Effect of strain rate on the deformation of 6061-T6 aluminum alloy at cryogenic temperature

Abstract

This study investigates the tensile behavior and strain rate sensitivity (SRS) of a commercial Al6061-T6 Al alloy at various temperatures (i.e., 20, −50, and −150 °C). Uniaxial tensile tests were performed at the quasi-static strain rates of 10−2, 10–3, and 10−4 s−1. A significant improvement in mechanical properties was found at the cryogenic temperature (CT), due to the enhanced work-hardening behavior during CT deformation. The SRS exponent changed from a slightly positive value at RT to a negative one as the temperature is lowered. The dislocation density was evaluated for the samples strained to 5 and 8% at 20 °C and −150 °C using X-ray diffraction to probe the strengthening mechanisms during RT and CT deformation. The dislocation density was higher for the strain rate of 10−2 s−1 at RT and 10−4 s−1 for CT. In addition, electron back scatter diffraction (EBSD) based kernel average misorientation (KAM) and coincidence site lattice (CSL) boundary distribution analysis were conducted on the initial and 8% strain-interrupted samples at both temperatures to explain the effect of temperature and strain rate on work hardening behavior. Transmission electron microscopy (TEM) was applied to investigate the dislocation structures under different strain rate and temperatures. The cryogenically deformed sample exhibited a distinctive dislocation tangling phenomenon in proximity of the grain boundary, In contrast, the room temperature deformed sample demonstrated dislocation penetration occurring at the grain boundary. The character of SRS and improved work-hardening behavior can be attributed to changing dislocation kinetics with decreasing stacking fault energy (SFE) at cryogenic temperature.