TEM study of S' hardening precipitates in the cold rolled and aged AA2024 aluminum alloy: influence on the microstructural evolution, tensile properties & electrical conductivity

Abstract

It is known that parameters such as deformation together with the temperature and time of aging heat treatment, play critical roles in changing the mechanical properties and the microstructure of AA2024 aluminum alloy. It is shown that a superior level of strength is achievable by applying cold deformation at an appropriate combination of temperature and time. In the present study, the effect of cold deformation on the various properties of artificially aged AA2024 aluminum alloy was assessed. Two cold rolling amounts of 25% and 50% were exerted after solution treatment at 500 °C for 1 h, followed by quenching. The samples were then aged at 180 °C for 2 h, 200 °C for 4 h and 220 °C for 6 h. The microstructure of the samples was examined by Optical microscope, Field Emission Scanning Electron Microscope (FESEM), and High Resolution Transmission Electron Microscope (HRTEM). The Mechanical properties were assessed by tensile and hardness tests. As well, electrical conductivity measurement was used to evaluate the state of precipitation. Results showed that if cold deformation is properly applied, the mechanical properties of the aged samples increase substantially at 180 °C for 2 h. This condition was recognized as the peak aging condition. In a sense, results showed that applying cold deformation would significantly influence on the aging treatment parameters. The cold rolled samples aged at 180 °C for 2 h showed the highest flow stress, but with slightly reduced elongation. Results also showed that aging at 200 °C and 220 °C reduced the strength to the level below the as cold rolled conditions due to the over aging process. High resolution TEM studies confirmed the aged hardening precipitates of the S′ type. Using HRTEM the early stages of these precipitates (i.e. Guinier–Preston–Bagaryatsky (GPB) zone) were explored. The grown state of the S′ precipitates in the form of S (Al2CuMg) type precipitates was also determined in the overaging conditions using the TEM approach. Finally, as an assessment of mechanical properties was performed, taking into account the three parameters of precipitation hardening, recovery and grain growth.

Keywords

• Aluminum
• Age hardening
• High resolution TEM
• Mechanical properties
• Structure
• Deformation