Mechanical properties and surface characteristics of an AA6060 alloy strained in tension at cryogenic and room temperature

Mechanical properties at 77 K and 295 K and surface characteristics of an AA6060 alloy have been studied. The alloy exhibited significant differences in mechanical properties and fracture mechanisms at 77 K in comparison with that at 295 K. When the temperature decreases, the yield strength, the ultimate tensile strength and ductility to fracture are drastically increased from 217.2 MPa, 252.4 MPa, 14.5% to 251.6 MPa, 335.9 Mpa, 21.5% respectively, and the average diameter of dimples decreases from ~8 µm to ~3 µm. It is proposed that the changed behavior at cryogenic temperature can be attributed to an increased accumulation of dislocations at low temperature. Detailed inspections of the deformed polished sample surfaces show that slip localization in wide slip-bands is not promoted at 77 K, indicating that crystallographic slip is homogeneous, while activation of surface slip systems forming wide slip bands is much easier at 295 K and therefore the surfaces were deformed more in-homogeneously and localized. The homogeneity in slip behavior introduced more topography at 77 K than observed at 295 K and the enhanced topography might be due to more 3-dimensional (3D) lattice rotation in grains having a high Schmid factor, hence introducing more accumulation of local displacements in the thickness direction.