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

• Fully dense and crack-free Nb-Si alloy with columnar grains. • Columnar microstructure impeding crack propagation. • Post heat treatment improves microstructure homogeneity and phase constitution. • Indentation fracture toughness increased by 128.3 % after heat treatment. 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 Nb 3 Si 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 Nb 5 Si 3 phases) with near-equiaxed grain shape due to the Nb 3 Si 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 .