高Zn含量Al-Zn-Mg-Cu系铝合金的凝固态显微组织

Solidification microstructure of high zinc-containing Al-Zn-Mg-Cu alloys

利用OM、SEM、XRD及TEM等分析方法,对几种高Zn含量Al-Zn-Mg-Cu系合金的凝固态组织进行研究,通过比较不同合金中一次凝固析出相的种类、形貌结构及合金元素在各相中的显微分布,揭示了不同凝固析出相的形成过程与机理。结果表明:法系7449、7056合金晶界处粗大析出相为T(Mg(Zn,Cu,Al)2)四元相共晶组织,且大部分共晶组织网层状结构发达,共晶组织特征明显;美系7136与7095合金晶界处粗大的网状结构第二相数目大大减少,主要以棒条状结构存在,且部分粗大第二相是以两相(T(Al Zn Mg Cu)与θ(Al2Cu))伴生的结构形态存在;不同合金凝固态显微组织的差异是由合金成分的不同而导致的凝固进程的差异造成的;其中,Cu元素含量对凝固态组织中一次凝固析出相的种类及结构形貌有较大影响。

The solidification microstructure of high zinc-containing Al-Zn-Mg-Cu alloys was studied by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The as-cast microstructure characteristics of alloys 7449, 7056, 7136 and 7095, including the once solidification precipitate type, morphology and the microdistribution of alloying elements, were investigated. And then the formation process and mechanism of different solidification precipitates were analyzed. The results show that the solidification precipitates in grain boundaries of alloy 7449 and alloy 7056 are non-equilibrium eutectic T(Mg(Zn,Cu,Al)2) phases, showing well-developed and obvious lamellar eutectic structure. Whereas, the number of coarse secondary phases that appears lamellar structure in as-cast alloy 7136 and alloy 7095 significantly decreases, and the non-equilibrium eutectic phases in grain boundaries mainly present rod-like morphology. Moreover, a association of T(Mg(Zn,Cu,Al)2) phase withθ(Al2Cu) phase is often observed in the rod-like eutectic phases. The as-cast microstructure is determined by solidification process, which is affected by alloy composition to a great extent. The mass fraction of element Cu plays a crucial role in controlling the type and morphology of the non-equilibrium eutectic phases in grain boundaries.