Damage accumulation of under-aged 7005 aluminum alloy sheet during high-cycle fatigue process

Abstract

In this study, we investigated the evolution of precipitate particles, dislocations, and other microstructural features in under-aged 7005 aluminum alloy sheets during high-cycle fatigue process, as well as the fatigue crack growth paths, to reveal the fatigue damage accumulation behavior of the alloy sheets under fatigue conditions of 20.5 MPa/205 MPa. As the fatigue cycle count increased to 7 × 106 cycles, the size of matrix precipitate particles in the under-aged 7005 aluminum alloy sheets gradually increased. Concurrently, the dislocation density in the alloy matrix monotonically increased from 0.07 × 1014 to 11.16 × 1014 m-2. The mechanical properties of the alloy sheets underwent two stages as the fatigue cycle increased, a fatigue hardening stage and fatigue stable stage. In the fatigue hardening stage (within 1 × 106 cycles), the yield strength and tensile strength of the alloy sheets increased by 15 MPa and 16 MPa, respectively, while the elongation decreased from 20.7% to 18.0%. Conversely, in the fatigue stable stage (beyond 1 × 106 cycles), the strength of the alloy sheets remained relatively unchanged, while the elongation marginally increased to 18.7%. During the fatigue process, the proliferating dislocations primarily accumulate in front of the dispersoid particles to form dislocation arrays. Subsequently, they transition to accumulating in front of the grain boundaries, forming dislocation networks that intersect the grain boundaries at approximately 45° and envelop the grains, which resulted in the fatigue damage in the alloy sheets, ultimately leading to an intergranular fatigue fracture.

Keywords: under-aged 7005 aluminum alloy sheets, fatigue hardening stage, fatigue stable stage, fatigue damage accumulation, intergranular fracture