|
Abstract
This study investigated the corrosion fatigue properties of 7050
aluminum alloys in different heat treatments. In particular,
the fatigue life and fatigue crack growth rate in air and
3.5 wt% NaCl solution for specimens in various tempers
were compared. The effect of the fatigue and stress
corrosion cracking mechanisms on the corrosion fatigue
behavior was characterized. Fractograph and microstructure
analyses with scanning electron microscoy (SEM) and
optical microscoy (OM) were conducted to determine
the corrosion fatigue crack initiation and propagation
mode.
Results showed that in all tempers both fatigue life and fatigue
crack growth rate tended to be worse in corrosive
environment than in air. The effect of NaCl solution
facilitates the fatigue crack initiation at corrosion
pits and enhances crack propagation by hydrogen
embrittlement as compared to air alone. 7050 Aluminum alloy in
T73 temper exhibited longer fatigue life in air but
shorter fatigue life in 3.5 wt% NaCl as compared to that in
T6 temper. This is attributed to the fact that fatigue
crack initiation is easier from corrosion pits in T73 than in
T6 under corrosive environment. Microstructural observations
indicate that corrosion fatigue cracking is transgranular type
and stress corrosion cracking is intergraular. Therefore,
improvement of grain boundary characteristics by
overaging treatment such as T73 to increase the stress
corrosion cracking resistance did not guarantee the
improvement of corrosion fatigue resistance.
Fractography analyses indicate that corrosion fatigue cracks
mostly initiated from corrosion pits followed by feathery
river line features and striations as the cracks stably grew
until final fracture. The striations observed in corrosive
environment are brittle type in contrast to the ductile
ones observed in air.
|