加载方向对Al—Zn—Mg合金型材应力腐蚀开裂行为的影响
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摘要
采用恒载荷拉伸应力腐蚀试验和电化学试验研究取向对Al-Zn-Mg合金型材的应力腐蚀(SCC)开裂的影响,腐蚀介质采用质量分数3. 5%的Na Cl溶液,容器温度维持在50±2℃,并通过光学金相显微镜(OM)、扫描电子显微镜(SEM)、电子背散射衍射(EBSD)等研究不同取向试样应力腐蚀前、后的微观形貌.结果表明横向试样在315 h时断裂,而纵向试样在整个加载过程中未发生断裂,纵向试样有更好的抗应力腐蚀开裂性能;纵截面(L-S面)的腐蚀电流密度为0. 980 m A·cm-2,约为横截面(T-S面)的5倍,腐蚀倾向于沿挤压方向发展;相比T-S面,L-S面晶粒间取向差较大,大角度晶界多,容易被腐蚀产生裂纹;在应力腐蚀加载过程中,试样先发生阳极溶解,形成腐蚀坑,聚集的腐蚀产物所产生的楔入力和恒定载荷的共同作用促使裂纹在腐蚀介质中加速扩展,两种取向试样均发生了明显的晶间腐蚀,存在应力腐蚀开裂的倾向.
Thick-section Al-Zn-Mg aluminum alloy extrusions are key materials for manufacturing rail transit vehicles,and stress corrosion cracking( SCC) is an important engineering application problem during the service life of these materials. The effect of sampling direction on the stress corrosion cracking behavior of Al-Zn-Mg alloys was investigated through constant load tensile stress corrosion and electrochemical tests. The microstructures of specimens were analyzed in different sampling directions both before and after stress corrosion via optical microscopy,scanning electron microscopy,and electron backscatter diffraction. Specimens with their tensile axes parallel or perpendicular to the extrusion direction of the extruded profiles were labeled as longitudinal specimens and transverse specimens,respectively. The specimens were completely immersed in a corrosive solution,a mixture of 35 g Na Cl and 1 L deionized water,with a constant unidirectional loading of 225 MPa for 360 h at 50 ± 2 ℃. The experimental results show that the transverse specimen is fractured at 315 h,whereas the longitudinal specimen does not break during the entire loading process. Thus,the transverse specimens have poor resistance to stress corrosion cracking. The corrosion current density of the longitudinal section( L-S) is0. 980 m A·cm-2,which is approximately 5 times that of the transverse section( T-S). Thus,corrosion tends to propagate along the longitudinal direction. The L-S is more susceptible to corrosion than the T-S owing to the larger misorientation difference and higher energy of the grain boundary. During the stress corrosion loading process,anodic dissolution occurs and forms corrosion pits. Then,the cooperation of the wedge force produced by the accumulation of corrosion products and constant load causes the crack to propagate along the grain boundary. Intergranular corrosion of the two types of samples is obvious under all immersion corrosion conditions. Different specimens exhibit the tendency to undergo stress corrosion cracking.
Thick-section Al-Zn-Mg aluminum alloy extrusions are key materials for manufacturing rail transit vehicles,and stress corrosion cracking( SCC) is an important engineering application problem during the service life of these materials. The effect of sampling direction on the stress corrosion cracking behavior of Al-Zn-Mg alloys was investigated through constant load tensile stress corrosion and electrochemical tests. The microstructures of specimens were analyzed in different sampling directions both before and after stress corrosion via optical microscopy,scanning electron microscopy,and electron backscatter diffraction. Specimens with their tensile axes parallel or perpendicular to the extrusion direction of the extruded profiles were labeled as longitudinal specimens and transverse specimens,respectively. The specimens were completely immersed in a corrosive solution,a mixture of 35 g Na Cl and 1 L deionized water,with a constant unidirectional loading of 225 MPa for 360 h at 50 ± 2 ℃. The experimental results show that the transverse specimen is fractured at 315 h,whereas the longitudinal specimen does not break during the entire loading process. Thus,the transverse specimens have poor resistance to stress corrosion cracking. The corrosion current density of the longitudinal section( L-S) is0. 980 m A·cm-2,which is approximately 5 times that of the transverse section( T-S). Thus,corrosion tends to propagate along the longitudinal direction. The L-S is more susceptible to corrosion than the T-S owing to the larger misorientation difference and higher energy of the grain boundary. During the stress corrosion loading process,anodic dissolution occurs and forms corrosion pits. Then,the cooperation of the wedge force produced by the accumulation of corrosion products and constant load causes the crack to propagate along the grain boundary. Intergranular corrosion of the two types of samples is obvious under all immersion corrosion conditions. Different specimens exhibit the tendency to undergo stress corrosion cracking.
引文
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[23] Liu J H,Hao X L,Li S M,et al. Resistance to stress corrosion cracking of new Ali-Mg--Cu alloy containing Sc. Chin J Nonferrous Met,2010,20(3):415(刘建华,郝雪龙,李松梅,等.新型含钪Al--Mg-Cu合金的抗应力腐蚀开裂特性.中国有色金属学报,2010,20(3):415)
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[25] Viswanadham R K,Sun T S,Green J A S. Grain boundary segregation in Al-Zn-Mg alloys———Implications to stress corrosion cracking. Metall Mater Trans A,1980,11(1):85
[2] Deng Y L,Wang Y F,Lin H Q,et al. Effect of extrusion temperature on strength and fracture toughness of an Al--Zn--Mg alloy.Chin J Mater Res,2016,30(2):87(邓运来,王亚风,林化强,等.挤压温度对Al-Zn-Mg合金力学性能的影响.材料研究学报,2016,30(2):87)
[3] Li Y,Yu X. Research on application of aluminum-extruded-profiles in military electronic equipment. Machine Build Autom,2015,44(3):68(李云,于新.铝型材在军用电子设备结构中的应用研究.机械制造与自动化,2015,44(3):68)
[4] Zhuang J J,Zhang X Y,Sun B,et al. Microarc oxidation coatings and corrosion behavior of 7050 aluminum alloy. Chin J Eng,2017,39(10):1532(庄俊杰,张晓燕,孙斌,等.微弧氧化对7050铝合金腐蚀行为的影响.工程科学学报,2017,39(10):1532)
[5] Chen Y Q,Deng Y L,Wan L,et al. Microstructures and properties of 7050 aluminum alloy sheet during creep aging. J Mater Eng,2012(1):71(陈愿情,邓运来,万里,等.蠕变时效对7050铝合金板材组织与性能的影响.材料工程,2012(1):71)
[6] Braun R. Environmentally assisted cracking of aluminum alloys.Materialwiss Werkstofftech,2007,38(9):674
[7] Hou L G,Zhao F,Zhuang L Z,et al. Retrogression and re-aging7B50 Al alloy plates based on examining the through-thickness microstructures and mechanical properties. Chin J Eng,2017,39(3):432(侯陇刚,赵凤,庄林忠,等.基于厚向组织性能考量的7B50铝合金中厚板回归再时效热处理.工程科学学报,2017,39(3):432)
[8] Liu J H,Li D,Guo B L. Investigation of stress corrosion cracking of 7xxx series aluminum alloys. Corros Sci Prot Technol,2001,13(4):218(刘继华,李荻,郭宝兰. 7xxx系列Al合金应力腐蚀开裂的研究.腐蚀科学与防护技术,2001,13(4):218)
[9] Jha A K,Murty S V S N,Diwakar V,et al. Metallurgical analysis of cracking in weldment of propellant tank. Eng Fail Anal,2003,10(3):265
[10] Rao A C U,Vasu V,Govindaraju M,et al. Stress corrosion cracking behaviour of 7xxx aluminum alloys:a literature review.Trans Nonferrous Met Soc China,2016,26(6):1447
[11] Ooro J. The stress corrosion cracking behaviour of heat-treated Al-Zn-Mg-Cu alloy in modified salt spray fog testing. Mater Corros,2010,61(2):125
[12] Heinz A,Haszler A,Keidel C,et al. Recent development in aluminium alloys for aerospace applications. Mater Sci Eng A,2000,280(1):102
[13] Chen K H,Huang L P. Strengthening toughening of 7xxx series high strength aluminum alloys by heat treatment. Trans Nonferrous Met Soc China,2003,13(3):484
[14] Yu B S,Xing S M,Ao X H,et al. Effect of pressures on macro-/microstructures and mechanical properties of A380 aluminum alloy. Chin J Eng,2017,39(7):1020(于佰水,邢书明,敖晓辉,等.压力对A380铝合金的铸造组织和力学性能的影响.工程科学学报,2017,39(7):1020)
[15] Lee E U,Taylor R,Lei C,et al. Stress corrosion cracking of aluminum alloys. Metall Trans A,1975,6(4):631
[16] Xiao Y P,Pan Q L,Li W B,et al. Influence of retrogression and re-aging treatment on corrosion behaviour of an Al-Zn-Mg--Cu alloy. Mater Des,2011,32(4):2149
[17] Wang D,Ma Z Y. Effect of pre-strain on microstructure and stress corrosion cracking of over-aged 7050 aluminum alloy. J Alloys Compd,2009,469(1-2):445
[18] Rometsch P A,Zhang Y,Knight S. Heat treatment of 7xxx series aluminium alloys—Some recent developments. Trans Nonferrous Met Soc China,2014,24(7):2003
[19] Speidel M O. Stress corrosion cracking of aluminum alloys. Metall Trans A,1975,6(4):631
[20] Fang H C,Chao H,Chen K H. Effect of recrystallization on intergranular fracture and corrosion of Al--Zn-Mg--Cu--Zr alloy. J Alloys Compd,2015,622:166
[21] Huang J,Peng G S,Song G S,et al. The effect of undissolved particles and the recrystallization on the resistance of SCC of Al--Zn-Mg-Cu alloys. J Qilu Univ Technol,2018,32(2):45(黄俊,彭国胜,宋广生,等.未溶相和再结晶对Al-Zn-MgCu合金应力腐蚀抗力的影响.齐鲁工业大学学报,2018,32(2):45)
[22] Shi Y J,Pan Q L,Li M J,et al. Effect of Sc and Zr additions on corrosion behaviour of Al-Zn--Mg-Cu alloys. J Alloys Compd,2014,612:42
[23] Liu J H,Hao X L,Li S M,et al. Resistance to stress corrosion cracking of new Ali-Mg--Cu alloy containing Sc. Chin J Nonferrous Met,2010,20(3):415(刘建华,郝雪龙,李松梅,等.新型含钪Al--Mg-Cu合金的抗应力腐蚀开裂特性.中国有色金属学报,2010,20(3):415)
[24] Song R G,Zeng M G. Hydrogen embrittlement of high strength aluminum alloys. J Mater Sci Eng,1995,13(1):63(宋仁国,曾梅光.高强度铝合金的氢脆.材料科学与工程,1995,13(1):63)
[25] Viswanadham R K,Sun T S,Green J A S. Grain boundary segregation in Al-Zn-Mg alloys———Implications to stress corrosion cracking. Metall Mater Trans A,1980,11(1):85