Effects of heat treatments on microstructures of TiAl alloys

Abstract

This study aims to investigate the effects of heat treatments on the microstructure of γ-TiAl alloys. Two Ti-47Al-2Cr-2Nb alloy ingots were manufactured by casting method and then heat-treated in two types of heat treatments. Their microstructures were studied by both optical and scanning electron microscopies. The chemical compositions of two ingots were determined as well. The ingot with lower Al content only obtains lamellar structures while the one higher in Al content obtains nearly lamellar and duplex structures after heat treatment within 1270 to 1185°C. A small amount of B2 phase is found to be precipitated in both as-cast and heat-treated microstructures. They are distributed at grain boundaries when holding at a higher temperature, such as 1260°C. However, B2 phase is precipitated at grain boundaries and in colony interiors simultaneously after heat treatments happened at 1185°C. Furthermore, the effects of heat treatments on grain refinement and other microstructural parameters are discussed.

Keywords

TiAl alloys / microstructure / heat treatment / casting

References

[1] Clemens H, Mayer S. Intermetallic titanium aluminides in aerospace applications-Processing, microstructure and properties. Mater. High Temp., 2016, 33(4–5): 560.

[2] Appel F, Clemens H, Fischer FD. Modeling concepts for intermetallic titanium aluminides. Prog. Mater. Sci., 2016, 81, 55.

[3] Dimiduk DM. Gamma titanium aluminide alloys-An assessment within the competition of aerospace structural materials. Mater. Sci. Eng. A, 1999, 263(2): 281.

[4] Kim YW. Intermetallic alloys based on gamma titanium aluminide. JOM, 1989, 41(7): 24.

[5] Kim YW, Kim SL. Advances in gammalloy materials. processes.application technology: Successes, dilemmas, and future. JOM, 2018, 70(4): 553.

[6] Bewlay BP, Nag S, Suzuki A, Weimer MJ. TiAl alloys in commercial aircraft engines. Mater. High Temp., 2016, 33(4–5): 549.

[7] J.K. Kim, J.H. Kim, J.Y. Kim, S.H. Park, S.W. Kim, M.H. Oh, and S.E. Kim, Producing fine fully lamellar microstructure for cast γ-TiAl without hot working, Intermetallics, 120(2020), art. No. 106728.

[8] Aguilar J, Schievenbusch A, Kattlitz O. Investment casting technology for production of TiAl low pressure turbine blades-Process engineering and parameter analysis. Intermetallics, 2011, 19(6): 757.

[9] Kelly TJ, Austin CM, Allen RE. Processing of Gamma Titanium-Aluminide alloy Using a Heat Treatment Prior to Deformation Processing. U.S. Patent, Appl. 08/376519, 1997

[10] Kelly TJ, Bewlay BP, Weimer MJ, Whitacre RK. Methods for Processing Titanium Aluminide Intermetallic Compositions. European Patent, Appl. 13159885.6, 2013

[11] Kelly TJ, Weimer MJ, Austin CM, London B, Larson DE, Wheeler DA. Heat Treatment of Gamma Titanium Aluminide Alloys. U.S. Patent, Appl. 08/262168, 2001

[12] Guillaume M, Jeanne MC, Marie MP. Heat Treatment of an Alloy Based on Titanium Aluminide. U.S. Patent, Appl. 15/302418, 2015

[13] Harding TS, Jones JW. Fatigue thresholds of cracks resulting from impact damage to γ-TiAl. Scripta Mater., 2000, 43(7): 623.

[14] Mercer C, Lou J, Soboyejo WO. An investigation of fatigue crack growth in a cast lamellar Ti-48Al-2Cr-2Nb alloy. Mater. Sci. Eng. A, 2000, 284(1–2): 235.

[15] Biamino S, Penna A, Ackelid U, Sabbadini S, Tassa O, Fino P, Pavese M, Gennaro P, Badini C. Electron beam melting of Ti-48Al-2Cr-2Nb alloy: Microstructure and mechanical properties investigation. Intermetallics, 2011, 19(6): 776.

[16] Mine Y, Takashima K, Bowen P. Effect of lamellar spacing on fatigue crack growth behaviour of a TiAl-based aluminide with lamellar microstructure. Mater. Sci. Eng. A, 2012, 532, 13.

[17] Zhu HL, Seo DY, Maruyama K, Au P. Effect of lamellar spacing on microstructural instability and creep behavior of a lamellar TiAl alloy. Scripta Mater., 2006, 54(12): 1979.

[18] Gao ZT, Yang JR, Wu YL, Hu R, Kim SL, Kim YW. A newly generated nearly lamellar microstructure in cast Ti-48Al-2Nb-2Cr alloy for high-temperature strengthening. Metall. Mater. Trans. A, 2019, 50(12): 5839.

[19] Ahmadi M, Hosseini SR, Hadavi SMM. Effects of heat treatment on microstructural modification of as-cast gamma-TiAl alloy. J. Mater. Eng. Perform., 2016, 25(6): 2138.

[20] Du YJ, Shen J, Xiong YL, Shang Z, Fu HZ. Stability of lamellar microstructures in a Ti-48Al-2Nb-2Cr alloy during heat treatment and its application to lamellae alignment as a quasi-seed. Intermetallics, 2015, 61, 80.

[21] Szkliniarz A. Grain refinement of Ti-48Al-2Cr-2Nb alloy by heat treatment method. Solid State Phenom., 2012, 191, 221.

[22] Kościelna A, Szkliniarz W. Effect of cyclic heat treatment parameters on the grain refinement of Ti-48Al-2Cr-2Nb alloy. Mater. Charact., 2009, 60(10): 1158.

[23] Novoselova T, Malinov S, Sha W. Experimental study of the effects of heat treatment on microstructure and grain size of a gamma TiAl alloy. Intermetallics, 2003, 11(5): 491.

[24] Wang JN, Yang J, Wang Y. Grain refinement of a Ti-47Al-8Nb-2Cr alloy through heat treatments. Scripta Mater., 2005, 52(4): 329.

[25] Zhang WJ, Chen GL, Evangelista E. Formation of α phase in the massive and feathery γ-TiAl alloys during aging in the single α field. Metall. Mater. Trans. A, 1999, 30(10): 2591.

[26] Tang HP, Yang GY, Jia WP, He WW, Lu SL, Qian M. Additive manufacturing of a high niobium-containing titanium aluminide alloy by selective electron beam melting. Mater. Sci. Eng. A, 2015, 636, 103.

[27] Schuster JC, Palm M. Reassessment of the binary Aluminum. Titanium phase diagram. J. Phase Equilib. Diffus., 2006, 27(3): 255.

[28] Han P, Kou HC, Yang JR, Yang G, Li JS. Solidification microstructure characteristics of Ti-44Al-4Nb-2Cr-0.1B alloy under various cooling rates during mushy zone. Rare Met., 2016, 35(1): 35.

[29] Liu Y, Hu R, Kou HC, Wang J, Zhang TB, Li JS, Zhang J. Solidification characteristics of high Nb-containing γ- TiAl-based alloys with different aluminum contents. Rare Met., 2015, 34(6): 381.

[30] Liu Y, Hu R, Yang G, Kou HC, Zhang TB, Wang J, Li JS. Widmannstatten laths in Ti48Al2Cr2Nb alloy by undercooled solidification. Mater. Charact., 2015, 107, 156.

[31] Huang SC, Hall EL. The effects of Cr additions to binary TiAl-base alloys. Metall. Trans. A, 1991, 22(11): 2619.

[32] Kim YW. Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy. Acta Metall. Mater., 1992, 40(6): 1121.