This paper investigates the precipitation microstructures in aluminium alloys 7040 and 7050 (Al -Zn -Mg -Cuvariants) as a function of quench rate and aging treatment, and the associated compromise between yield strength and fracture toughness. The precipitate microstructures are quantitatively characterised by a combination of techniques covering the different scales involved: transmission electron microscopy (TEM), eld emission gun scanning electron microscopy (FEG-SEM) and small angle X-ray scattering (SAXS), and fracture toughness is estimated using the Kahn tear test. Yield strength and strain hardening behaviour are determined by conventional tensile tests. It is shown that the composition modi cation from alloy 7040 to alloy 7050 results in a better compromise between yield strength and toughness. The strength decrease in alloy 7040 following a slow quench is much reduced compared to alloy 7050 because of a lower sensitivity to quench-induced precipitation on dispersoids. Both alloys show a large decrease in toughness upon slow cooling, however this decrease is much more pronounced in the case of alloy 7050 because of the presence of bands of dispersoid nucleated quench-induced precipitates, promoting low energy transgranular fracture. Further improvement of toughness values in the slowly quenched materials would require a decrease in the quench sensitivity of grain boundary precipitation.MST/5778

The influence of recrystallization and quench rate after solution treatment on the fracture toughness of 7010 aluminum plate has been studied in longitudinal-transverse (L-T) and short-longitudinal (S-L) orientations for T76-type heat treatments. Extensive fractographic analysis was carried out to identify the failure mechanisms, including simultaneous scanning electron microscope (SEM) observation of fracture surfaces and underlying microstructures. A slow quench rate was strongly detrimental because it modified the dominant failure mode from a relatively high energy primary void growth mechanism to lower energy transgranular shear and grain boundary ductile failure in the L-T and S-L orientations, respectively. Low energy failure was associated with coarse precipitation during the quench in both L-T and S-L orientation tests, with intragranular and intersubgranular particles contributing to L-T quench sensitivity, and intergranular particles contributing to S-L sensitivity. Partial recrystallization was generally detrimental, with recrystallized grains being shown to be a preferential crack path. The commonly supposed susceptibility of recrystallized grains to intergranular failure did not explain this behavior, particularly in fast quench materials, as recrystallized grains primarily failed by transgranular void growth from the large intermetallics with which they were intrinsically associated. Exceptional S-L orientation quench sensitivity was observed in unrecrystallized material and attributed to a synergistic interaction between heterogeneous boundary precipitation and the specific location of coarse intermetallics along grain boundaries in the unrecrystallized condition. Quantitative assessment of individual contributions to overall fracture resistance is discussed for cases where multiple failure mechanisms occur, highlighting the importance of interacting and noninteracting mechanisms.

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Damage tolerance of aerospace grade aluminum alloys was studied in relation to a new design philosophy in skin and stringer geometries. Systematic thickness variations (crenellations) were introduced onto the skin and stringers of the laser beam welded (LBW) stiffened Al2139-T8 large flat panels in order to modify the stress intensity factor (SIF) distribution on center cracked panels in such a way that fatigue life can be improved. Fatigue crack propagation (FCP) tests (on panels with crenellations) with crack growing perpendicular to the welded stringers were conducted under constant amplitude and spectrum loading conditions. Results were compared with the results of "classical" LBW stiffened panels (with no crenellations) having equal weight and tested under the same conditions. The new panel design with crenellations showed substantially longer fatigue lives under constant amplitude loading. This gain significantly improved under spectrum (Mini-Twist) loading fatigue tests. This paper presents the first FCP test results of a comprehensive ongoing program which investigates the benefits and potential role of crenellations on welded Alalloy and steel structures. Further issues including microstructural examinations, numerical investigations, fitness for service analysis and residual strength aspects will be topics of another communication.

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