The ill-defined π(AlFeMgSi) phase intermetallics formed in an automotive Al-Si-Mg alloy

Fe-containing quaternary π(AlFeMgSi) phase is one of the most important harmful intermetallic compounds in Al-Si-based automotive alloys. Understanding its structure and relation with another intermetallic phase AlFeSi is critical for improving the toughness of the alloys. In the present study, advanced electron microscopy was employed to unveil the defective or ill-defined structures of the π(AlFeMgSi) phase intermetallics. Experimental atomic-resolution images from two projection directions confirmed that the perfect structure of π(AlFeMgSi) phase has a composition of Al9FeMg3Si5 and a space group of P6¯2m. Interesting is that massive planar defects exist in π(AlFeMgSi) intermetallic compounds. It is revealed that these defects are poor of Mg. Our study suggests a probable mechanism for the π(AlFeMgSi) phase to transform into the AlFeSi phase, which may provide a clue for tuning the quantity or amount of harmful Fe-containing intermetallic compounds in Al alloys.