Application and Process Metallurgical Aspects of Niobium, Tantalum, and Hafnium

Abstract

Among the family of reactive and refractory groups of metals, niobium, tantalum, and hafnium command considerable interest on account of their unique combination of widely differing properties, which makes them ideal or potential candidate materials for a variety of applications in the conventional and frontier areas of technology. Both niobium and tantalum have high melting points and show good resistance to high-temperature oxidation (the oxidation rates for tungsten and molybdenum are about 4 times and 20 times more at any given temperature) and are hence widely used in heat-shields and high-temperature structures. Addition of titanium, cobalt, and nickel further enhances their oxidation resistance, and alloys such as Ta-Ti-Co and Ta-Ti-Ni (commercial production of these alloys has not yet been established), are reported to have been used in advanced ram jet engines. Another important area of application for tantalum and niobium is in the chemical industry, where because of their superior resistance to corrosion attack in several types of chemical media, they are used for making reaction vessels or serve as reactor lining material. Tantalum is perhaps the only metal that can safely be used for containing the highly corrosive molten rare earth metals and their alloys. The metals also find extensive use in the electronics industry for capacitor manufacture owing to their ability to form exceptionally stable surface adherent oxide film. Moreover, niobium finds use in nuclear and magnetics industries because of its attractive nuclear and superconducting properties. The steel industry also needs a significant quantity of niobium in the form of ferro-alloys. Hafnium is another metal that finds use in the nuclear industry, but in a different role, as a control rod material because of its high neutron absorption cross section. The metal also has good potential for use in a jet engine turbine alloy. In the carbide form, all the three metals are widely used in the cutting tool industry. In the Bhabha Atomic Research Centre, a considerable amount of development work has been carried out on niobium, tantalum, hafnium, and other reactive and refractory metals, and flow sheets have been evolved for the production of metals, some of their carbides, alloys, and ferro-alloys. Based on these processes, indigenous production facilities have also been set up. The paper briefly presents the work that has been carried out on the production technology of the reactive and refractory groups of metals with particular reference to niobium, tantalum, and hafnium.