淬火速率对7136铝合金应力腐蚀开裂敏感性的影响

Effect of Quenching Rate on Stress Corrosion Cracking Susceptibility of 7136 Aluminum Alloy

针对7xxx系铝合金的抗应力腐蚀性能对淬火速率敏感的问题,通过浸入式末端淬火和慢应变速率拉伸实验研究了淬火速率对7136铝合金应力腐蚀开裂(SCC)敏感性的影响规律。结果显示,随着淬火速率的减小,合金的SCC敏感性先增高后降低,淬火速率约为5.3℃/s时的SCC敏感性最高;SCC裂纹扩展方式由穿晶扩展转变为沿晶扩展。淬火速率越小淬火析出相越多,尺寸越大,晶界和亚晶界附近的无沉淀析出带越宽。淬火速率大于5.3℃/s时,晶界析出相中Zn、Mg元素含量随淬火速率减小迅速增加,而淬火速率小于5.3℃/s后,Cu元素含量迅速增加。晶界和亚晶界析出相形貌特征以及晶界析出相化学成分的变化是SCC敏感性随淬火速率减小先增加后降低的主要原因。

7xxx series aluminum alloys are well-known structural materials,and they have been used in various fields,such as aerospace and vehicle,owing to their low density,high strength,and good formability.However,they are susceptible to stress corrosion cracking(SCC).SCC reduces the service life of these alloys and limits their application.With the development of the aerospace and vehicle industries,high-strength 7xxx series aluminum alloys are manufactured as semiproducts with large sections,such as thick plates,to avoid welding and jointing defaults.Quenching is a critical step for producing thick plates because their properties,such as SCC susceptibility,are sensitive to the quenching rate,and the quenching rate is generally lower at the center layer than at the surface layer during quenching.In this study,the effect of quenching rate on the SCC susceptibility of 7136 aluminum alloy was investigated using immersion end-quenching technique and slow strain rate tensile(SSRT)test.The strength,elongation,and fracture time of the samples after SSRT in oil and NaCl solution were obtained,and the crack features on and near the fracture surface were examined using SEM.SCC susceptibility was evaluated using the reduction rates of strength,elongation,fracture time,and stress corrosion sensitivity index(I).The mechanism is discussed herein based on microstructural examination using SEM,EBSD,STEM in a HAADF mode,and EDS.Results show that the SCC susceptibility of the alloy first increases and then decreases with the decrease in the quenching rate.The SCC susceptibility is the highest at the quenching rate of approximately 5.3℃/s with the Ivalue of approximately 0.049.With the decrease in the quenching rate,the crack propagation changes from a transgranular to a intergranular mode;the number and size of quench-induced precipitates increase gradually,and the precipitate free zones(PFZs)near grain boundary(GB)and subgrain boundary(SGB)widen;and the contents of Zn and Mg in precipitates at grain boundaries increase rapidly when the quenching rate is greater than 5.3℃/s,and Cu content increases rapidly when the quenching rate is lower than 5.3℃/s.The quench-induced changes in the morphology and chemical composition of precipitates at GB/SGB are the main reasons for the SCC susceptibility to increase first and then decrease with the decrease in the quenching rate.