GoKawiilThis Article From Issue January-February 2015 Volume 103, Number 1 Page 30 DOI: 10.1511/2015.112.30
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The very first flight powered by a jet engine took place in Germany on August 27, 1939. Now most of the 19,400 airplanes in the global air-transportation fleet are jets, with about 5 million passengers boarding them every day. On heavily traveled North Atlantic routes between North America and Europe, there are about 800 flights daily; it is possible for a passenger to reach almost any part of the planet within a day. Yet the jet engine remains largely unsung as both a masterpiece of energy conversion and a means of modern transit.
These turbine blades have had their surfaces etched with acid to reveal their inner structure. The pair at left are single crystals, whereas the pair in the middle are directionally solidified, with all the crystal boundaries going in one direction. The pair at right are made up of small crystal grains, with numerous boundaries. Blades of single crystal have significantly increased life spans under extreme temperature conditions. Image courtesy of Alcoa Howmet.
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Since it was invented, the aviation version of the gas turbine (a common workhorse for the generation of electricity) has been continuously upgraded by legions of engineers. Following the laws of thermodynamics, one of the most fruitful paths toward better performance has been finding ways to increase thermal efficiency—the amount of fuel that actually turns into the desired output and not waste heat—by raising the temperature at which the jet engine operates. Creating turbine parts that can survive extreme heat has been a major engineering challenge. Meeting it has required fundamentally rethinking the material structure of the turbine blades, making metals do things that they do not normally do in nature. The result has been a largely invisible revolution, but one that is responsible for much of the ongoing success of the jet age.
Superalloys Beat the Heat
All turbines operate on similar principles: A gas or other fluid turns a rotor, which can do useful work. In a jet engine, air is taken in and compressed, then fuel is added and combusted to heat the air, which then turns the rotor blades of a turbine. The hot exhaust is then expelled through a nozzle to create thrust. (See "The Adaptable Gas Turbine," Technologue, July–August 2013.)
A jet engine operates by first taking air in and compressing it. Fuel is added and combusted to heat the air, which then turns the rotor blades of a turbine. The hot exhaust is expelled through a nozzle to create thrust. Wikimedia Commons
Gas turbine thermal efficiency increases with greater temperatures of gas flow exiting the combustor and entering the turbine. In modern, high-performance jet engines, the temperature of this gas can exceed 1,650 degrees Celsius (nonaviation gas turbines operate at 1,500 degrees or lower, whereas military jet engines can reach 2,000 degrees, which exceeds the boiling point of molten silver). Since the 1950s, in high temperature regions of the turbine, special blades and vanes are made from a combination of metals based on high-melting-point nickel. This material is called a “superalloy” because it retains strength and resists oxidation at extreme temperatures. The nickel in this superalloy has a crystal structure called a face-centered cubic, meaning it’s a cube with an atom at each corner and one at the center of each side. Other metallic elements are alloyed with nickel to produce a microstructure with two variant types, or phases, of crystals, one of which contains different elements at specific locations in the cubic crystal. When this phase is equally distributed in the larger nickel alloy, it helps stabilize the microstructure at elevated temperatures, resulting in high strength and corrosion resistance. Such superalloys, when they are cast using conventional methods in a vacuum furnace to prevent oxidation, soften and melt at temperatures between 1,250 and 1,400 degrees. This temperature limit means blades and vanes closest to the engine combustor may be operating in gas path temperatures far exceeding their melting point, and thus must be cooled to typically eight- to nine-tenths of the melting temperature to maintain integrity. To maintain these temperatures, turbine airfoils subjected to the hottest gas flows must be cast with intricate internal passages and surface hole patterns needed to channel and direct cooling air (bled from the compressor) within and over their exterior surfaces. After casting, the working surface can be sprayed with ceramic thermal barrier coatings to increase life and act as a thermal insulator (allowing inlet temperatures a few hundred degrees higher). To create blades that can endure these extreme conditions, engineers began digging deeper into the structure of the blades themselves starting in the 1960s. Conventionally cast turbine airfoils are polycrystalline, consisting of a three- dimensional mosaic of small metallic crystals, or grains, formed during solidification in the casting mold. Each grain has a different orientation of its crystal lattice from its neighbors’. The interfaces between these crystals are most often not aligned along the crystals’ axes, resulting in what are called grain boundaries. Untoward events happen at grain boundaries, such as increased chemical activity, slippage under stress loading, and the formation of voids. Among other problems, these conditions can lead to creep, an insidious life limiter: the tendency of blade material to deform at a temperature-dependent rate under stresses well below the yield strength of the material. Corrosion and cracks also start at grain boundaries. Thus grain boundaries greatly shorten turbine vane and blade life, and require lowered turbine temperatures with a concurrent decrease in engine performance. One can try to gain sufficient understanding of grain boundary phenomena so as to control them. But in the early 1960s, researchers at jet engine manufacturer Pratt & Whitney Aircraft (now called simply Pratt & Whitney, and owned by United Technologies Corporation) set out to deal with the problem by eliminating grain boundaries from turbine airfoils altogether. Its researchers invented techniques to cast single-crystal turbine blades and vanes, and designed alloys to be used exclusively in single-crystal form.
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