The microstructure of ZA27 pressure die-castings was examined by scanning and transmission electron microscopy after ageing for 5 years at ambient temperatures. Solidification began with the formation of compact aluminium-rich alpha' dendrites and tiny rounded alpha' particles, followed by the peritectic reaction whereby a zinc-rich beta phase formed around the edges of the primary phases. The extremely high cooling rate during solidification reduced the extent of the peritectic reaction so that the liquid became highly enriched with zinc and solidification was completed by eutectic formation of beta and eta phases, the beta joining the peritectic beta and the eta remaining in the interdendritic regions. On rapid cooling after casting through the eutectoid transformation temperature, the beta phase decomposed eutectoidally into well-formed lamellae or semi-particulate irregular particles of alpha and eta, and some lamellar colonies spread into the low-aluminium alpha'-phase cores of the dendrites to form coarse lamellar products. The bulk of the alpha', however, decomposed into a very fine mixture of zinc-rich phases in an aluminium matrix. These structures are consistent with solidification under conditions of high undercooling. Enclosed within the alpha constituent of the decomposed peritectic and eutectic beta phases were small particles of a phase which was identified as the transitional alpha'(m) phase containing 30.2% Al or 14.8% Al, with an fee crystal structure and lattice parameter (at 14.8% AI) of about 0.395 nm. It had a symmetrical cube/cube orientation relationship with the surrounding alpha phase. This metastable phase was probably stabilized by copper. Copper became concentrated in the eutectic liquid during the first stages of solidification, and was rejected from the liquid in the form of discrete irregular particles, 1-2 mu m in diameter, during eutectic solidification. After solidification, copper was also rejected from solid solution in the zinc-rich eta phase in the form of a dense precipitation of small particles of 70-120 nm diameter and 2-3 nm thick. Both of these particles were identified as the metastable c p h epsilon-phase (CuZn4) with lattice parameters a = 0.274 nm, c = 0.429 nm, and c/a = 1.566. [References: 34]
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机译：在环境温度下老化5年后，通过扫描和透射电子显微镜检查ZA27压铸件的微观结构。凝固开始于形成致密的富铝α'树枝状晶体和微小的圆形α'颗粒，然后发生包晶反应，从而在一次相的边缘形成富锌的β相。凝固过程中极高的冷却速度降低了包晶反应的程度，从而使液体高度富集锌，并通过共晶形成β相和η相来完成凝固，β与包晶β相连接，而η保留在树突间区域中。在通过共析转变温度浇铸后进行快速冷却后，β相共析分解为形成良好的薄片或半粒状不规则的α和eta颗粒，一些片状菌落扩散至树突的低铝α'相核心形成粗糙的层状产品。然而，大部分的α'在铝基质中分解成非常细的富锌相混合物。这些结构与高过冷条件下的凝固相一致。在分解的包晶和共晶β相的α成分内包裹着一个小颗粒，该相被确定为过渡α'（m）相，其中含有30.2％的Al或14.8％的Al，具有微晶结构和晶格参数（在约0.395 nm的14.8％AI）。它与周围的α相具有对称的立方/立方取向关系。该亚稳态相可能由铜稳定。铜在凝固的第一阶段就集中在共晶液体中，并在共晶凝固过程中以离散的不规则颗粒（直径为1-2微米）的形式从铜中排出。固化后，铜也以固相沉淀的形式从直径70-120 nm且厚度为2-3 nm的小颗粒中以浓锌形式从富锌eta相中排除。这两个粒子均被确定为亚稳态c p hε相（CuZn4），晶格参数为a = 0.274 nm，c = 0.429 nm和c / a = 1.566。 [参考：34]
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