Mechanisms governing synergistic enhancement of strength-plasticity and α₂ thermal stability in TiAl alloys fabricated via selective electron beam melting

The incoordination of strength-plasticity and the low thermal stability of α₂ phases seriously restrict the development of TiAl alloys. To overcome these shortcomings, Ti-48Al-2Cr-2Nb (at.%) alloys with superior mechanical performance and high thermal stability of α₂ phases were prepared by reasonably optimizing the printing strategy of selective electron beam melting (SEBM) in this work. The alloys exhibit the room-temperature compressive strength of 2716.08 MPa with 58.71 % fracture strain, and maintain compressive strength of 749.43 MPa at 850 °C. The improvement of strength-toughness and enhanced α₂-phase thermal stability are mainly attributed to the influence of long-period stacking ordered (LPSO) structures introduced via SEBM. High temperature and stress will induce the generation of high density 9R-type LPSO structures, and 9R structures can promote the orientation transformation of γ phases. Abundant 9R structures and deformation twins play a key role in enhancing strength and toughness. Furthermore, this study reveals the transformation mechanism of 9R-type and 6H-type LPSO structures and first proposes three reaction pathways for the transformation from γ to α₂ phases, with 6H phases serving as the intermediate structures. The three reaction processes are γ→6H→α₂, γ→9R→6H→α₂ and γ→γ_T→6H→α₂, in which 9R and twin structures can transform into 6H configurations, promoting the transformation of γ to α₂ phases. Due to γ→α₂ transformation induced by LPSO structures, the conventional decomposition reaction of α₂ phase is effectively inhibited, thus improving the stability of α₂ phases.