Mechanisms influencing the cryogenic fracture-toughness behavior of aluminum-lithium alloys

Cryogenic strength-toughness relationships for advanced aluminum-lithium alloys 2090, 8090, 8091 and 2091 are examined as a function of microstructure, plate orientation and wrought-product form (plate vs sheet), with specific emphasis on the underlying micro-mechanisms associated with crack advance. It is found that, with decrease in temperature from 298 K to 77 and 4 K, strength, tensile elongation and strain-hardening exponent are increased for all alloy chemistries, microstructures and product forms; however, the longitudinal (L-T, T-L) fracture toughness may increase or decrease depending upon the prevailing microscopic mechanism (microvoid coalescence vs transgranular shear) and macroscopic mode (plane strain vs plane stress) of fracture. In general, alloy microstructures that exhibit changes in either the fracture mechanism or mode at low temperatures show a decrease in L-T toughness. Conversely, when the fracture mechanism is unchanged between ambient and 4 K, observed variations in toughness with temperature are a strong function of the degree of local stress-triaxiality that develops at the crack tip. In very thin sheets, where the fracture mode remains one of plane stress (“slant” fracture), the elevation in toughness at low temperatures is associated with the concurrent increase in tensile strength and ductility; conversely, in thick plate, the increased occurrence of through-thickness delaminations (due to the weak short-transverse properties) at low temperatures locally promotes plane-stress conditions, thereby enhancing toughness by relaxing triaxial constraint. In sheets of intermediate thickness, however, the absence of such through-thickness delaminations permits the expected transition from plane-stress to plane-strain conditions, with the result that the toughness now decreases with reduction in temperature.