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In order to improve mechanical properties of TiAlNb alloys, different contents of silicon were added into Ti48Al6Nb alloy. The Ti48Al6NbxSi (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5, at.％) alloys were prepared by vacuum arc melting. The phase constitution, microstructure evolution and mechanical properties of the alloys were studied. Results show that the Ti48Al6NbxSi alloys consist of γ-TiAl phase, α2-Ti3Al phase and B2 phase, and Ti5Si3 silicide phase is formed when the addition of silicon is higher than 0.3at.％. The addition of silicon leads to the decrease in γ phase and increase in α2 phase. The formation of silicide decreases the amount of Nb dissolved in the TiAl matrix, and therefore decreases B2 phase. Compressive tests show that the ultimate strength of the alloys increases from 2,063 MPa to 2,281 MPa with an increase in silicon from 0 to 0.5at.％, while the fracture strain decreases from 34.7％ to 30.8％. The increase of compressive strength and decrease of fracture strain can be attributed to the decrease of B2 phase and the formation of Ti5Si3 phase by the addition of silicon. The strengthening mechanism is changed from solid solution strengthening when the addition of silicon is less than 0.3at.％ to combination of solid solution strengthening and secondary phase strengthening when the addition of silicon is higher than 0.3at.％.