Grain refinement by low boron additions in niobium-rich TiAl-based alloys

The grain refinement achieved with low boron additions to selected TiAl-based alloys was investigated in unidirectional solidified samples by means of EBSD-orientation mapping of the α(Ti) phase, retained by quenching. The analysis shows that excellent grain refinement is achieved during the solid state transformation of the body centred β(Ti) into the hexagonal close packed α(Ti). It leads to a fairly random orientation of the α(Ti) grains which indicates that heterogeneous nucleation of α(Ti) on borides is at the origin of refinement. The borides were identified to be orthorhombic (Ti,Nb)B monoboride ribbons with the crystal structure of NbB. The refinement is severely impeded by peritectic growth of α(Ti): peritectic α(Ti) grains almost completely invade the microstructure by imposing their crystal orientation through all subsequent solid state transformations.