Coupling effect of Nb and BN on microstructural evolution and mechanical properties of TiAl alloy

To improve mechanical properties of high-Nb TiAl alloys, boron nitride (BN) reinforcement was introduced, and an optimum Nb content was determined. Ti46Al2Cr0.5BN- x Nb (at.%) alloys were fabricated through vacuum arc melting. The microstructural evolution of Ti46Al2Cr0.5BN- x Nb alloys was studied, including lamellar colony size, lamellar spacing, element distribution, and the characteristics of borides and nitrides. Results show that when Nb content increases from 0 to 8at.%, the lamellar colony size of Ti46Al2Cr0.5BN- x Nb alloys is refined from 49 µm to 28 µm, and the lamellar spacing decreases from 463 nm to 239 nm. The increase of Nb content leads to an increase in the volume fraction of TiB 2 precipitates and a concomitant decrease in the volume fraction of Ti 2 AlN precipitates. The compression strength at 800 °C shows an increase from 1,062 MPa to 1,658 MPa as the Nb content increases from 0 to 8at.%. The dominant fracture mechanisms of TiAl composites were revealed, including grain boundary fracture, crack deflection, interlamellar fracture, and pull-out of reinforcing phases.