¹Institution of Russian Academy of Sciences A. A. Baikov Institute of Metallurgy and Materials Science, Moscow, Russia ; ²National University of Science and Technology “MISiS”, Moscow, Russia:

T. K. Akopyan, Researcher^1,2, e-mail: nemiroffandtor@yandex.ru

National University of Science and Technology “MISiS”, Moscow, Russia:

N. O. Korotkova, Post-Graduate Student, Engineer of a Chair “Metal forming”
A. P. Dolbachev, Master's Degree Student, Engineer of a Chair “Casting Technologies and Art Treatment of Materials”

Interdepartmental Analytical Center, Moscow, Russia:

N. I. Dashkevich, Area Manager

Using the calculation and experimental methods, we analyzed the phase composition and structure after solidification, cooling and subsequent thermal treatment, including hot isostatic pressing (HIP) of the γ-alloy TNM. Thermodynamic analysis, carried out in the Thermo-Calc program, revealed the β-solidification of the investigated TNM+C alloy (at. %: 43.00 Al, 4.46 Nb, 1.06 Mo, 0.83 Cr) and the basic TNM alloy (Ti_43.5Al₄Nb₁Mo_0.1B) (i. e. its solidification in equillibrium conditions is finished in the β-area). This analysis was followed by the solid-state transformations: polymorphic β→α transformation, segregation of secondary γ-crystals and eutectoid transformation α→α₂+β+γ. Using the Sheil-Gulliver model, we analyzed the non-equilibrium solidification of the investigated alloy. Non-equilibrium solidification proceeded via two peritectic reactions: L+β and L+α→γ, which led to the appearance of the non-equilibrium α- and β-phases in addition to β-phase in the as-cast structure of the alloy. In contrast to the equilibrium conditions, the non-equilibrium crysiallization ended at a lower temperature, which corresponds to non-equilibrium solidus, reaching 1250 ^oС. Investigations in scanning and transmission electron microscope revealed that the microstructure of the alloy TNM+C in the as-cast state and after vacuum annealing at 1250 ^oC consists predominantly of lamellar γ/α₂-colonies with the thickness of the separate plates of α₂- and γ-phases less than 250 nm after annealing. The lamellar γ/α₂-colonies are surrounded by areas of β and γ-phase. Qualitative and quantitative analysis of the alloy microstructures after HIP-treatment at 1250 ^oC and an excess pressure of 170 MPa showed several significant differences compared to the microstructures after vacuum annealing at the same temperature. The alloy structure got coarser, expressed by the eutectic colonies size increase with the thickness of the separate plates of α₂- and γ-phases of 300–350 nm. In addition the phase ratio significantly changed. In particular, the amount of γ-phases increased by more than 5 times (from 6 to 34 vol.%) while the amount of α-phase reduced by almost half (82 to 40 vol.%). The higher content of the softest &-globular decreased the alloy hardness from 420 HV to 350 HV.

1. Appel F., Paul J. D. H., Oehring M. Gamma titanium aluminide alloys. Wiley, 2011. 746 р.
2. Wu X. Review of alloy and process development of TiAl alloys. Intermetallics. 2006. Vol. 14. pp. 1114–1122.
3. Hu D., Wu X., Loretto M. H. Advances in optimization of mechanical properties in cast TiAl alloys. Intermetallics. 2005. Vol. 13. pp. 914–919.
4. Güther V., Rothe C., Winter S., Clemens H. Metallurgy, microstructure and properties of intermetallic TiAl ingots. Berg and Hüttenmännische Monatshefte. 2010. Vol. 155. pp. 325–329.
5. Clemens H., Wallgram W., Kremmer S., Güther V., Otto A., Bartels A. Design of novel β-solidifying TiAl alloys with adjustable β/B2-phase fraction and excellent hot-workability. Advanced Engineering Materials. 2008. Vol. 10. pp. 707–713.
6. Clemens H., Mayer S. Design, processing, microstructure, properties, and applications of advanced intermetallic TiAl alloys. Advanced Engineering Materials. 2013. Vol. 15. pp. 191–215.
7. Hongsheng Ding, Ge Nie, Ruirun Chen et al. Influence of oxygen on microstructure and mechanical properties of directionally solidified Ti – 47Al – 2Cr – 2Nb alloy. Materials & Design. 2012. Vol. 41. pp. 108–113.
8. Wang Y., Wang J. N., Yang J., Zhang B. Control of a fine-grained microstructure for cast high-Cr TiAl alloys. Materials Science and Engineering: A. 2005. Vol. 392. pp. 235–239.
9. Bartels A., Koeppe C., Mecking H. Microstructure and properties of Ti – 48Al – 2Cr after thermomechanical treatment. Materials Science and Engineering: A. 1995. Vol. 192, 193. pp. 226–232.
10. Huang Z. W., Voice W., Bowen P. Thermal exposure induced α₂+γ→β₂(ω) and α_2→β2(ω) phase transformations in a high Nb fully lamellar TiAl Alloy. Scripta Materialia. 2003. Vol. 48. pp. 79–84.
11. Jin Y., Wang J. N., Yang J., Wang Y. Microstructure refinement of cast TiAl alloys by solidification. Scripta Materialia. 2004. Vol. 51. pp. 113–117.
12. Hao Y. L., Yang R., Cui Y. Y., Li D. The influence of alloying on the α₂/(α₂+γ) phase boundaries in TiAl based systems. Acta Materialia. 2000. Vol. 48. pp. 1313–1324.
13. Ilin A. A., Kolachev B. A., Polkin I. S. Titanium alloys. Composition, structure, properties : reference book. Moscow : VILS-MATI, 2009. 520 p.
14. Witusiewicz V. T., Bondar A. A., Hecht U., Velikanova T. Ya. The Al – B – Nb – Ti system: IV. Experimental study and thermodynamic re-evaluation of the binary Al – Nb and ternary Al – Nb – Ti systems. Journal of Alloys and Compounds. 2009. Vol. 472. pp. 133–161.
15. Hao Y. L., Yang R., Cui Y. Y., Li D. The influence of alloying on the α₂/(α₂+γ) phase boundaries in TiAl based systems. Acta materialia. 2000. Vol. 48. pp. 1313–1324.
16. Kainuma R., Fujita Y., Mitsui H. et al. Phase equilibria among α(hcp), β(bcc) and γ(L₁₀) phases in Ti – Al base ternary alloys. Intermetallics. 2000. Vol. 8. pp. 855–867.
17. Kuang J. P., Harding R. A., Campbell J. The effects of HIP pore closure and age hardening on primary creep and tensile property variations in a TiAl XD™ alloy with 0,1 wt.% carbon. Materials Science and Engineering: A. 2002. Vol. 31. pp. 329–331.
18. Huang A., Hu D., Loretto M. H., Mei J., Wu X. The influence of pressure on solid-state transformations in Ti – 46Al – 8Nb. Scripta Materialia. 2007. Vol. 56. pp. 253–256.
19. State Standard GOST 2999–75. Metals and alloys. Vickers hardness measurement. Introduced: 1976–07–01.
20. Belov N. A., Akopyan T. K., Belov V. D., Gershman J. S., Gorshenkov M. V. The effect of Cr and Zr on the structure and phase composition of TNM gamma titanium aluminide alloy. Intermetallics. 2017. Vol. 84. pp. 121–129.
21. Kartavykh A. V., Asnis E. A., Piskun N. V., Statkevich I. I., Stepashkin A. A., Gorshenkov M. V., Akopyan T. K. Complementary thermodynamic and dilatometric assessment of phase transformation pathway in new β-stabilized TiAl intermetallics. Materials Letters. 2017. Vol. 189. pp. 217–220.
22. Schwaighofer E., Clemens H., Mayer S., Lindemann J., Klose J., Smarsly W., Güther V. Microstructural design and mechanical properties of a cast and heattreated intermetallic multi-phase &-TiAl based alloy. Intermetallics. 2014. Vol. 44. pp. 128–140.