Superalloys alloys

Such alloys - superalloys - have been developed. The largest applications of superalloys are in aircraft and industrial gas turbines, rocket engines, space vehicles, submarines, nuclear reactors, and landing apparatus. The nickel-based alloys possess many attractive properties for structural appUcations, such as high-temperature creep and excellent oxidation resistance. The superalloys are used in a temperature interval approximately from 1023 to 1273 K. 

Using rapid solidification technology molten metal is quench cast at a cooling rate up to 10 °C/s as a continuous ribbon. This ribbon is subsequently pulverized to an amorphous powder. RST powders include aluminum alloys, nickel-based superalloys, and nanoscale powders. RST conditions can also exist in powder atomization. 

An alloy of molybdenum containing 1.2% hafnium with carbon at the level of 0.08—0.10% has a slight advantage over TZM. This alloy has been produced in small quantities for special extmsion dies and ejector pins in the isothermal forging of superalloys. 

Because of constitutional complexity, the exact chemistries of nickel-base superalloys must be controlled carehiUy in order to avoid the precipitation of deleterious topologically close-packed (TCP) phases and extraneous carbides after long-term high temperature exposure. Heat-treatment schedules and thermomechanical treatments in the case of wrought alloys also are important to provide optimum strength and performance. 

Niobium is important as an alloy addition in steels (see Steel). This use consumes over 90% of the niobium produced. Niobium is also vital as an alloying element in superalloys for aircraft turbine engines. Other uses, mainly in aerospace appHcations, take advantage of its heat resistance when alloyed singly or with groups of elements such as titanium, tirconium, hafnium, or tungsten. Niobium alloyed with titanium or with tin is also important in the superconductor industry (see High temperature alloys Refractories). 

Any major materials development programme, such as that on the eutectic superalloys, can only be undertaken if a successful outcome would be cost effective. As Fig. 20.10 shows, the costs of development can be colossal. Even before a new material is out of the laboratory, 5 to 10 million pounds (8 to 15 million dollars) can have been spent, and failure in an engine test can be expensive. Because the performance of a new alloy cannot finally be verified until it has been extensively flight-tested, at each stage of development risk decisions have to be taken whether to press ahead, or cut losses and abandon the programme. 

Nickel and its alloys form another important class of non-ferrous metals (Table 1.3). The superb creep resistance of the nickel-based superalloys is a key factor in designing the modern gas-turbine aero-engine. But nickel alloys even appear in a model steam engine. The flat plates in the firebox must be stayed together to resist the internal steam pressure (see Fig. 1.3). Some model-builders make these stays from pieces of monel rod because it is much stronger than copper, takes threads much better and is very corrosion resistant. 

Until the late 1990s, Waspaloy was still the best alloy available for the majority of hot gas turboexpanders used in industry and, until recently, it continued to offer the many special characteristic needed for hot gas expander applications. However, a new development followed in 2000 when the Ebara Corporation (Japan) released data on a nickel-base superalloy. 

The nature of this ereep depends on the material, stress, temperature, and environment. Limited ereep (less than 1%) is desired for turbine blade applieation. Cast superalloys fail with only a minimum elongation. These alloys fail in brittle fraeture—even at elevated operating temperatures. 

Temperature limits of flight engine alloys have been steadily inereasing about 20 °F (11 °C) per year sinee 1945. Transpiration and internally eooled metal blades have resulted in higher temperatures and more effieient operation. But the direet eorrelation between effieieney and fabrieation eost has resulted in a situation of diminishing returns for the superalloys. As more and more eooling air is needed for the superalloy eomponents, the effieieney of the engine drops to a point where turbine inlet temperatures around 2300 °F (1260 °C) are the optimum and, at that point, they are uneeonomie for automotive use. 

Creep becomes an important factor with different metals and alloys at different temperatures. For example, lead at room temperature behaves similarly to carbon steel at 1,000°F and to certain of the stainless steels and superalloys at 1,200°F and higher. 

The evolution of superalloys has been splendidly mapped by an Ameriean metallurgist, Sims (1966, 1984), while the more restrieted tale of the British side of this development has been told by Pfeil (1963). I have analysed (Cahn 1973) some of the lessons to be drawn from the early stages of this story in the eontext of the methods of alloy design it really is an evolutionary tale... the survival of the fittest, over and over again. The present status of superalloy metallurgy is eoneisely presented by MeLean (1996). 

As we learn from Sims s reviews, many other improvements have been made to superalloys and to their exploitation in recent decades. Solid-solution strengthening, grain-boundary strengthening with carbides and other precipitates, and especially the institution, some twenty years ago, of clean processing which allows the many unwanted impurities to be avoided (Benz 1999) have all improved the alloys to the point where (McLean 1996) the best superalloys now operate successfully at a Kelvin temperature which is as much as 85% of the melting temperature this shows that the prospect of significant further improvement is slight. 

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