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作者简介:

余施佳,女,1996年出生,硕士研究生。主要研究方向为表面工程和材料服役行为。E-mail:719039119@qq.com;

蔡振兵(通信作者),男,1981年出生,博士,研究员,博士研究生导师。主要研究方向为摩擦学、表面工程和材料服役行为。E-mail:caizb@swjtu.cn

中图分类号:TG156;TB114

DOI:10.11933/j.issn.1007−9289.20211009003

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目录contents

    摘要

    为了研究锆合金表面 Cr 涂层的循环热冲击行为,使用自研的热冲击设备模拟循环热冲击环境,针对多弧离子镀技术制备的 Cr 涂层进行不同循环次数的热冲击试验。通过 X 射线衍射仪(XRD)和显微硬度计分别分析热冲击前后的物相变化和硬度变化,采用扫描电镜(SEM)和能谱仪(EDS)探究 Cr 涂层的表面破坏行为。循环热冲击过程中,Cr 涂层生成的氧化层有“自愈”效果,可以有效阻挡 O 进入锆合金基体,同时诱发 Cr-Zr 中间层的非均匀扩散。大量 Cr 元素的内部扩散会促使 α-Zr(O)的生成。热冲击生成的裂纹大量分布于外部氧化层、非均匀扩散的中间层以及 α-Zr(O)层。经过 N =24 次循环热冲击后,残余的 Cr 涂层仍然可以有效保护锆合金基体,避免 Zr 与大量 O 反应。通过将锆合金表面 Cr 涂层的循环热冲击行为分为三个阶段,进一步揭示了循环热冲击作用下锆合金 Cr 涂层的组织结构和抗热冲击性能的演变规律。

    Abstract

    In order to study the cyclic thermal shock behavior of the Cr coating on the surface of zirconium alloy, the self-developed thermal shock equipment is used to simulate the cyclic thermal shock environment, and the thermal shock test of the Cr coating prepared by the multi-arc ion plating technology is carried out with different cycles. The phase change and hardness change before and after thermal shock are analyzed by X-ray diffractometer (XRD) and microhardness tester, and the surface damage behavior of Cr coating is explored by scanning electron microscope (SEM) and energy spectrometer (EDS). During the cyclic thermal shock process, the oxide layer generated by the Cr coating has a “self-healing” effect, which can effectively block O from entering the zirconium alloy matrix and induce non-uniform diffusion of the Cr-Zr intermediate layer. The internal diffusion of a large amount of Cr element will promote the generation of α-Zr(O). A large number of cracks generated by thermal shock are distributed in the outer oxide layer, the non-uniformly diffused intermediate layer and the α-Zr(O) layer. After N =24 cycles of thermal shock, the residual Cr coating can still effectively protect the zirconium alloy matrix and prevent Zr from reacting with a large amount of O. By dividing the cyclic thermal shock behavior of the Cr coating on the surface of the zirconium alloy into three stages, this paper further reveals the evolution of the microstructure and thermal shock resistance of the Cr coating on the zirconium alloy under the action of the cyclic thermal shock.

  • 0 前言

  • 在2011年日本福岛核事故中,由于堆芯冷却剂的流动中断,发生失水事故(Loss of coolant accident,LOCA),燃料件的温度迅速升高。锆合金包壳管与高温水蒸汽剧烈反应产生大量的氢气引发氢爆,导致了大量放射性物质外泄,最终造成了严重的核安全事故。为了弥补锆合金材料在失水事故状态下的不足,对耐事故材料(Accident tolerant fuel,ATF)的研究成为了进一步提升核电站安全性和可靠性的重要举措。

  • 由于Zr具有低的中子吸收截面,锆合金的替代材料的开发极为困难,因此在锆合金包壳管表面制备涂层,成为最有可能提高其高温抗氧化性的途径之一[1]。该涂层材料应具备良好的耐磨性、耐腐蚀性以及抗高温氧化性,SiC、TiN等陶瓷材料以及FeCrAl、Cr等金属材料受到广泛研究[2]。研究表明, SiC、TiN等陶瓷材料的可加工性差,且长期暴露在高温环境中易发生脆性断裂和热冲击断裂[3]。 FeCrAl合金虽然拥有良好的高温抗氧化性能,但不同元素扩散速率的差异性,导致外部生成的起保护作用的铝氧化层易破损[4-5]。而纯金属Cr的熔点高、抗氧化性能好、热膨胀系数与锆相近的同时,避免了多元素的扩散问题,在LOCA环境中能更有效地提高包壳管的耐事故能力,拥有较强的发展前景[6]

  • 目前在锆合金表面制备Cr涂层的工艺主要分为熔覆与喷涂技术、物理气相沉积技术两大类[1]。熔覆与喷涂技术的沉积率高,但较难实现涂层厚度、均匀性的精确控制[7],同时激光熔覆的快速加热与急速冷却会在熔覆层中残留极大的热应力[8]。传统的物理气相沉积技术中,磁控溅射技术因沉积速率可控、工艺重复性好等优势,在锆合金包壳涂层制备中得到了较大的应用与研究,但由于磁控溅射技术的溅射能量低,靶材的离化率较差,制备的Cr涂层表面会存在大量孔洞,涂层结构的稳定性以及隔绝氧的能力较差[9]。多弧离子镀技术作为在溅射镀膜基础上发展起来的一种新的镀膜方式,通过电弧放电在阴极靶材上直接蒸发金属,制备的涂层具有强度高、耐性好、附着力高等优点[1]。PARK等采用电弧离子镀在锆合金上沉积了致密均匀的Cr涂层,试验表明Cr涂层有效地保护锆合金免受高温蒸汽氧化,成功起到了防腐层的作用[10]。国内针对锆合金基体Cr涂层进行了大量高温氧化行为的研究,但对于高温氧化与热冲击相结合的研究较少。

  • 1 试验准备

  • 1.1 试验材料

  • 试样由中国核动力研究设计院提供。试样采用多弧离子镀技术在10mm×10mm×1mm的Zr-4合金表面制备了约20 μm厚的纯Cr金属涂层。镀膜选用纯度为99.99%的Cr,基体温度为300℃,弧电流为120A,负偏压为160V,工作气压为0.8Pa,制备8.5h。采用多弧离子镀技术在Zr-4基体上,

  • 图1a所示电弧离子镀制备的Cr涂层表面显示,高能粒子轰击阴极靶材,熔融的金属液滴飞溅并在基材上沉积、凝固后呈现为大量大颗粒[11]。从图1b的截面形貌可以看出,涂层致密且高度均匀,制备的Cr涂层厚度约为20 μm。电弧离子镀制备的Cr涂层与锆合金基体的结合性能良好[12],有助于试样更好地抵抗热冲击。

  • 图1 原始试样表面和截面形貌

  • Fig.1 Surface and cross-sectional morphologies of original sample.

  • 1.2 试验方法

  • 本试验采用的自研热冲击试验装置如图2a所示,设备主要分为四个区域:升降功能区、快速冷却区、温度隔绝区、加热区。试验通过升降功能区将试样放入恒温1 100℃的加热区中,氧化10min后将试样上升至快速冷却区中,通过在快速冷却区通入−30℃的冷空气,实现试样快速冷却。同时依靠温度隔绝区,隔绝来自加热区的高温。

  • 图2b为试样的载荷示意图,试验以1 100℃氧化10min后快速冷却至室温为一个循环,分别进行循环次数 N=1、2、3、6、9、12、15、18、21、 24次。试验探究Cr涂层的形貌和组织结构随循环热冲击次数的演变关系,深入分析氧化膜物相结构的转变过程,揭示锆合金表面Cr涂层循环热冲击行为机理。

  • 图2 热冲击试验装置及载荷示意图

  • Fig.2 Schematic diagram of thermal shock test device and load diagram

  • 1.3 表征与分析

  • 采用扫描电子显微镜(SEM,JSM 7800F) 观察涂层表面和截面形貌特征,并结合能谱仪 (EDS, TESCAN MIRA 3XH)对氧化后试样截面的元素分布进行表征。采用KELITI000ZB型显微硬度计对比涂层的表面硬度变化,加载载荷9.8N,保压15s。利用X射线衍射仪(XRD, DX-2700B)对涂层的物相组成进行分析,扫描角度为30°~90°,扫描速度为8 (°)/min。采用JJ324BC型电子天平测量试样的氧化增重情况,精度为0.1mg。

  • 2 结果与讨论

  • 2.1 表面形貌及物相分析

  • 图3 显示了不同次数循环热冲击后样品的增重情况。随着热冲击循环次数的上升,由于涂层的保护性能降低,整体的氧化动力学加快[13],试样持续增重,且增重幅度平稳上升,热冲击过程中无涂层大量剥落。

  • 图4a所示原始试样表面在光镜中呈现为灰黑色,经过 N=3次循环热冲击后涂层表面显示为绿色。由图4b热冲击前后维氏硬度对比,显示表面维氏硬度出现急剧上升,由初始的255.43HV上升为433.10HV。经图4c中XRD数据表明,原始试样表面仅有Cr相存在。经过 N=3次循环热冲击后,表面检测出Cr相及大量Cr2O3 相同时存在。由此可知,表面硬度的上升是由Cr涂层高温氧化生成Cr2O3 造成的,其化学反应式为:

  • Cr+34O212Cr2O3

  • 图3 不同次数循环热冲击后样品的增重情况

  • Fig.3 Mass gain of samples after different cycles of thermal shock

  • 随着循环热冲击次数的上升,在 N=24的XRD数据中仍未检测出Zr相的存在,表明生成的Cr2O3 可以有效地充当O的扩散阻挡层[14]

  • 图4 热冲击前后涂层的表面形貌、显微硬度及XRD变化情况

  • Fig.4 Changes of surface morphology, microhardness and XRD of the coating before and after thermal shock.

  • 如图5a循环次数 N=3的表面形貌所示,表面均匀地出现大量小尺寸氧化层鼓包,且几乎所有鼓包带有独立的裂纹。鼓包区域易产生局部应力集中[15],而裂纹会在应力集中的部位形成和扩展[16]。同时,高温有氧环境中物相组成的变化对于裂纹的产生起到促进作用,例如高温下生成的氧化物Cr2O3 属于脆性相,试样通过在氧化层产生裂纹释放热应力[17-18]

  • 图5b循环热冲击次数 N=12时,氧化层表面鼓包的密度显著上升,热冲击后产生的裂纹变细。表1的EDS点化学成分分析显示,随着热冲击次数的上升,氧化层表面的氧含量逐渐增大,Cr与O的原子数百分比逐渐趋于1∶3,这可能是由于Cr2O3在900℃温度以上会继续与O反应生成CrO3 [19],化学反应式为:

  • 12Cr2O3+34O2CrO3

  • 图5c循环热冲击次数 N=24的表面形貌出现了显著差异,试样表面呈现为大隆起状态,且放大表面形貌显示有大量粗晶状氧化物产生。粗晶状氧化物的形成是少量Cr元素沿氧化层细裂纹析出后的氧化结晶,粗晶状氧化物的形成作为Cr的一种消耗方式[20-21]。表1显示 N=24时,粗晶状氧化物上的氧含量略低于表面氧化层,表明粗晶状氧化物的氧化时长较短。

  • 图5 不同循环次数热冲击试验后的表面形貌

  • Fig.5 Surface morphology after thermal shock test with different cycles.

  • 表1 图5的EDS化学成分分析(at.%)

  • Table1 Chemical composition analysis of EDS in Fig.5 (at.%).

  • 2.2 截面形貌及成分分析

  • 在1 100℃高温的作用下,Cr涂层表面氧化的同时,在Cr涂层与锆合金的交界处会出现Cr、Zr元素的相互扩散[22]。如图6的截面形貌显示,经不同循环次数热冲击试验后,试样截面形成了与[23]中相似的四层结构:外部氧化层、Cr层、Cr-Zr中间层、Zr基体。Cr涂层具有致密的微观结构与足够的厚度,可以保护锆合金避免被氧化。

  • 图6a、6b的截面形貌显示,氧化层厚度分布均匀且存在大量孔洞,呈现为疏松多孔的形貌特征,这是由于O2− 负离子向内与Cr3+正离子向外部的传输速率不一致[13]。图6c循环次数 N=24的截面形貌显示,随着热冲击循环次数的上升,外部氧化层的孔洞依然存在,但其厚度不再均匀。后文会对造成该现象的原因进行进一步说明。

  • 图6 的截面线扫数据显示,Cr层中有少量均匀分布的氧。该现象证明O很快扩散至Cr中达到饱和,且Cr涂层深度没有造成明显的氧分布不均匀[24]。同时,在Zr基体中O的含量降到接近零,再次表明Cr涂层能有效地充当O的扩散阻挡层。

  • 图6 中Cr-Zr中间层的元素分布显示,随着热冲击次数的增加,Cr、Zr元素的相互扩散会导致Cr-Zr中间层增厚,且Cr、Zr元素的分布不再均匀, Cr(6.5×10−6/K)、Zr(5.77×10−6 ~7.62×10−6/K) 的热膨胀失配会导致大量裂纹在中间层处萌生[18, 21]

  • 图6 不同循环次数热冲击试验后的截面形貌及元素分析

  • Fig.6 Cross section morphology and elemental analysis after thermal shock test with different cycles.

  • 从截面扫描电镜图像量化外部氧化层、残余Cr涂层与Cr-Zr中间层的厚度演变(图7)。外部氧化层随着O的进入不断增厚,当厚度达到一定值时,氧化层厚度不再上升,而是随着循环热冲击次数的上升,其厚度分布不再均匀。由于Cr在Cr-Zr中间层中的扩散系数比在Zr基体中小5个数量级[1],随着热冲击次数的上升,Cr-Zr中间层不断增厚的同时,大量Cr向内扩散并溶解在Zr基体中,因此有助于Cr涂层的整体变薄[23,25]

  • 图7 不同次数循环热冲击后Cr涂层厚度的演变情况

  • Fig.7 Evolution of Cr coating thickness after different cycles of thermal shock.

  • 图8a循环次数 N=3的截面形貌显示,在外部氧化层和残余Cr层之间有大量空洞生成。结合图8b所示的空洞处氧化层下方的Cr元素分布均匀,未受空洞生成的影响,判断氧化层与Cr涂层界面的空洞缺陷产生于快速冷却过程。因此,空洞形成的主要原因是外部氧化层与残余Cr层热膨胀系数差异过大,Cr2O3 的膨胀系数为9.6× 10−6/K,Cr为6.5×10−6/K[24]。胡小刚等[26]的分析表明,氧化层与Cr涂层间的小空洞往往会聚集形成较大的空洞。

  • 截面空洞的生成形成了图5a、5b所示的氧化层表面鼓包形貌。由上述分析可知,氧化层鼓包处会生成独立裂纹释放热应力,由于氧化层疏松多孔的特性,生成的裂纹会贯穿氧化层,但Cr涂层能有效阻止裂纹的延展,如图8a截面形貌所示。因为图5a显示的表面形貌非常完整,所以图8a显示的外部氧化层破裂剥落不是由热冲击造成的,而是发生在截面样品制备磨、抛过程中。

  • 图8 循环热冲击次数 N=3后截面形貌及元素分析

  • Fig.8 Section morphology and element analysis after cyclic thermal shock times N=3.

  • BRACHET等[27]的研究表明,由于氧化层裂纹 “自愈”,Cr涂层上预先存在的裂纹对样品整体氧化的影响非常有限。如图9a所示,裂纹的存在为O向内部转移提供了条件,同时高温会促进Cr元素沿裂纹向外扩散与O反应生成新的Cr2O3。新的Cr2O3 的生成能有效地阻止O继续向内部扩散,达到“自愈”的效果。

  • Cr、Zr两多晶材料之间具有晶体连续性,因此Cr-Zr中间层的结合性能良好[9]。随着Cr元素的向外扩散,Zr元素在应力作用下同时沿Cr晶界向外延伸,如表2点扫数据所示。由于Zr元素的延伸大部分在Cr-Zr中间层边界处,因此形成下粗上细的“鱼刺”形貌[13]

  • 随着循环热冲击次数上升,氧化层经历反复“开裂→自愈”过程,形成如图10a所示的厚度不均匀的氧化层(2.18~7.33 μm)、残余Cr层(4.56~7.65 μm)和Cr-Zr中间层,该截面形貌在图5c氧化层表面形貌中呈现为大隆起状态。

  • 图9 循环热冲击次数 N=12后截面形貌及元素分析

  • Fig.9 Section morphology and element analysis after cyclic thermal shock times N=12.

  • 表2 图9的EDS化学成分分析(at.%)

  • Table2 Chemical composition analysis of EDS in Fig.9 (at.%).

  • 已知1 000℃以上,β-Zr是唯一稳定的相[28]。但由于Zr比Cr有更高的氧亲和力,溶解入锆合金基体的Cr元素携带的O促使部分 β-Zr向 α-Zr进行转变,并反应生成稳定的 α-Zr(O)相[24]。因为 α-Zr(O) 是脆性的,而 β-Zr将保持其延展性,在热冲击的作用下会生成如图10a所示的裂纹[19]。表3点扫数据中氧含量的变化佐证了上述推测。

  • 表3 图10的EDS化学成分分析(at.%)

  • Table3 Chemical composition analysis of EDS in Fig.10 (at.%)

  • 如图10b所示,Cr-Zr中间层的非均匀扩散会影响Cr涂层在高温下的整体机械稳定性[20],在Cr-Zr中间层上可以观察到大量裂纹产生。由于电弧离子镀的Cr涂层具有足够高的结合性能[12],内部的裂纹并未导致涂层的剥落,残余Cr元素依然分布于Zr基体上起到了氧扩散阻挡层的作用。在截面没有明显的氧化锆层生成,证实电弧沉积设备制备厚度约为20 μm的Cr涂层可以为燃料包壳锆合金提供足够的事故容限[24]

  • 图10 循环热冲击次数 N=24后截面形貌及元素分析

  • Fig.10 Section morphology and element analysis after cyclic thermal shock times N=24.

  • 2.3 循环热冲击分析

  • 通过上述研究和分析,锆合金基体表面Cr涂层的循环热冲击行为机理如图11所示。可以将Cr涂层循环热冲击行为机理分为三个阶段。

  • 图11 锆合金基体Cr涂层循环热冲击行为过程示意图

  • Fig.11 Schematic diagram of cyclic thermal shock behavior of Cr coating on zirconium alloy substrate.

  • (1)Cr涂层表面氧化阶段。如图6a所示,Cr涂层表面与O反应生成Cr2O3,涂层内部与Zr相互扩散形成Cr-Zr中间层。由于氧化层与Cr涂层的膨胀系数不同,氧化层与Cr涂层间产生如图8a显示的大量空洞。在热冲击作用下,空洞处生成独立裂纹释放热应力。

  • (2)热冲击裂纹“自愈”阶段。图7数据显示,外部氧化层生长到一定厚度后不再增厚,但表1的EDS数据表明试样表面氧化物的氧含量持续增加。部分Cr元素沿粗裂纹向外扩散与O反应生成新的氧化层,愈合氧化层因热冲击生成的裂纹,达到“自愈”的效果。局部Cr元素的向外扩散会带动Zr元素沿Cr晶粒间隙向外转移,在Cr-Zr中间层上形成图6b的“鱼刺”形貌;

  • (3)Cr、Zr不均匀扩散阶段。如图6c截面形貌显示,外部氧化层、残余Cr层、Cr-Zr中间层的厚度不均匀。在此阶段,涂层的表面呈现出如图5c所示的大隆起形貌,表面氧化物Cr2O3在1 100℃高温下继续被氧化成CrO3 [19]。热冲击形成的裂纹变细小,少量Cr元素沿细裂纹析出并与氧反应生成粗晶状氧化物。由于Cr、Zr不均匀扩散,热冲击下Cr-Zr中间层会形成图6c所示的大量裂纹。如图6c的线扫数据显示,部分Cr元素携少量O溶解入锆基体中。β-Zr转变为 α-Zr的过程中,会与这部分O反应生成稳定的 α-Zr(O)相[24],脆性的 α-Zr(O)在热冲击作用下易生成裂纹,如图10b所示。

  • 3 结论

  • 研究了锆合金Cr涂层的循环热冲击行为,开展了不同循环次数的热冲击试验,得出结论如下:

  • (1)Cr涂层循环热冲击行为机理可分三个阶段:Cr涂层表面氧化阶段、热冲击裂纹“自愈”阶段和Cr、Zr不均匀扩散阶段。

  • (2)在高温作用下,Cr元素会沿氧化层裂纹向外扩散,并与O反应生成新的氧化层,达到裂纹的 “自愈”效果。同时,Zr元素会沿Cr晶粒间隙向外扩散,诱发Cr-Zr中间层的非均匀扩散。

  • (3)Cr的内部扩散会促使 α-Zr(O)的生成,降低Cr涂层的抗热冲击性能。热冲击生成的裂纹大量分布于外部氧化层、非均匀扩散的中间层以及 α-Zr(O)层。

  • 本文的研究重点是将高温氧化与热冲击相结合,探究锆合金表面Cr涂层在模拟循环热冲击环境下组织结构、抗热冲击性能的演变规律,为ATF包壳涂层结构完整性和稳定性的研究提供了数据支撑。但是包壳管在核事故条件下的运行工况为1 100~1 300℃的高温蒸汽环境,故在高温蒸汽环境中ATF包壳涂层的表面破坏行为值得进一步研究。

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