Failure mode transition of Nb phase from cleavage to dimple/tear in Nb-¹⁶Si-based alloys prepared via spark plasma sintering

The influence of Nb powder sizes and morphologies (equiaxed and flaky powders) on fracture behaviour of the Nb-¹⁶Si alloy and a Nb-¹⁶Si-²²Ti-²Al-²Hf-⁷Cr alloy with a Nb/Nb₅Si₃ microstructure fabricated via spark plasma sintering (SPS) was investigated. A decrease in the Nb powder size from 78.8 μm to 4.9 μm led to a fine-grain (9.9 μm) Nb matrix plus a Nb₅Si₃ island microstructure, which changed fracture mode of Nb from cleavage to a mixed mode of dimple, tear and cleavage, the fracture toughness (K_Q) of the Nb-¹⁶Si samples was significantly improved from 8.2 MPa·m^1/2 to 12.4 MPa·m^1/2. A further improvement of K_Q to 15.8 MPa·m^1/2 was achieved in the Nb-¹⁶Si-²²Ti-²Al-²Hf-⁷Cr alloy owing to full dimples on the finer Nb grains (11.2 μm) through the addition of 22 at.% Ti and 2 at.% Hf. When the flaky Nb powders (123.6 μm in diameter and 15.1 μm in thickness) were used, two fracture modes were observed in one flaky Nb grain, i.e., the cleavage mode in the radial plane and the dimple/tear mode in the thickness plane of the flaky Nb grain. These fracture modes in one flaky Nb grain are also beneficial to improving K_Q of the bulk Nb-¹⁶Si sample.