Precipitation Mechanism of coherent precipitates using HAADF-STEM

我们利用原子分辨率高角环形探测器暗场扫描透射电子显微镜（HAADF-STEM）和常规高分辨率透射电子显微镜（HRTEM）对时效Al-Cu合金中的Guinier-Preston （GP）带进行了研究Al-Cu体系：除了常见的单层GP-I外，我们还发现GP-I区由两层Cu组成。鉴定了GP-II的结构，其中Cu单层被三个Al层隔开；而GP-II的变体确实存在，例如，双Cu层被单Al层隔开。此外，在GP-I阶段，我们偶尔发现被三层Al层隔开的Cu单层，这可能是GP-II的初始阶段。这些结果与以前用小角度散射和场离子显微镜观察到的结果基本一致。同时，我们的结果表明GP-II区可以作为θ′相的成核位点Al-Li-Cu体系：在含2.4wt%-Li的淬火试样中存在δ′相（Al_3Li, Ll_2）组织，而在含1.6wt%-Li的淬火试样中没有析出。在100℃时效3hr后，两种试样的GP-I区均成核；而1.6wt%-Li合金在此阶段未检测到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区两侧有一对透镜状δ′粒子的复合结构稳定。系统地发现在gp - 1区的面对透镜状δ′粒子彼此是反相的。

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.