Effect of B addition on the microstructures and mechanical properties of Nb–16Si–10Mo–15W alloy

Nb-silicide based, hypoeutectic Nb–16Si–10Mo–15W alloys doped with various amounts of boron from 0 to 10 at.% were prepared by arc melting and annealing. Microstructures and mechanical properties of these alloys were investigated by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA), Vickers hardness measurement and compression test. The eutectic reaction L → Nbss (Nb solid solution) + β silicide (W5Si3-type, D8m) exists in B contents less than 1 at.%, while a new eutectic reaction L → Nbss + α silicide (Cr5B3-type, D8l) does in B contents higher than 1 at.%. The α silicide is also found in 0.44 at.% (500 wt. ppm) and 1 at.% B-added alloys. β silicide completely transforms into α silicide after annealing treatment at 1700 °C for 48 h. The lattice parameter of α silicide decreases with increasing boron content. Boron addition increases the hardness of silicide phase, but yield no appreciable change of hardness in Nbss. Moreover, boron addition greatly improves the high-temperature strength.