Precipitation Mechanism of coherent precipitates using HAADF-STEM

我们研究了Guinier-Preston（GP）区在老化的Al-Cu合金使用原子分辨率高角度环形探测器暗场扫描透射电子显微镜（HAADF-STEM），连同传统的高分辨率透射电子显微镜（HRTEM）。[1]Al-Cu系统：除了常见的单层GP-Ⅰ外，我们还发现了由两个Cu层组成的GP-Ⅰ带。GP-II的结构，其中Cu单层被三个Al层分开，被鉴定;而GP-II的变体确实存在，例如，双层Cu层由单层Al层隔开。此外，在GP-I阶段，我们偶尔会发现由三层Al层分隔的Cu单层，这可能是GP-II的初始阶段。这些结果与以前的小角散射和场离子显微镜观察结果基本一致。此外，我们的研究结果表明，GP-Ⅱ区可以作为一个形核网站的θ'相。[2]Al-Li-Cu系：含2.4wt%Li的淬火试样中存在δ'相（Al_3Li，Ll_2），而含1.6wt%Li的淬火试样中没有析出相。在100℃时效3小时后，两种试样均出现GP-Ⅰ形核区，而1.6wt%-Li合金中未发现δ'相。随着时效的进行，δ'相在GP-Ⅰ区形核长大。在180-200℃范围内，GP-Ⅰ区两侧为一对透镜状δ'颗粒的复合结构是稳定的。系统地发现GP-Ⅰ带上相向的透镜状δ'粒子彼此反相。

We have examined Guinier-Preston (GP) zones in aged Al-Cu alloys using the atomic resolution high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM), together with conventional high-resolution transmission electron microscopy (HRTEM).[1] Al-Cu system: We have found the GP-I zones consisting of two Cu layers, in addition to the commonly-known monolayered GP-I. The structure of GP-II, in which Cu monolayers are separated by three Al layers, was identified; whereas a variant of GP-II does exist, e.g., double Cu layers separated by single Al layer. In addition, in the GP-I stage, we have occasionally found Cu monolayers separated by three Al layers, which can be the incipient stage of GP-II. These results are in general agreement with previous observations by small angle scattering and field-ion microscopy. Also, our results indicate that the GP-II zone can act as a nucleation site for the θ' phase.[2] Al-Li-Cu system: The δ' phase (Al_3Li, Ll_2 structure was found to exist in the as-quenched specimen containing 2.4wt%-Li, whereas no precipitation was observed in the as-quenched specimen with 1.6wt%-Li. After aging at 100℃ for 3hr, GP-I zones nucleated homogeneously in both specimens; but no δ' phase was detected in the 1.6wt%-Li alloy at that stage. As the aging proceeds, the δ' phase nucleates and grows on the GP-I zones. The composite structure, in which the GP-I zone is flanked by a pair of lenticular δ' particles, is stable in 180-200℃. The facing lenticular δ' particles on a GP-I zone were systematically found to be anti-phase with respect to each other.