Early-Stage Precipitation in Al-Li, Al-Cu, Al-Li-Cu and Complex Al Alloys Using Atom Probe Tomography

Aluminum-based alloys play a key role in modern engineering and are widely used in construction components in aircraft, automobiles and other means of transportation due to their lightweight and superior mechanical properties. Additions of Li and/or Cu to Al have been identified as an efficient strategy to improve the strength in age-hardenable or heat-treatable Al alloys via the precipitation hardening phenomenon. The introduction of different nano-structure features can improve the service and the physical properties of such alloys. An improvement of an Al-based alloy has been performed based on the understanding of the relationships among compositions, processing, microstructural characteristics and properties. Knowledge of the decomposition process of the microstructure during the precipitation reaction is particularly important for future technical developments. The first objective of this study is to investigate the nano-scale chemical composition in the Al-Cu, Al-Li and Al-Li-Cu alloys during the early stage of the precipitation sequence and to describe whether this compositional difference correlates with variations in the observed precipitation kinetics. Al-Li-Cu alloys typically exhibit high strengths and good mechanical properties. Investigation of the fine-scale segregation effects of dilute solutes in aluminum alloys which experienced different heat treatments by using atom probe tomography has been achieved. The results show that an Al-1.7 at.% Cu alloy requires a long ageing time of approximately 8 h at 1600C to allow the diffusion of Cu atoms into the Al matrix. For the Al-8.2 at.% Li alloy, a combination of both the natural ageing condition (48 h at room temperature) and a short artificial ageing condition (5 min at 1600C) induces increasing in the number density of the Li clusters and hence increases the number of precipitated particles. Applying this combination of natural ageing and short artificial ageing conditions onto the ternary Al-4 at.% Li-1.7 at.% Cu alloy induces the formation of a Cu-rich phase. Increasing the Li content in the ternary alloy up to 8 at.% and increasing the ageing time to 30 min resulted in the precipitation processes ending with
' particles. To achieve the second objective of this study, a complex and industrial Al alloy AA2195 has been selected and characterized by atom probe tomography. Applying plastic deformation on such alloy has been highlighted as an important engineering tool for manipulation with second-phase precipitates in the microstructure. The findings of the characterization analysis were translated to construct a robust microstructure with an excellent hardness behavior (hardness value of 209 HV) by applying a low-energy-consumption, cost-effective method. The results obtained in this chapter prove the possibilities for the development of specially optimized alloys by obtaining a deep comprehension of the involved microstructural processes.