Effects of cooling rate and cryogenic temperature on the mechanical properties and deformation characteristics of an Al-Mg-Si-Fe-Cr alloy

The Al-Mg-Si-Fe-Cr alloy was subjected to water quenching (WQ) and air quenching (AQ) after solution treatment for 30 min at 843 K followed by a peak artificial aging treatment at 453 K for 8 h. Precipitates of different sizes and precipitate free zone (PFZ) of different widths were obtained. Uniaxial tensile tests were conducted to investigate the effect of the microstructure and deformation temperature on the mechanical properties and deformation behavior of the WQ and AQ samples at 298 K and 77 K. The results showed that the yield strength and work hardening rate of the AQ samples were lower than those of the WQ sample. In addition, the strength and ductility of the two samples are significantly improved at cryogenic temperatures. Furthermore, quasi-in situ electron back scatter diffraction (EBSD) was conducted to investigate the orientation evolution of grains with strains of 1%, 3%, 7% and 10% for the two samples at 298 K and 77 K. The different orientation evolution of the two samples was caused by the nonuniform deformation in the PFZ and grain interior. The dislocation characteristics and fracture morphology were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Because the rod-shaped precipitates hinder the dislocation movement, no obvious slip traces could be observed on the deformed surface of the AQ sample. In addition, two slip systems are activated for most grains of the WQ sample at 77 K, but only one slip system is activated at 298 K. Due to the weak grain boundaries deformed at 298 K, the stress applied to grains is consistent with the tensile direction. However, the grain boundaries are strengthened when tested at 77 K, and the stress field of the grain boundaries is more complex, resulting in the stress not only in the tensile direction. More slip systems are activated to coordinate the deformation, improving the ductility. TEM images reveal that the deformation of the PFZ is more severe for the AQ sample at 298 K than at 77 K. The deformation in the precipitate-free zone and within the grain was more homogeneous in the AQ sample at 77 K, which increased its ductility compared to the AQ sample at 298 K.