Intermetallic precipitation in rare earth-treated A413.1 alloy: A metallographic study

Abstract The present study was performed mainly on A413.1 alloy. Measured amounts of La, Ce or La+Ce, Ti and Sr were added to the molten alloy in the form of master alloys. Samples sectioned from castings obtained from thermal analysis experiments were used for preparing samples for metallographic examination. The results show that addition of rare earth ( RE ) metals to Al–Si alloys increased the α-Al nucleation temperature and depressed the Al-Si eutectic formation temperature, thereby increasing the solidification range. Depending upon the alloying elements/additives, a large number of RE -based intermetallics could be formed: Al 4 (Ce,La), Al 13 (Ce,La) 2 Cu 3, Al 7 (Cu,Fe) 6 (Ce,La) 6 Si 2, Al 4 La, Al 2 La 5 Si 2, Al 2 Ce 5 Si 2, Al 2 (Ce,La) 5 Si 2. Under an electron microscope, these phases appear in backscatter imaging mode in the form of thin grayish-white platelets on the dark gray Al matrix. The average thickness of these platelets is about 1.5 μm. When the alloy is grain refined with Ti-based master alloys, precipitation of a gray phase in the form of sludge is observed: Al 12 La 3 Ti 2 , or Al 12 (Ce,La) 3 Ti 2. Regardless the alloy composition, the RE /Al ratios remain constant in each type of intermetallic. Rare earth metals have a strong affinity to react with Sr (resulting in partial modification of the eutectic Si particles) as well as some transition elements, in particular Ti and Cu. Iron has a very low affinity for interaction with RE metals. It is only confined to Fe-based intermetallics.