Thermal stability evaluation of nanostructured Al6061 alloy produced by cryorolling

Grain growth of nanostructured Al6061 produced by cryorolling and aging process was investigated during isothermal heat treatment in 100-500 degrees C temperature range. Transmission electron microscopy (TEM) observations demonstrate that after cryorolling and aging at 130 degrees C for 30 h, the microstructure contains 61 nm grains with dispersed 50-150 nm precipitates and 0.248% lattice strain. In addition, an increase in tensile strength up to 362 MPa because of formation of fine strengthening precipitation and nano-sized grains was observed. Thermal stability investigation within 100-500 degrees C temperature range showed release of lattice strain, dissolution of precipitates and grain growth. According to the X-ray diffraction (XRD) analysis, Mg2Si precipitates disappeared after annealing at temperatures higher than 300 degrees C. According to the results, due to the limited grain growth up to 200 degrees C, there would be little decrease in mechanical properties, but within 300-500 degrees C range, the grain growth, dissolution of strengthening precipitates and decrease in mechanical properties are remarkable. The activation energies for grain growth were calculated to be 203.3 kJ/mol for annealing at 100-200 degrees C and 166.34 kJ/mol for annealing at 300-500 degrees C. The effect of precipitation dissolution on Al lattice parameter, displacement of Al6061 (111) XRD peak and Portevin-LeChatelier (PLC) effect on stress-strain curves is also discussed.