Effects of alkali-metal impurities on fracture toughness of 2090 Al-Li-Cu extrusions

Abstract

The effects of alkali-metal impurity (AMI) content, temperature, and crack-mouth-opening displacement (CMOD) rate on the fracture toughness of 2090-T8 Al-Li-Cu alloy extrusions were studied, particularly for short-transverse (S-L) orientations. Decreasing AMI content resulted in increasing room-temperature fracture toughness, especially for underaged S-L and T-L specimens. Unlike most Al-Li based alloys, material with very low (<2 wt. ppm) AMIs produced by vacuum refining had a high S-L fracture toughness (up to 38 MPa√m for proof strengths ∼440 MPa) as well as high toughness in other orientations. The increase in room-temperature fracture toughness with decreasing AMI content was associated with a decrease in the proportion of brittle intergranular and cleavagelike islands, and a corresponding increase in the proportion of high energy dimpled fracture modes, on fracture surfaces. Both the present and previous studies indicate that the brittle islands result from liquid-metal embrittlement due to the presence of discrete sodium-potassium rich liquid phases. For medium to high AMI contents (5 to 37 wt ppm), S-L fracture toughness increased with decreasing temperature due to solidification of these phases and a consequent decrease in the mobility of embrittling atoms. The ability of embrittling atoms to keep up with crack tips also depended on crack velocity so that CMOD rate influenced fracture toughness. The grain structure (degree of recrystallization) appeared to be another important parameter affecting fracture toughness.