Effect of microstructure on pit-to-crack transition of 7075-T6 aluminum alloy.

摘要

Corrosion is a significant issue for the aircraft community to combat. Aircraft are mainly comprised of aluminum alloys, which are susceptible to pitting corrosion. Pits often act as fatigue crack nucleation sites.; The main focus of this research was to gain an increased understanding of how microstructure influences pit growth, pit-to-crack transition, and critical crack propagation to fracture. Two thicknesses and one extruded form of 7075-T6 aluminum alloy were etched and then subjected to concomitant corrosion fatigue in a 3.5% NaCl environment. Testing was interrupted at various intervals to obtain information on pit generation, growth, and potential cracking. In situ inspections were performed regularly throughout each interval. Critical pits and cracks were linked to microstructural features when possible. Each specimen's evolving discontinuity state (EDS) was documented. Characterization of the EDS was performed using metallurgical and scanning electron microscopy.; Results indicated that microstructure has a significant influence on pit-to-crack transition and fatigue crack propagation. Fatigue cracks in a concomitant corrosion fatigue environment propagated only when the crack growth rate was greater than the pit growth rate. Short cracks (100 mum) in a corrosion fatigue environment were strongly influenced by grain boundaries and crystallographic grain orientation. Constituent particles competed with corrosion pits as critical crack nucleation sites. Fractography confirmed the presence of noncritical cracks within the corroded region, related to pitting and constituent particles.