This invention relates to nickel-based superalloys, and, more particularly, to such alloys that are coated with aluminide coatings to enhance their resistance to environmentally induced degradation.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a compressor, and mixed with fuel. The compressed mixture is burned in a combustor, and the hot combustion gases flow through a turbine that turns the compressor. The hot gases then flow from the rear of the engine.
The turbine includes stationary turbine vanes that deflect the hot gas flow sideways, and turbine blades mounted on a turbine Wheel that turns as a result of the impingement of the hot gas stream. The turbine vanes and blades experience extreme conditions of high temperature, thermal cycling when the engine is turned on and off, oxidation, corrosion, and, in the case of the turbine blades, high stress and fatigue loadings. The higher the temperature of the hot combustion gas, the greater the efficiency of the engine. There is therefore an incentive to push the materials of the engine to ever-higher temperatures and loadings.
Nickel-based superalloys are widely used as the materials of construction of gas turbine blades and vanes. These superalloys contain primarily nickel, and a variety of alloying elements such as cobalt, chromium, tungsten, aluminum, tantalum, rhenium, hafnium, and others in varying amounts carefully selected to provide good mechanical properties and physical characteristics over the extremes of operating conditions experienced by the engine.
In designing alloys for use in turbine components, it has been observed that no single superalloy discovered to date has an optimal combination of good mechanical properties and good resistance to environmental damage such as oxidation and high-temperature corrosion. The primary approach that has evolved as a result of this observation is to utilize as the basic structure of the turbine blade or vane a superalloy having good mechanical properties, and to coat the superalloy with an environmentally resistant coating of another material to protect the blade or vane from oxidation and corrosion damage.
One type of coating is an aluminide coating. Aluminum is diffused into the surface of the nickel-based superalloy article to form a nickel-aluminide layer, which then oxidizes to form an aluminum oxide surface coating during treatment or in service. (Optionally, platinum may also be diffused into the surface. ) The aluminum oxide surface coating renders the coated article more resistant to oxidation and corrosion, desirably without impairing its mechanical properties. Aluminide coating of turbine blades and vanes is well known and widely practiced in the industry, and is described, for example, in U.S. Pat. Nos. 3,415,672 and 3,540,878.
Recently it has been observed that, when some advanced nickel-based superalloys are coated with an aluminide coating and then exposed to service or simulated-service conditions, a secondary reaction zone (SRZ) forms in the underlying superalloy. This SRZ region is observed at a depth of from about 50 to about 250 micrometers (about 0.002-0.010 inches) below the original superalloy surface that has received the aluminide coating. The presence of the SRZ reduces the mechanical properties in the affected region, because the material in the SRZ appears to be brittle and weak.
The formation of the SRZ is a major problem in some types of turbine components, because there are cooling channels located about 750 micrometers (about 0.030 inches) below the surface of the article. Cooling air is forced through the channels during operation of the engine, to cool the structure. If the SRZ forms in the region between the surface and the cooling channel, it significantly weakens that region and can lead to reduced strength and fatigue resistance of the article.
To date, there has been proposed no approach for avoiding the formation of, or reducing the adverse effects of, the secondary reaction zone. There is a need for such a solution to the SRZ problem, in order to permit the affected superalloys to be used in gas turbine blades and vanes over extended service lives when the SRZ can form. The present invention fulfills this need, and further provides related advantages.