Effect of N addition on microstructure and high temperature mechanical properties of Ti-48Al-2Cr-2Nb (at. %) intermetallic alloy. (December 2020)

• Record Type: Journal Article
• Authors: Ghorbani, Hamid Reza; Kermanpur, Ahmad; Rezaeian, Ahmad; Ahmadi Siahboumi, Abbas; Ali Taebi, Mohammad
• Is Part Of: Materials today communications. Volume 25(2020)
• Publication Date: 2020-12
• Subjects: Titanium aluminide -- Ti-48Al-2Cr-2Nb -- Nitrogen -- High temperature mechanical properties -- Small punch test; Materials science -- Periodicals; 620.11
• DOI: 10.1016/j.mtcomm.2020.101494
• Languages: English
• ISSNs: 2352-4928

Abstract

Highlights: N addition prevented massive gamma phase transformation. N-bearing alloys had similar mechanical property despite different structures. High-T mechanical properties of N-bearing alloys were lower than 4822 alloy. Formation of Ti2 AlN particles at γ/α lamella interface was failure mechanism. Presence of Ti2 AlN particles at colony boundaries also caused lower strength. The 1N alloy showed more uniform deformation behavior. Abstract: Microstructures and high temperature mechanical properties of the Ti-48Al-2Cr-2Nb (4822) intermetallic alloy containing low content of N (up to 2 at. %) were investigated. The alloys were fabricated by vacuum arc re-melting followed by hot isostatic pressing and homogenization treatment. Phase diagram and critical temperature data of the alloys were evaluated by differential thermal analysis and Thermo-Calc™ calculations. Microstructures were characterized by optical microscopy, secondary electron microscopy and X-ray diffraction. High temperature (800 ℃) small punch testing (SPT) was used to investigate mechanical properties of the homogenized samples. Results showed that alpha transus transformation temperature increased by N addition. A fully lamellar microstructure with nitride precipitates was formed in the furnace cooled (FC) microstructures, while a complex microstructure of massive, feathery, and widmanstätten gamma and lamellar structure with some nitride precipitates was achieved in the air cooled (AC) alloys. Increasing N resulted in a finer lamellar structure in the FC alloys and more lamellar structure in the AC ones. The SPT results of the FC alloys revealed that the maximum force and mean displacement of 4822 alloy decreased by N addition. However, despite different N content and various microstructures of the N-bearing alloys, their SPT curves were approximately similar. Precipitation of the coarse, blocky primary Ti2 AlN precipitates at colonies boundaries and the fine, elongated secondary Ti2 AlN precipitates at the α/γ lamella interface were suggested as the main cause for premature failure of the N-bearing alloys. The 4822 alloy containing 1 at. % N showed more uniform deformation behavior during SPT. Inter-lamellar and trans-lamellar fracture and delamination of lamellas were found the most involved fracture mechanisms in the N-containing alloys.