Study of the ductility enhancement of 5A90 Al-Mg-Li alloy sheets with stress relaxation

Stress relaxation tests were conducted on 5A90 (Al-Mg-Li) sheets using in-plane tension along various loading directions with different pre-strain levels to investigate the effects of stress relaxation on the mechanical properties. It is found that considerable ductility enhancement is achieved via the proposed stress relaxation approach. To investigate the mechanisms of ductility enhancement, in-situ X-ray diffractometer (XRD) and electron backscatter diffraction (EBSD) were conducted. The evolution of statistically stored dislocation (SSD) density and geometrically necessary dislocation (GND) density was investigated. A decrease in GND and SSD densities is observed, which is driven by internal stress homogenization and annihilation of dislocations after stress relaxation. Such decrease in dislocation density allows the rearrangement of GNDs and SSDs, and therefore improves the ductility with possible shifts of the strain localization during subsequent reloading. Meanwhile, the ductility enhancement is found to be strongly dependent on loading direction, which is related to the distinctive Schmid factor distribution. For instance, for loading direction with more 'soft grains' (i.e. crystal orientation with high Schmid factors), only a slight increase of ductility was observed owing to a lower possibility of shifting strain localization caused by much larger proportion of soft grain boundaries.