Refractory metals properties

Physical Properties. Molybdenum has many unique properties, leading to its importance as a refractory metal (see Refractories). Molybdenum, atomic no. 42, is in Group 6 (VIB) of the Periodic Table between chromium and tungsten vertically and niobium and technetium horizontally. It has a silvery gray appearance. The most stable valence states are +6, +4, and 0 lower, less stable valence states are +5, +3, and +2. 

Ruthenium and osmium have hep crystal stmetures. These metals have properties similar to the refractory metals, ie, they are hard, britde, and have relatively poor oxidation resistance (see Refractories). Platinum and palladium have fee stmetures and properties akin to gold, ie, they are soft, ductile, and have excellent resistance to oxidation and high temperature corrosion. 

More than half of the elements in the Periodic Table react with silicon to form one or more silicides. The refractory metal and noble metal silicides ate used in the electronics industry. Silicon and ferrosilicon alloys have a wide range of applications in the iron and steel industries where they are used as inoculants to give significantly improved mechanical properties. Ferrosilicon alloys are also used as deoxidizers and as an economical source of silicon for steel and iron. 

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. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

You may be wondering why we did not mention the pure refractory metals Nb, Ta, Mo, W in our chapter on turbine-blade materials (although we did show one of them on Fig. 20.7). These metals have very high melting temperatures, as shown, and should therefore have very good creep properties. 

Molybdenum is a high-strength refractory metal, although recrystallizes above 950°C with accompanying reduction in mechanical properties. It is easily fabricated. Its properties are summarized in Table 6.6. CVD is commonly used for the production of molybdenum coatings and free-standing shapes. 

Niobium (also known as columbium) is a soft, ductile, refractory metal with good strength retenti on at high temperature, and a low capture cross-section for thermal neutrons. Itis readily attacked by oxygen and other elements above 200°C. CVD is used to produce coatings or free standing shapes. The properties of niobium are summarized in Table 6.8. 

Tantalum is a refractory metal with a high melting point and excellentchemical resistance, especiallytoacidcorrosion.Itisductilewhen pure. Its properties are summarized inTable6.1 l.t lltisreadily produced byCVD. 

The refractory-metal borides have a structure which is dominated by the boron configuration. This clearly favors the metallic properties, such as high electrical and thermal conductivities and high hardness. Chemical stability, which is related to the electronic... 

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. 

The intercalation of C or N into the metal thus involves an increased refractoriness with preservation of metallic properties. 

Because several of the superalloys contain very little iron, they are closely related to some of the non-ferrous alloys. Some of the second- and third-row transition metals possess many of the desirable properties of superalloys. They maintain their strength at high temperatures, but they may be somewhat reactive with oxygen under these conditions. These metals are known as refractory metals, and they include niobium, molybdenum, tantalum, tungsten, and rhenium. 

It should be noted that it is difficult to obtain models that can accurately predict thermal contact resistance and rapid solidification parameters, in addition to the difficulties in obtaining thermophysical properties of liquid metals/alloys, especially refractory metals/al-loys. These make the precise numerical modeling of flattening processes of molten metal droplets extremely difficult. Therefore, experimental studies are required. However, the scaling of the experimental results for millimeter-sized droplets to micrometer-sized droplets under rapid solidification conditions seems to be questionable if not impossible,13901 while experimental studies of micrometer-sized droplets under rapid solidification conditions are very difficult, and only inconclusive, sparse and scattered data are available. 

In the twelfth century there appeared in certain Latin works alleged to be translations from the Arabic the theory of the principles of metals namely mercury, which confers metallic properties, and sulfur, which causes the loss of these properties on roasting. Another principle, salt, which imparted refractoriness or fixity in the fire, was added later by the famous popularizer of medical chemistry, Paracelsus (85). 

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