Precipitation Mechanism of coherent precipitates using HAADF-STEM

点击翻译按钮获取中文摘要

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.