Quantitative study on the microstructural evolution and dimensional stability mechanism of 2024 Al alloy during long-term thermal cycling

During the start-stop process of high-precision instruments in service, the critical materials of instruments undergo thermal cycling, resulting in changes in microstructure and dimension. In this paper, the evolution and coupling effects of dislocations and precipitates within 2024 Al alloy during 500 cycles were investigated, and their influence on the dimensional change was quantitatively characterized and decoupling analyzed. Transmission Electron Microscopy (TEM)/aberration-corrected scanning TEM (CS-TEM), X-ray diffraction (XRD), and Three-Dimensional Atom Probe Tomography (3D-APT) were used for microstructural evolution analysis and quantitative statistics. The dislocation density increased from 3.32 x 1014 m- 2 to 5.75 x 1014 m- 2 after 500 cycles, which accelerated elemental diffusion and provided nucleation sites of precipitates, contributing to an increase in the number density of the S'/S phase from 1.04 x 1024 m- 3 to 1.41 x 1024 m- 3. Based on the quantitative statistics of precipitate types and corresponding volume fractions, the dimensional change induced by precipitate evolution was -1.25 x 10-4. The dimensional change caused by the free volume introduced by dislocation density was 3.32 x 10-6. Comparing these values with the experimental value of -1.94 x 10-4, it is clear that the precipitate evolution is the main factor that triggers the dimensional change.