Microstructure and Properties of Ultrafine-Grained Pure Copper Based on Equal Channel Angular Pressing Deformation
-
摘要:
在室温下采用BC路径(每道次挤压时将试样绕其轴线沿同一方向旋转90°)对T2纯铜进行4道次等通道转角挤压(ECAP),研究了不同道次ECAP变形前后纯铜的显微组织、力学性能和耐腐蚀性能。结果表明:经过1道次ECAP变形后,纯铜内部形成粗大的条带状组织,与剪切应力方向呈30°~45°;经过4道次ECAP变形后,纯铜晶粒发生显著破碎与细化,出现了大量的超细等轴晶组织,平均晶粒尺寸约为0.96 μm;4道次ECAP变形后纯铜的显微硬度与抗拉强度较未变形显著提高,增幅分别为43.8%和51.5%;随着ECAP道次的增加,纯铜的溶液电阻、电荷转移电阻和极化电阻均增大,开路电位和自腐蚀电位均升高,自腐蚀电流密度减小,耐腐蚀性能提升。
Abstract:Four-pass equal channel angular pressing (ECAP) was performed on T2 pure copper by using the BC pathway (the specimen was rotated 90° in the same direction around its axis at each ECAP) at room temperature, the microstructure, mechanical properties and corrosion resistance of the pure copper before and after ECAP deformation for different passes (one-pass, four-pass) were studied. The results show that after one-pass ECAP deformation, a large banded structure was formed in the pure copper, and the direction of the band was at 30°–45° to the direction of shear stresses. After four-pass ECAP deformation, the pure copper grains were broken and refined significantly, a large number of ultrafine-equiaxed grained structures appeared, and the average grain size was about 0.96 μm. The microhardness and tensile strength of the pure copper after four-pass ECAP deformation significantly increased by 43.8% and 51.5%, respectively. With the increase of ECAP pass, the solution resistance, charge transfer resistance and polarization resistance of the pure copper increased, the open circuit potential and self-corrosion potential increased, and the self-corrosion current density decreased, indicating the corrosion resistance was improved.
-
0. 引言
纯铜因具有优良的导电性、导热性、延展性和耐腐蚀性而被广泛用于通信、电子电气和船舶军工等领域[1]。随着现代产业的升级,传统生产方法制备的铜制构件在服役过程中常因强度不足而发生脆性断裂[2],铜材料性能的进一步提升成为研究热点。
金属材料的性能与其组织密切相关[3],当晶粒细化至亚微米甚至纳米级尺度时将表现出更优异的综合性能[4-5]。杨泽洲等[6]研究发现,超细晶D6A钢具有较高的动态拉伸强度和良好的延展性。雷漫江等[7]研究发现,超细晶钛在质量分数3.5%NaCl溶液中的疲劳极限高于粗晶钛,并且耐腐蚀疲劳性能也更优异。剧烈塑性变形技术是制备块体超细晶材料的重要方法之一,其中等通道转角挤压(equal channel angular pressing,ECAP)是最有效的方法。ECAP技术通过在模具通道转角处设定特定角度,使坯料转角时受到剧烈剪切变形,从而达到晶粒细化的目的[8-9]。在ECAP过程中,每道次挤压时可将试样绕其轴线旋转一定角度,形成不同的挤压路径,其中采用BC路径(每道次挤压时将试样绕其轴线沿同一方向旋转90°)制备的材料晶粒细化效果最佳[10]。近年来,国内外研究人员针对ECAP制备的超细晶纯铜的显微组织和力学性能开展了大量研究[11-12],但对其耐腐蚀性能的研究却鲜有报道。作者以T2纯铜为研究对象,在室温下采用BC路径对其进行4道次ECAP试验,研究了不同道次ECAP变形后T2纯铜的显微组织、力学性能和腐蚀行为,以期为纯铜材料的改性研究和工程应用提供技术指导与理论依据。
1. 试样制备与试验方法
试验材料为均匀化退火态T2纯铜圆棒,直径为15 mm,化学成分(质量分数/%)为99.960 0Cu,0.000 8Fe,0.000 9S,0.000 9Zn,0.000 7P,余Ni等。采用线切割加工出尺寸为ϕ15 mm×80 mm的圆柱形坯料,采用Y32-100型液压机在室温下对坯料进行4道次ECAP试验,通道直径为15 mm,内角为120°,坯料与模具通道间用石墨-机油润滑,以减小摩擦对坯料变形的不利影响,挤压路径为BC路径,挤压速度为1 mm·s−1。
采用线切割在不同道次(1,4道次)ECAP变形前后的坯料上切取若干块状试样,将试样进行研磨、机械抛光和振动抛光后,用无水乙醇超声清洗,采用MIRA LMS型扫描电子显微镜(SEM)的电子背散射衍射(EBSD)模式观察显微组织,加速电压为20 kV,利用Channel 5软件分析晶粒分布及尺寸。采用HVS-1000A型显微维氏硬度计测试显微硬度,载荷为9.8 N,保载时间为10 s,测17个点取平均值,测试点分布见图1。采用ETM-104C型电子万能试验机进行室温拉伸试验,拉伸试样形状和尺寸如图2所示,拉伸速度为2 mm·min−1。
采用CHI600E系列电化学工作站进行电化学试验,采用三电极测试系统,工作电极为块状试样,参比电极为饱和甘汞电极(SCE),辅助电极为铂电极,腐蚀介质为质量分数3.5%NaCl溶液。块状试样的尺寸为5 mm×5 mm×5 mm,在其背面缠绕一根导线,并用环氧树脂将测试面以外部分进行封装,仅留出面积25 mm2的测试表面。将待测试样在腐蚀介质中浸泡24 h,待系统稳定后分别测定开路电位、交流阻抗谱(EIS)和动电位极化曲线。交流阻抗谱测试时的频率为0.1 Hz~100 kHz,动电位极化曲线扫描范围为−0.3~0.3 V,扫描速率为0.002 V·s−1。使用ZSimpWin软件对EIS进行拟合,利用Tafel外推法对极化曲线进行拟合。采用SEM观察试样腐蚀表面的微观形貌。
2. 试验结果与讨论
2.1 显微组织
由图3可见:未经ECAP变形的T2纯铜的组织为粗大的等轴晶,晶粒尺寸为30~40 μm,晶粒内伴有少许孪晶;经过1道次ECAP变形后,原始等轴晶在剪切应力的作用下被明显拉长,形成了粗大的条带状组织,与剪切应力方向呈30°~45°,剪切带宽度为10~15 μm;经过4道次ECAP变形后,剪切带逐渐消失,纯铜内部除了存在少量粗大晶粒外,还出现了大量以细小均匀的等轴晶为主的超细晶组织,平均晶粒尺寸约为0.96 μm。采用BC路径时每道次挤压后试样都会沿同一方向旋转90°,使得材料受到交变剪切应力作用,不同方向上的剪切带相互交叉,导致大量晶粒发生破碎细化;随着ECAP道次的增加,材料内部的应变累积量增大,且在多个方向上受到剪切应力作用,晶粒尺寸逐渐减小且趋于均匀[13]。
2.2 力学性能
由图4可以看出,不同道次ECAP变形前后的T2纯铜在拉伸过程中均没有明显的屈服阶段。由表1可见:随着ECAP道次的增加,T2纯铜的抗拉强度和显微硬度增大,硬度分布逐渐均匀,断后伸长率降低;经过4道次ECAP变形后,T2纯铜的显微硬度和抗拉强度相较于原始退火态分别提高了43.8%和51.5%,断后伸长率略有降低,但总体上表现出较好的塑韧性。这是由于在多道次ECAP过程中,剧烈的塑性剪切变形使得材料内部累积大量应变,晶粒显著细化,变形均匀性得到改善[14],在位错强化和细晶强化等机制的共同作用下,材料的力学性能得到提升。
表 1 不同道次ECAP变形前后T2纯铜的力学性能Table 1. Mechanical properties of T2 pure copper before and after ECAP deformation for different passes条件 屈服强度/MPa 抗拉强度/MPa 断后伸长率/% 平均显微硬度/HV 未变形 295.6 328.90 18.9 91.89±11.39 1道次 316.6 382.85 17.8 109.79±4.74 4道次 476.4 498.47 15.1 132.16±2.98 2.3 耐腐蚀性能
由图5可知,未变形以及经1,4道次ECAP变形后,T2纯铜的开路电位分别为−0.253,−0.247,−0.238 V。可见,随着ECAP道次的增加,T2纯铜的开路电位不断升高,表明其耐腐蚀性能得到改善。
由图6可知,随着ECAP道次的增加,T2纯铜在高频区的容抗弧半径逐渐增大,表明T2纯铜的电化学活性降低,抗腐蚀能力提高[15]。采用等效电路对交流阻抗谱进行拟合,等效电路中:Clf为吸附电容元件;Rlf为吸附电阻元件;Rt为电荷转移电阻元件;Rs为溶液电阻元件;Qdl为双电层电容元件。拟合结果见表2,表中Clf为吸附电容;Rlf为吸附电阻;Rt为电荷转移电阻;Rs为溶液电阻;Qdl为双电层电阻。溶液电阻与电荷转移电阻是衡量材料耐腐蚀性能的重要指标[16]:溶液电阻越高或电荷转移电阻越大,意味着电荷穿过腐蚀介质与电极两相界面的难度增大,因此材料耐腐蚀性能越好[17]。由表2可知,随着ECAP道次的增加,溶液电阻和电荷转移电阻均增大,说明T2纯铜的耐腐蚀性能得到了提升。
表 2 不同道次ECAP变形前后T2纯铜腐蚀时的电化学阻抗拟合参数Table 2. EIS fitting parameters of T2 pure copper in corrosion before and after ECAP deformation for different passes条件 Rs/(Ω·cm2) Qdl/(F·cm−2) Rt/(Ω·cm2) Clf/(F·cm−2) Rlf/(103 Ω·cm2) 未变形 4.926×10−5 2.089×10−4 6.031 5.401×10−6 4.044 1道次 3.563×10−1 8.134×10−5 6.486 9.192×10−7 6.754 4道次 3.037 1.580×10−5 3.453×103 1.543×10−3 4.330 由图7可以看出,未变形和经过ECAP变形后T2纯铜在电化学腐蚀过程中均出现钝化现象。腐蚀后的T2纯铜表面形成了一层钝化膜,阻碍了T2纯铜与腐蚀介质间的电子流动,抑制了T2纯铜的进一步腐蚀[18-20]。
由表3可知:随着ECAP道次的增加,T2纯铜的自腐蚀电位Ecorr升高,自腐蚀电流密度Icorr减小,说明T2纯铜在腐蚀过程中单位面积通过的电流逐渐减小,极化电阻Rp显著增加,说明阳极反应受到的阻碍增大。可见,相较于未变形和1道次ECAP,4道次ECAP变形后腐蚀形成的钝化膜对T2纯铜腐蚀的抑制能力最强。
表 3 不同道次ECAP变形前后T2纯铜腐蚀时的电化学拟合参数Table 3. Electrochemical fitting parameters of T2 pure copper in corrosion before and after ECAP deformation for different passes条件 Ecorr/V ba bc Icorr/(μA·cm−2) Rp/(Ω·cm2) 未变形 −0.184 0.016 −0.055 2.203 4 453.386 1道次 −0.183 0.010 −0.035 1.932 3 150.651 4道次 −0.166 0.016 −0.135 0.151 52 261.154 由图8可知:腐蚀后退火态T2纯铜表面发生严重且不均匀的局部腐蚀;随着ECAP道次的增加,腐蚀坑的大小减小,数量减少,分布变均匀。ECAP变形后T2纯铜的耐腐蚀性能得到显著提升,这是因为在ECAP变形过程中,T2纯铜累积了大量塑性应变,内部位错密度增加,晶粒显著细化;晶粒的细化使得位错运动能在更多的晶粒中进行,塑性变形变得更均匀,应力集中现象减弱[21],从而减小了裂纹萌生的倾向,降低了材料在腐蚀环境下发生腐蚀的可能性。同时,T2纯铜的晶粒细化使晶界数量大幅增加,晶界密度提高。晶界处往往因活性较强而先发生腐蚀,晶粒细化使得晶界数量增加,材料反应活性增强,促使T2纯铜表面生成更加均匀、稳定和致密的钝化膜;致密钝化膜的形成可以更有效地阻碍腐蚀介质中电子、Cl−等穿过,从而使得极化电阻增大,自腐蚀电流密度减小[22-23]。
3. 结论
(1)经过1道次ECAP变形后,T2纯铜由于受到剧烈的剪切变形,内部形成了粗大的条带状组织,与剪切应力方向呈30°~45°。经过4道次ECAP变形后,在交变剪应力和应变累积的综合作用下,T2纯铜的晶粒显著细化,变得破碎且均匀,并且出现了大量的超细等轴晶粒,平均晶粒尺寸约0.96 μm。
(2)经过4道次ECAP变形后,在位错强化与细晶强化等的综合作用下,T2纯铜的显微硬度与抗拉强度相比于原始退火态均得到显著提高,增幅分别为43.8%和51.5%,断后伸长率略有降低,T2纯铜整体上表现出良好的塑韧性。
(3)随着ECAP道次的增加,T2纯铜的溶液电阻、电荷转移电阻和极化电阻均增大,开路电位和自腐蚀电位均升高,自腐蚀电流密度减小,耐腐蚀性能显著提升。
-
表 1 不同道次ECAP变形前后T2纯铜的力学性能
Table 1 Mechanical properties of T2 pure copper before and after ECAP deformation for different passes
条件 屈服强度/MPa 抗拉强度/MPa 断后伸长率/% 平均显微硬度/HV 未变形 295.6 328.90 18.9 91.89±11.39 1道次 316.6 382.85 17.8 109.79±4.74 4道次 476.4 498.47 15.1 132.16±2.98 表 2 不同道次ECAP变形前后T2纯铜腐蚀时的电化学阻抗拟合参数
Table 2 EIS fitting parameters of T2 pure copper in corrosion before and after ECAP deformation for different passes
条件 Rs/(Ω·cm2) Qdl/(F·cm−2) Rt/(Ω·cm2) Clf/(F·cm−2) Rlf/(103 Ω·cm2) 未变形 4.926×10−5 2.089×10−4 6.031 5.401×10−6 4.044 1道次 3.563×10−1 8.134×10−5 6.486 9.192×10−7 6.754 4道次 3.037 1.580×10−5 3.453×103 1.543×10−3 4.330 表 3 不同道次ECAP变形前后T2纯铜腐蚀时的电化学拟合参数
Table 3 Electrochemical fitting parameters of T2 pure copper in corrosion before and after ECAP deformation for different passes
条件 Ecorr/V ba bc Icorr/(μA·cm−2) Rp/(Ω·cm2) 未变形 −0.184 0.016 −0.055 2.203 4 453.386 1道次 −0.183 0.010 −0.035 1.932 3 150.651 4道次 −0.166 0.016 −0.135 0.151 52 261.154 -
[1] 刘琴,刘广义,牛晓雪,等. 铜表面三唑硫酮衍生物的自组装及其抗盐酸腐蚀的电化学性能[J]. 中国有色金属学报,2019,29(5):1102-1117. LIU Q ,LIU G Y ,NIU X X ,et al. Self-assembled monolayer of triazole-thione derivatives and its electrochemical property of corrosion resistance of hydrochloric acid[J]. The Chinese Journal of Nonferrous Metals,2019,29(5):1102-1117.
[2] 符彩涛,刘芳,许光丽,等. 超细晶纯铝微观组织及力学和腐蚀性能研究[J]. 材料科学与工艺,2018,26(1):47-53. FU C T ,LIU F ,XU G L ,et al. Researches on microstructure,mechanical and corrosion properties of ultra-fine grained pure aluminum[J]. Materials Science and Technology,2018,26(1):47-53.
[3] 梁宁宁块体超细晶铜及铜铬锆合金的制备及其冲击韧性、热稳定性和电导性研究南京南京理工大学2018梁宁宁. 块体超细晶铜及铜铬锆合金的制备及其冲击韧性、热稳定性和电导性研究[D]. 南京:南京理工大学,2018. LIANG N NImpact toughness,thermal stability and electrical conductivity of bulk ultrafine-grained Cu and CuCrZr alloyNanjingNanjing University of Science and Technology2018LIANG N N. Impact toughness,thermal stability and electrical conductivity of bulk ultrafine-grained Cu and CuCrZr alloy[D]. Nanjing:Nanjing University of Science and Technology,2018.
[4] 罗雷,段西明,杨西荣,等. 生物医用超细晶钛及钛合金的研究进展[J]. 中国材料进展,2022,41(4):314-320. LUO L ,DUAN X M ,YANG X R ,et al. Research progress of biomedical ultrafine-grain titanium and titanium alloy[J]. Materials China,2022,41(4):314-320.
[5] 石凤健. ECAP法制备T2纯铜的组织与性能[J]. 江苏科技大学学报(自然科学版),2006,20(2):85-88. SHI F J. Microstructure and properties of pure copper fabricated by equal channel angular pressing[J]. Journal of Jiangsu University of Science and Technology (Natural Science Edition),2006,20(2):85-88.
[6] 杨泽洲,申勇峰,冯晓伟,等. 超细晶D6A钢动态拉伸力学特性实验研究[J]. 爆炸与冲击,2022,42(4):65-73. YANG Z Z ,SHEN Y F ,FENG X W ,et al. Investigation on dynamic tensile properties of an ultrafine grained D6A steel[J]. Explosion and Shock Waves,2022,42(4):65-73.
[7] 雷漫江,杨西荣,刘晓燕,等. 超细晶工业纯钛在3.5%NaCl溶液中的高周疲劳[J]. 稀有金属,2022,46(3):402-408. LEI M J ,YANG X R ,LIU X Y ,et al. High cycle fatigue of UFG CP-Ti in 3.5%NaCl solution[J]. Chinese Journal of Rare Metals,2022,46(3):402-408.
[8] 石凤健,汪建敏,许晓静. ECAP法制备超细晶铜的再结晶行为研究[J]. 热加工工艺,2005,34(12):24-26. SHI F J ,WANG J M ,XU X J. Study on recrystallization behavior of ultra-fine grain copper fabricated by equal channel angular pressing[J]. Hot Working Technology,2005,34(12):24-26.
[9] 宋丹,马爱斌,江静华,等. 等通道转角挤压超细晶ZL203合金的晶间腐蚀行为[J]. 机械工程材料,2010,34(3):27-30. SONG D ,MA A B ,JIANG J H ,et al. Intergranular corrosion behavior of ultra-fine grained ZL203 alloy fabricated by equal-channel angular pressing[J]. Materials for Mechanical Engineering,2010,34(3):27-30.
[10] IWAHASHI Y ,HORITA Z ,NEMOTO M ,et al. The process of grain refinement in equal-channel angular pressing[J]. Acta Materialia,1998,46(9):3317-3331. [11] 王军丽,史庆南,钱天才,等. 大变形异步叠轧技术制备高强高导超细晶铜材研究[J]. 航空材料学报,2010,30(3):14-18. WANG J L ,SHI Q N ,QIAN T C ,et al. Ultra-fine-grained copper with high strength and conductivity fabricated by severe deformation of asymmetrical accumulative roll bonding[J]. Journal of Aeronautical Materials,2010,30(3):14-18.
[12] 崔浩,史庆南,张坤华,等. 异步叠轧制备超细晶铜微观组织与力学性能[J]. 新技术新工艺,2006(8):18-20. CUI H ,SHI Q N ,ZHANG K H ,et al. Microstructure and mechanical properties of ultra-fine grained copper prepared by asymmetrical accumulative roll-bonding(AARB) process[J]. New Technology & New Process,2006(8):18-20.
[13] 吴世丁,安祥海,韩卫忠,等. 等通道转角挤压过程中fcc金属的微观结构演化与力学性能[J]. 金属学报,2010,46(3):257-276. WU S D ,AN X H ,HAN W Z ,et al. Microstructure evolution and mechanical properties of fcc metallic materials subjected to equal channel angular pressing[J]. Acta Metallurgica Sinica,2010,46(3):257-276.
[14] 王庆娟,王清平,杜忠泽. 等径弯曲通道变形制备超细晶铜的力学行为[J]. 特种铸造及有色合金,2011,31(10):887-890. WANG Q J ,WANG Q P ,DU Z Z. Mechanical properties of ultra-fine grain copper produced by equal channel angular pressing (ECAP)[J]. Special Casting & Nonferrous Alloys,2011,31(10):887-890.
[15] MIYAMOTO H ,HARADA K ,MIMAKI T ,et al. Corrosion of ultra-fine grained copper fabricated by equal-channel angular pressing[J]. Corrosion Science,2008,50(5):1215-1220. [16] 刘新凯,韦庆朕,陈东旭,等. 深海环境下静水压力与浸泡时间对304不锈钢电化学行为影响[J]. 辽宁科技大学学报,2022,45(3):179-182. LIU X K ,WEI Q Z ,CHEN D X ,et al. Effects of hydrostatic pressure and soaking time on electrochemical behavior of 304 stainless steel in deep sea environment[J]. Journal of University of Science and Technology Liaoning,2022,45(3):179-182.
[17] 王爱玲,李开鸿,张哲,等. 基于等效电路模型的X90钢交流腐蚀动力学行为[J]. 腐蚀与防护,2022,43(7):57-62. WANG A L ,LI K H ,ZHANG Z ,et al. AC corrosion electrode process of X90 steel based on equivalent circuit model[J]. Corrosion & Protection,2022,43(7):57-62.
[18] 陆春慧,郑元俐,邱憬. 镍铬、钴铬合金和纯钛在人工唾液中的耐腐蚀性评价[J]. 上海口腔医学,2010,19(2):168-172. LU C H ,ZHENG Y L ,QIU J. Study of dental alloys in the artificial saliva using an electrochemical impedance spectroscopy[J]. Shanghai Journal of Stomatology,2010,19(2):168-172.
[19] 耿敏,马洋,张文东,等. 柔性还原氧化石墨烯薄膜的制备及电化学性能研究[J]. 安康学院学报,2020,32(6):115-118. GENG M ,MA Y ,ZHANG W D ,et al. Synthesis and electrochemical investigation of flexible reduction oxidation graphene film[J]. Journal of Ankang University,2020,32(6):115-118.
[20] 郭绘芬,李子岩,李秋睿,等. ECAP对Mg-9Al-1.5Bi-0.5In合金组织及电化学性能的影响[J]. 中国有色金属学报,2022,32(6):1695-1707. GUO H F ,LI Z Y ,LI Q R ,et al. Effect of ECAP on microstructure and electrochemical properties of Mg-9Al-1.5Bi-0.5In alloy[J]. The Chinese Journal of Nonferrous Metals,2022,32(6):1695-1707.
[21] 李明山等通道转角挤压制备超细晶铜的疲劳性能西安西安建筑科技大学2008李明山. 等通道转角挤压制备超细晶铜的疲劳性能[D]. 西安:西安建筑科技大学,2008. LI M SThe fatigue property of ultrafine-grained copper produced by ECAPXi´anXi´an University of Architecture and Technology2008LI M S. The fatigue property of ultrafine-grained copper produced by ECAP[D]. Xi´an:Xi´an University of Architecture and Technology,2008.
[22] 曾许多,杨丽景,张阳明,等. 等通道转角挤压Zn-22Al合金在NaCl水溶液中的腐蚀行为[J]. 中国腐蚀与防护学报,2014,34(2):178-184. ZENG X D ,YANG L J ,ZHANG Y M ,et al. Corrosion behavior of equal-channel-angular-pressed Zn-22Al alloys in NaCl aqueous solution[J]. Journal of Chinese Society for Corrosion and Protection,2014,34(2):178-184.
[23] 李鑫,董月成,淡振华,等. 等通道角挤压制备超细晶纯Ti的腐蚀性能研究[J]. 金属学报,2019,55(8):967-975. LI X ,DONG Y C ,DAN Z H ,et al. Corrosion behavior of ultrafine grained pure Ti processed by equal channel angular pressing[J]. Acta Metallurgica Sinica,2019,55(8):967-975.