From the mid-1950s to the early 1960s, niobium alloys with excellent oxidation resistance and high strength were mainly developed. Later, niobium alloys with good processing and welding properties were developed, and the research of niobium alloy antioxidation protective coating was strengthened. China began the niobium production process research in 1958, and began the industrial production of niobium in 1963.
Niobium alloy has good plasticity at low temperature (-196℃). Compared with molybdenum and tungsten, niobium has many kinds of alloy elements and high addition amount. There are more than ten kinds of niobium alloys produced on industrial scale. Niobium alloys as structural materials are mainly divided into three categories: high-strength alloys (such as nb-30w-1zr, nb-17w-4hf-0.1c, nb-20ta-15w-5mo-1.5 zr-0.1c), medium-strength alloys and low-strength high-plastic alloys.
The main ways of strengthening niobium are solid solution, precipitation and deformation heat treatment. In niobium alloy, the high temperature and low temperature strength can be significantly improved by containing alloy elements tungsten and molybdenum, but too much content will reduce the technological properties of the alloy. Tantalum is a medium strengthening element and can reduce the ductile - brittle transition temperature of alloys. Another way to strengthen niobium alloy is to add titanium, zirconium, hafnium and a certain proportion of carbon to form dispersed carbide phase and conduct 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 alloy to molten alkali metal. Hafnium can significantly improve the oxidation resistance and welding properties of alloys. High-strength niobium alloys generally contain a large number of solid solution elements (tungsten, molybdenum, tantalum, etc.) for solid solution strengthening, as well as precipitation strengthening to make the alloys have 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 containing carbide precipitation strengthening phase, other varieties are added with medium content of solid solution strengthening elements to ensure excellent comprehensive properties. Low strength alloys can be guaranteed to have excellent machining performance as long as appropriate amount of titanium, zirconium and hafnium are added.
Niobium has poor oxidation resistance at high temperature, 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 coating for protection. The coatings of si-cr-fe, cr-ti-si and al-cr-si have better protection effect on niobium alloy.
