摘要
采用扫描电镜(SEM)、透射电镜(TEM)及晶间腐蚀测试等测试手段,研究了冷却预处理对7055铝合金板材第二相、微区成分和耐晶间腐蚀性能的影响,旨在探明微区成分差异对其合金耐腐蚀性能的影响规律。结果表明:冷却预处理会引起合金元素的局部富集,从而改变局部区域合金元素的分布范围,随着冷却预处理温度降低,晶界溶质原素的含量先增加后降低。当冷却预处理温度为455℃时,晶界溶质元素含量最高,Zn,Mg和Cu元素含量分别为3.97%,3.63%和3.15%;当冷却预处理温度为445℃时,由于少量S-Al_(2)MgCu相析出与合金元素团簇现象的发生,晶界溶质元素含量有所降低。与常规固溶处理结合峰时效相比,455℃冷却预处理结合峰时效处理合金的晶间腐蚀抗性得到一定程度的增强,这是由于晶界析出η相的断续分布使得腐蚀行为在晶界发生时的通道被打断,以及晶界η相Cu元素含量升高使得晶界析出的η相与之相邻区域的电位差减小。
7055 aluminum alloy treated by traditional peak ageing(T6)had the highest tensile strength and a poor corrosion resistance.The high alloying element content and extremely complicate precipitation behavior during heat treatments of 7055 aluminum alloy led to the diversity of micro-area composition,which showed a great difference of corrosion resistance.High Cu content at grain boundaries could improve the corrosion resistance 7055 aluminum.In order to improve the corrosion resistance of 7055 aluminum alloys treated by traditional peak ageing(T6),7055 aluminum alloy plates prepared by snake rolling were used as experiment materials for different heat treatment processes.The equilibrium phase diagram of 7055 aluminum alloy was calculated by JMat Pro software and the precipitate temperature of the main precipitates of Mg Zn2(ηphase),Al2Cu Mg(S phase)and Al3Fe were 430,454 and 570℃,respectively.According to the precipitate temperature of S phase,three cooling pretreatment temperatures(465,455 and 445℃)were selected to heat-treat 7055aluminum alloy plates,respectively.The solid solution(480℃×1 h)and cooling pretreatment of 7055 aluminum alloy plate were carried out on Gleeble-3500 thermal simulation test machine,and ageing process were performed in the air circulating furnace.EIectrochemical Impedance Spectroscopy(EIS)of different heat-treated samples was measured by Zennium electrochemical workstation.According to GB/T 7998-2005 test standard,the accelerated corrosion test of the aged samples for 12 h was carried out.The corrosion environment was neutral corrosion solution(35 g Na Cl+10 ml H2O2+100 ml H2O)at(35±3)℃in aqueous solution.The corrosion depths of the samples were observed and measured by Axio imager microscope.The effects of cooling pre-treatment on the precipitate behavior,micro-zone composition of 7055 aluminum alloy plates were studied by means of scanning electron microscopy(SEM),scanning transmission electron microscopy(STEM).The cooling pre-treatment could cause the local enrichment of alloying elements,and the precipitation of S phase was closely related to the content of Cu elements at the grain boundary.After water quenching,the content of solute atoms at grain boundaries firstly increased and then decreased with the decrease of pre-treatment temperature.When the pretreatment temperature was455℃,the solute content of grain boundary was the highest,and the contents of Zn,Mg and Cu elements were 3.97%,3.63%and3.15%,respectively.When the pre-treatment temperature was 445℃,S phase precipitated,the solute content of Mg and Cu at grain boundary decreased and the contents of Zn,Mg and Cu elements were 4.13%,2.83%and 2.47%,respectively.The sample pretreated by 455℃was aged by peak ageing process(Sample CPt455+T6),and then was compared with sample treated by traditional T6(Sample SS+T6).Compared with Sample SS+T6,the size of intragranular precipitates of Sample CPt455+T6 slightly increased,while the precipitates at grain boundary was obviously different.In the cooling pretreatment stage at 455℃,alloying elements quickly segregated to grain boundaries,which shortened the diffusion time of alloying elements gathering to grain boundaries during aging.Therefore,the discontinuousηphase formed at the grain boundaries of Sample CPt455+T6,while Sample SS+T6 had a continuousηphase at grain boundaries.Micro-area composition analysis presentedη-Mg Zn2 phases at grain boundary in Sample CPt455+T6 had higher Cu content than that of Sample SS+T6.After intragranular corrosion test,the corrosion depth of Sample SS+T6 was(68±5)μm,while the corrosion depth of Sample CPt455+T6 was(154±5)μm.The larger the diameter of circular arc in electrochemical impedance spectroscopy(EIS)curves,the greater the impedance and the better the corrosion resistance.The circular arc diameter of Sample CPt455+T6 was larger than that of Sample SS+T6.Therefore,the corrosion resistance of Sample CPt455+T6 was better than that of Sample SS+T6.η-Mg Zn2 phases at the grain boundaries were regarded as an anodic during corrosion.For continuousη-Mg Zn2 phases in Sample SS+T6,the formed corrosion channel could continuously expand along the grain boundaries.The discontinuousηphase in Sample CPt455+T6 could cut off corrosion channels,which made the corrosion only occur in the local area of grain boundaries and the integrity of grain boundaries were retained.Thus,Sample CPt455+T6 had better intergranular corrosion resistance than Sample SS+T6.the probability of Cu atoms replacing Zn atoms in grain boundariesηof Sample CPt455+T6 was larger than that of Sample SS+T6,it led to the positive shift of the corrosion potential ofηphase,which reduced the potential difference between the grain boundariesηphase and the precipitate free zone.Based on the test and characterization results,the conclusion we could get were the content of grain boundaries solute atoms increased with the cooling pretreatment temperature decreased.When the cooling pretreatment temperature was 445℃,S phase precipitated and the solute content at grain boundary decreased.Compared with SS+T6 process,CPt455+T6 process caused discontinuousηphase with high Cu content to precipitate at grain boundaries of 7055 aluminum alloy.Furthermore,compared with SS+T6 process,CPt455+T6 process could enhance AC impedance spectroscopy and intergranular corrosion resistance of 7055 aluminum alloy.
作者
王文波
闫亮明
胡强
杜青春
王一凡
Wang Wenbo;Yan Liangming;Hu Qiang;Du Qingchun;Wang Yifan(School of Materials Science and Engineering,Inner Mongolia University of Technology,Hohhot 010051,China;Inner Mongolia Key Laboratory of Light Metal Materials,Hohhot 010051,China;Baotou Shengtai Auto Parts Manufacturing Co.,Ltd.,Baotou 014016,China)
出处
《稀有金属》
EI
CAS
CSCD
北大核心
2024年第10期1378-1387,共10页
Chinese Journal of Rare Metals
基金
国家自然科学基金项目(51764043)
内蒙古自治区科技创新引导项目(KCBJ2018017)
内蒙古自治区高校“青年科技英才”支持计划项目(NJYT-20-A16)
内蒙古自治区留学回区人员科技活动项目
内蒙古自然科学基金项目(2020MS05061)资助。
关键词
铝合金
冷却预处理
晶界
析出相
元素分布
aluminum alloy
cooling pre-treatment
grain boundary
precipitation
element distribution
