RATE-LIMITING PROCESSES IN ENVIRONMENTAL FATIGUE CRACK PROPAGATION IN 7000-SERIES ALUMINUM ALLOYS

摘要

The objective of this research is to understand the mass transport and reaction processes that govern cycle-time dependent environment enhanced fatigue crack propagation (EFCP) in 7000-series aluminum alloys. The loading frequency (f) dependence of EFCP in peak aged alloys in aqueous-chloride solution is governed by diffusion of embrittling hydrogen (H) to sites within the fracture process zone, as supplied by rapid-surface reaction kinetics. The relationship between the plateau crack growth rate and the critical frequency for f-dependent EFCP is predicted by a H diffusion model, but several model parameters are uncertain. Chromate addition to neutral NaCl solution reduces da/dN for AA7075-T651 and alters the fdependence of da/dN for the S-L crack orientation. The mechanism of inhibition appears to be reduced-H uptake, traced to either reduced H~+ activity or a H-barrier film at the crack tip. This beneficial film is destabilized mechanically, above a crack tip strain rate that is Na_2CrO_4-concentration dependent. Hydrogen diffusion in the process zone can rate-limit EFCP rate in 7000-series alloys stressed in pure water vapor, particularly at higher water vapor pressures and lower f. Growth rates for water vapor and chloride solution correlate with an exposure parameter related to the ratio of crack-surface H concentration to loading frequency, but this parameter cannot be quantified.