Tailored microstructure creation of TiAl-based refractory alloys within VGF solidification

• Columnar-dendrite and granular structures created by directional solidification in Ti–46Al–8Nb ingots.
• Ti–44Al–7Nb–2B (at.%) grain refined alloy produced at TiB2 alloying.
• TiB2 dissolved and re-precipitated into (Ti,Nb)B phase, where Ti/Nb ratio is 1:1.
• Hypothesis: Nb from heavily alloyed melt can substitute Ti in TiB precipitates up to NbB.
• State and nucleation efficacy of (Ti,Nb)B can depend on Nb content through Ti/Nb ratio.

The work is aimed at the study of the formation and refinement of primary microstructure appearing in the refractory lightweight structural TiAl-based intermetallics during their solidification from the melt. The alloy with Ti–46Al–8Nb (at.%) nominal composition is selected for applied study in microstructure engineering. For tailored microstructure development, the Directional Solidification (DS) of pre-synthesized alloy was performed in the vertical multizone electro-furnace by Vertical Gradient Freezing (VGF) power-down technique in pure argon environment. Both columnar-dendrite, and equiaxed-granular reproducible as-cast microstructures have been produced in DS ingots, basing onto Columnar-to-Equiaxed Transition (CET) diagram exploration. Particular attention was paid further to equiaxed microstructure improvement by combination of modifying doping of alloy with boron grain refiner and VGF processing. As a result the perfect inoculated microstructure of Ti–44Al–7Nb–2B (at.%) ingots was produced with 120 μm mean grain diameter, low scattering of dimensional characteristics and high tolerance to DS process parameters variation. DS samples were examined by SEM microanalysis along with EBSD and Auger spectrometry of boride particles in the alloy matrix. The nature, state and exact composition of precipitated borides are discussed along with the nucleation mechanism in relation to the literature data.