Effect of T6I6 tempering on the coherence between core/shell precipitates and a solid solution in the 2024 aluminium alloy

Abstract

Heat treatment of materials, including—in particular—two-stage T6I6 treatment, enables the nucleation of precipitates with a two-phase structure: core and shell. The above-mentioned treatment can be used to harden materials by selecting suitable treatment parameters and the chemical composition of the material that initiates a selective diffusion of the alloying elements. In this paper, tests of the T6I6 treatment were carried out on the EN AW-2024 alloy. The aluminium alloy was subjected to T6I6 treatment, and then the element distribution map was examined within the precipitate using the EDS method. The mechanical properties of the precipitate were tested using the nanoindentation method, and the kinetics of the formation of the precipitate were analysed using DSC. The impact of two-phase precipitates on the mechanical properties was determined through a static strength test, whereas the impact of the structure on the above-mentioned mechanical properties was determined based on HR-TEM observations. The results of the study confirmed the formation of a precipitate with a two-phase structure (shell; core) in the EN AW-2024 alloy, as well as the kinetics of their formation as a function of the parameters of the T6I6 process, such as temperature, duration and cooling rate. Strength tests confirmed an increase in the strength of the alloy to Rm = 520 MPa, with elongation remaining the same at A = 13.2%. HR-TEM observations, in turn, proved that the shell forming on the core is coherent with solid solution α, which means that the hardening of the alloy may occur as a sum of the Mott-Nabarro, Friedel and Ashby-Orowan mechanisms.