Recently the American Society of Mechanical Engineers (ASME) designated the Pratt & Whitney single-crystal turbine blade an historic mechanical engineering landmark. These blades, which can be found in many modern, high-performance gas turbines and larger electricity-producing turbines, are grown from a single crystal in a vacuum casting furnace from superalloys. They increase the turbine’s efficiency and individual blades’ service life, while giving unmatched resistance to high-temperature creep and fatigue.
About 60 years ago, a small group of Pratt & Whitney gas turbine industry mechanical engineers and metallurgists set out to develop single-crystal blades, and did so relatively quickly, going from concept to manufactured product in about 10 years.
The superalloys — The thermal efficiency of gas turbines increases with the temperatures of gases leaving the combustor and entering the turbine, the work-producing component. Turbine inlet temperatures in the gas path of modern, high-performance jet engines can exceed 3,000°F (1,649°C) while non-aviation turbines operate at 2,700°F (1,482°C) or lower.
In the turbine’s high-temperature regions, high-melting-point, nickel-based superalloy blades and vanes retain their strength and resist oxidation at extreme temperatures. These superalloys, when conventionally vacuum cast, soften and melt between 2,200° and 2,500°F (1240° to 1371°C.) This means the blades and vanes closest to the engine combustor may operate in temperatures far exceeding their melting points and must be cooled to 80% to 90% of their melting temperatures to keep working. (Military jet engines can reach 3,600°F (1,982°C), which exceeds the boiling point of molten silver.)
Thus, engineers needed to design cast turbine blades with intricate internal passages and surface hole patterns that channel and direct cooling air (bled from the compressor) within and over exterior blade’s. To eliminate the harmful effects of impurities, blades are investment cast in vacuum chambers. After casting, the blade’s working surface is often coated with a ceramic thermal barrier that increases life and acts as a thermal insulator, letting inlet temperatures be 100° to 300°F or higher.
Exploring, reducing grain boundaries — Conventionally cast turbine airfoils are polycrystalline, consisting of a three-dimensional mosaic of small metallic equiaxed crystals, or “grains,” formed when the metal solidifies in the mold. Each grain differs from its neighbors in how its crystal lattice is oriented. These lattice misalignments form interfaces called grain boundaries.
Damaging events happen at grain boundaries, such as intergranular cavitation, voids form, chemical activity increases, and grains slip passed one another under stress loading. These conditions lead to creep, shorten cyclic strain life, and decrease overall ductility. (Creep is the tendency of blade materials to deform at a temperature dependent rate under stresses well below the materials’ yield strength.) Corrosion and cracks also start at grain boundaries. In short, physical activities that begin at grain boundaries in superalloys shorten turbine vane and blade life considerably, and are accelerated by high temperatures. But, lowering temperatures, decreases engine efficiency.
In the early 1960s, researchers at Pratt & Whitney Aircraft (now Pratt & Whitney, a United Technologies Corp. company) set out to deal with the problem by eliminating altogether the grain boundaries from turbine airfoils. Their goal was to invent techniques that would let them cast single-crystal blades and vanes, and design alloys that would only be used in single-crystal form.
Most of this work was done at the Advanced Materials R&D Laboratory (AMRDL) in Middletown, CT, for Pratt & Whitney in the 1960s. AMRDL was an excellent example of industry using fundamental and applied research to create and bring to market a superior product within a decade. At its peak, ARMDL staff numbered over 200 scientists, engineers, and technicians, conducting research and development on all aspects of single-crystal technology from casting, alloy development, coatings, joining, and repair. Over its subsequent 10-year life, AMRDL pioneered single-crystal superalloy technology.
At AMRDL’s start, Frank VerSnyder from jet engine manufacturer, General Electric, had developed a concept that transverse (spanwise) grain boundaries could be eliminated in the casting of turbine blades and this would make them stronger. (General Electric never exploited or patented VerSnyder’s concept.)
Directional solidification — By 1966, VerSnyder invented and patented the directionally-solidified columnar-grained turbine blade., the first major step toward a single-crystal blade.
Directional solidification is done in a vacuum chamber and involves pouring molten superalloy metal into a vertically mounted mold heated to metal melt temperatures, and filling the turbine airfoil mold cavity from root to tip (bottom to top.) The bottom of the mold is formed by a water-cooled copper chill plate having a knurled surface exposed to the molten metal. At the chill plate surface, crystals form from the liquid superalloy, and the solid interface advances until it reaches the top.
The ceramic mold is surrounded by a temperature-controlled enclosure, which keeps the sides of the mold at a constant temperature distribution and all the latent heat of solidification is removed by the chill plate. As solidification climbs through the mold, it is slowly lowered out of the enclosure.
The result is a turbine airfoil composed of columnar crystals or grains running spanwise. For rotating turbine blades, where spanwise centrifugal forces can see accelerations of 20,000 g, the columnar grains are aligned with the major stress axis. This alignment strengthens the blade and effectively eliminates destructive intergranular crack initiation normal to blade span. In gas turbines, directionally solidified (DS) blades have improved ductility and thermal fatigue life. They also provide more tolerance to localized strains (such as at blade roots), and have allowed small increases in turbine temperature and, hence, performance.
Once material properties were measured and manufacturing technique perfected, DS turbine blades and vanes were ready for engine application. Their first use by Pratt & Whitney in a production engine came in 1969, in the J58, two of which powered the SR-71 Blackbird supersonic aircraft. Commercial jet engines began using DS blades in 1974. The success of the DS blades set the stage for the invention of single-crystal turbine airfoils.
Single-crystal development — While casting DS crystals, AMRDL researchers found that if they put a right-angle bend in the casting mold just a short distance above the chill plate (called the “starter” chamber), the number of crystals climbing above the bend would be reduced. Putting in two such bends reduced the number even more. Later, it was found that a helical channel with smooth continuous turning was a natural filter, admitting columnar crystals from the starter and letting just one crystal rise above the helix and start to form the entire blade. This single-crystal selector was dubbed the “pigtail”.
As the single-crystal structure forms, the mold must be kept at the right temperature as the mold is taken from the heat-controlled enclosure. Any heat conducted to the sides of the mold lateral surfaces could lead to localized crystallization, which disrupts the single-crystal structure with secondary grains.
In the 1970s, Pratt & Whitney developed techniques to manufacture single-crystal (SX) turbine airfoils to overcome casting defects (such as secondary grains, recrystallized regions, and freckle chains.) This early pioneering work was carried on by several manufacturers and improved upon over the past 50 years. Companies now see prosecution yields greater than 95% in casting of single-crystal blades for aviation gas turbines. This helps offset cost of SX casting compared to equiaxed casting.
The first SX castings were made from conventionally-cast polycrystalline alloys, such as Mar-M200 and IN100. These alloys contain carbon, boron, and zirconium, three elements which segregate preferentially to grain boundaries and provide high-temperature grain-boundary strength and ductility for creep resistance. But in SX castings, which have longer solidification times and no grain boundaries, these three elements generated primary carbides, which hurt the blades high and low fatigue properties.
By the early 1970s, SX alloys were developed that eliminated carbon, baron, and zirconium, giving the alloys higher melting points, higher creep strength and greatly improved high and low cycle fatigue resistance in SX blades and vanes.
SX alloys consist of many metals, each serving a specific purpose. For example, alloy PWA 1484, which P&W developed in the early 1980s, consists (by weight) of nickel (59%), cobalt (10%), tantalum (9%), aluminum (6%), tungsten (6%) and others (10%.) One of the others, is rhenium (3%), which lets the final alloy endure higher temperatures before weakening and melting. Rhenium is a “by-product of a by-product”, derived from specific copper-molybdenum ores, and a costly element in limited supply.
Before committing to using PWA 1484, P&W management wanted assurance rhenium could be supplied over time at a known, acceptable price. The solution was that P&W entered a long-term contract with a Chilean mining company to provide rhenium to Pratt for its internal inventory. Rhenium would then be delivered to casting vendors in sufficient quantities to meet Pratt’s requirements.
In the 1970s, within 10 years of the start of the single-crystal program, SX turbine airfoils were installed in P&W F100 production engines to power the F-15 and F-16 jet fighters. The first commercial aviation use was in P&W’s JT9D-7R4 jet engine, which received flight certification in 1982 and powered the Boeing 767 and Airbus A310.
In 1986, Pratt & Whitney earned the ASM International Engineering Materials Achievement Award for the development of single-crystal turbine blades.
In jet engines, single-crystal turbine airfoils have proven to have as much as nine times the relative life in terms of creep strength and thermal fatigue resistance, and over three times the relative life for corrosion resistance, compared to equiaxed crystal blades. Modern jet engines, with their high turbine-inlet temperatures and long lives (on the order of 25,000 hrs. of operation between overhauls) would be impossible without those single-crystal airfoils. By eliminating grain boundaries, the developers have extended the thermal and fatigue life of jet engines and provided more corrosion resistance. In addition, the blades use less material because they are hollow, and therefore reduce the weight of jet engines. They also have higher melting points. All these improvements contribute to more efficient gas turbines.
The next chapter of the SX story concerns their introduction to large gas turbines used in electric power plants. These units, as big as 500 MW, use supersized SX blades and vanes for both corrosion resistance and increased operating temperatures.
Their first use was for corrosion resistance (using PWA 1483 alloy) in the 163-MW Siemens V84.3A electric-power gas turbine, introduced to the market in 1995. In more recent years, to increase thermal efficiency, these gas turbines’ inlet turbine temperatures have been increased to aviation levels, so SX airfoils with higher temperature capacity are needed for long life.
General Electric’s 9H, a 50 Hz combined-cycle gas turbine, is one of the world’s largest. The first model went into service in 2003 in Wales, feeding as much as 530 MW into the U.K.’s electric grid at a combined-cycle thermal efficiency just under 60%. The 404-ton 9H uses first-stage turbine vanes that measure almost a foot long and blades close to 18 inches long. Each comes from a finished casting that weighs about 30 lbs., and each is a single-crystal airfoil.
Lee S. Langston is Professor Emeritus of Mechanical Engineering at the University of Connecticut. An earlier version of his report appeared in Machine Design.
