Abstract: Based on the design combination of Ni-Nb binary alloy and Ti-Mn-Fe ternary alloy, and the calculated results of liquidus temperature for the quinary system alloys, the optimum composition of Ti-Mn-Fe-Ni-Nb system medium-entropy alloy brazing filler metal was proposed for TiAl joining. The filler alloy consisted of Ti-based solid solutions, TiFe and TiMn phases with Nb and Ni dissolved, showing a liquidus temperature of 1084.2 °C. As the dynamic wettability test result, the filler alloy exhibited an equilibrium contact angle of 24.5° on the TiAl substrate after being held at 1150 °C for 5 min. The TiAl/TiAl joint brazed with the brazing filler powder was mainly composed of B2, γ-TiAl, and α₂-Ti₃Al as well as residual brazing filler metal reaction phase with a hexagonal close-packed structure. The presence of nanotwins, dislocations, and their interactions within the γ phase, as well as 6H and 18R type long periodic stacking ordered structures within the brazing seam, had a beneficial effect on joint strength. A transition region with a thickness of approximately 10 nm was observed at the B2/α₂ interface by means of a three-dimensional atom probe. Increasing brazing temperature or prolonging dwell time led to a gradual increase in both the joint thickness and the volume fraction of the γ-TiAl phase within the joint, yet it was accompanied by a remarkable decrease in the thickness of the residual brazing filler metal reaction layer. Under the brazing condition of 1180 °C/45 min, the joints offered a volume fraction of γ-TiAl phase of 31.6 %, a joint thickness of 244 µm and a maximum tensile strength of 463 MPa at ambient temperature, representing 78.5 % of the base alloy strength. The brazed joints exhibited tensile strength of 497 MPa at 750 °C and 469 MPa at 800 °C, corresponding to the joint strength coefficient of 88.3 % and 90.7 %, respectively. In this case, the fracture occurred in the brazing seam with a flexural pathway but still presented typical brittle characteristics.

Key words: Medium-entropy alloy filler metal, Ti-Mn-Fe-Ni-Nb system alloy, Brazing, TiAl-based alloy, Microstructure