Al-Zn-Mg基合金多级时效不同热处理阶段的析出行为研究

采用X射线衍射仪(XRD)和附带能谱仪(EDAX)的扫描透射电子显微镜(STEM)对Al-4.6Zn-0.9Mg合金三级时效工艺过程中强化相析出行为进行了系统研究。结果表明,100℃单级时效合金强度优于120℃单级时效;三级时效的再时效过程中,由于二级时效温度不同,合金内析出相的种类、尺寸及分布有显著差异;多级时效过程中如果二级时效温度位于300℃～400℃之间,第一级时效形成的析出相会溶解,且在再时效前期合金元素在晶界处显著偏聚,尽管在该回归温度区间内时效,很多析出相会溶解,但其中的合金元素并没有完全扩散开,依然留下了某种成份偏聚。这些偏聚的原子集团不能在后续再时效过程中重新形成GPII区。这使得再时效时析出相成核数量很少但粗化速度非常快,材料强度偏低。

Study of the precipitation behaviors in a triple-step-aged AI-Zn-Mg alloy

The precipitation behaviors in a triple-step-aged A1ZnMg alloy were studied using scanning transmission electron microscopy （STEM）, energy dispersive x-ray （EDX） analysis, and x-ray diffraction in association with thermal treatments and hardness measurements. Results show that for one-step ageing higher hardness values can be achieved if the alloy is aged at 100℃ rather than 120 ℃. The retrogression, i. e. , the second step of the triple-step-ageing process, plays a crucial role in determining the precipitate microstructure of the alloy. It is demonstrated that if the retrogression temperature is in the range between 300℃ and 400 ℃ nearly all the precipitates formed in the first-step ageing are dissolved and further more will lead to more severe boundary segregation of solute atoms in the next step of re-ageing. However, the precipitate dissolution only destroys the precipitate structure and dissolves some of the solute atoms of the original precipitates. The remained solute atoms can form the solute cluster that would not re-transform back to original precipitate structures. As such the number of effective nuclei of the hardenning precipitates during re-ageing will decrease, but they can grow （or coarsen） rapidly. Upon re-ageing, leading to a triple-step-aged alloy with low strong.