Crystallographic study of grain refinement of Al by Nb addition

The grain refinement of Al by the addition of a small amount of peritectic-forming solute, Nb, has been studied from the crystallographic point of view. Combining the observations of optical microscopy and scanning electron microscopy with the results of energy-dispersive X-ray spectroscopy and X-ray diffraction, it is confirmed that the particles observed at or near the grain centres of refined Al alloys are pro-peritectic Al₃Nb particles. The crystallographic matching between the Al₃Nb particles and Al grains has also been evaluated using an edge-to-edge matching model and further verified using electron backscatter diffraction and transmission electron microscopy. It is found that there are reproducible crystallographic orientation relationships between the Al₃Nb particles and Al grains, and the experimental results are consistent with the predictions of the edge-to-edge matching model. This implies that the pro-peritectic Al₃Nb particles are favourable nucleation sites for Al grains from the crystallographic point of view. Furthermore, the analysis of the size distribution of Al₃Nb particles reveals that the Al₃Nb particles at the grain centres have relatively large particle size, which also corroborates the high potency of Al₃Nb according to the free growth model. It is therefore concluded that the significant grain refinement resulting from the addition of Nb is predominantly attributed to the in situ formed Al₃Nb particles which promote grain refinement via enhanced heterogeneous nucleation.