High temperature deformation behaviour of an aluminum-magnesium-silicon-copper alloy and its relation to the microstructural characteristics

The microstructural evolution and mechanical properties at elevated temperatures of a recently fabricated fine-grained AA6xxx aluminium sheet were evaluated and compared to the commercially fabricated sheet of the same alloy in the T4P condition. The behaviour of the fine-grained and T4P sheets was compared at elevated temperatures between 350°C and 550°C, as well as room temperature. The materials were tested at various strain-rates in the range of 5.0×10-4s-1 to 6.7×10 -1s-1. Static ageing at elevated temperatures was conducted to examine the precipitate evolution when no deformation was involved, and tensile tests were conducted at elevated temperatures to study both the deformation behaviour and the microstructural evolution during testing. The grain structure was examined before and after deformation with optical microscopy. The level of damage due to cavitation was measured and the fracture surfaces of the samples were examined after deformation using optical and scanning electron microscopy. Static exposure to elevated temperatures revealed that the precipitate structure of the fine-grained material did not change extensively. The T4P material, however, underwent extensive growth of precipitates, including a large amount of grain boundary precipitation. At room temperature, the T4P material deformed at much higher stresses than the FG material. The FG material, however, achieved greater elongations to failure than the T4P material. The greater elongation to failure of the FG material at room temperature was related to the lower stresses which delayed the onset of void formation and changed the mechanism of failure. Deformation at elevated temperatures revealed that the fine-grained material achieved significantly larger elongations to failure than the T4P material in the temperature range of 350°C-450°C. Both materials behaved similarly at 500°C and 550°C. At temperatures below 500°C, deformation resulted in elongation of the grains. Above 500°C, the grain size was greatly reduced in the T4P material, and only a slightly increased in the fine-grained material. The final grain size after deformation in both materials was found to be smaller at high strain-rates than at low strain-rates. At temperatures above 450°C, the elongation to failure in both materials generally increased with increasing strain-rate. Cavitation played a large role in the failure of both materials, particularly at the highest temperatures and lowest strain-rates. The poor performance of the T4P material at these temperatures was attributed to the precipitate characteristics of the sheet, which lead to elevated stresses and increased cavitation. The deformation mechanism of both materials was found to be controlled by dislocation climb, accommodated by the self diffusion of aluminium at 500°C and 550°C. The deformation mechanism in the fine-grained material transitioned to power law breakdown at lower temperatures. At 350°C to 450°C, the T4P material behaved similarly to a particle hardened material with an internal stress created by the precipitates. The reduction in grain size of the T4P material after deformation at 500°C and 550°C was suggested to be caused by dynamic recovery/recrystallization. The grain size evolution of the fine-grained material may have been caused by the same mechanism and/or grain boundary sliding effects, however, no clear conclusion could be drawn. The role of a finer grain-size in the deformation behaviour at elevated temperatures was mainly related to enhanced diffusion through grain boundaries, while grain boundary sliding in the FG material at the highest deformation temperatures and the lowest strain-rates was considered as a possibility. The differences in the behaviour of the two materials were mainly attributed to the difference in the precipitation characteristics of the materials; when precipitates were present, they behaved differently, and when precipitates dissolved at high temperatures, the materials with similar reduced grain-sizes behaved similarly.