Crystallographic studies on the iron-containing intermetallic phases in the 319-type aluminium casting alloys

Abstract

The iron-containing intermetallic compounds in type 319 aluminium casting alloys were investigated using convergent beam electron diffraction (CBED) and electron backscatter diffraction (EBSD). The Chinese script α-phase was identified as having the cubic Im space group consistent with the Al₁₉Fe₄MnSi₂ phase reported in the literature. However, the atomic ordering of this phase results in complex Kikuchi lines in the CBED and EBSD patterns, which makes phase and orientation identification difficult. In fact, it is shown that the EBSD patterns in the scanning electron microscope and Kikuchi patterns in the transmission electron microscope from this phase are closely related to the structurally similar m icosahedral quasicrystals that form in numerous aluminium–transition metal alloys by both conventional and rapid solidification. The other iron-containing phase in these alloys was determined to be plate shaped and to have the orthorhombic Cmcm space group consistent with the β-Al₅FeSi phase, which has sometimes been confused with having a tetragonal or monoclinic structure. The nature of this structure is such that it frequently grows with a high density of faults that separate equivalent variants; these will be described and related to the previously incorrect interpretations of this being a tetragonal phase.

Acknowledgements

The authors would like to acknowledge the General Motors Powertrain Division for providing financial support for this study. The authors also acknowledge the Center for Advanced Research and Technology at the University of North Texas for access to the experimental facilities used for this study.