Super-alloys

Nickel is well-known as an essential alloying element in stainless steels, Ni-Cu alloys, Ni-Fe alloys, Ni-Cr-Fe alloys, super alloys, as nickel-chromium alloys and special corrosion-resistant and high temperature alloys. Nickel ferromagnetic with a density of 8.9 g/cm. It is ductile and malleable like steel. Nickel alloys are well-knovm for their high temperature strength and good resistance to corrosion. 

Low Expansion Alloys. Binary Fe—Ni alloys as well as several alloys of the type Fe—Ni—X, where X = Cr or Co, are utilized for their low thermal expansion coefficients over a limited temperature range. Other elements also may be added to provide altered mechanical or physical properties. Common trade names include Invar (64%Fe—36%Ni), F.linvar (52%Fe—36%Ni—12%Cr) and super Invar (63%Fe—32%Ni—5%Co). These alloys, which have many commercial appHcations, are typically used at low (25—500°C) temperatures. Exceptions are automotive pistons and components of gas turbines. These alloys are useful to about 650°C while retaining low coefficients of thermal expansion. Alloys 903, 907, and 909, based on 42%Fe—38%Ni—13%Co and having varying amounts of niobium, titanium, and aluminum, are examples of such alloys (2). 

Nickel alloys include monels (Ni-Cu alloys), nichromes (Ni-Cr alloys), nimonics and nickel-based super-alloys (Ni-Fe-Cr-Al-Co-Mo alloys). 

These requirements severely limit our choice of creep-resistant materials. For example, ceramics, with their high softening temperatures and low densities, are ruled out for aero-engines because they are far too brittle (they are under evaluation for use in land-based turbines, where the risks and consequences of sudden failure are less severe - see below). Cermets offer no great advantage because their metallic matrices soften at much too low a temperature. The materials which best fill present needs are the nickel-based super-alloys. 

Fig. 20.3(a) A piece of a nickel-based super-alloy cut open to show the structure there are two sizes of precipitates in the alloy - the large white precipitates, and the much smaller black precipitates in between. 

TaC to obstruct the dislocations and (c) to form a protective surface oxide film of Cr203 to protect the blade itself from attack by oxygen (we shall discuss this in Chapter 22). Figure 20.3 (a and b) shows a piece of a nickel-based super-alloy cut open to reveal its complicated structure. 

These super-alloys are remarkable materials. They resist creep so well that they can be used at 850°C - and since they melt at 1280°C, this is 0.72 of their (absolute) melting point. They are so hard that they cannot be machined easily by normal methods, and must be precision-cast to their final shape. This is done by investment casting a precise wax model of the blade is embedded in an alumina paste which is then fired the wax bums out leaving an accurate mould from which one blade can be made by pouring liquid super-alloy into it (Fig. 20.4). Because the blades have to be made by this one-off method, they are expensive. One blade costs about UK 250 or US 375, of which only UK 20 (US 30) is materials the total cost of a rotor of 102 blades is UK 25,000 or US 38,000. 

Slides Turbofan aero-engine super-alloy turbine blades, showing cooling ports [3] super-alloy microstructures [4] DS eutectic microstructures [3, 5] ceramic turbine blades. 

Ceramies are quite tolerant of sueh eontaminants as sodium and vanadium, whieh are present in low-eost fuels and highly eorrosive to eurrently used niekel alloys. Ceramies are also up to 40% lighter than eomparable high-temperature alloys—another plus in applieation. But the biggest plus is material eost. Ceramies eost around 5% the eost of super alloys. 

Nickel and ils alloy s ai c expensive and used mainly either for (lieir liigfi[Pg.205]

In the 1970s and 1980s alloys with higher contents of aluminium have become more important, especially for casting using metal or graphite moulds. Table 4.30 has been modified to cover these alloys. A super-plastic alloy containing about 22% aluminium has also been developed, but only has very limited use. 

In the chemical process industry molybdenum has found use as washers and bolts to patch glass-lined vessels used in sulphuric acid and acid environments where nascent hydrogen is produced. Molybdenum thermocouples and valves have also been used in sulphuric acid applications, and molybdenum alloys have been used as reactor linings in plant used for the production of n-butyl chloride by reactions involving hydrochloric and sulphuric acids at temperatures in excess of 170°C. Miscellaneous applications where molybdenum has been used include the liquid phase Zircex hydrochlorination process, the Van Arkel Iodide process for zirconium production and the Metal Hydrides process for the production of super-pure thorium from thorium iodide. 

Miscellaneous Niobium also finds use in satellite launch vehicles and spacecraft and one of the principal applications for niobium-base alloys is in the production of super-conducting devices. 

The ash deposits resulting from the combustion of solid and oil fuels often contain appreciable quantities of other corrodants in addition to vanadium pentoxide. One of the more important of these is sodium sulphate, and the effects of this constituent in producing sulphur attack have been mentioned. The contents of sodium sulphate and vanadium pentoxide present in fuel oil ash can vary markedly and the relative merits of different materials depend to a great extent upon the proportions of these constituents. Exposure of heat-resisting alloys of varying nickel, chromium and iron contents to ash deposition in the super-heater zones of oil-fired boilers indicated a behaviour pattern depending on the composition of the alloy and of the ash... 

Super austenitic, high nickel, stainless steels, containing between 29 to 30 per cent nickel and 20 per cent chromium, have a good resistance to acids and acid chlorides. They are more expensive than the lower alloy content, 300 series, of austenitic stainless steels. 

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