The Precipitates Microstructures Of AlCuMg Alloys In Relation With Their Mechanical Properties And Thermal Processes

Al-Cu-Mg alloys are among one of the important structural materials in aerospace engineering industry. Their good mechanical properties such as high strength and fracture toughness largely arise from nanosized hardenig precipitates. Thus it is of great significance to know the microstructures、types、dimensions of precipitates, and the relationships among different types of precipitates, as well as the precipitate evolution with ageing time, in order to modify the properties of the alloys with thermal processes. In recent years, with advanced characterisation tools, quite a few progresses have been made about the precipitate microstructures in the alloys, but some related issues still need to be further clarified. In this present study, using various mechanical property characterisation tools and advanced atomic resolution transmission electron microscope(TEM) and scanning transmission electron microscope(STEM) techniques, in association with first principles calculations, we have systematicly investigated the intrinsic relations among ageing treatments、 mechanical properties and microstructure properties of the targetted Al-Cu-Mg alloys. Our results demonstrate the following conclusions.(1) For the AlCuMg alloy one-step aged at 180℃, there are primarily two kinds of hardening precipitate and their precursors, which are the S phase and GPB zones, together with their metastable precursers. The plate-like S phase precipitate has a fundmental orientation relationship with Al matrix as [100]s//[100]Al, [010]s//[021]Al, [001]s//[012]Al.The experimental results show that an S phase precipitate can slightly rotate around the<100>s axis in order to reach minimum energy state of the system. Such rotation is accompanied by an adjustment of its lattice parameters and mophology, as well as its interface with the Al-matrix. This phenomenon usually takes place at high ageing temperature or at low ageing temperature but for long ageing time, when the rotated S-phase particles become mostly coarse and therefore have limited contribution to the hardness of the alloy.(2) As another major hardening precipitates, the GPB zones can form either together with the formation of the S phase at higher ageing temperature or after the formation of S-phase precipitate at lower ageing temperature for a long ageing time.The GPB zone is a one-dimensional crystal without periodic structure in its cross-section.(3) In the alloy samples aged for short times at 180℃, a great amount of second phase particles and dislocations can be observed in the AA2024 alloy. It is shown that the interface between the second phase and the Al matrix provide favourable nucleation site for the S phase. Meantime, the dislocations has been identified to be helical disloations by TEM diffraction contrast analysis instead of dislocation loops as having been found in the Al-Cu-Mg alloys from previous reports. What’s more, such helical disloations may result from the mismatch at the interface beween the S phase and the Al matrix where usually alloying elements tend to segregate.(4) The experimental observations on the samples aged at various temperatures and for different ageing times show that the nucleation and growth of S phase strongly depend upon ageing temperature and exhibit anisotropic features. Since the S phase has an orthormbic crystal structure, it grows anisotropically. On one hand, the S phase precipitate grows thick through its precursor or the GPB2-II zone, which is attached to the side of the S phase. On the other hand, the growth of GPB2-II along the width of the S phase can be blocked by the rapidly formed GPB structural units at the end of the GPB2-Ⅱ at high temperature.As such the average width-to-thickness ratio of an S-phase precipitate fomed at higher ageing temperature is smaller than that formed at at lower ageing temperature. Although the GPB zone or GPB units can grow together with the GPB-S complexes, a GPB zone cannot directly transform to an S-phase precipitate, and vice versa.(5) In order to improve the strength and fracture toughness of the alloy, multi-steps ageing treatments (T6I4 and T6I6, where I stands for interruption were applied and studied. The results show that the fracture toughness of the alloy can be improved without too much loss of strength by the T6I4 treatment, as coMPared with the T6 treatment (single-step ageing), due to the probably formation of atomic clusters in the interruption stage. The T6I6 treatments are found to be unfavourable for the enhancement of alloy’s mechanical properties. This is due to additional S phase precipitates cannot be formed in the interruption stage, and meanwhile the previously formed S phase precipitates grow coarser in the final ageing stage.