A study aimed at determining and understanding the fracture behaviour of an Al–Li–Cu–Mg–Zr alloy 8090 |
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Authors: | A C Bregianos A G Crosky P R Munroe A K Hellier |
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Affiliation: | (1) Department of Mechanical Engineering, The Applied Research Center, Old Dominion University, Norfolk, VA 23529, USA;(2) National Institute of Aerospace, Hampton, VA 23666, USA;(3) NASA Langley Research Center, Hampton, VA 23681, USA |
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Abstract: | The S-L fracture toughness of aluminium–lithium 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 34–45mm 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, 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. |
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