Clean, defect-free superalloy castings have been the objective in the gas turbine industry since it is well known that premature mechanical failure in superalloy castings primarily is attributable to the presence of non-metallic inclusions in the casting microstructure. Over the years, internally cooled high temperature cast turbine blades have been developed for use in the turbine section of the gas turbine engine. As a result, turbine blades have become more complex and airfoil wall cross-sections have become thinner and thinner. Unfortunately, microscopic inclusions which were relatively innocuous in simpler, relatively thick walled blade castings have become a limiting factor in the design of new complex, internally cooled, thin walled turbine blade castings.
Over this same time period, prior art workers also developed unidirectional casting techniques to produce single crystal turbine blade castings which exhibit improved mechanical properties at high temperatures as a result of the elimination of grain boundaries that were known to be the cause of high temperature equiaxed casting failure. Single crystal turbine blade castings are in widespread use today as a result.
Since single crystal castings do not include grain boundaries, prior art workers initially believed that elements, such as carbon, that form grain boundary strengthening precipitates in the microstructure would not be necessary in single crystal superalloy compositions. As a result, the concentration of carbon in single crystal superalloys was limited so as not to exceed relatively low maximum levels. For example, the carbon content of a certain nickel base superalloys, such as MAR-M200 and UDIMET 700, was controlled so as not exceed 100 ppm (0.01 weight %) in U.S. Pat. No. 3,567,526 to avoid formation of MC-type carbides that were believed to reduce the fatigue and creep resistance of the alloy castings. Similarly, U.S. Pat. No. 4,643,782 discloses controlling trace elements, such as C, B, Zr, S, and Si, so as not to exceed 60 ppm (0.006 weight %) in the hafnium/rhenium-bearing, single crystal nickel base superalloy known as CMSX-4.
However, the reduction of the carbon concentration to the low levels set forth above in single crystal superalloys ignored the role that carbon was known to play in vacuum induction melted superalloys where oxygen was known to be a chief source of contamination. For example, oxygen is present in the raw materials from which the alloys are made and in the ceramic crucible materials in which the alloys are melted. In particular, superalloy castings are generally produced by vacuum induction melting a superalloy charge and then vacuum investment casting the melt into suitable investment molds. In both of these processing stages, ceramic crucibles are used to contain the superalloy melt and are known to contribute to oxygen contamination of the alloy. Oxygen will react with elements, such as aluminum, present in the superalloy compositions to form harmful dross which can find its way into the casting as inclusions.
In particular, the major role of carbon in the vacuum induction melting and refining process (during master alloy formulation) was to remove oxygen from the melt. This refining action is conducted by what is called the "carbon boil" wherein carbon combines with oxygen in the melt to form carbon monoxide which is removed by the vacuum present during the induction melting operation. However, the low carbon levels present in single crystal superalloys at the heat formulation stage substantially negated the carbon boil previously present in the production of superalloys.
One single crystal nickel base superalloy was found to develop a problem of cleanliness in its production for single crystal turbine blade casting applications. This superalloy is described in U.S. Pat. Nos. 4,116,723 and 4,209,348 (designated ALLOY A hereafter) and comprised, in weight %, about 5.0% Co, 10.0% Cr, 4.0% W, 1.4% Ti, 5.0% Al, 12.0% Ta, 0.003% B, 0.0075% Zr, 0.00-0.006% C., and the balance Ni at the time the cleanliness problem was observed. In response to the cleanliness problem, the carbon content of the superalloy at the heat formulation stage was increased to 200 ppm (0.02 weight %) in an attempt to provide a carbon boil during the heat formulation stage. This was found to improve the cleanliness of single crystal superalloy castings produced from the modified alloy formulation. An alloy carbon content of 400 ppm yielded further improvement in alloy cleanliness. The carbon content of the alloy ingot and investment casting of this superalloy is now specified by the gas turbine manufacturer to be acceptable if in the range from 0 to 500 ppm maximum. The upper or maximum limit on carbon is specified by the manufacturer on the basis of preventing formation of carbide precipitates or particles in the single crystal investment casting.
It is an object of the present invention to provide nickel base superalloy compositions having carbon concentrations optimized for the particular alloy compositions involved, especially with respect to the concentrations of the strong carbide formers, titanium, tantalum, and tungsten present in a particular alloy composition.