Abstract
Al-alloys with Si as the main alloying element constitute the vast majority of Al castings used commercially today. The eutectic Si microstructure in these alloys can be modified from plate-like to coral-like by the addition of a small amount of a third element to improve ductility and toughness. In this investigation the effects of Eu and Yb are studied and their influence on the microstructure is compared to further understand this modification. The two elements impact the alloy differently, where Eu modifies Si into a coral-like structure while Yb does not. Atom probe tomography shows that Eu is present within the Si phase in the form of ternary compound Al2Si2Eu clusters, while Yb is absent in the Si phase. This indicates that the presence of ternary compound clusters within Si is a necessary condition for the formation of a coral-like structure. A crystallographic orientation relationship between Si and the Al2Si2Eu phase was found, where the following plane normals are parallel: 011Si//0001Al2Si2Eu, 111Si//6bar{7}10Al2Si2Eu and 011Si//6bar{7}10Al2Si2Eu. No crystallographic relationship was found between Si and Al2Si2Yb. The heterogeneous formation of coherent Al2Si2Eu clusters inside the Si-phase is suggested to trigger the modification of the microstructure.
Highlights
The use of light-weight castings in the automotive sector results in higher energy efficiency and reduced fuel consumption
Eu addition results in a homogeneously modified coral-like Si structure, while Yb addition shows less drastic morphological change with a coarser microstructure similar to the plate-like structure of the unmodified alloys
The presence of the intermetallic compounds was confirmed by standardless quantitative EDS and their electron back-scattered diffraction (EBSD) Kikuchi patterns were successfully indexed to match the crystallographic structure www.nature.com/scientificreports of Al2Si2Eu and Al2Si2Yb, respectively[32,33,34]
Summary
The use of light-weight castings in the automotive sector results in higher energy efficiency and reduced fuel consumption. A well accepted hypothesis by Lu and Hellawell[19] for the multiplication of defects is the so called impurity induced twinning (IIT) They proposed poisoning of step sources across the closely packed {111} planes in Si by the adsorption of atoms of the modifying agent at the solidification front. Ytterbium (Yb), which is the element with the best fit to the IIT-model (rYb/rSi = 1.66) shows, similar to many other rare earth metals, only a refinement of the Si plate-like structure This shows that the atomic radius alone is not capable of predicting the morphological transition of the eutectic structure and the reason why they behave differently remains an open question
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