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 duc tility and toughness at elevated temperatures. The mechanism is contro versial. Experimental results for cryogenically milled powder metallur gy Al extrusion (with 3 vol.% of 20 nm Al2O3, a 0.5 mu m grain size, b ut 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 degr ees C. Fracture is by microvoid processes at all temperatures; reduced toughness correlates with changed void shape from spherical to irregu lar with some faceted walls. Strain-based micromechanical modeling pre dicts fracture toughness, and shows that temperature-dependent decreas es in K-JICi and T-R are due to reduced yield strength, elastic modulu s, and intrinsic fracture resistance. Since CM Al does not contain sol ute such as Fe, dynamic strain aging is not necessary for low-toughnes s fracture at elevated temperature. Rather, increased temperature redu ces work and strain rate hardening between growing primary voids, lead ing to intravoid instability and coalescence at lowered strain. Decrea sed strain rate hardening is attributed to increased mobile dislocatio n density due to dislocation emission and detrapping from dispersoids in dynamically recovered dislocation-source-free grains.