DOI: 10.11817/j.ysxb.1004.0609.2021-37905
TiC对Fe43Ni35Co22中熵合金微观组织与性能的影响
李睿锴1,陈维平1,王 浩1,陈 强2,曾大海1,付志强1
(1. 华南理工大学,广东省金属新材料制备与成形重点实验室,广州 510640;
2. 西南技术工程研究所,重庆400039)
摘 要:本文系统研究了TiC含量对Fe43Ni35Co22中熵合金微观组织、力学性能以及磁性能的影响。添加5%和10%(摩尔分数)Ti至中熵合金Fe43Ni35Co22,通过原位自生反应形成TiC/Fe43Ni35Co22合金块体,制备方法为“机械合金化(MA)+放电等离子烧结(SPS)”。研究结果表明:经40h球磨后, Fe43Ni35Co22粉末相的组成为FCC主相+少量BCC,两种TiC/Fe43Ni35Co22粉末的相组成为BCC主相+FCC相。经SPS烧结后,Fe43Ni35Co22块体为单相FCC和少量的杂质;两种TiC/Fe43Ni35Co22块体均由FCC+TiC两相组成,其中FCC相呈现“微米晶+超细晶”构成的多尺度结构,且随着TiC含量的增加,超细晶区增多。性能上,TiC的添加大幅提高了Fe43Ni35Co22的压缩屈服强度和矫顽力,同时也导致了材料的塑性和饱和磁化强度的降低。Ti添加量为5%的TiC/Fe43Ni35Co22综合性能最优异,其压缩屈服强度为1352 MPa,压缩断裂应变为24.5%,矫顽力为12.4 Oe,饱和磁化强度为138.7 emu/g。
关键词:中熵合金;机械合金化;放电等离子烧结;力学性能;软磁性能
文章编号:1004-0609(2021)-xx-- 中图分类号:TG113 文献标志码:A
引文格式:李睿锴, 陈维平, 王 浩, 等. TiC对Fe43Ni35Co22中熵合金微观组织与性能的影响[J]. 中国有色金属学报, 2021, 31(x): xxxx-xxxx. DOI: 10.11817/j.ysxb.1004.0609.2021-37905
LI Rui-kai, CHEN Wei-ping, WANG Hao, et al. Influence of TiC on microstructure and properties of Fe43Ni35Co22medium-entropy alloy[J]. The Chinese Journal of Nonferrous Metals, 2021, 31(x): xxxx-xxxx. DOI: 10.11817/j.ysxb.1004.0609.2021-37905
随着科学技术的飞速发展,对材料的性能提出了更高的要求。例如,在混合动力汽车或纯电动汽车中,为了提高系统的工作效率,需要用到微型、高效的感应电动机以及对其充电的功率转换器[1]。而电机的微型化与效率的提高,关键在于电机中的软磁材料。一方面,软磁材料需要更高的饱和磁化强度,以减少电机中软磁材料的体积;另一方面,软磁材料需要更高的强度,以抵抗高频带来的更大的扭转力。此外,一定的塑性,既可以保证材料良好的加工成型性能,降低生产成本,也是材料服役性能要求。目前,现有的软磁材料在不同方面存在瓶颈,如:非晶合金较脆,晶化温度较低,使用温度受限[2];FeCo合金也较脆,且合金中Co含量高,价格昂贵[3];大块纳米晶软磁材料加工工艺复杂且热稳定性差[4]。因此,亟需开发新型软磁材料。
20 世纪 90 年代,YEH 等[5]提出了一个全新的合金设计理念—— 多主元高熵合金,极大地丰富了金属材料的设计策略。与传统合金的设计理念不同,高熵合金一般由4种及以上的主要元素组成,且每种元素的原子百分含量为 5%~35%。较高的混合熵抑制了金属间化合物的形成,使高熵合金形成简单的固溶体相[6-9]。同时,高熵合金具有许多优异的性能,如:高的强度和硬度、良好的抗辐射性、优异的低温断裂韧性、耐蚀性以及抗高温软化和特殊的磁性能[10-15],使得材料学家对其进行了广泛的研究。而关于高熵合金磁性材料的研究,主要集中在含Fe、Co、Ni的合金系统,力求兼具良好的力学性能与软磁性能[16-20]。作者前期[17]采用MA+SPS工艺制备TiC/Fe30Ni30Co29Mn5.5Cu5.5高熵合金,通过添加Ti元素,实现了TiC的原位自生,并在FCC基体中成功构建了多尺度结构,实现了优异力学性能和良好软磁性能的组合。此外,由于含Fe、Co、Ni的高熵合金在高温条件下有着良好的热稳定性,因而有望用作高温软磁材料[21-23]。
众所周知,铁磁性元素Fe,Co,Ni占比较大的高熵合金,饱和磁化强度通常会比较高[23, 24],并且根据FENG等的研究[25],Fe和Co可以更好地增强材料的磁饱和强度。GUO等研究发现[27],降低Ni的含量,会导致高熵合金中出现BCC相,将降低材料的塑性。此外,考虑到钴的价格昂贵,故本文适当提高Fe的含量、降低Co的含量,设计出具有单相FCC结构的Fe43Ni35Co22中熵合金。然后,通过添加Ti元素至该合金体系,采用MA+SPS的制备工艺,在Fe43Ni35Co22中原位生成TiC,并使FCC相构建多尺度结构,实现强韧化。通过改变Ti元素的添加量(0、5%、10%),实现TiC含量的调控,系统地研究TiC对Fe43Ni35Co22中熵合金微观组织、力学性能以及软磁性能的影响。
1 实验
本实验采用质量分数≥99.7%、粒度≤45μm的Fe、Co、Ni、Ti四种单质金属粉末为原料,按摩尔配比配制 (Fe43Ni35Co22)1-xTix(x=0、5、10)原始粉末,再将原始粉末置于混粉机混合5 h;之后,再将混合均匀的粉末置于不锈钢球磨罐中球磨40h,球料比为10:1,转速为300 r/min,使用环己烷作过程控制剂,并在球磨罐中冲入氩气进行保护。整个球磨过程在QM-3SP4型行星式球磨机上进行,并对球磨0h、20h、40 h的粉末进行XRD测试。最后,将球磨40 h的粉末置于真空干燥箱干燥,并使用SPS(Dr. Sinter 825, Sumitomo Coal Mining Co., Ltd., Japan)将干燥好的粉末烧结成d 20 mm×10 mm的块体材料。烧结过程中程序设置如下:首先,加热 1 min 将温度由室温升至100 ℃;随后,加热 8 min 将温度升高到 900 ℃;最后,再用 2 min 加热到 1000 ℃。并且在 1000 ℃下保温 10 min,烧结过程中保持恒压,压强为30 MPa。为方便阅读,下面将(Fe43Ni35Co22)1-x Tix(x=0、5%、10%)依次简写为HEA1、HEA2、HEA3。
本研究采用X 射线衍射仪(XRD,Bruker-D8 Advance, Germany)对不同球磨时间的粉末及烧结后的块体进行物相分析;采用扫描电子显微镜(SEM, NOVA NANOSEM 430, USA) 对合金块体的微观组织进行观察及成分分析;采用透射电镜(200 kV TEM, JEM-2100, JEOL, Tokyo, Japan)进一步的分析高熵合金的微观组织特征、晶体结构及成分;采用万能试验机(AG-100kNX, Shimadzu Corporation, Japan)进行压缩性能测试,样品尺寸为d 3 mm×4.5 mm,压头下压速率为 0.27 mm/min;采用振动样品磁强计(VSM,Lake Shore 7407, USA)测试材料的室温磁滞回线,试样尺寸为d 3 mm×1 mm。
2 结果与分析
2.1 XRD图谱分析
图1所示为不同球磨时间(Fe43Ni35Co22)1-xTix粉末的XRD谱。图 1(a)为HEA1粉末的 XRD谱。0 h时,各个元素的衍射峰清晰可见。当球磨时间增加到20 h,各元素衍射峰强度急剧降低,表明材料发生了合金化[28]。球磨时间增加至40 h时,衍射峰强度进一步降低。此时,粉末由FCC主相+少量BCC相组成。
图 1(b)、(c)所示为HEA2与HEA3粉末的XRD谱。与HEA1相比,其合金化行为略有不同。0 h时,各个元素的衍射峰清晰可见。随着球磨时间增加到20 h,衍射峰强度急剧降低,且衍射峰略微宽化,表明材料发生合金化和晶粒细化。当球磨时间增加至40 h,HEA2和HEA3衍射峰均明显宽化,表明材料的晶粒进一步细化[29];同时,各元素的衍射峰消失,表明此时的材料,已完全合金化,合金粉末均由FCC相和BCC主相组成。为了更好的表征球磨40 h之后粉末的相组成,对球磨40 h的合金粉末进行XRD慢扫,图谱如图1(d)所示,各个合金粉末中的FCC相与BCC相的特征峰清晰可见。
图1 不同球磨时间(Fe43Ni35Co22)1-x Tix粉末的XRD谱
Fig. 1 XRD patterns of (Fe43Ni35Co22)1-xTix powders with different milling times
图2所示为1000 ℃烧结后三种合金块体的XRD谱。可以观察到,烧结后,三种合金中BCC相的衍射峰均消失。表明在SPS烧结过程中发生了相变。这是因为粉末的机械合金化是一个非平衡状态下的工艺过程,球磨后的粉末是一个亚稳态的过饱和固溶体;此外,在球磨的过程中产生了大量的纳米晶,导致晶界体积分数增大,而在晶界处储存了大量的畸变能,降低了相变所需的自由能。最终,高熵合金粉末在 SPS 烧结成形的过程中,在高温以及磁场、电场、等离子体场等多场效应的耦合作用下,转变为相对更稳定的FCC相。同时,在HEA2和HEA3中,还观察到了TiC的衍射峰,表明形成了TiC相。TiC主要是SPS烧结过程中原位自生形成的,其化学反应方程为Ti+C→TiC。其中,C主要来源于干燥后残留的过程控制剂环己烷的分解。Ti和C原子对的混合焓为-109 kJ/mol[30],在SPS烧结过程中,两者很容易发生原位自生成反应得到TiC。在HEA3中的TiC的峰强比增大,表明随着Ti含量的增加,HEA3中形成了更多的TiC。最终,HEA1为单相FCC结构,HEA2与HEA3的相组成相同,均为FCC+TiC,但HEA3中TiC的含量比HEA2高。
图2 (Fe43Ni35Co22)1-xTix块体的XRD谱
Fig. 2 XRD patterns of bulk (Fe43Ni35Co22)1-xTix
2.2 块体的相组成和微观组织
图3所示为三种高熵合金的背散射电子(BSE)照片。图3(a)~(b)所示为HEA1,可以看出,HEA1存在灰色区域(A),深色颗粒(B)以及白色颗粒(C),并且FCC基体与第二相颗粒的结合处存在孔洞。图3(c)~(d)所示为HEA2,材料的组织由白色区域(D)与灰色区域(E)组成,且存在少量的孔洞。图3(e)~(f)所示为HEA3,其组织与HEA2相似,由白色区域(F)与灰色区域(G)组成,存在较多的微观缺陷;但在HEA3中,灰色区域的占比明显提高。为了进一步分析三个材料中各个区域所表示的相,对图3中各个区域做能谱分析(EDS),每一块区域做5次能谱分析,取平均值,结果列于表1。
根据能谱分析结果, A区域含Fe、Co、Ni三种元素,其组成成分接近HEA1的名义成分;B区域含有氧和铁两种元素;C区域的主要元素为钨。由此推断,在HEA1中,灰色区域A为FCC基体,深色颗粒B与白色颗粒C,分别对应铁的氧化物和WC,是材料制备过程中引入的杂质。其中,铁的氧化物可能来源于粉末在空气中混粉时原始粉末的氧化[31];WC则是来自碳化钨磨球,在FeCoNi三元合金中也有出现[32]。对于HEA2,白色区域(D)与灰色区域(F)的Ti元素分布存在明显的差别;其中区域D的Ti元素含量较少,仅为1.16%,而区域(E)的Ti元素的含量较多,达到6.26%。在HEA3中,元素的分布与HEA2有相似规律。对于HEA2与HEA3,其相组成有待进一步的分析。
图3 (Fe43Ni35Co22)1-xTix的BSE图像
Fig. 3 BSE images of (Fe43Ni35Co22)1-xTix
表1 (Fe43Ni35Co22)1-xTix的EDS/BSE分析结果
Table 1 Chemical compositions of bulk (Fe43Ni35Co22)1-xTix HEAs analyzed by EDS/BSE (mole fraction, %)
图4所示为HEA1的TEM明场像,以及晶粒1沿[011]轴的衍射斑点。可以确定晶粒1的晶体结构为FCC。HEA1的EDS/TEM结果列于表2,晶粒1中各元素都接近HEA1的名义成分,晶粒2主要含铁和氧,可以确定晶粒1为FCC基体,晶粒2为铁的氧化物,结合HEA1的SEM图像分析结果,可知HEA1包含FCC基体相、少量铁的氧化物和极微量的WC。其中,铁的氧化物与WC均为污染物。因此,HEA1的相组成可以描述为FCC相+少量杂质(铁的氧化物+WC)。
图5(a)为HEA3的明场像,以及晶粒3沿[011] 轴的衍射斑点,可知晶粒3为FCC结构;同时,可以观察到,在HEA3中形成了由超细晶区+微米晶区构成的多尺度结构。结合表2中 HEA3的EDS/TEM分析结果,可得出微米晶区为富Fe、Co、Ni的FCC相。图5(b)为HEA3超细晶区的明场像,结合EDS/TEM结果,可以判断细晶区中存在两个物相,即富Fe、Co、Ni的FCC相与TiC相。HEA3的相组成为FCC相+TiC,其中FCC相表现出多尺度结构。HEA2与HEA3有着相似的相组成和微观组织。由此推断,HEA2的相组成亦为FCC相+ TiC,且FCC为多尺度微观结构;与HEA3的区别是HEA2中的TiC更少,因此在烧结过程中阻碍晶粒长大的效果相对较弱,从而微米晶FCC相相对较多,并且超细晶的尺寸可能更大一点。
图4 HEA1的TEM照片及晶粒1的衍射斑点
Fig. 4 Bright-feld (BF) TEM images of HEA1 and SAED pattern corresponding to grain 1 (FCC)
图6所示为HEA3晶粒中微米晶区和超细晶区中晶粒尺寸分布的统计直方图。根据统计结果,超细晶区晶粒的平均直径为123 nm,微米晶区晶粒的平均直径1.65 μm。与HEA1相比,HEA3的晶粒明显细化,并且出现了多尺度的结构。这主要是因为HEA3中原位生成了TiC。在烧结过程中,粉末颗粒相互接触,在粉末颗粒内部,由于TiC的存在,内部纳米晶粒的长大受阻,最终得到超细晶区。粉末颗粒之间,由于接触表面小,承受的电流过大而形成局部高温,促使晶粒快速长大,最终形成微米晶区。
表2 (Fe43Ni35Co22)1-x Tix的EDS/TEM分析结果
Table 2 Chemical compositions of bulk (Fe43Ni35Co22)1-x Tix HEAs analyzed by EDS/TEM
图5 HEA3的TEM照片及晶粒3的衍射斑点
Fig. 5 Bright-field (BF) TEM images of HEA3
图6 HEA3的晶粒粒径分布统计直方图
Fig. 6 Statistical grain diameter distribution histogram of bulk HEA3
2.3 高熵合金的力学性能与磁性能
图7(a)为(Fe43Ni35Co22)1-xTix在室温下的压缩工程应力-应变曲线。从图中可以看出,HEA1塑性最好,但屈服强度仅为420 MPa。随着TiC含量的增加,材料的塑性降低,但屈服强度得到大幅提升。当Ti含量为5%时,材料的屈服强度提升221%,达到1352 MPa;当Ti含量为10%时,材料的屈服强度提升319%,高达1762 MPa。这主要归因Ti的添加使得材料中原位生成TiC颗粒,细小的TiC颗粒弥散分布于基体中,变形过程中会阻碍位错的运动,有着弥散强化的效果;同时,在烧结过程中,细小的TiC颗粒阻碍了晶粒的长大,使得材料出现大量的超细晶,晶粒的细化增加了晶界的体积分数,使得位错运动阻力增加,强度提升(晶界强化)。随着Ti含量的添加,原位生成TiC的量增加,对基体的强化效果更加显著。此外,由于材料同时存在微米晶区和超细晶区,会使得材料获得背应力强化,进一步提高材料的强度,这种强化现象在多尺度(异构)材料中都有报道[33, 34]。
图7(b)为(Fe43Ni35Co22)1-xTix室温下的磁滞回线。由磁滞回线可知,HEA1具有最高的饱和磁化强度以及最低的矫顽力,即HEA1的软磁性能最好。随着TiC含量的增加,材料的饱和磁化强度略有下降,同时材料的矫顽力急剧增加,导致软磁性能恶化。添加5%Ti使得材料的矫顽力提升327%,为12.4 Oe;当添加10%Ti时,材料的矫顽力提升438%,为15.6 Oe。这主要是因为饱和磁化强度是一个本征参数,其值主要取决于材料的组成元素及晶体结构等,而矫顽力受晶粒大小、位错、第二相等因素的影响[35, 36]。在(Fe43Ni35Co22)1-xTix中,随着Ti元素的添加,材料中原位生成TiC含量增加,铁磁性元素占比降低,导致饱和磁化强度略有下降。同时,原位生成的TiC促使晶粒细化,并形成了微米晶区和超细晶区。一方面,晶粒细化有助于畴壁的运动,使得材料的矫顽力降低;另一方面,第二相颗粒的添加,会阻碍畴壁的运动,使得材料的矫顽力增大[3, 37-38]。显然,本文中TiC颗粒对畴壁运动阻碍效果更为显著,使得材料的矫顽力增加。综合考虑,HEA1的强度过低,HEA3的矫顽力太大而导致磁性能不佳,故HEA2具有最为优异的综合性能,其压缩屈服强度为1352 MPa,矫顽力为12.4 Oe,饱和磁化强度为138.7 emu/g,压缩断裂应变为24.5%,抗压强度为1969 MPa。
图7 (Fe43Ni35Co22)1-x Tix的室温压缩性能曲线与磁滞回线
Fig. 7 Room-temperature compressive curve (a) and hysteresis loop (b) of (Fe43Ni35Co22)1-x Tix
表3 (Fe43Ni35Co22)1-x Tix的性能总结
Table 3 Compressive and soft magnetic properties of (Fe43Ni35Co22)1-x Tix
3 结论
1) HEA1高熵合金粉末在球磨40 h粉末的相组成为FCC主相+少量BCC相;HEA2和HEA3高熵合金粉末球磨40 h后粉末的相组成为BCC主相+FCC相。
2) 经过1000 ℃的SPS烧结后,HEA2和HEA3原位生成了TiC,且随着Ti添加量的增加,TiC的含量增多。最终HEA1的相组成为FCC相+少量杂质;HEA2和HEA3则由单相FCC+TiC组成,其中FCC相为超细晶+微米晶的多尺度结构。
3) HEA2表现出最优异的综合性能。HEA1具有良好的塑性和软磁性能,但屈服强度较低。随着TiC含量的增加,材料的强度与矫顽力显著提高,当Ti添加量为5%时,材料的屈服强度提升221%,达到1352 MPa;矫顽力提升327%,达到12.4 Oe;当Ti添加量为10%时,材料的屈服强度提升319%,达到1762 MPa,矫顽力则提升至15.6 Oe。强度的提升,主要来源于弥散强化、细晶强化以及背应力强化;矫顽力的增高则是因为TiC颗粒对磁畴的阻碍作用。
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Influence of TiC on microstructure and properties of Fe43Ni35Co22medium-entropy alloy
LI Rui-kai1, CHEN Wei-ping1, WANG Hao1, CHEN Qiang2, ZENG Da-hai1, FU Zhi-qiang1
(1. Guangdong Key Laboratory for Advanced Metallic Materials Processing, South China University of Technology, Guangzhou 510640, China;
2. Southwest Institute of Technology and Engineering, Chongqing 400039, China)
Abstract: This paper systematically studied the influence of TiC on the microstructure, mechanical and magnetic properties of Fe43Ni35Co22 medium-entropy alloy (MEA). Adding 5% or 10% (mole fraction) Ti to theFe43Ni35Co22 MEA, the bulk TiC/Fe43Ni35Co22composite was achieved via in-situ reaction, using the combination of mechanical alloying (MA) and spark plasma sintering (SPS). The results showed that after 40hof ball milling, Fe43Ni35Co22 MEA powder was composed of a major face-centered cubic (FCC) phase and a small amount of body-centered cubic (BCC) phase, and the two TiC/Fe43Ni35Co22 powders were composed of a primary BCC phase and an FCC phase. Following by SPS, the bulk Fe43Ni35Co22MEA showed a single FCC phase with a small amount of contamination, and the two bulk TiC/Fe43Ni35Co22 consisted of a primary FCC phase with some TiC. Note that the FCC phase in the two bulk TiC/Fe43Ni35Co22 exhibited a multi-scale structure consisting of micron grains and ultra-fine grains. In addition, the volume fraction of ultra-fine grains increased as the TiC increased. In terms of properties, the addition of TiC evidently improved the compressive yield strength and coercivity of Fe43Ni35Co22, there by leading to a decrease in the plasticity and magnetic saturation of the material simultaneously. The bulk TiC/Fe43Ni35Co22 with 5% Ti addition showed the best performance, in detail, showing a compressive yield strength of 1352 MPa and a compressive fracture strain of 24.5%, along with a coercivity of 12.4 Oe and a saturation magnetization of 138.7emu/g.
Key words: medium-entropy alloys; mechanical alloying; spark plasma sintering; mechanical properties;
soft magnetic properties
Foundation item: Project(2020A1515111104) supported by Guangdong Basic and Applied Basic Research Foundation; Project supported by the Guangzhou Science and Technology Bureau
Received date: 2021-03-04; Accepted date: 2021-07-19
Corresponding author: FU Zhi-qiang; Tel: +86-20-87113832; E-mail: zhiqiangfu2019@scut.edu.cn
(编辑 )
基金项目:广东省基础与应用基础研究研究联合基金-青年基金项目(2020A1515111104);广州市科技局支持
收稿日期:2021-03-04;修订日期:2021-07-19
通信作者:付志强;电话:020-87113832;E-mail:zhiqiangfu2019@scut.edu.cn