A study aimed at determining and understanding the fracture behaviour of an Al–Li–Cu–Mg–Zr alloy 8090

Abstract

The S-L fracture toughness of aluminiumlithium based alloys is generally poor and this has limited their applicability, particularly in aerospace where good damage tolerance is required. The low S-L toughness has been attributed variously to grain boundary precipitation, planar slip and lithium segregation. This work examined the S-L fracture behaviour of an 8090 Al-Li-based alloy in 34-45 mm plate. The results confirmed that the S-L fracture toughness decreases from the centre to the surface of plate material and increases with double ageing. Fracture is principally intergranular, with both ductile and brittle components occurring, but some transgranular fracture also occurs and this produces steps in the fracture plane. Changes in the relative proportions of brittle and ductile intergranular fracture, as well as in the amount of transgranular fracture, accompany the changes in toughness. However, A. C. Bregianos the decrease in fracture toughness across the plate is accompanied principally by an increase in the relative proportion of brittle intergranular fracture, while the toughening produced by double ageing is accompanied principally by an increase in the amount of transgranular fracture. Evidence of coarse slip, indicative of slip planarity, was seen from slip steps and dislocation structures. However, planar slip was seen only towards the centre of the plate and not towards the surface. The level of planar slip was not reduced markedly by double ageing. Texture varied with position across the plate. There were also a larger number of low angle boundaries towards the centre of the plate than towards the surface. The hardness did not change across the plate. The results could not be explained fully in terms of either the grain boundary precipitate theory or the planar slip model, but were generally consistent with the lithium segregation model. However, the basic tenet of this model is that the level of embrittlement is influenced by the grain boundary structure, and the results did not indicate a substantial difference between the boundaries that failed in a ductile manner and those which failed by brittle fracture. This suggests that the factors which affect lithium segregation may be more complex than originally envisaged.Keywords Short transverse · S-L orientation · Fracture behaviour · Toughness · 8090 Aluminiumlithium based aerospace alloy 123 142 A. C. Bregianos et al.

Strengthening mechanisms, deformation behavior, and anisotropic mechanical properties of Al-Li alloys: A review

“…Besides, the cost of Al-Li alloys is higher than that of traditional Al alloys because of the ageing conditions and comparable strength. Therefore, various studies have been carried out to investigate metal forming technologies (i.e., hydroforming, impact hydroforming, stamping, bending, and superplastic forming) under different working conditions (i.e., cold, warm, and hot deformation) to identify an alternative manufacturing route and to optimize the working conditions to decrease the higher costs related to the addition of Li and the manufacturing of sound, complex shape components from Al-Li alloys [19] , [20] , [21] , [22] , [23] , [24] , [25] , [26] , [27] , [28] , [29] , [30] , [31] , [32] , [33] , [34] , [35] , [36] , [37] , [38] , [39] , [40] , [41] , [42] , [43] , [44] , [45] , [46] , [47] , [48] , [49] .…”

Stationary shoulder friction stir welding of military aircraft sheet alloys

“…Examples of the differences in these two grain structures (of recrystallised and unrecrystallised AA8090) are shown in Figure 5. Bregianos, Crosky, Munroe and Hellier (2010) describe the dispute within the literature as to the reason for low fracture toughness. It has been attributed to: slip planarity (Blankenship, Hornbogen, & Starke, 1993), i.e.…”

“…Lithium segregation involves brittle intergranular failure when the lithium concentration at the grain boundaries is above a certain level, and ductile intergranular failure occurs below it. While Bregianos et al (2010) found evidence to support aspects of each model in their experimental work, indicating that the dispute remains today. When Al-Li alloys are compared with conventional aluminium alloys in notched highcycle fatigue testing, the results are generally favourable (Wanhill, 1994).…”