Cryogenic temperature deformation behaviour of two Al-Mg-Si alloys
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A comprehensive study was carried out to clarify the effects of cryogenic temperature on mechanical properties and microstructure of two peak-aged Al-Mg-Si profiles, i.e., fully recrystallized AA6060 and fibrous AA6082. Attention was paid to several topics: Strength, ductility, failure characteristics, slip events, grain rotation and surface roughening. The relationships between them were established. For comparison, some selected topics were also addressed for room temperature deformation. Both alloys exhibited significant differences in mechanical properties and fracture mechanisms at 77K in comparison with that at 295K. When the temperature decreased, the yield strength, the ultimate tensile strength and elongation to fracture were significantly increased. Simultaneously, the average diameter of fracture surface dimples decreased at 77K. Especially, transgranular failure was more likely at 77K for the alloy AA6060. The changed behaviour at cryogenic temperature can be attributed to an increased accumulation of dislocations and a more homogeneous deformation at low temperature. As a consequence of temperature assisted microstructure evolution, pre-stretching at 77K and subsequent tensile testing at room temperature developed a weaker yield point than pre-stretched at 295K for the AA6082 alloy. Further annealing after pre-stretching improved ductility. For the AA6060, detailed investigations of slip traces and Schmid factor distributions after tensile deformation at 77K, revealed relations between formation of slip bands, lattice rotations and surface roughening. The cryogenic temperature enhanced surface roughening due to strain incompatibility among grains, where both the maximum Schmid factor and the number of activated slip systems played vital roles in the crystal plasticity description (i.e., Taylor or Sachs). Grains with various orientations responded differently to the same applied load in terms of activation of slip systems at cryogenic temperature. Hence a hierarchical microstructure formed in the severely cryorolled AA6082 alloy. Therefore, a combination of high strength and high ductility was obtained.