7055铝合金二次时效特征研究—(Ⅱ)显微组织与断口形貌特征

Investigation of Secondary Ageing Characteristics of 7055 Aluminum Alloy —(Ⅱ) Microstructures and Fractography

利用透射电镜和扫描电镜研究了二次时效处理后7055合金显微组织及Kahn撕裂断口形貌特征。结果表明,高温预时效时未析出的溶质原子,在低温二次时效时继续析出造成强度提高;由于二次时效温度降低,析出相长大和粗化速度降低,析出相密度高,可以获得高于单级峰值时效的强度和硬度。提高二次时效温度促进比GP区强化效果更高的η’相析出,从而获得比低温二次时效更高的强度。缩短预时效时间以及降低预时效温度,都有利于残留固溶原子和空位的保留,二次时效时硬化响应更快。晶界析出相的体积分数和形貌变化对7055合金断裂韧性存在显著的影响。低温预时效及低温二次时效获得少量且断续分布的球形沿晶析出相,断裂韧性较高。而提高二次时效温度则促进晶界析出相析出,不利于断裂韧性的改善。

Based on TEM and SEM observation, the effect of secondary ageing on microstructures and Kahn tear fractographs of 7055 Al alloy was investigated. The experimental results show that the solute atoms which did not precipitated during preliminary ageing precipitate again during secondary ageing at lower temperature. The growth and coarsening rate of the precipitates became slower due to lower secondary ageing temperatur, which resulted in higher precipitate density and strength than that of single step ageing. During secondary ageing at higher temperature higher strength was received due to the precipitation of η’ phases. Shorter preliminary ageig time and lower preliminary ageing temperature was beneficial to the retention of remnant vacancies and solute atoms, which contributed to quicker age hardening response. Preliminary and secondary ageing at lower temperature resulted in small amount of discontinuous grain boundary precipitates. As a result, the fracture toughness of the alloy is improved. However, increasing secondary ageing temperature promotes the precipitation of grain boundary precipitates resulted in bad fracture toughness.