Microstructure and mechanical properties of Al-Zn-Cu-Mg-Sc-Zr alloy after retrogression and re-aging treatments

The mechanical properties and stress corrosion cracking (SCC) resistance of an Al-Zn-Cu-Mg-Sc-Zr alloy under different aging conditions were investigated. The dependence of microstructure and mechanical properties on aging parameters was evaluated by tensile test, hardness test and conductivity measurement. The results show that for the alloys with retrogression and re-aging treatment (RRA), the conductivity increases with the retrogression time and temperature, while the tensile strength decreases. The transmission electron microscopy (TEM) results show that the precipitates η(MgZn₂) at grain boundary aggregate apparently with retrogression time and the precipitates inside the matrix exhibit the similar distribution to T6 temper, which comprises fine GP zones, large η′(MgZn₂) and η(MgZn₂) phases. According to the mechanical properties and microstructure observations, the optimal RRA regime is recommended to be 120 °C, 24 h + 180 °C, 30 min + 120 °C, 24 h. The strength level of the alloy after the optimum RRA treatment is similar to that in T6 condition and the SCC resistance is improved obviously in contrast to T6 condition.     

含Sc超高强Al-Zn-Cu-Mg-Zr合金均匀化过程中的组织演变

用光学显微镜（OM）、扫描电子显微镜（SEM）、能谱分析（EDS）和X射线衍射技术（XRD）分析了不同均匀化条件下Al-Zn-Cu-Mg-Sc-Zr铝合金的组织演变与成分分布。结果表明：Al-Zn-Cu-Mg-Sc-Zr铝合金铸锭中存在严重的枝晶偏析,晶界有许多共晶相,主要元素在枝晶内部区域呈周期性分布;晶界的主要共晶相为Al7Cu2Fe相和T（Al2Mg3Zn3）相;在合金均匀化过程中,随着温度的升高和时间的延长,残留相逐渐溶入基体,这个过程可以用一指数方程来描述;合金的过烧温度为473.9°C;最佳的均匀化处理制度为470°C,24h,这一结果与均匀化动力学方程测算的结果相符。     

含Sc超高强Al-Zn-Mg-Cu-Zr合金均匀化处理

采用光学显微镜、扫描电镜、能谱分析、X射线衍射分析和DSC差热分析等研究了含Sc超高强Al-Zn-Mg-Cu-Zr合金铸态与均匀化态的显微组织和成分分布。研究了均匀化动力学过程,导出了均匀化动力学方程。结果表明:合金铸态组织存在晶界偏析,Cu、Zn和Mg元素存在严重的晶界偏析。470℃×24h均匀化处理后,合金晶界变得细小稀疏,晶界上非平衡低熔点共晶相基本消除。由均匀化动力学方程,可得出合金的均匀化处理制度为470℃×22h,与实验结果(470℃×24h)基本相符。因此合金适宜的均匀化处理工艺为470℃×24h。     

时效工艺对Al-Zn-Cu-Mg-Sc-Zr合金组织与性能的影响

通过透射电镜分析、力学性能和电导率测试,研究了单级时效、双级时效和回归再时效（RRA）工艺对含Sc超高强Al-Zn-Cu-Mg-Zr合金组织与性能的影响。结果表明：经（120℃×8 h＋160℃×16 h）双级时效处理后,合金的强度大幅下降,而电导率显著升高;其晶内组织开始粗化,晶界析出相呈断续状分布,无沉淀析出带（PFZ）形成。经（120℃×24 h预时效＋180℃×30min回归处理＋120℃×24 h终时效）RRA处理后,合金既能保持接近T6态的强度,也能获得较高的电导率;其晶内析出组织与T6态的组织类似,而晶界析出相则聚集、粗化,与过时效的组织相似。