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The mechanism underlying the grain refinement of cast aluminium by zirconium has been studied through examination of a range of Al alloys with increasing Zr contents. Pro-peritectic Al₃Zr particles are reproducibly identified at or near the grain centres in grain-refined alloy samples based on the observations of optical microscopy, scanning electron microscopy and X-ray diffraction. From the crystallographic study using the edge-to-edge matching model, electron backscatter diffraction and transmission electron microscopy, it is substantiated that the Al₃Zr particles are highly potent nucleants for Al. In addition, the effects of Al₃Zr particle size and distribution on grain refinement has also been investigated. It has been found that the active Al₃Zr particles are bigger than previously reported other types of active particles, such as TiB₂ for heterogeneous nucleation in Al alloys. Considering the low growth restriction …