Effects of cold temperatures, strain rates and anisotropy on the mechanical behavior and fracture morphology of an Al–Zn–Mg–Cu alloy

In order to investigate flow stress behavior of Al–Zn–Mg–Cu alloy at below-zero temperature, uniaxial tensile experiments have been conducted at various temperatures (-75 °C–25 °C) and strain rates (10-4 s-1 to 10-1 s-1). Specimens have been extracted from different orientations (0°, 45°, 90°) to detect anisotropy effect on flow stress. Peak stress increases with fluctuations as temperature decreases at fixed strain rate for all specimens. The 0° specimens exhibit the highest peak stress, while the value decreases slightly for 90° specimens. The 45° specimens display the lowest peak stress, which is about 60 MPa lower than its counterparts at each temperature. Coupled effects of dynamic strain aging (DSA) and work hardening (WH) determine strain rate sensitivity (SRS) value. Deformation process of 0° specimens is mainly controlled by DSA (negative SRS value) with some exceptions at specific temperatures and strain rates, while deformation processes of 45° and 90° specimens are dominated by DSA. Fracture morphologies have been observed by scanning electron microscopy (SEM). All specimens deformed at RT demonstrate ductile fracture mode featured as transgranular fractured grains, and then transform into brittle-ductile mixed fracture mode at below-zero temperatures characterized as intergranular fractured grains and cleavage steps with river pattern. Anisotropy demonstrated on fracture morphologies has also been thoroughly discussed.