NIOBIUM AND ALLOYS

Note should be made of the fact that in the United States this element was originally called columbium (symbol Cb). The Nomenclature Committee of the International Union of Pure and Applied Chemistry in 1951 adopted a recommendation to name this element niobium (symbol Nb). American chemists use this name, but the metallurgists and metals industry still use the name columbium. Most niobium is used in special stainless steels, high-temperature alloys, and superconducting alloys such as Nb3Sn. The low cross-section capture of niobium for thermal neutrons of only 1.1 barn makes it suitable for use in nuclear piles.

Niobium is a tough, shiny, silver-gray, soft, ductile metal that somewhat resembles stainless steel in appearance. Niobium is relatively low in density, yet can maintain its strength at high temperatures. It has excellent corrosion resistance to liquid metals, and can be easily fabricated into wrought products.

Over 95% of all niobium is used as additions to steel and nickel alloys for increasing strength. Only 1 to 2% of niobium is in the form of niobium-base alloys or pure niobium metal. Superconducting niobium-titanium alloys account for over half of that, and high-temperature and corrosion applications account for the remainder.

The density of niobium at 8.57 g/cm3 is moderate compared with most other high melting point metals. It is less than molybdenum at 10.2 g/cm3 and half that of tantalum at 16.6 g/cm3.

Alloys

Commercial niobium alloys are relatively low in strength and extremely ductile, and can be cold-worked over 70% before annealing becomes necessary. The resulting ease of fabrication into complex parts combined with relatively low density frequently favors the selection of niobium alloys over other refractory metals such as molybdenum, tantalum, or tungsten.

High-temperature niobium alloys were developed in the 1960s for nuclear and aerospace applications and today serve in communications satellites, human body imaging equipment, and a variety of high-temperature components. Although niobium alloys have useful strength at temperatures hundreds of degrees above nickel-base superalloys, applications have been limited by their susceptibility to oxidation and to long-term creep.

Production

Typical production electron-beam furnaces generate 500 to 5000 kW of power, and are capable of purifying ingots with diameters of 305 to 508 mm and lengths over 2 m. "Drip-melting" is the current standard electron-beam method, but hearth melting may eventually become practical as higher-power furnaces are developed.

Alloys of niobium are made by vacuum-arc remelting with the appropriate elemental additions. The most common alloy additions are zirconium, titanium, and hafnium, which readily go into solution during arc melting.

Properties

The most common high-temperature niobium alloys are listed in Table N.2. All are hardened primarily by solid solution-strengthening; however, small amounts of second-phase particles are present. The composition of these particles varies, but they are generally associated with interstitial impurities that form oxides, nitrides, and carbides. These particles are important because the size and distribution of second phases can often have a strong influence on mechanical properties and recrystallization behavior. For example, a variation of the Nb-1% Zr alloy, commonly known as PWC-11, contains an intentional addition of 0.1 wt% carbon, specifically to form carbide precipitates that significantly improve high-temperature creep properties.

Another alloy listed, WC-3009, normally contains ~0.10 wt% oxygen, which is approximately five times more oxygen than other niobium alloys. This high level of oxygen, which is introduced during powder processing, is not deleterious to mechanical properties because the oxygen combines with hafnium in the alloy to form stable hafnium oxide precipitates. The WC-3009 alloy is unique in that it exhibits an oxidation rate less than one tenth that of most other niobium alloys. When WC-3009 was developed, it was speculated that such an alloy could survive a short supersonic mission even if its protective coating failed.

TABLE N.2

Commercially Available Niobium Alloys for High-Temperature Service

		Thermal	Thermal	Total				Coefficient
		Conductivity	Conductivity	Emissivity	Total Emissivity		Melting	of Thermal
		at 800°C (1470°F),	at 1200°C (2190°F),	at 800°C	at 1200°C	Density, g/cm3	Point,	Expansion
Alloy	Composition, wt%	W/m °C	W/m °C	(1470° F)	(2190° F)	(lb/in.3)	°C (°F)	at 20°C x 10-6/°C
Pure niobium	Nb	—	—	—	—	8.57	2468	7.1
C-103	Nb-10Hf-1Ti	37.4	42.4	0.28	0.40	8.85 (0.320)	2350±50	8.73
					0.70-0.82		(4260 ± 90)
					(silicide coated)
Nb-1Zr	Nb-1Zr	59.0	63.1	0.14	0.18	8.57 (0.310)	2410 ±10	6.8
							(4365 ± 15)
PWC-11	Nb-1Zr-0.1C	—	—	—	—	8.57 (0.310)	—	6.8
WC-3009	Nb-30Hf-9W	—	—	—	—	10.1 (0.365)	—	7.5
FS-85	Nb-28Ta-10W-1Zr	52.8	56.7	—	—	10.6 (0.383)	—	7.1

TABLE N.3

Typical Room-Temperature Tensile Properties of Niobium Alloys

	Yield Strength,	Ultimate Tensile Strength,	Elongation,	Elastic Modulus at 20°C,	Elastic Modulus at 1200°C,
Alloy	MPa (ksi)	MPa (ksi)	%	GPa (Msi)	GPa (Msi)
C-103	296 (42.93)	420 (60.91)	26	90 (13.1)	64 (9.3)
Nb-1Zr	150 (21.75)	275 (39.88)	40	80 (11.7)	28 (4.1)
PWC-11	175 (25.38)	320 (46.41)	26	80 (11.7)	28 (4.1)
WC-3009	752 (109.06)	862 (125.02)	24	123 (17.9)	NA
FS-85	462 (67.00)	570 (82.67)	23	140 (20.4)	110 (16.0)

In general, niobium alloys are much less tolerant of impurity pickup than other reactive metals such as titanium and zirconium. Alloys containing second-phase particles that form a continuous boundary between grains can exhibit drastically reduced tensile elongation. This condition is usually caused by contamination or improper heat treatment. Copper, which can accidentally be introduced during welding, is particularly disastrous to mechanical properties. In fact, the total permissible interstitial oxygen, hydrogen, carbon, and nitrogen content of niobium alloys is typically one fifth to one tenth that of titanium or zirconium alloys.

All the commercial alloys are quite ductile at room temperature. Tensile properties of the common alloys at 20°C are shown in Table N.3. The highest tensile strength at room temperature and elevated temperature is exhibited by the WC-3009 alloy. Even though WC-3009 clearly exhibits the highest tensile strength, FS-85 has superior creep strength. Its high creep strength is due to its higher melting point, which is elevated by its high concentration of tantalum and tungsten.

Elastic modulus, thermal conductivity, coefficient of thermal expansion, and total hemispherical emissivity are also listed in Tables N.2 and N.3. The emissivity data is for smooth and nonoxidized surfaces, which exhibit much lower emissivity values than oxidized material. Also shown is an emissivity value of 0.7 to 0.82 for silicide-coated C-103. This value is for a common Si-20% Fe-20% Cr coating applied by the "slurry coat and fusion" method.

Applications

The most common application for niobium alloys is in sodium vapor lamps. The Nb-1% Zr alloy demonstrates excellent formability, weldability, and long life in a sodium vapor environment.

These bulbs are used throughout the world for highway lighting, because of their high electrical efficiency and long life, which is typically in excess of 25,000 h. The high-carbon version of this alloy, PWC-11, has a creep rate approximately five times lower than that of Nb-1% Zr at 1100°C, because of the effects of carbide precipitates.

For aerospace applications at 1100 to 1500°C, alloy C-103 has been the workhorse of the niobium industry because of its higher strength. Excellent cold-forming and welding characteristics enable fabricators to construct very complex shapes, such as thrust cones and high-temperature valves. Closed die forgings are also easily produced. Most of these components in propulsion systems are exposed for relatively short times to temperatures between 1200 and 1400°C.

The service environment for propulsion systems often is less oxidizing than the normal atmosphere. Because C-103 has virtually no oxidation resistance, components are extensively coated with silicides.

Another very successful application for coated C-103 is thrust augmenter flaps in a turbine engine. These flaps, placed at the tail end of the engine to form a high-temperature liner in the afterburner section, typically reach 1200 to 1300°C and last for ~100 h of afterburner time.

Niobium alloys have also been evaluated for various high-temperature components of the National Aerospace Plane. Hypersonic leading edges and nose cones were fabricated to function as heat-pipe thermal management systems. The heat-pipe concept was designed to transport extreme heat away from hot spots, such as hypersonic leading edges, to cooler areas where heat could be expelled by radiation. A typical 500 g niobium heat pipe can dissipate over 10 kW of heat and operate isothermally at 1250 to 1350°C. These devices were successfully tested in combustion torches, high-velocity jet fuel burners, tungsten-quartz lamps, and even electric welding arcs at heat fluxes well over 1000 W/cm2.