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 alpha(Ti) phase, retained by quenching. The analysis shows that excellent grain refinement is achieved during the solid state transformation of the body centred beta(Ti) into the hexagonal close packed alpha(Ti). It leads to a fairly random orientation of the alpha(Ti) grains which indicates that heterogeneous nucleation of alpha(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 alpha(Ti): peritectic alpha(Ti) grains almost completely invade the microstructure by imposing their crystal orientation through all subsequent solid state transformations. (C) 2008 Elsevier Ltd. All rights reserved.