加载方向对Al—Zn—Mg合金型材应力腐蚀开裂行为的影响

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. 

Effect of sampling direction on the stress corrosion cracking behavior of Al-Zn-Mg alloy

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.