Niobium alloy still has good ductility at low temperature (-196℃). Compared with molybdenum and tungsten, niobium has more kinds of alloying elements and higher amount of addition. There are more than ten kinds of niobium alloys produced on an industrial scale. As structural materials, niobium alloys are mainly divided into three categories: high strength alloys (such as Nb-30W-1Zr, Nb-17W-4Hf-0.1C, Nb-20Ta-15W-5Mo-1.5Zr-0.1C), medium strength alloys and low strength and high plasticity alloys.
The main strengthening ways of niobium are solution, precipitation and deformation heat treatment. In niobium alloy, the alloy elements tungsten and molybdenum can significantly improve the high and low temperature strength, but too much content will reduce the alloy process performance. Tantalum is a moderately strengthening element and can reduce the plastic-brittle transition temperature of alloys. Another way to strengthen niobium alloy is to add titanium, zirconium and hafnium as well as a certain proportion of carbon to form a dispersed carbide phase for precipitation strengthening. In addition, these active elements can improve other properties, such as titanium can obviously improve the alloy oxidation resistance and process properties; Hafnium and zirconium can improve the corrosion resistance of molten alkali metals. Hafnium can significantly improve the oxidation resistance and weldability of the alloy. High strength niobium alloys generally contain a large number of solid solution elements (tungsten, molybdenum, tantalum, etc.) for solution strengthening, while precipitation strengthening is also used to give the alloys high strength (see metal strengthening). The plastic processing of these alloys is difficult and the deformation parameters need to be strictly controlled. In addition to Nb-10W-1Zr-0.1c alloy which contains carbide precipitation strengthening phase, other alloys have medium content of solid solution strengthening element to ensure excellent comprehensive properties. As long as appropriate amount of titanium, zirconium and hafnium are added to low strength alloys, excellent machining performance can be guaranteed.
Niobium has poor oxidation resistance at high temperature and begins to oxidize rapidly at about 600℃. Although some high strength niobium alloys such as WC-3015 (Nb-15W-4Ta-28Hf-2Zr-0.1c) have been developed, they are far from meeting the practical requirements and still need to rely on high temperature oxidation resistance coating to protect them. Si-cr-fe, Cr-Ti-Si and Al-Cr-Si coatings are the best coatings for niobium alloy.
