Formation of (Nb, X)ss/(3-(Nb, X)5Si3 eutectic and its effect on mechanical properties of Nb-Si-Ti alloys by addition of Mo

Nb-22Si-20Ti-xMo (x = 0, 2, 4, 6, 8 at%) alloys with different content of Mo were prepared by vacuum nonconsumable arc melting method, and effects of Mo content on phase constitution, microstructure and mechanical properties of Nb-22Si-20Ti-xMo (x = 0, 2, 4, 6, 8 at%) alloys were investigated. Results show that addition of Mo changes phase composition of Nb-22Si-20Ti-xMo alloys. (Nb, X)3Si phase disappears and some block-like primary (3-(Nb, X)5Si3 phase form in the alloys with addition of Mo. In addition, fine (Nb, X)ss/(3-(Nb, X)5Si3 eutectic structure can be observed in the alloys with addition of Mo, and volume fraction of eutectic structure increases with increasing of Mo. The average diameter of primary (3-(Nb, X)5Si3 phase is decreased, and the microstructure of Nb-Si-Ti alloys is significantly refined by addition of Mo. Solubility of Mo element in both (Nb, X)ss and (3-(Nb, X)5Si3 silicides increases, and Mo element is mainly dissolved in the (Nb, X)ss phase with increasing of Mo element in alloys. Microhardness of (3-(Nb, X)5Si3 silicides in Nb-Si-Ti alloys increases from 1298.5 HV to 1489.9 HV when the addition of Mo content is increased from 0 to 8 at%, and microhardness of eutectic structure in Nb-Si-Ti alloys with addition of Mo increases from 865.1 HV for NST-2Mo alloy to 964.1 HV for NST-8Mo alloy. Room-temperature compressive ultimate strength of Nb-Si-Ti alloys is greatly improved by addition of Mo, and it is improved from 1972.5 MPa to 2384.2 MPa with increasing content of Mo from 0 to 8 at %. Fracture strain of Nb-Si-Ti alloys first increase and then decrease with increasing of Mo, and the highest is 9.9 % for NST-6Mo alloy with addition of 6 at% Mo. Fracture of Nb-Si-Ti alloys is brittle-fracture dominated brittle-ductile mixed fracture to ductile-fracture dominated brittle-ductile mixed fracture by increasing addition of Mo from 0 to 8 at%. Strength and microhardness improvement of Nb-Si-Ti alloys is contributed to solid solution strengthening and grain refinement strengthening by addition of Mo.