Abstract
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 light weight and superior mechanical properties. 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 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. Investigation of the fine scale segregation effects of dilute solutes in aluminum alloys which were 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 160°C to allow the diffusion of Cu atoms into 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 160°C) induces increasing on the number density of the Li clusters and hence increase 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 δ' particles. Thus the results contribute to the understanding of Al-alloy design.
Highlights
Aluminum of high purity in an annealed condition has a low strength near 10 MPa [1]
The results show that an Al-1.7 at.% Cu alloy requires a long ageing time of approximately 8 h at 160 ̊C to allow the diffusion of Cu atoms into Al matrix
Al-Cu Alloy Because atom probe tomography (APT) is capable of resolving nano-scale microstructures including atomic clusters and other fine scale segregation effects, we used APT to analyze the microstructure of the three Al-1.7 at.% Cu alloys, which have been subjected to the three heat treatment conditions shown in Table 1 (i.e., 1 S, 2 S and 3 S)
Summary
Pure metals are strengthened by the introduction of obstacles (i.e., any inhomogeneity) in their microstructures to pin dislocations. The introduction of solute atoms into the Al matrix is considered to be an important method to improve conventional Al alloys. The solute must have an appreciable solid solubility in the matrix at the annealing temperature, and has to remain in the solid solution after slowly cooling and must not be removed by reacting with other elements in the alloy [1]. 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
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