Niobium Alloys

alloys based on niobium.

The first industrial niobium alloys appeared in the early 1950’s, when new branches of technology required materials capable of operating at temperatures above 1000°C. In addition to a high melting point, niobium alloys have good technological properties and low density relative to alloys composed of other high-melting metals (molybdenum, tungsten, and tantalum). The cold-brittleness limit of low niobium alloys is below the temperature of liquid nitrogen. These properties make possible the use of niobium alloys for thermally stressed parts in rockets, spacecraft, and special-purpose aircraft. A low thermal neutron capture cross section and good durability in contact with liquid-metal heat-transfer agents make niobium alloys valuable construction materials for atomic reactors.

Niobium alloys are resistant to many acids and other chemical reagents. However, they oxidize upon heating above 400°C in air and other oxidizing media; therefore, they must have protective coatings for operation under such conditions. At 1100°C, the rate of oxidation of niobium alloys in air is 30–120 g/(m² · hr); for unalloyed niobium the rate is 300–350 g/(m² · hr). Niobium alloys with silicide protective coatings oxidize at 1100°C at a rate of 0.2–0.4 g/(m² · hr).

The physical properties of niobium alloys differ little from those of unalloyed niobium. Large niobium-alloy parts with a protective coating are highly resistant to thermal fatigue because of their combination of a low thermal coefficient of linear expansion (8.42 × 10^-6 upon heating from 20° to 1100°C) and high thermal conductivity (about 59 watts per [m · °K], or 0.14 cal/[sec · cm · °C], at 1100°C).

The main alloying elements of niobium alloys are molybdenum, tungsten, and vanadium, which form a continuous series of solid solutions with niobium. The strength of the solid solutions is greater than that of unalloyed niobium. In addition, niobium alloys are alloyed with zirconium, hafnium and carbon, or nitrogen. The highly stable carbides and oxides—and, in some cases, oxycarbonitrides—thus formed, which are only slightly soluble in the solid solution, lead to increased strength of the alloy because of mechanical inhibition of creep.

The elastic modulus of niobium alloys is low (see Table 1) but does not decrease with an increase in temperature to 1100°C. The long-term ultimate strength over 100 hr at 1100°C for medium niobium alloys (5–10 percent tungsten or 3–5 percent molybdenum and 1–2 percent zirconium or hafnium) is 100–150 mega-newtons per sq m (MN/m²), or 10–15 kilograms-force per sq mm (kgf/mm²); for high niobium alloys (15–20 percent tungsten or 10–15 percent molybdenum, 1–2 percent zirconium or hafnium, and 0.1–0.4 percent carbon), it is 280–300 MN/m² (28–30 kgf/mm²).

Niobium alloys are produced by melting in vacuum arc furnaces with a consumable electrode or in electron-beam and plasma furnaces, which provide sufficient purity of the metal (mainly in terms of inclusion elements—oxygen, nitrogen, hydrogen, and carbon) for retention of resilience. Primary deformation of niobium alloys is performed at 1200°–1600°C (heating in a neutral medium, in vacuum, or in the ordinary furnace atmosphere, with application of special protective enamels to the semifinished products being heated). Deformation of the semifinished products is performed mainly in air (at 800°–1200°C). For homogenization and degassing, the niobium alloy ingots undergo vacuum annealing at 1500°–2000°C for 5–10 hr, with subsequent annealing at 1300°–1350°C for 10 hr under vacuum (≧ 1 × 10^-4 mm Hg). To remove stress, the deformed semifinished products are heated to 1000°–1100°C for 30–60 min; for recrystallization, they are heated to 1350°–1400°C for the same period. Vacuum rolling of sheets has been put into practice.

Medium niobium alloys readily undergo pressure shaping. They are used to make forged, pressed, and stamped pieces, as well as sheet, foil, and tubing of various sizes (down to capillary size). Such alloys are satisfactorily worked by machining and welded by argon-arc contact welding and electron-beam welding. The strength of the welded seam is not less than 90 percent of the strength of the base metal in the recrystallized state. The resilience of welded joints is expressed by the bending angle at which the first crack appears (on a mandrel with a radius equal to the thickness of the welded sheet); it is 120°–180° for argon-arc welding in a chamber with a neutral medium. Medium niobium alloys are welded with low copper, titanium, and zirconium alloys and soldered with other metals, using special solders.

In addition to heat-resistant niobium alloys, superconducting alloys of niobium with zirconium, tin, and titanium are acquiring great significance. The critical current density of niobium alloys depends on the type of deformation, the conditions of heat treatment, and the direction of the magnetic field. Superconducting niobium alloys are used in large accelerators, lasers, and hot plasma reflectors in thermonuclear installations. The technology of production of semifinished products (wire, strips, and tubing) from superconducting niobium alloys is similar to that of heat-resistant niobium alloys.