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-Si-16 alloy and a Nb-Si-16-Ti-22-Al-2-Hf-2-Cr-7 alloy with a Nb/Nb5Si3 microstructure fabricated via spark plasma sintering (SPS) was investigated. A decrease in the Nb powder size from 78.8 mu m to 4.9 mu m led to a fine-grain (9.9 mu m) 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-Si-16-Ti-22-Al-2-Hf-2-Cr-7 alloy owing to full dimples on the finer Nb grains (11.2 mu m) through the addition of 22 at.% Ti and 2 at.% Hf. When the flaky Nb powders (123.6 mu m in diameter and 15.1 mu m 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-Si-16 sample. (C) 2016 Elsevier Ltd. All rights reserved.