Enhanced room temperature fracture toughness of directionally solidified Nb_ss/Nb₅Si₃in situ composites via Rhenium addition

Using directional solidification technique, a series of Nb-Si in situ composites with nominal compositions of Nb-16Si-23Ti-4Cr-2Al-2Hf-xRe (x = 0, 0.1, 0.4, 0.6, at.%) were manufactured to investigate the phase constituent, microstructural evolution and room temperature fracture toughness. These four different alloys all consisted of Nbss, αNb5Si3 and γNb5Si3, while Rhenium was primarily partitioned in Nbss matrix. With further addition content of Re, the silicide got refinement firstly, its average size minimized to 4.68 μm in 0.4Re alloy; but that coarsened again in 0.6Re alloy which resulting in the maximum micro-hardness of 1627 HV. The ductile Nbss matrix in 0.4Re alloy exhibited best continuity and its volume fraction rose to 63.1%; that triggered more obvious microcrack, fracture step and crack bifurcation, especially interface decohesion during loading process, thus increasing the room temperature fracture toughness from 8.4 MPa m1/2 to 13.1 MPa m1/2. But the 0.6Re alloy had an adverse effect on room temperature plasticity due to higher proportion and larger size of silicide, then exerted perceptible constraint for ductile Nbss matrix. In order to better optimize the mechanical performance of Nb-Si based alloys, the alloying amount of Rhenium should be properly selected to circumvent its concentration-sensitive effect.