Mechanical properties and fracture behavior of an NbSS/Nb5Si3 in-situ composite modified by Mo and Hf alloying

An NbSS/Nb5Si3 in-situ composite whose nominal composition is Nb–18Si–5Mo–5Hf was fabricated. The microstructure consists of an Nb solid solution (NbSS) and α-type niobium silicide (Nb5Si3). This in-situ composite exhibits good balance in its strength at ambient and high temperatures; the UTS is 550 and 530 MPa at room temperature and 1470 K, respectively. The tensile fracture behavior is dominated by cleavage of the Nb5Si3 phase at 1470 K and lower temperatures. However, the behavior is governed by ductile rupture of NbSS at 1570 K and higher temperatures, which would be ascribed to both the increased ductility of NbSS and the decreased NbSS/Nb5Si3 interface strength comparable to or lower than the Nb5Si3 transgranular fracture strength. The room temperature fracture toughness is 8.6±0.9 MPa m1/2 which is not very high. NbSS incorporated into Nb5Si3 contributes to fracture toughness improvement due to a crack bridging mechanism, however, the constraint imposed on NbSS by surrounding Nb5Si3 seems to suppress the toughening effect.