Study of creep crack growth in 2618 and 8009 aluminum alloys

Creep crack growth (CCG) has been investigated in an 8009 (Al-Fe-V-S) P/M alloy at 175 °, 250 °, and 316 ° and in a 2618 ingot alloy at 150 °, 175 °, and 200 °. Under sustained load, subcritical crack growth is observed at stress intensity levels lower thanK _ic ; for 2618, at 200 °, crack growth is observed at stress intensities more than 40 pct lower thanK _ic . Alloys 8009 and 2618 exhibit creep brittle behavior,i.e., very limited creep deformation, during CCG. The CCG rates do not correlate with CCG parameters C* and C but correlate with the stress intensity factor,K, and theJ integral. Generally, crack growth rates increase with increasing temperature. Micromechanisms of CCG have been studied with regard to microstructural deg-radation, environmental attack, and creep damage. Although theoretical estimation indicates that CCG resistance decreases with second-phase coarsening, such coarsening has not been observed at the crack tip. Also, no evidence is found for hydrogen- or oxygen-induced crack growth in comparing test results in moist air and in vacuum. Creep deformation and cavitation ahead of crack tip are responsible for observed time-dependent crack growth. Based on the cavitation damage in the elastic field, a micromechanical model is proposed which semiquantitatively explains the correlations between the creep crack growth rate and stress intensity factor,K.