7N01铝合金应力腐蚀行为研究

Stress Corrosion Behavior of 7N01 Aluminum Alloy

通过对7N01铝合金进行T4和T7热处理,选取材料的挤压方向(ED)和垂直挤压方向(ND)试样,利用透射电子显微镜(TEM)、扫描电子显微镜(SEM)、X射线衍射(XRD)、慢应变速率拉伸(SSRT)和电化学分析方法,研究了7N01铝合金的应力腐蚀(SCC)行为。结果表明:7N01-T4晶界无明显析出相,晶内存在少量粗大析出相,7N01-T7的晶间无析出带(PFZ)宽度为38 nm,晶内有大量弥散分布的细小析出相,主要为η(MgZn_(2))相;T7态的腐蚀速率高于T4态,LS面(long-short,长-短面,垂直于ND方向的面)的腐蚀速率高于ST面(short-transverse,短-横面,垂直于ED方向的面);在6.67×10^(-6)s^(-1)的应变速率下,T4态的SCC行为不明显,而T7态的SCC敏感性高于T4态,ED与ND的SCC敏感性相同;SCC敏感性来自于由腐蚀导致的性能损失和应力与腐蚀介质交互作用产生的附加敏感性,7N01-T7的ED与ND的附加敏感性指数分别为0.011和0.148;合金在3.5%NaCl溶液中慢拉伸后断口氢浓度为295×10^(-6),氢元素会降低材料的点蚀电位和增大其腐蚀电流密度,从而降低耐蚀性;氢造成T7态ED和ND试样的力学性能损失在I_(SCC)(应力腐蚀敏感性指数)中占比分别为84.7%和66.7%,且充氢断口特征与SCC断口相同,7N01-T7的主要应力腐蚀机制为氢致开裂。

7N01 aluminum alloy is a kind of aluminum alloy used in the structure of rail transport train body.It has been widely used because of its good workability and weldability.However,7N01 aluminum alloy has inferior resistance to exfoliation corrosion and stress corrosion,which leads to many failure accidents.Therefore,it is necessary to understand the stress corrosion behavior of 7N01aluminum alloy.For investigating the stress corrosion mechanism of 7N01 aluminum alloy is controlled by the mechanism of anodic dis‐solution(Al atoms converse to Al ion,leading to corrosion pits)or hydrogen-induced cracking(H atoms combine to H_(2),leading to Hy‐drogen bubbling or crack).The effects of structure of 7N01 aluminum alloys on its stress corrosion mechanism were studied with elec‐trochemical hydrogen charged and non-hydrogen charged T7(Overaging,475℃/1 h+120℃/96 h)and T4(underaging,475℃/1 h+25℃/15 d)samples.By means of transmission electron microscope(TEM),scanning electron microscopy(SEM),X-ray diffraction(XRD),slow strain rate tension(SSRT)and electrochemical analysis,stress corrosion cracking(SCC)behavior of 7N01 aluminum alloy was studied.TEM and XRD results showed that there was no obvious precipitation at grain boundaries of 7N01-T4.A few coarse precipitates could be found in the grains.The PFZ width of 7N01-T7 was 38 nm,and there were a large number of dispersed precipi‐tates in grains,mainlyη(MgZn_(2))phase;The electrochemical analysis results showed that corrosion rate of 7N01-T7 was higher than that of 7N01-T4,and that of LS(longtitudinal-short)surface was higher than that of ST(short-transverse)surface.SSRT results showed that at the strain rate of 6.67×10^(-6)s^(-1),T4 showed no obvious SCC behavior,while T7 had higher SCC susceptibility than T4.SCC susceptibility of ED(parallel to the extrusion direction)was equal to that of ND(normal to extrusion direction);SCC susceptibil‐ity consists of the susceptibility caused by corrosion,and the additional susceptibility caused by the interaction between stress and cor‐rosion.The susceptibility caused by corrosion,I_(corr)could be characterized by the plasticity and strength loss of the specimens which preimmersed in 3.5%NaCl aqueous solution.The additional SCC susceptibility,I_(c-σ),which could be caculated by the difference of I_(SCC )and I_(corr),indiced of ED and ND of 7N01-T7 are 0.011 and 0.148,respectively.SSRT in 3.5%sodium chloride solution could make the material contain high content of hydrogen,which reduced pitting potential and increased corrosion current density,thus reducing the corrosion resistance of the material.The mechanical performance loss of ED-T7 and ND-T7 specimens caused by hydrogen in I_(SCC)was84.7%and 66.7%,respectively.The fracture characteristics of hydrogen charged specimens were the same as that of SCC specimens.The main stress corrosion mechanism of 7N01-T7 was hydrogen-induced cracking.