Ho添加对Al-Zn-Mg-Cu合金均匀化热处理过程中微观组织和性能演变的影响

Microstructure and Property Evolution of Al-Zn-Mg-Cu Alloy with Ho Addition During Homogenization

摘要:

通过SEM、OM和DSC，研究添加Ho的Al-Zn-Mg-Cu合金均匀化热处理制度，测试不同均匀化热处理过程中合金的电导率和硬度变化。结果表明，铸态合金中存在4种第二相：T(AlZnMgCu)，Al₇Cu₂Fe，Al₈Cu₄Ho 及S (Al₂CuMg)，第二相导致合金元素分布存在严重微观偏析。合金在475 ℃均匀化热处理20 h后，T相完全回溶基体且未观察到S相，仅剩余Al₇Cu₂Fe和Al₈Cu₄Ho。硬度和电导率随T相的回溶而变化，T相的回溶使得合金硬度升高，电导率降低。同时，在475 ℃均匀化热处理5~20 h过程中，Al₃Ho相析出，这一现象引起硬度和电导率的升高。结合均匀化动力学分析，确定合金适宜的均匀化热处理制度为470~475 ℃/20~25 h。

Abstract:

Homogenization treatment for Al-Zn-Mg-Cu alloy with Ho addition was investigated by scanning electron microscope (SEM), optical microscope (OM), and differential scanning calorimeter (DSC). Electrical conductivity and micro-hardness were tested during different homogenization processes. The results show that there are four kinds of second phases in as-cast alloy: T(AlZnMgCu), Al₇Cu₂Fe, Al₈Cu₄Ho and S (Al₂CuMg), which cause severe micro-segregation in microstructure. After homogenization treatment at 475 °C for 20 h, T phase is completely dissolved back into matrix and no S phase is observed, with Al₇Cu₂Fe and Al₈Cu₄Ho remaining. Micro-hardness and electrical conductivity are associated with vibration of T phase, dissolution of which can increase micro-hardness and decrease electrical conductivity. Besides, precipitation of Al₃Ho also contributes to the increase in both micro-hardness and electrical conductivity during homogenization at 475 °C for 5~20 h. Finally, through homogenization kinetic analysis, the appropriate homogenization parameter is determined as 470~475 °C/20~25 h.