Refractory metals compositions

Dr. Gerhard Leichtfried received his Hi.D from the Montanuniversitat Leoben and is qualified for lecturing in powder metallurgy. For 20 years he has been working in various senior positions for the Plansee AktiengeseUschaft, a company engaged in refractory metals, composite materials, cemented carbides and sintered iron and steels. 

Hafnium has been successfully alloyed with iron, titanium, niobium, tantalum, and other metals. Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory of all known metal nitrides (m.p. 3310C). At 700 degrees C hafnium rapidly absorbs hydrogen to form the composition HfHl.86. 

In the case of refractory metals, coatings generaHy are sHicides, appHed by pack cementation or slurry processes. Typical sHicide compositions are... 

Both molybdenum and tungsten can be worked in air without ductiHty loss. AH refractory metals can be made into tubing by extmsion, and most refractory metals, except chromium, are available as wine. Tungsten wines were attempted as fiber reinforcement for experimental nickel-base composites. 

A more extensive comparison of many potential turbine blade materials is available (67). The refractory metals and a ceramic, sHicon nitride, provide a much higher value of 100 h stress—mpture life, normalised by density, than any of the cobalt- or nickel-base aHoys. Several intermetaHics and intermetaUic matrix composites, eg, aHoyed Nb Al and MoSi —SiC composites, also show very high creep resistance at 1100°C (68). Nevertheless, the superaHoys are expected to continue to dominate high temperature aHoy technology for some time. 

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Lubricants. TeUurides of titanium, 2irconium, molybdenum, tungsten, and other refractory metals are heat- and vacuum-stable. This property makes them useful in soUd self-lubricating composites in the electronics, instmmentation, and aerospace fields (see Lubrication and lubricants). Organic teUurides are antioxidants in lubricating oUs and greases. 

Non-ferrous Metals and Alloys Diaphragm material, metal composite, refractory metal. 

Structure. The structure of the refractory-metal carbides increases in complexity with increasing group number. Thus the carbides of Group IVa are characterized by a single cubic monocarbide. In those of Group Va, a M2C phase exists as well as the monocarbide. The carbides of Group Via are far more complex and have several compositions. 

Modem machining deals with an increasingly wide range of materials which includes, in addition to the traditional metals, high-chromium and nickel stainless steels, titanium, intermetallics, refractory metals, ceramics, glasses, fiber-reinforced composites, and many others. These materials have widely different properties. They react differently to machining and each presents a special machining problem. 

Aluminum. Aluminum, as A O, is a nuisance dust. There may be instances where elemental composition of the nuisance dustis desired therefore, Al is included in P CAM 173. Aluminum is difficult to dissolve in nitric acid and should be treated as a refractory metal. Since the nitrous oxide/acetylene flame is subject to many interferences, both 1000 ppm Cs and 1000 ppm La, a releasing agent, should be added to the final solution. 

The most important polymeric matrices are linear and cross-linked polyesters, epoxy resins and linear and cross-linked polyimides the most important reinforcements are high-performance polymeric fibres and filaments (for polymeric composites), filaments of refractory metals and inorganic materials (E-glass, A12C>3, B, BN, SiC and Carbon) and whiskers (fibrillar single crystals of A1203, B4C, WC, SiC and C, exclusively for reinforcement of metals). 

By definition, brazes have a different composition from the components they are used to join and hence interdiffusion will occur during and after interface creation. Reference has been made already to the detrimental effects of the growth of thick reaction products at metal-ceramic interfaces and similar effects can occur with metal-metal systems. Thus it is not good practice to use A1 brazes for the joining of steel or Cu components or to use Ni brazes containing Si for the joining of refractory metal components because of the rapid formation of fragile layers of intermetallic compounds. 

The performance of a composite is strongly influenced by the nature of the counterface and the application conditions, and a development programme is essential before any critical application is attempted. Probably the best indication that a particular composite has a useful performance is when it is available commercially, and commercial products are available under the trade names Bemol, Sinite, Sinitex and Molalloys. Bhushan and Gupta refer to commercially-available compacts of molybdenum disulphide in refractory metals, which can be used at specific loads up to 70 GPa at 500 C and 7 GPa at 800 C in vacuum. These may be the Molalloy... 

Palme H., Hutcheon 1. D., and Spettel B. (1994) Composition and origin of refractory-metal-rich assemblages in a Ca,Al-rich Allende inclusion. Geochim. Cosmochim. Acta 58, 495-513. 

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