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The Effect of Chromium on the Evolution of Dispersoids in Al-Mg-Si Alloys
[Thesis]. Manchester, UK: The University of Manchester; 2018.
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Abstract
Aluminium is increasingly being used in the automotive industry to reduce the weight of vehicles. It is the additions of transition elements such as Mn and Cr that can be picked up during recycling, that can form dispersoid particles during homogenisation. Dispersoids play a significant role in the recrystallization and texture development for wrought Al-Mg-Si alloys by inhibiting grain boundary motion. It is therefore important to understand the precipitation kinetics of such particles. The Mn+Cr dispersoid phases are currently thought to nucleate on ï¢`-Mg1.8Si particles via an intermediate semi-coherent precipitate denoted the u-phase. In this study, Al-Mg-Si alloys with additions of Fe and varying levels of Cr were cast to study the effect of different homogenisation regimes on the dispersoid precipitation mechanisms and final characteristics. Electron Probe Micro Analysis (EPMA) was conducted to study the inhomogeneity of elements in the cast structure and through heating to the homogenisation temperature. It was found that Mg, Si and Fe segregate towards the dendrite edges during solidification while Cr segregates towards the dendrite centre. During heating, the matrix composition of both Mg and Si decrease and increase due to precipitation of Mg+Si phases. Cr and Fe stay segregated during the heating process due to the slower diffusion rates in the face centred cubic Al matrix. Dispersoid free regions have also been observed in the microstructure correlating to the elemental segregation in the as-cast condition. Optical, scanning and transmission electron microscopy was utilised in order to study the change in dispersoid characteristics with varying homogenisation regimes and as a function of distance through a grain. With an increase in homogenisation temperature, the mean size of dispersoids increased but number density decreased. For a longer dwell time, the dispersoids remained approximately the same size but increased in volume fraction and density. Increasing the heating rate did not significantly change the dispersoid size, volume fraction or density. The dispersoids size and number density was also studied as a function of distance through a number of grains with the interplay of nucleation, growth and coarsening discussed. Both ï¡-Al(FeCr)Si and ï¡`-AlCrSi dispersoids were found to exist with a variety of morphologies while the ï¡`-AlCrSi dispersoids were found to have a larger effective diameter.
Keyword(s)
Al-Mg-Si alloys; Dispersoids; Electron Probe Micro Analysis; Microscopy