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

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 Ti2AlN precipitates at colonies boundaries and the fine, elongated secondary Ti2AlN 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.