Structural transformation behavior in a high Nb-TiAl alloy additively manufactured by laser powder bed fusion

High Nb-TiAl alloys fabricated using laser powder bed fusion (LPBF) exhibit a metastable alpha 2 phase due to the rapid cooling rate of the LPBF process. However, the understanding of the structural transformation behavior in LPBF-fabricated high Nb-TiAl alloys remains limited. In this work, a high Nb-TiAl alloy with a nominal composition of Ti-48Al-2Cr-8Nb (at. %) was additively manufactured using LPBF. The structural transformation from metastable alpha 2 phase to gamma phase was then studied by the in-situ high-temperature x-ray diffraction (XRD), in-situ heating transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Furthermore, the phase composition and microstructure of the samples annealed at different temperatures were determined by XRD and electron backscatter diffraction (EBSD). Our findings demonstrate that the alpha 2 phase in LPBF-fabricated high Nb-TiAl alloy remains stable up to 873 K, after which it decomposes and forms fine gamma lamellar colonies along with a disordered gamma ' phase as an intermediate phase. Notably, the transition from alpha 2 to gamma ' phase is reversible. In contrast, the formation of the gamma phase is irreversible and persists down to room temperature. It is also found that the changes in grain size and phase composition of the LPBF-fabricated high Nb-TiAl alloy have a significant impact on its hardness. This study provides valuable insights into the structural transformation behavior of additively manufactured high Nb-TiAl alloys by LPBF, offering guidance for regulating their structure and properties.