Effect of Ti or Fe Addition and Annealing on Microstructural Evolution and Mechanical Properties of Hypereutectic Nb-Si-Mo Alloys

Microstructural evolution, elastic properties, hardness, and indentation fracture toughness of arc-melted hypereutectic ternary Nb-19Si-5Mo (NSM) have been compared with Nb-19Si-5Mo-20Ti (NSM-20Ti) and Nb-19Si-5Mo-4Fe (NSM-4Fe) alloys in the as-cast or annealed conditions (1500 °C for 60 or 100 h) to examine the effect of quaternary alloying with Ti and Fe. Characterization of microstructure has revealed the presence of primary β or α -5-3-silicide ((Nb,X) 5 Si 3 , X=Ti,Fe) along with eutectic comprising Nb ss +( β or α )-5-3-silicide, and additionally β -Ti ss in NSM-20Ti and Nb 4 FeSi in NSM-4Fe. The lamellar eutectic in the as-cast NSM is replaced by non-lamellar morphology partially on Fe addition, and completely on Ti addition, suggesting decoupled growth during solidification. Annealing has not only altered the phase volume fractions, but also changed the eutectic morphology from lamellar to non-lamellar by spheroidization and coarsening of constituents. The dynamic Young’s modulus of NSM (163 ± 2 GPa) is increased to 176 ± 4 GPa and 222 ± 4 GPa on Ti and Fe additions, respectively, and these values have increased by 9-21 pct on annealing due to β → α transformation of 5-3 silicides, altered phase volume fractions, and partitioning of alloying elements. The microhardness of primary 5-3-silicide found as greater than that of eutectic is lowered on annealing of NSM and NSM-20Ti, but is increased in the annealed NSM-4Fe due to solute partitioning. The hardness is lowered with Ti or Fe addition, or on annealing of NSM and NSM-20Ti, but is higher in annealed NSM-4Fe due to altered microstructure and solute concentrations. The indentation fracture toughness of NSM (~9.2 MPa.m 1/2 ) is increased by >2 times in annealed NSM-20Ti to ~18.8 MPa.m 1/2 through ductile-phase toughening mechanisms.