Correlative characterization of primary Al3(Sc,Zr) phase in an Al-Zn-Mg based alloy

Three-dimensional electron backscatter diffraction, focused ion beam, transmission electron microscopy and energy filtered transmission electron microscopy were employed to investigate the structural information of primary Al3(Sc,Zr) phase, i.e. size, shape, element distribution and orientation relationship with the α-Al matrix. It was found that (i) most primary Al3(Sc,Zr) phases have a cubic three-dimensional morphology, with a size of about 6-10 μm, (ii) most primary Al3(Sc,Zr) phases are located within the α-Al matrix, and exhibit a cube to cube orientation relationship with the α-Al matrix, and (iii) a layer by layer growth was observed within primary Al3(Sc,Zr) phases. Al, Cu, Si and Fe are enriched in the α-Al matrix between the layers of cellular eutectic Al3(Sc,Zr) phase, while Sc, Ti and Zr are enriched in small Al3(Sc,Zr) phases. A peritectic reaction and subsequent eutectic reaction between Al3Sc and Al was proposed to interpret the observed layer by layer growth. This paper demonstrates that the presence of impurities (Fe, Si, Cu, Ti) in the diffusion field surrounding the growing Al3(Sc,Zr) particle enhances the heterogeneous nucleation of Al3(Sc,Zr) phases.