Continuous-Cooling-Transformation (CCT) Behaviors and Fine-Grained Nearly Lamellar (FGNL) Microstructure Formation in a
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GAMMA titanium aluminide (c-TiAl)-based alloys have great potential in aerospace industry due to their low density, good balanced mechanical properties, and oxidation resistance at elevated temperatures.[1,2] The peritectic high-Al composition, such as cast Ti-48Al-2Cr-2Nb (4822) alloy, has been successfully applied to aero-engine turbine blades for service temperature up to 700 C.[3,4] However, low strength limits its further application. Many attempts were made to achieve the high-temperature (HT) capability and balanced properties.[4,5] One of the most attractive ways is the further addition of Nb, which can not only offer
JIEREN YANG is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, Shaanxi, China and also with Shaanxi Key Laboratory of HighPerformance Precision Forming Technology and Equipment, Northwestern Polytechnical University, X’an, 710072, China Contact e-mail: [email protected] ZITONG GAO, XIAOGAN ZHANG, and RUI HU are with the State Key Laboratory of Solidification Processing, Nordthwestern Polytechnical University. Manuscript submitted February 24, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
good strength but elevate the potential service temperature to 800 C as well.[6] However, a large amount of Nb addition (~ 8 at. pct) would cause severe segregation and brittle B2 phase that are harmful to mechanical properties.[7,8] It is worth pointing out that a mid-Nb (~ 4 at. pct) addition can avoid these problems and maintain good solute strengthening. Meanwhile, high-Al content (~ 48 at. pct) can result in a peritectic route and avoid the formation of brittle b/B2 phase during cooling, which prevents the fracture in castings and improves the room-temperature (RT) ductility.[9] Indeed, our previous work reveals that the cast microstructure of a Ti-48Al-4Nb alloy presents a peritectic feature with low segregation and no brittle B2 phase.[10] Based on these, a high-Al and mid-Nb-containing cast Ti-48Al-4Nb-2Cr (4842) alloy was designed and it may offer promising HT properties compared with the current commercial 4822 alloy. As for the newly designed 4Nb alloy, it is important to clarify its phase transformation characteristics and microstructure evolution in order to achieve the microstructural control. In general, the formation of RT microstructure of TiAl alloys is via continuous cooling. It is well known that the CCT diagram has been used to control the phase transformation and
mechanical properties of alloys.[11] Similarly, CCT can effectively guide the microstructural design and predict the formation of harmful microstructure in TiAl alloys. Some CCT work for TiAl alloys has been done. And these alloys can be roughly divided into two categories: (1)
(2)
The peritectic compositions. The CCT features of a binary Ti-48Al alloy and a Ti-47Al2Nb-1Mn-1W-0.2Si alloy indicated that the transformation product of a phase could be massive, feathery, lamellar, or granular, depending on the cooling rate.[12,13] Seo et al. found t
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