Creep crack growth in AI–Li alloy 8090 sheet

Abstract

Recrystallised and unrecrystallised Al–Li–Cu–Mg–Zr alloy 8090 sheet and Al–Cu alloy 2024 sheet have been tested to determine their relative resistance to creep cracking. Creep cracking in recrystallised 8090 sheet occurred at much lower stress intensity factors K, at higher rates for a given value of K, and at lower temperatures, compared with unrecrystallised 8090 or 2024 sheet. For recrystallised sheet, significant rates of creep cracking were observed at temperatures as low as 60°C, at K values of ∼10%K_c (the critical value of K for overload fracture). Creep crack growth was predominantly intergranular for recrystallised 8090 and 2024 sheet, and a mixture of intersubgranular and transgranular for unrecrystallised 8090 sheet. The fractographic features, as well as other observations, suggest that 8090 alloys contain low melting point sodium-rich phases which are not found in conventional alloys. It was concluded that the presence of these phases and the continuous, intergranular crack paths approximately normal to the applied stress in recrystallised 8090 sheet were responsible for its poor resistance to creep cracking.

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