs. SCE / mV | Icorr/ μA·cm-2 |
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As-cast | -184.10 | 4.86 × 10-2 |
700 | -191.10 | 2.55 × 10-1 |
800 | -80.67 | 2.06 × 10-1 |
900 | -72.02 | 7.40 × 10-2 |
3 讨论
3.1 组织结构
表3 Fe、Ni、Cr、Al、Nb二元合金的混合焓[19]
Table 3
Element | Fe | Ni | Cr | Al | Nb |
---|---|---|---|---|---|
Fe (0.127 nm) | - | -2 | -1 | -11 | -16 |
Ni (0.125 nm) | -2 | - | -7 | -22 | -30 |
Cr (0.127 nm) | -1 | -7 | - | -10 | -7 |
Al (0.143 nm) | -11 | -22 | -10 | - | -18 |
Nb (0.146 nm) | -16 | -30 | -7 | -18 | - |
计算出Fe35Ni30Cr20Al10Nb5合金的δ为5.17%,低于高熵合金的平均原子尺寸差,式中ri为第i个元素的原子半径,ci为第i个元素的原子分数,
3.2 力学性能
由于晶格畸变效应会对位错运动造成阻碍,所以高熵合金的屈服强度往往比传统合金要高[20]。在压缩试验中,样品的制备状态往往会影响其屈服强度。铸态合金由于冷却速率较高,会在合金内部产生内应力。对合金进行热处理后,内应力会得到释放,屈服强度会有所降低。但是,在700℃退火的样品其屈服强度却异常地提高。图6给出了不同状态Fe35Ni30Cr20Al10Nb5合金的压缩断口形貌。由图6可见,在700℃退火样品其断口形貌发生显著的变化:不同于铸态、在800和900℃退火的样品,断口表面凹凸不平而没有形成明显的滑移痕迹。这表明,裂纹在扩展过程中受到了较大的阻碍,使合金的屈服强度明显提高。在800和900℃退火的样品中fcc基体发生分解和有序B2相和Fe2Nb金属间化合物的含量提高,使大量的Laves相连接成片。于是,在压缩过程中极易萌生裂纹且很难阻止其扩展,使合金的强度和塑性降低。
图6
图6
不同状态Fe35Ni30Cr20Al10Nb5高熵合金的准静态压缩断口形貌
Fig.6
Quasi-static compression fracture morphology of Fe35Ni30Cr20Al10Nb5 high-entropy alloy in different states (a) as-cast, (b) 700oC, (c) 800oC, (d) 900oC
3.3 耐腐蚀性能
高熵合金的耐腐蚀性能与合金中相结构有关,还与相含量有密切的关系。各种相的共同作用,决定了合金的耐腐蚀性能。图7给出了不同状态Fe35Ni30Cr20Al10Nb5高熵合金的电化学腐蚀形貌。从图7a可以看出,铸态合金表面腐蚀后出现了点蚀,是一些小尺寸孔隙而不是尺寸较大的腐蚀孔,圆形点蚀的最大直径约20 μm。在700℃退火的样品腐蚀后枝晶相中出现了很多腐蚀坑,因为枝晶相中Nb元素含量较低不能生成稳定的钝化膜,使其耐腐蚀性能较低。在800和900℃退火的样品其腐蚀表面较为平整,生成了较为严密的钝化膜。其原因是,在这两种合金的枝晶相中都析出了大量的点状Fe2Nb金属间化合物。这些金属间化合物生成一层较厚的钝化膜使基体不受腐蚀离子的侵蚀,因此其耐腐蚀性能较高。这表明,高温退火后在合金的枝晶相中析出的Fe2Nb金属间化合物在电化学腐蚀过程中生成一层致密的钝化膜降低了腐蚀溶液对基体的侵蚀,从而使其耐腐蚀性能提高。
图7
图7
不同状态Fe35Ni30Cr20Al10Nb5高熵合金的电化学腐蚀形貌
Fig.7
Electrochemical corrosion morphology of Fe35Ni30Cr20Al10Nb5 high-entropy alloy in different states: (a) as-cast, (b) 700oC, (c) 800oC, (d) 900oC
4 结论
(1) 铸态Fe35Ni30Cr20Al10Nb5高熵合金含有fcc和少量bcc双相结构的富Fe-Cr相和B2-NiAl相,并在枝晶间析出网状Fe2Nb金属间化合物。随着退火温度的提高合金中富Fe-Cr相的体积分数逐渐减少,B2-NiAl相和Fe2Nb金属间化合物的体积分数增加。
(2) 在准静态下铸态合金的塑性变形能力良好,塑性变形量接近20%,但是屈服强度随退火温度的提高先提高后降低,退火温度为700℃时其屈服强度最高(达到1247.7 MPa),是合金中Fe2Nb金属间化合物与基体共同作用的结果。退火温度为900℃时合金的屈服强度最低(为611.8 MPa),其原因是在高温下基体fcc相分解。
(3) 在900℃退火的合金在3.5%NaCl(质量分数)溶液中的耐腐蚀性能最高,腐蚀电位为-72.02 mV,其次是在800℃退火的合金、铸态合金和在700℃退火的合金。其原因是,在高温退火过程中枝晶相中析出了大量的Fe2Nb金属间化合物。富含Nb元素的枝晶相在表面生成一层致密的钝化膜,从而使合金的耐腐蚀性提高。
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