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

Abstract

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 78.8μm to 4.9μm led to a fine-grain (9.9μm) 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 (KQ) of the Nb-16Si samples was significantly improved from 8.2MPa·m1/2 to 12.4MPa·m1/2. A further improvement of KQ to 15.8MPa·m1/2 was achieved in the Nb-16Si-22Ti-2Al-2Hf-7Cr alloy owing to full dimples on the finer Nb grains (11.2μm) through the addition of 22at.% Ti and 2at.% Hf. When the flaky Nb powders (123.6μm in diameter and 15.1μm 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 KQ of the bulk Nb-16Si sample.