Strengthening of alloy 8090 and its fracture mechanics parameters

An investigation has been made of the tensile properties, impact-, initial fracture toughness and fracture mode of an aluminium-lithium 8090 alloy at room temperature and 77 K, depending upon the heat treatment and orientation. The peak-aged material exhibited an excellent combination of strength and toughness, equal to or exceeding that shown by the high-strength aluminium alloys of the 2000 and 7000 series. The superior strength and toughness of peak-aged plates, including that of 3% stretched material, compared to underaged material seems to be associated with the lower content of coarse insoluble precipitates, a higher density of S-precipitates in a matrix ligament (grain) which promote ductile fracture. The impact toughness of the peak-aged specimens increased at 77 K only in the L-T plate orientation, while in the T-L orientation it was somewhat lower or remained the same. The toughness increase at 77 K is discussed in terms of the role of the matrix and (sub)grain-boundary precipitates, freezing of low-melting point impurities of sodium and potassium alkaline metals at (sub)grain boundaries and the occurrence of the fine crack divider delamination toughening. The yield strength, R_o.2, increase on ageing was accompanied by a corresponding increase in initial crack divider fracture toughness, K_lc, opposite to the trends obtained for some traditional high-strength aluminium alloys. Changes of K_lc versus R_o.2 depending on orientation are discussed using models for ductile fracture toughness behaviour of aluminium alloys, based on the criterion that a critical width of the heavily strained zone at the crack tip approximates the average ligament width, d_p, i.e. the thickness of the elongated grain in the L-T and T-L plate orientations. It was also found that, for constant chemical composition and fabrication practice of the alloy, a critical plate thickness exists B 0.1 6 t_i, where _iis the initial thickness of the rolling ingot, for which the tensile strength properties in the L-T orientation are the same as that in the T-L orientation, while the plasticity (measured by elongation to failure) of the plate is a maximum. Two types of laminated cracks were observed on fracture surfaces of the specimens: large, >1 mm deep (the number of these cracks remains the same as the number of hot-rolling passes), and fine <0.4 mm (shallow laminated cracks, the number of which significantly increases with decreasing temperature, 77 K).