利用小角度X射线散射(SAXS)技术系统、定量地研究了AA 7055铝合金在120和160 ℃时效过程中析出相的演变规律。结果表明,在120 ℃时效时,随时效时间延长析出相半径不断增加,时效5 h后半径基本稳定,约为3.3 nm,且尺寸分布基本不变;析出相的体积分数随着时效时间的延长不断增加,由时效5 h时的2.4%增加到时效60 h时的5.2%。在160 ℃时效时,随时效时间延长析出相半径不断增加,由时效0.5 h时的3.1 nm增加到时效72 h时的11.7 nm,尺寸分布范围也随着时效时间的延长而逐渐增加;析出相的体积分数随着时效时间的延长先不断增加,由时效0.5 h时的1.4%增加到时效16 h时的5.4%,时效16 h后趋于平衡。在120和160 ℃时效时,析出相均属于轴比介于0.2~0.3之间的扁椭球状,即为盘状。
AA 7055 aluminum alloy is a newly advanced Al-Zn-Mg-Cu alloy. It has been wide applied in aviation and aerospace field due to its attractive combined properties, such as high strength, high fracture toughness, good resistance to the growth of fatigue cracks and good stress corrosion resistance, and so on. It is generally believed that the optimum ageing precipitates are responsible for these good properties. However, the detailed information, such as size and its distribution, volume fraction, and morphology of precipitate in this alloy is still not clear. Although TEM is used to determine these information, the results are mostly qualitative. Small angle X-ray scattering (SAXS) provides a direct technique to determine the size, morphology and volume fraction of nano-scale particles and the sampling size is much larger than that in TEM. In this work, the evolution of the precipitates during ageing at 120 and 160 ℃ in AA 7055 aluminum alloy were investigated systematically and quantitatively by SAXS technique. The results show that, when ageing at 120 ℃, the average radius of the precipitates increases with increasing the ageing time. After ageing for 5 h and later, the average radius of the precipitates is 3.3 nm, and its distribution almost keeps stably. The volume fraction of the precipitates is also increased with increasing the ageing time. When ageing from 5 h to 60 h, the volume fraction increases from 2.4% to 5.2%. When ageing at 160 ℃, however, the average radius of the precipitates increases from 3.1 nm to 11.7 nm with increasing the ageing time from 0.5 h to 72 h. The volume fraction of the precipitates increases from 1.4% to 5.4% with increasing the ageing time from 0.5 h to 16 h. After ageing for 16 h and later, the volume fraction of the precipitates keeps stably. Both ageing at 120 and 160 ℃, the morphology of the precipitates is similar to a flat ellipsoid with an axis ratio between 0.2 and 0.3. Based on these quantitative results of the precipitates, the strength models during ageing will be built possibility.