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 77K, 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 Al 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 77K 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 77K is discussed in terms of the role of the matrix and (sub)grain-boundary precipiates, 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 sub 0,2 , increase on ageing was accompanied by a corresponding increase in initial crack divider fracture toughness, K sub Ic , opposite to the trends obtained for some traditional high-sterngth Al alloys. Changes of K sub Ic vs. R sub 0,2 depending on orientation are discussed using models for ductile fracture toughness behaviour of Al allos, based on the criterion that a critical width of the heavily strained zone at the crack tip approximates the average ligament width, d sub 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 approx 0.16 t sub i , where t sub i is 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 lamianted 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 lamianted cracks, the number of which significantly increases with decreasing temperature, 77K).