A Newly Generated Nearly Lamellar Microstructure in Cast Ti-48Al-2Nb-2Cr Alloy for High-Temperature Strengthening
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INTRODUCTION
GAMMA titanium aluminide (c-TiAl)-based alloys, called ‘‘Gammalloys,’’ offer a good combination of lightweight, oxidation resistance, and mechanical properties, which is needed to replace Ni-base superalloys in the service temperature range from 650 C to 1000 C.[1–3] Over the last nearly four decades, great deal of efforts has been made in compositional exploration, alloying fundamentals, and process development. As the results, useful alloys have been developed, which include 4822,[4] XD,[5] TNB/TNM,[6,7] K5/ 09C,[1,3] and high-Nb alloys.[8] Among them, alloy 4822 (nominally Ti-48Al-2Cr-2Nb at. pct) in a specific casting duplex (CDP) microstructure form[9] was successfully implemented and used in the commercial GEnx-1B engines as low-pressure turbine (LPT) blades since 2012.[9,10] This first engineering gammalloy material form possesses relatively low strength levels but a good balance of mechanical properties, enabling its ZITONG GAO, JIEREN YANG, YULUN WU, and RUI HU are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, Shaanxi, P.R. China. Contact e-mail: [email protected] SANG-LAN KIM and YOUNG-WON KIM are with the Gamteck LLC, Dayton, OH, 45431. Contact e-mail: [email protected] Manuscript submitted April 19, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
service temperatures up to 700 C.[9] Alloy TNM (Ti-43Al-4Nb-1Mo-0.1B at. pct) in a wrought nearly lamellar (WNL) microstructure form began to be used in 2016 for GTF engine LPT blades in the temperature range up to near 750 C.[11,12] Alloy 45XD (Ti-48Al-2Nb-2Mn-1B at. pct) LPT blades having a stabilized lamellar (XDL) microstructure form are to be used in RR engines in the temperature range up to 700 C. The current three engineering gammalloys setting their own service temperature limit (700 C to 750 C) are not in the best microstructure forms. Therefore, people try to develop optimized fully lamellar (FL) microstructures; however, the refined FL microstructure seems not to be practically possible in some alloy compositions.[3,13,14] Enhancing higher-temperature mechanical behavior is feasible by controlling the microstructures to contain increased lamellar components (4822 and TNM)[13,14] and increasing lath anisotropy (XD).[3] Nonetheless, efforts in this area have been relatively ineffective primarily due to our unawareness or overlook of its importance or potentials. Instead, efforts to meet increased demand of greater-temperature (> 750 C) gammalloys were directed toward developing higher-order gammalloys[3,7,8] that are solid-solution strengthened as well as environmentally more resistant with refractory elements such as Nb and W.[15]
Unfortunately, controlled fully lamellar (FL) microstructures have not been identified yet for balance in enhanced properties in high alloying system.[3,16,17] Recently, advanced beta gammalloys exhibiting finegrained FL (FGFL) microstructures were designed utilizing the emerging DKI (data, knowledge, and information)-based concepts.[3] Qu
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