In-situ EBSD study on the microstructure evolution of an Al–Mg–Si–Fe alloy with different precipitation free zones during tension at cryogenic and ambient temperatures

Abstract

AA6101 (Al-0.44Mg-0.39Si-0.12Fe) extrudate was subjected to solid solution treatment followed by either water quenching (WQ) or air quenching (AQ) before a peak artificial aging (T6) to produce different precipitation free zone widths. These samples are then tested using uniaxial tension at 298 K and 77 K to study the effect of the initial microstructure and cryogenic temperature on the mechanical properties. The yield strength, ultimate tensile strength and ductility of the sample at 77 K are higher than those at 298 K. The yield strength of the air quenching sample is lower than that of the water quenching sample at the same temperature. The microstructure evolution was further characterized by in-situ tensile tests performed at both room and cryogenic temperatures. Based on the in-situ SEM/EBSD results with increasing strain, the deformation mechanisms at both room and cryogenic temperatures are revealed in terms of the accumulation of geometrically necessary dislocations (GNDs), activation of slip systems and lattice rotations. In-situ EBSD results show that with increasing strain, the GND distribution becomes more uniform at 77 K, resulting in a higher work hardening rate than that at 298 K. Wider precipitate free zone (PFZ) in air quenching sample leads to higher stress mismatch resulting in intergranular fracture. The dislocation pile-up at the edge of PFZ leads to strain relaxation at grain boundaries and increased ductility. The dislocations at the edge of PFZ and in grains are uniformly distributed resulting delayed crack initiation and further ductility improvement at 77 K.