UBC Theses and Dissertations
Microstructure evolution during homogenization and its effect on the high temperature deformation behaviour in AA6082 based alloys Liu, Chenglu
There is a current trend in increased use of aluminum extrusion alloys in automotive applications. This trend is driven by the need to reduce the vehicle weight, which in turn, is to decrease energy use and/or emissions of the vehicles. In this work, Al-Mg-Si alloys, and especially variants of AA6082 with different Mn and/or Cr additions have been studied. The objectives of the study are to i) experimentally characterize the evolution of the constituent particles (phase, volume fraction and size) and dispersoids (chemistry, crystal structure, size and volume fraction), ii) rationalize the mechanisms of dispersoid evolution and iii) develop a physically based constitutive law for the high temperature flow stress. The characterization of microstructure evolution during homogenization was done using a combination of i) transmission electron microscopy, ii) field emission gun scanning electron microscopy, iii) electron microprobe microanalysis and iv) electrical resistivity measurements. The high temperature flow stress was characterized by uniaxial compression tests. The main results on microstructure evolution during the process of homogenization show that i) there is a transformation of the constituent particles from the β to α phase during homogenization and a concurrent speriodization of the particles, ii) dispersoids with the size range of 20-200 nm and a volume fraction of 0.25 – 1.3 % are initially formed during homogenization but they eventually dissolve as Mn is transported to the constituent particles and iii) a steady-state flow stress of between 20 and 45 MPa was measured for the test temperature between 550 °C and 580 °C with strain rates of 0.1 – 10 s-¹. The evolution of dispersoids during homogenization was rationalized by considering their nucleation, growth and coarsening. It is proposed that dispersoid coarsening initially involves long-range diffusion of Fe from the constituent particles to dispersoids and later Mn and Fe diffuse to the constituent particles. The Kocks-Chen constitutive model was extended to include the role of dispersoids on high temperature flow stress using an Orowan type model for precipitation hardening. This was found to predict the flow stress ± 5 % in ≈ 95 % of the cases that were studied.
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