Additively manufactured Nb-Ti-Si based alloy: As-built and heat-treated conditions

This work aims to fully characterise the Nb-Ti-Si based alloy (Nb-26Ti-16Si-2.2Al-2Cr), processed by the electron-beam powder-bed-fusion in both as-built and heat-treated conditions, to elucidate the microstructure-property relationships. The as-built condition has [001]-oriented columnar grains of the Nb₃Si phase with the Nbss phase dispersed throughout the microstructure. The microhardness is 645.2 ± 6.7 HV_0.5, and the indentation fracture toughness shows distinct directionality: 7.7 MPa·m^1/2 in the horizontal direction compared to 5.3 MPa·m^1/2 in the vertical direction. Both properties are comparable to the cast version. The directionality is attributed to the underlying mechanisms such as crack bridging, arrest, and micro-crack formation. By contrast, in the heat-treated condition, the alloy exhibits a dual-phase microstructure (Nbss and Nb₅Si₃ phases) with near-equiaxed grain shape due to the Nb₃Si phase decomposition. The fracture toughness increases to 12.1 MPa·m^1/2, at the expense of a reduced microhardness of 564.4 ± 15.0 HV_0.5.