Precipitation in magnesium alloy WE54 during isothermal ageing at 250°C
Precipitation in magnesium alloy WE54 during isothermal ageing at 250°C
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DOI：
10.1016/s1359-6462(99)00084-6
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发表时间：
1999-04
影响因子：
6
通讯作者：
J. Nie;B. Muddle
中科院分区：
文献类型：
--
作者：
J. Nie;B. Muddle
人们对开发用于汽车和航空航天工业的高强度、抗蠕变的镁合金的兴趣迅速增长(1-4)。到目前为止，开发的最成功的镁合金是基于Mg-Y-ND系(5-9)的镁合金，被鉴定为WE54(Mg-5.1wt%Y-3.3wt%RE-0.5wt%Zr)和WE43(mg-4.0wt%Y-3.3wt%RE-0.5wt%Zr)，其中RE代表稀土元素。这些合金的强度基本上是通过沉淀强化获得的。据报道，根据时效温度的不同，WE合金中的析出顺序(5-7)涉及到相的形成，这些相指定为？、？和？亚稳相具有D019晶体结构(六方，a 0.642 nm，c 0.521 nm)(5)。中间相为碱心正交晶系(a 0.640 nm，b 2.223 nm，c 0.521 nm)，平衡相为面心立方(a 2.223 nm)(5)。相的最高溶解温度据报道分别低于200℃和300℃(5-7)。因此，在250℃等温时效过程中，已报道的析出序列(5-7)只涉及连续形成的？相和？相。所有这三个析出相都被描述为在镁基相(5-7)的{1100}α面上形成的片状，并且很难根据形态区分它们。这些棱柱状的沉淀板被认为在强化这种镁合金方面起着重要的作用。然而，对这些阶段的详细描述很少，也缺乏实验证据来支持所报道的降水序列。亚稳中间相(S)在连续中间相或平衡析出相形核中的作用尚待证实。本文对WE54合金在250℃等温时效过程中的析出顺序进行了实验观察，特别注意了中间相(S)在相继亚稳和平衡析出产物形核中的作用。
There has been a rapid growth in interest in the development of higher strength, creep-resistant magnesium alloys for the automotive and aerospace industries (1–4). The most successful magnesium alloys developed to date have been those based on the Mg-Y-Nd system (5–9), identified as WE54 (Mg-5.1 wt% Y-3.3 wt% RE-0.5 wt% Zr) and WE43 (Mg-4.0 wt% Y-3.3 wt% RE-0.5 wt% Zr), where RE represents rare earth elements. The strength of these alloys is achieved essentially via precipitation strengthening. Depending on the ageing temperature, the precipitation sequence in WE alloys has been reported (5–7) to involve formation of phases designated ß, ß and ß. The metastable phase ß has a D019 crystal structure (hexagonal, a 0.642 nm, c 0.521 nm)(5). The intermediate phase ß has a base-centred orthorhombic structure (a 0.640 nm, b 2.223 nm, c 0.521 nm), while the equilibrium phase ß is face-centred cubic (a 2.223 nm)(5). The maximum solvus temperatures for the ß and ß phases have been reported to be below 200 and 300 C, respectively (5–7). Thus during isothermal ageing at 250 C, the precipitation sequence has been reported (5–7) to involve successive formation of the ß and ß phases only.All three precipitate phases have been described to form as plates on {1100} α planes of the magnesium matrix phase (5–7), and it has proven difficult to distinguish them on the basis of morphology. These prismatic precipitate plates have been suggested (10) to play an important role in strengthening such magnesium alloys. However, there has been little detailed characterisation of these phases and there is a lack of experimental evidence to support the reported precipitation sequence. The role of the metastable intermediate precipitate phase (s) in the nucleation of successive intermediate or equilibrium precipitate phases remains to be established. It is thus the purpose of the present paper to report experimental observations of the precipitation sequence in a WE54 alloy during isothermal ageing at 250 C, with particular attention given to the role of intermediate precipitate phase (s) in nucleating successive metastable and equilibrium precipitation products.