Carbides in AZ91 and their role in the grain refinement of magnesium

Abstract

Carbon inoculation is a well-known method to grain refine Mg-Al-based alloys, but the underlying mechanisms remain unclear. Here we study carbides and their relationship with the surrounding magnesium in inoculated Mg-9Al-0.7Zn-0.2Mn (wt.%, AZ91) by combining controlled solidification experiments with analytical electron microscopy. Up to three carbides formed depending on the solidification conditions: T1-Al2MgC2, T2-Al2MgC2 and Al4C3. All carbides grew with their basal planes as the largest facet and the trigonal carbides, T2-Al2MgC2 and Al4C3, often contained basal planar growth faults and growth twins. In many cases there was evidence of eutectic-like Al4C3 growing off T2-Al2MgC2, indicating that Al4C3 formed later in the solidification sequence after α-Mg had nucleated, consistent with recent phase diagram calculations. A basal-to-basal OR was measured between α-Mg and T2-Al2MgC2 for approximately 10% of particles, indicating that T2-Al2MgC2 is the heterogeneous nucleant for α-Mg. Particles with the OR had diverse locations including near grain boundaries and near the centre of grains, suggesting that the OR can form by pushing and engulfment as well as by heterogeneous nucleation.