Stress corrosion crack propagation in an age-hardened Cu-Be alloy

Abstract

Stress corrosion crack (scc) propagation in a fully age-hardened Cu—1.8Be alloy in aqueous ammoniacal environments has been investigated by measurements of crack velocities and by fractography. At low values of stress intensity factor the fracture is intergranular. As the value of the stress intensity factor increases, the increasing size of the plastic zone results in increasing amounts of slip line attack and close to K Io the fracture becomes partly transgranular along the {112} plane, which is the habit plane of the intragranular γ′ precipitate. In over-aged specimens transgranular cracking occurs along the {111} plane which is the habit plane of the equilibrium γ precipitate. Dynamic loading experiments show that the transition is a function of the rate of change of stress intensity factor. Separate investigations of crack velocity as a function of dissolved Cu content, ammonia concentration and solution pH indicate that crack velocity is related to the rate of formation of the tarnish film which is CuO. Propagation is thought to occur by slip-assisted rupture of the tarnish film which forms preferentially in regions containing a high density of precipitates.