Elevated temperature fracture toughness of Al-Cu-Mg-Ag sheet: Characterization and modeling

The plane-strain initiation fracture toughness (K-JICi) and plane-stress crack growth resistance of two Al-Cu-Mg-Ag alloy sheets are characterized as a function of temperature by a J-integral method. For AA2519 + Mg + Ag, K-JICi, decreases from 32.5 MPa root m at 25 degrees C to 28.5 MPa root m at 175 degrees C while K-JICi for a lower Cu variant increases from 34.2 MPa root m at 25 degrees C to 36.0 MPa root m at 150 degrees C. Crack-tip damage in AA2519 + Mg + Ag evolves by nucleation and growth of voids from large undissolved Al2Cu particles, but fracture resistance is controlled by void sheeting coalescence associated with dispersoids. Quantitative fractography, three-dimensional (3-D) reconstruction of fracture surfaces, and metallographic crack profiles indicate that void sheeting is retarded as temperature increases from 25 degrees C to 150 degrees C, consistent with a rising fracture resistance. Primary microvoids nucleate from smaller;constituent particles in the low Cu alloy, and fracture strain increases. A strain-controlled micromechanical model accurately predicts K-JICi as a function of temperature, but includes a critical distance parameter (l*) that is not definable a priori. Nearly constant initiation toughness for AA2519 + Mg + Ag is due to rising fracture strain with temperature, which balances the effects of decreasing flow strength, work hardening, and elastic modulus on the crack-tip strain distribution. Ambient temperature toughnesses of the low Cu variant are:comparable to those of AA2519 + Mg + Ag, despite increased fracture strain, because of reduced constituent spacing and l*.