Localized deformation and elevated-temperature fracture of submicron-grain aluminum with dispersoids

Advanced aluminum alloys with thermally stable submicron grains, fine dispersoids, and metastable solute are limited uniquely by reduced ductility and toughness at elevated temperatures. The mechanism is controversial. Experimental results for cryogenically milled powder metallurgy Al extrusion (with 3 vol.% of 20 nm Al2O3, a 0.5 mu m grain size, but no solute) establish that uniaxial tensile ductility, plane strain crack initiation fracture toughness K-JICi and tearing resistance T-R decrease monotonically with increasing temperature from 25 to 325 degrees C. Fracture is by microvoid processes at all temperatures; reduced toughness correlates with changed void shape from spherical to irregular with some faceted walls. Strain-based micromechanical modeling predicts fracture toughness, and shows that temperature-dependent decreases in K-JICi and T-R are due to reduced yield strength, elastic modulus, and intrinsic fracture resistance. Since CM Al does not contain solute such as Fe, dynamic strain aging is not necessary for low-toughness fracture at elevated temperature. Rather, increased temperature reduces work and strain rate hardening between growing primary voids, leading to intravoid instability and coalescence at lowered strain. Decreased strain rate hardening is attributed to increased mobile dislocation density due to dislocation emission and detrapping from dispersoids in dynamically recovered dislocation-source-free grains.