This invention relates to nickel-based superalloys, and, more particularly, to compositions of single crystal superalloys and articles made from such compositions.
One of the most demanding materials applications is found in turbine components used in aircraft jet engines; the higher the operating temperature of an engine, the greater its efficiency, and the more power it can produce from each gallon of fuel. There is, therefore, an incentive to operate such engines at as high a temperature as possible. Among the critical limitations on the operating temperature of an engine are the materials used for applications in the hottest regions of the engine, such as turbine airfoils.
There has been a great deal of effort over the past 40 years to develop materials that can be used in high temperature gas turbine components. The most popular and successful of such materials are the nickel-based superalloys, which are alloys of nickel with additions of a number of other elements such as chromium, molybdenum, titanium, tungsten, cobalt, aluminum, tantalum and many others. The compositions of these superalloys are carefully engineered to maintain their strength and other desirable properties during extended use at the high temperatures of engine operations, which are in the neighborhood of 2000.degree. F. and higher.
In addition to the selection of the best composition for use in each component, high operating temperatures of gas turbine components are attained by other techniques. For example, cooling passages may be provided through the components so that cooling air bled from the engine may be passed through the components to cool them internally and also to cool the surfaces as by film cooling.
In another approach, the gas turbine components may be cast as single crystals of the selected superalloy. Particular crystallographic directions, such as [001] in face-centered cubic superalloys, exhibit a balance of mechanical properties superior to those measured in other directions and in polycrystalline articles. The turbine component is therefore cast as a single crystal of that orientation, with the result that the mechanical properties, particularly low cycle fatigue, are improved as compared with other structures made of the same alloy. The alloy compositions of such single crystal alloys are beneficially adjusted because of the absence of grain boundaries that would otherwise require strengthening. There is an ongoing need for making superalloy compositions useful in single crystal articles having improved mechanical and other properties of interest in high-temperature superalloy applications. The present invention fulfills this need, and further provides related advantages.
The term "balance essentially nickel" is used to include, in addition to nickel in the balance of the alloy, small amounts of impurities and incidental elements, which in character and/or amount do not affect the advantageous aspects of the alloy.