Unravelling formation mechanism, phase stability and transitions of metastable α2 and B2 phases in laser additively manufactured Ti-48Al-2Cr-2Nb alloy

The solidification route and microstructure evolution of TiAl alloys are strongly dependent on cooling rate, leading to a significant presence of ordered α₂ and B2 phases in those fabricated using laser powder bed fusion (LPBF). However, the formation mechanisms, thermal stability and decomposition behaviors of this special phase composition remain unclear. In this work, we studied a Ti-48Al-2Cr-2Nb alloy fabricated by LPBF to understand its unique ordered phase structure. It was found that both the high-temperature α and β phases can nucleate and grow from the liquid TiAl, and subsequently transform to metastable α₂ and B2 phases through ordered transition. The minimum cooling rate for the transition of β→B2 was determined as 3.28 × 10⁴ K/s, with the content of the B2 phase increasing at higher cooling rate. Based on in-situ characterization and annealing treatment, both the ordered α₂ and B2 phases were revealed to exhibit weak thermal stability, which were attributed to their special chemical composition deviated from that in equilibrium solidification due to insufficient atomic diffusion. This phenomenon also results in the formation of disordered γ phase and early precipitation of high-temperature α and β phases during heat treatment. This study provides valuable insights into the microstructural characteristic of Ti-48Al-2Cr-2Nb alloys fabricated at high cooling rate, offering guidance for the LPBF building and post heat treatment process.