Electron microscopy investigation of the TiBAl and TiCAl grain refiner master alloys

The present work brings a detailed statistical analysis of the microstructural characteristics and chemistry of the second phase particles present in the Al-5Ti-1B and Al- 3Ti-0.15C (wt.%) commercial grain refiner rods used in the Al casting industry. The investigation was performed using the TEM, EDS, STEM and HREM techniques. Both refiner rods contained large, blocky Al₃Ti DO₂₂ particles. Borides present in the TiBAl rod, known to be main α-Al nucleation substrates, exhibited a facetted, hexagonal platelet morphology and largely represented a mixed (Ti,Al)B₂ phase, with Ti and Al contents changing continuously across the particle dimensions. There were two distinct families of small particles observed in the TiCAl rod. The coarser, facetted particles corresponded to cubic TiC crystal structure and these are believed to be major α-Al nucleation sites. The family of finer, round particles appeared to correspond to orthorhombic Al₆Fe phase and their role in the nucleation process remains to be clarified. The results obtained may be used in optimising the grain refiner rod microstructures that would ensure improved refining performance during casting.