Niobium alloy still has good plasticity at low temperature (-196℃). Compared with molybdenum and tungsten, niobium has more types of alloying elements and a higher amount of addition. There are more than ten kinds of niobium alloys produced on an industrial scale. As a structural material, 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 high-plastic alloys.
The main ways of strengthening niobium are solution, precipitation and deformation heat treatment. In niobium alloys, the alloy elements tungsten and molybdenum can significantly improve the high and low temperature strength, but too much content will reduce the process performance of the alloy. Tantalum is a medium strengthening element and can reduce the plastic-brittle transition temperature of the alloy. Another way of strengthening niobium alloy is to add titanium, zirconium and hafnium and a certain proportion of carbon to form a dispersed carbide phase for precipitation strengthening. In addition, these active elements can also improve other properties, such as titanium can significantly improve the oxidation resistance and process properties of the alloy; Hafnium and zirconium can improve the corrosion resistance of molten alkali metal; 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 solid solution strengthening, and precipitation strengthening is also used to make the alloy have high strength (see strengthening of metals). 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 containing carbide precipitation strengthening phase, other varieties are added medium content of solid solution strengthening elements to ensure excellent comprehensive properties. As long as the appropriate amount of titanium, zirconium and hafnium is added to the low strength alloy, excellent processing properties can be guaranteed.
The high temperature oxidation resistance of niobium is very poor, and it begins to oxidize rapidly at about 600℃. Although high-strength niobium alloys with certain antioxidant properties such as WC-3015 (Nb-15W-4Ta-28Hf-2Zr-0.1C) have been developed, they are far from meeting the actual requirements and still need to rely on high-temperature antioxidant coatings to protect them. The coatings of Si-Cr-Fe series, Cr-Ti-Si series and Al-Cr-Si series have better protection effect on niobium alloys.
