Reactive alloying elements

Several attempts have been made to improve wetting of Al on different kinds of C substrates by use of alloying elements, but no promising results have been obtained (Manning and Gurganus 1969, Nicholas and Mortimer 1971, 

Eustathopoulos et al. 1974). In certain cases, limited improvement in wetting has been observed due to an effect of the alloying element on the stability of the oxide layer covering Al rather than to an effect on intrinsic interfacial energies of the system. A typical example of this is provided by Mg additions (Eustathopoulos et al. 1974). 

Non-reactive systems of graphite and Ag, Au, Cu, Pb, Sn, etc. are non-wetting with contact angles of about 120° due to the action of only van der Waals 

Pure Al and Si react with graphite substrates to form wettable Al and Si carbides. However, the reaction kinetics are so slow at temperatures close to the melting point of Al that Al behaves as a non-reactive and non-wetting metal. 

These conclusions are also valid for vitreous carbon and diamond although some differences exist in the final contact angle for non-reactive metals and the spreading kinetics for reactive metals. 

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In the first Section, attention is paid to distinguishing between reactive and non-reactive systems from the point of view of wettability. Then, after describing wetting and bonding of non-reactive couples, we discuss the effect on these characteristics of oxygen, which is the most common impurity in solid/liquid/vapour systems, as well as the effect of reactive and non-reactive alloying elements. Finally, in a short Section, we consider some results for the wetting of fluorides which like oxides are very ionic. 

Lead and its alloys are generally melted, handled, and refined in cast-iron, cast-steel, welded-steel, or spun-steel melting ketdes without fear of contamination by iron (qv). Normal melting procedures require no dux cover for lead. Special reactive metal alloys require special alloying elements, duxes, or covers to prevent dross formation and loss of the alloying elements. 

No fewer than 14 pure metals have densities se4.5 Mg (see Table 10.1). Of these, titanium, aluminium and magnesium are in common use as structural materials. Beryllium is difficult to work and is toxic, but it is used in moderate quantities for heat shields and structural members in rockets. Lithium is used as an alloying element in aluminium to lower its density and save weight on airframes. Yttrium has an excellent set of properties and, although scarce, may eventually find applications in the nuclear-powered aircraft project. But the majority are unsuitable for structural use because they are chemically reactive or have low melting points." ... 

Figure 6.28. a) Effect of small additions of an alloying element on the interfacial or liquid surface energies of a non-reactive binary alloy/ceramic system for very positive and very negative values of adsorption energy, b) A very negative value of the slope of 0 implies a negligible slope... 

Fora non-reactive and non wetting pure metal on an oxide, improved wetting can be achieved either by adsorption at metal/oxide interface of oxygen supplied by a gas with a controlled P02 or by introducing certain specific alloying elements... 

Niobium is similar in nature to the other psissivating reactive-refractory metals (titanium, zirconium, and tantalum) and has an inherent resistance to a wide range of chemicals. In general, compared to Zr and Ti, Nb has better corrosion properties in acids with small amounts of metal or organic contaminants. Niobium alloys with alloying elements such as Zr and Ti have been evaluated surd have shown increased reactive tendencies in rough proportion to their compositional content as might be expected with solid solution alloys. 

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