On the state of boride precipitates in grain refined TiAl-based alloys with high Nb content

The work is aimed at the refinement of primary microstructure appearing in the refractory lightweight structural TiAl-based intermetallics during their solidification from the melt. The green alloy with Ti-46Al-8Nb (at.%) nominal composition is selected for applied study in microstructure engineering. For grain refining, the metallurgical alloying of the material was performed with microcrystalline titanium diboride TiB2 additive, along with subsequent Directional Solidification (DS) in the vertical multizone electro-furnace by power-down technique in pure argon environment. As a result the uniform equiaxed microstructure of Ti-44Al-7Nb-2B (at.%) ingots was produced with low dimensional scattering and characteristic grain diameter of 110-130 μm. DS samples were examined by SEM microanalysis along with EBSD and Auger spectrometry of boride particles in the alloy matrix. It was stated that initial diboride TiB2 additive was completely dissolved and has been afterwards re-precipitated in the course of DS into orthorhombic (Ti,Nb)B monoboride single-crystalline micro-particles of B27 structure, acting as randomly distributed point seeds for inoculated solid grains nucleation. The nature, exact composition and nucleation potency of newly formed borides are discussed along with the nucleation mechanism in relation to the literature data. The addition of trace amount (below 0.1 at.%) of beryllium solute is proposed for the enhancement of β-grain refinement at solidification stage.