Dispersoid Structures and Properties of Cr and Zr-containing 6xxx Extrusion Alloys
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The overall objective of this work has been to study the dispersoid structures introduced by adding Cr and Zr to an AA6082 alloy and other features which may influence and improve the mechanical properties of the alloy. Four alloys are added different amounts of Cr and Zr in addition to Mn which are dispersoid forming elements. The alloys are homogenized at two different temperatures (i.e. 530ºC and 575ºC) and dispersoid structures are studied by means of size and distribution using scanning electron microscopy. The influence of the alloying elements on as-cast and extruded structure is investigated by light optical microscopy. Chemical analysis of the dispersoids have been carried out by energy dispersive spectroscopy both in the scanning electron microscope and in the transmission electron microscope. The aging behavior after extrusion has been studied and tensile tests were conducted at peak hardness. The results from this study show that the density of the dispersoids is greatly affected by the homogenization temperature and by the alloy additions. Low homogenization temperature (530ºC) promoted high density of small dispersoids whereas high homogenization temperature (575ºC) promoted low density of coarser dispersoids. Additions of Cr and Zr was both found to increase the dispersoid density. The majority of the dispersoids found by energy dispersive spectroscopy were Al(MnCrFe)Si dispersoids with what seem to be different morphologies while some small dispersoids were found to be (Al,Si)3Zr. The microstructure of the extruded samples was for all alloys found to be fibrous in the bulk of the material, while only two of the alloys showed a recrystallized surface layer. The recrystallization resistance was attributed the high density of the dispersoids. The strength of an AA6082 alloy was found to increase with addition of Cr, Zr and extra addition of Si. A relationship between hardness and yield strength was not found, as the hardness of the reference sample AA6082 with the lowest amount of dispersoid forming elements was highest.