车体用挤压态Al-6.5Zn-1.55Mg-0.25Cr-0.1Zr合金晶粒取向及应力腐蚀性分析

对含有不同显微组织形态的Al-6.5Zn-1.55Mg-0.25Cr-0.1Zr铝合金开展耐应力腐蚀表征,利用EBSD和TEM处理方法深入探讨了试样发生应力腐蚀开裂的特性。研究结果表明:晶粒形成了许多小角度晶界,大角度晶界基本都是由纤维状晶界构成。以挤压工艺制备的Al-Zn铝合金可以对再结晶过程起到抑制作用,由此减小大角度晶界的比例。有大量亚稳态球形Mg-Zn2颗粒分布于等轴晶组织以及纤维组织晶粒中,还有大量纤维组织出现在亚晶结构中;在纤维组织中还可以观察到一些非常细小的亚晶界析出相,颗粒尺寸只有12 nm。原始组织会减小应力耐腐性,在不同时间的裂纹扩展方向也存在较大差异,在临界深度处变成横向排布的裂纹。大角度晶界成为应力腐蚀裂纹的高效扩展通道,当形成小角度亚晶界后则能够对裂纹扩展发挥抑制作用。

Analysis on grain orientation and stress corrosion for extruded Al-6.5Zn-1.55Mg-0.25Cr-0.1Zr alloy used in vehicle body

The stress corrosion resistance of Al-6.5Zn-1.55Mg-0.25Cr-0.1Zr aluminum alloy with different microstructures was characterized,and the stress corrosion cracking characteristics of sample were investigated by EBSD and TEM treatment.The results show that many grain boundaries of small angle are formed in grain,while the grain boundaries of large angle are basically composed of fibrous grain boundaries.However,Al-Zn aluminum alloy prepared by extrusion process inhibits the recrystallization process,and the proportion of grain boundary of large angle reduces.There are a lot of metastable spherical MgZn2 particle distributing in the isometric tissue and fibrous tissue of grain,and there are many fibrous tissue appeared in the crystal structure.In the fibrous tissue,some very small subgrain boundary precipitates with the particle sizes of 12 nm are observed.Furthermore,primary grain microstructure reduces the stress corrosion resistance,and the direction of crack propagation at different times also has great difference to become transverse crack at the critical depth.Thus,the grain boundary of large angle is the efficient propagation channel of stress corrosion crack,and the subgrain boundary of small angle inhibits the crack propagation.