Microstructure and phase evolution in γ-TiAl/Ti₂AlNb dual alloy fabricated by direct metal deposition

As a potential material used in aero-engine, a γ-TiAl/Ti₂AlNb dual alloy was fabricated by depositing γ-TiAl alloy on a Ti₂AlNb alloy plate using direct metal deposition (DMD) technique. Due to re-melting of previous layer during DMD, a transition zone with gradual change of compositions and microstructures was generated between γ-TiAl and Ti₂AlNb alloy. In this paper, formation mechanism of the transition zone was analyzed after considering solidification route and compositional variations. Results reveal that the transition zone mainly consists of three layers, and compositions are comparatively uniform in layers but change stepwise between each other. Microstructures of layers are dominantly controlled by their compositions and cooling schedules, and the first layer possesses columnar grains with α₂ phase and γ phase dispreading in B2 matrix. While for the second layer, a mixed microstructure of (α₂+γ) lamella, B2 phase and γ phase is formed. The third layer exhibits equiaxial (α₂+γ) colony with fully lamellar microstructure, which is similar to microstructure of γ-TiAl alloy but without elemental segregations. Results of room temperature tensile test indicate that fracture takes place at γ-TiAl alloy side, and the fracture morphology is a mixture of interlamellar fracture and translamellar fracture.