Microstructure and fracture behaviours of Nb–16Si ultra-high temperature alloy fabricated by laser directed energy deposition

We prepared Nb–16Si ingot via the laser directed energy deposition (L-DED) using mixed pure Nb and Nb5Si3 alloy powders, and investigated the microstructure and fracture behaviour of the L-DEDed samples along directions that were parallel and perpendicular to the built direction, as well as the fracture behavior of the arc melting Nb–16Si sample for comparison. The L-DED molten pool approximately showed a scaly-like morphology, and the fine microstructure containing primary dendritic NbSS arm and eutectic colony of NbSS + Nb3Si was obtained, which oriented to the built direction and the scanning direction. The multiformity of orientation relationships (ORs) with the small Miller indices in the NbSS/Nb3Si couple was confirmed, while the ORs, such as (0 1 1‾) NbSS//(0 2 0) Nb3Si, [0 1 1] NbSS//[0 0 1] Nb3Si, and (1‾ 1 0) NbSS//(2 0 0) Nb3Si, [1 1 2] NbSS//[0 0 1] Nb3Si were the dominant. The fine Nb phase in the L-DEDed sample fractured in a mixed dimple, tear and cleavage mode, resulting in higher fracture toughness KQ of approximately 7.6–8.6 MPa m1/2. As for the arc-melting Nb–16Si sample with coarsened microstructure, the NbSS phase usually failed in a single cleavage mode, leading to a low KQ value of 6.8 MPa m1/2.