Effects of Tantalum on Microstructure Evolution and Mechanical Properties of High-Nb TiAl Alloys Reinforced by Ti ₂ AlC

Abstract

<jats:p>Experiments have been carried out to study the relationship between the addition of tantalum and microstructure, especially the formation of the B2 phase in lamellar colonies. The mechanical properties, with different contents of Ta, were also measured. Ti46Al8Nb2.6C<jats:italic>x</jats:italic>Ta alloys were prepared by casting with the content of Ta varying from zero to 1.0 at.%. Experimental results show that the B2 phase forms in lamellar colonies with the addition of Ta, and its content increases when the content of Ta increases. Meanwhile, the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mi>γ</mml:mi></mml:math> phase decreases and the lattice parameter of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:msub><mml:mrow><mml:mi>α</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math> phase increases. The size of the lamellar colony decreased from 29.9 to 21.6 <jats:italic>μ</jats:italic>m. Ta dissolves into Ti<jats:sub>2</jats:sub>AlC by substitution, and its solubility is more than 1.1% tested by EDS. Nb, which is necessary for the formation of the B2 phase, comes from two aspects. The first is that Ta dissolves into the Ti<jats:sub>2</jats:sub>AlC and partly replaces the Nb atom and the second is the decrease in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M3"><mml:mi>γ</mml:mi></mml:math> phase because it has higher solid solubility for Nb. The increase in Nb in the liquid phase increases the composition supercooling and heteronucleation at the solidification front, which accounts for refining the lamellar colony. Room temperature compressive testing showed that the compressive strength and the strain increased when the Ta content increased up to 0.8% and then decreased. Improvement of the compressive properties resulted from the grain boundary strengthening and their decrease induced by more content of the B2 phase. Tensile properties, at elevated temperature, were improved with testing temperature increasing from 750 to 950°C, because solid solution strengthening is a major influence factor.</jats:p>

Characteristic effects of alloying elements on β solidifying titanium aluminides: A review

“…This is because Ta hinders the dispersion of Nb atoms from dissolving further within the lamellar colonies. Furthermore, Ta is described to refine lamellar colony sizes of TiAl alloys [36]. This was attributed to the heterogeneous nucleation particles formed due to Ta additions.…”

“…Ta can equally substitute Nb since the positions of Ta in the periodic table suggests they both would exhibit similar chemical behaviours. Likewise, high melting point of Ta makes it offer prospects of increasing TiAl-based alloys’ operating temperature [ 20 , 36 , 55 ]. Research works on the effects of Ta in the TiAl-based alloys are usually on improving oxidation resistance.…”

“…In an experiment conducted to examine the correlation between microstructure and addition of Ta to produce B2-phase within the lamellar colonies, Fang et al. [ 36 ], prepared Ti46Al8Nb2.6CxTa alloys (Ta = 0–1.0 at.%) through casting. It was observed in the lamellar colonies that B2-phase formation increases with increased Ta content.…”

“…Furthermore, Ta is described to refine lamellar colony sizes of TiAl alloys [ 36 ]. This was attributed to the heterogeneous nucleation particles formed due to Ta additions.…”

“…The ordered γ-phase has low ductility (up to 750 °C), thereby, decreases its plastic deformation capability likewise does the ordered α 2 -phase have low ductility (up to 600 °C) [ 30 ]. This is because of the limited number of slip systems, the occurrence of planar slip [ 30 , 36 ] and lack of twinning [ 32 ] which makes these phases brittle.…”