Stress relaxation aging of 7050 aluminum alloy under elastic and plastic loadings: A perspective from the geometrically necessary dislocations

• Published in: Journal of alloys and compounds 2025-01, Vol.1010, p.177066, Article 177066
• Main Authors: Zhang, L.W., Bian, T.J., Li, H., Yang, X.D., Lei, C.
• Format: Article
• Language: English
• Subjects: 7050 aluminum alloy; Geometrically necessary dislocation; Plastic loading; Precipitate; Stress relaxation aging
• ISSN: 0925-8388
• DOI: 10.1016/j.jallcom.2024.177066

Aerospace integral panels with large curvature often experience not only elastic loading but also plastic loading during creep age forming. However, the stress relaxation aging behaviors and mechanisms under loading stresses ranging from elastic to plastic regions, especially from the perspective of geometrically necessary dislocations (GNDs), have not been fully explored. The stress relaxation aging (SRA) behavior of 7050 aluminum alloy under elastic (200/250/300 MPa) and plastic loadings (350/400 MPa) are studied by uniaxial SRA, hardness and tensile tests, and corresponding mechanisms are clarified by Electron Back Scatter Diffraction (EBSD) tests and transmission electron microscopy (TEM) observations. Regardless of the loading stress, the stress relaxation of 7050 alloy shows a typical two-stage behavior (rapid stress reduction and stable stress relaxation rate). The second stage of stress relaxation under elastic loading shows an abnormal stress rebound, while the stress continuously decreases under plastic loading, and hardness and yield strength at 350 MPa are lower than that at 200 MPa during SRA. These results are attributed to strong dislocation pinning from small and dense precipitates at elastic loading and dislocations shearing large and sparse precipitates at plastic loading, respectively. Furthermore, both the stress reduction and its ratio increase with the increase of initial stress. GNDs contribute to the formation of low-angle grain boundaries (LAGBs) and accumulate at the LAGBs intersection to coordinate grain orientation for creep deformation. Compared to elastic loading, plastic loading promotes global growth of intragranular precipitates by elastic stress field and local heterogeneous nucleation and coarsening of precipitates at subgrain boundaries by more GNDs. •The second stage of stress relaxation aging, abnormal stress rebound occurs under elastic loading, whereas under plastic loading, the stress continues to decrease.•Geometrically necessary dislocations contribute to the formation of low-angle grain boundaries and accumulate their intersections, coordinating the grain orientation.•Compared to elastic loading, plastic loading promotes global growth of intragranular precipitates and local heterogeneous nucleation and coarsening of precipitates at subgrain boundaries by geometrically necessary dislocations•Dislocation pinning and shearing cause stress rebound and sustained reduction in the second stage of stress relaxation aging