EFFECTS OF ALKALI-METAL IMPURITIES ON FRACTURE-TOUGHNESS OF 2090-AL-LI-CU EXTRUSIONS

The effects of alkali-metal impurity (AMI) content, temperature, and c rack-mouth-opening displacement (CMOD) rate on the fracture toughness of 2090-T8 Al-Li-Cu alloy extrusions were studied, particularly for sh ort-transverse (S-L) orientations. Decreasing AMI content resulted in increasing room-temperature fracture toughness, especially for underag ed S-L and T-L specimens. Unlike most Al-Li based alloys, material wit h very low (<2 wt. ppm) AMIs produced by vacuum refining had a high S- L fracture toughness (up to 35 MPa root m for proof strengths similar to 440 MPa) as well as high toughness in other orientations. The incre ase in room-temperature fracture toughness with decreasing AMI content was associated with a decrease in the proportion of brittle intergran ular and cleavage-like islands, and a corresponding increase in the pr oportion of high energy dimpled fracture modes, on fracture surfaces. Both the present and previous studies indicate that the brittle island s result from liquid-metal embrittlement due-to the presence of discre te sodium-potassium rich liquid phases. For medium to high AMI content s (5 to 37 wt ppm), S-L fracture toughness increased with decreasing t emperature due to solidification of these phases and a consequent decr ease in the mobility of embrittling atoms. The ability of embrittling atoms to keep up with crack tips also depended on crack velocity so th at CMOD rate influenced fracture toughness. The grain structure (degre e of recrystallization) appeared to be another important parameter aff ecting fracture toughness.