On strengthening phases in the Nb-Si based alloys

The effect of the type of a strengthening phase on the structure and properties of cast alloys Nb-15 at.% Si-x at.% Al (x = 0 and 3) was studied by scanning electron microscopy, X-ray phase analysis, and energy-dispersive analysis. The indentation method was used to measure the mechanical properties of the obtained alloys: fracture toughness and hardness. The formation of two-phase structures consisting of a solid solution based on niobium (NbSS) and niobium silicides was observed in the obtained alloys. NbSS solid solutions of different compositions were formed in binary and ternary alloys – Nb(Si) and Nb(Si, Al), respectively. The lattice parameters of the solid solutions were larger compared to the lattice parameter of pure niobium, which was associated with the dissolution of silicon in niobium. Changing the volume fractions of the matrix and strengthening phase, as well as the formation of different types of niobium silicide affected the mechanical characteristics of the studied alloys. The formation of Nb3Si silicide and the increase in the proportion of the silicide phase in the binary alloy compared to the ternary alloy leads to a higher hardness value of the binary alloy. The formation of a high-temperature modification of the Nb5Si3 silicide in the ternary alloy leads to an increase in the value of the fracture toughness due to an increase in the volume fraction of NbSS.