In gas turbine engines turbine section is located at the rear of the engine where it can extract energy from the hot exhaust gases coming out of the combustion chamber. After the combustion chamber turbine section is regarded as the hottest section of the gas turbine engine. The temperature in the turbine section is around 1000 which is much higher as compared to the temperature in the compressor of the gas turbine engine and thus both of these sections contains different materials because of their different working environment. Turbine section of the gas turbine engine plays a vital role in defining the efficiency and performance of the engine. Turbine converts the energy of the burned fuel from the combustion chamber into a mechanical power to derive the compressor of the engine. Turbine section is basically made up of number of turbine blades mounted on a disk attached to a shaft. Turbine blades are arranged in the form of stage in the turbine section where each stage consist of stationary vanes followed by the rotating blades.
The turbine blades are often considered as the critical component of the gas turbine engine because of their operation in the extremely high temperature zone of the engine. Therefore to sustain the harsh environment special design procedures were adopted for the turbine blades. The aim in designing the aircraft is to make them as light as possible with increased performance. By keeping this in mind the blades of compressor and turbines are designed to be light as possible. This can be achieved in case of compressor blades but in turbine blades itââ‚¬â„¢s still a concern due to the different working environment. Therefore it is important to work on the best suitable material for the compressor and turbine blades. Specially for turbine blades it is necessary to take in account the high temperature corrosion that occur due to the presence of contamination and chemical reactions from the exhaust fumes coming out of the combustion chamber at a high temperature .
In the beginning selection of material for high temperature section of the gas turbine with an enhanced performance was a challenge. A much need for a desired material stimulated the research in the field of alloys and thus results are achieved in the form of presenting new materials and processing methods for the modern gas turbine engines . The turbine blades operating at elevated temperatures are at a risk of development of metallurgical alloys. There are three most common types of damages that drastically affect the turbine blades namely creep, fatigue and high temperature corrosion . These three types of damages are interrelated on each other. It has been seen that in the presence of the high temperature corrosion the creep rate drastically increases and cause failure of the turbine blades. Therefore special care has to be taken in choosing the material that will have the ability to resist all the above mentioned damages. For example it is not feasible to choose a light material for the turbine blades as it will not resist creep at in a high temperature environment.
High temperature corrosion or hot corrosion is a type of corrosion that takes place where the environment temperature is too high containing certain containments . As the gas turbine engine technology is advancing, more and more steps have been taken to increase the gas turbine efficiency for which they need to operate the engine at much higher temperature. Increased temperature leads to increase high temperature corrosion which can cause the failure of engine in certain cases if proper measures have not been taken . In most cases vanadium and sulfur contents present in the fuel can cause hot corrosion.
Hot Corrosion by Sulfur
There are usually two types of sulfate hot corrosion, one is Type I hot corrosion occur above the melting point of sodium sulfates typically between (825 Co to 950 Co) and the second is Type II hot corrosion that occurs below the melting point of sodium sulfates typically between (700 Co to 800 Co). In Type I case if steel was used as a material in hot temperature area then the protective coating of the oxide will be dissolved because of the sulfur diffusion and will prevent steel from rebuilding the new oxide layer. This is the same problem in case of turbine blades where sulfur at higher temperature helps in decaying the protective oxide layer so that the corrosion will start.
The reaction of high temperature corrosion can be acidic or basic in nature. The reaction is of the type:
And for sodium ion the reaction is:
The equilibrium between, indicates the basicity or acidity of the reaction condition.
After this reaction the protective oxide layer react with deposit due to a fluxing process  as follows:
And also it can react as:
This type of fluxing reactions can only occur at high temperature (> 900 Co) and thus it is known as Type I hot corrosion.
Hot Corrosion by Vanadium
Vanadium is present as small traces in the fuel and can be oxidized to different vanadates which help in diffusing oxygen in the metal. Vanadium oxides are dangerous in hot corrosion because at higher temperature vanadium pentoxide formed which has the highest corrosion rate at high temperature. To lower the corrosion rate by vanadium either a refractory coating has been used or high chromium alloys can be used like 50 % Nickel and 50 % Cr alloy.
Hot Corrosion by Lead
Lead can also cause hot corrosion by creating a low melting slag which can damage the oxide layer . Hot corrosion can also be caused due to the presence of other salts like Ca, K and Mg.
These types of attacks can seriously damage the turbine blades and thus superalloys have been designed to provide best protection from to corrosion. Alloys that contain high corrosion contents are found to resist the hot corrosion very effectively but on the other hand Cr is not good with the alloys having high creep resistance and also excess Cr can reduce alloy stability as well. Thus there is a need for compromise among selection of different materials to form the alloy and thatââ‚¬â„¢s why in case of turbine blades special emphasis is given to the right alloy that will not only resist the hot corrosion but also resists creep and fatigue.
NICKEL BASED SUPER ALLOYS
It has been seen that to survive the difficult environment in the turbine section, materials like the nickel base alloys have been used most often in the manufacturing of the turbine blades [1,3] for the prevention of high temperature corrosion. Nickel-based alloys usually have 10-20% Cr, 8% Al & Ti, 5-10% Co, and some few percentages of B, Zr, and C . Nickel based alloys have a high density but because of their excellent mechanical properties the weight increment of the engine is accepted, which was further controlled by making hollow turbine blades with the cooling channel passage to maintain the turbine blade temperature. For the turbine blade the most used material is nickel-based ââ‚¬Å“super-alloyââ‚¬Â materials. These alloys have excellent properties to sustain the high temperature environment and to resist the stresses while exposing in the high temperature environment.
Superalloy contains excellent mechanical strength and also corrosion and creep resistance at higher temperature. Development of supperalloys were started in 1940s  and the main alloying element of superalloy is usually cobalt, nickel-iron or nickel. Superalloys have a vast range of use in aerospace and marine turbine applications especially in the turbine section part of the gas turbine engine which is exposed to high temperature . A list of some of the alloying materials used in turbine blades are shown in table :
Table: List of some turbine blade materials
Nickel is regarded as most appropriate for alloying because of its high tendency to form stable alloys without going into instability phase. With the chromium addition it form Cr2O3 oxide layer which protect the motion of metallic element. Similarly nickel also form aluminum oxide Al2O3 layer which is strongly resistant to oxidation at a high temperature and thus helps in preventing corrosion. Each of these oxides affects the oxidation growth. Initially the oxidation process grows in a parabolic way and after some time the rate of oxidation slows down because of the presence of alumina layer .
Different phases present in the Nickel Superalloy are :
Gamma () Phase:
Gamma phase is a face centered cubic (FCC) nickel based austenitic phase which holds a much greater percentage of solid solutions like Cr, Mo and Co.
Gamma Prime () Phase:
Gamma prime () is the principle strengthening phase in Ni based alloys.
Carbides have an FCC crystal structure and is beneficial in superalloys where it can increase rupture strength at elevated temperatures.
Topologically Close-Packed Phases:
These are brittle phases and usually form during heat treatment. They have close packed atoms in a cell structure. These close packed atoms diffuse from one another by larger atoms and thus creates a topology.
Usually the strength of most materials decreased due to increase temperature because of the dislocation of the atoms due to thermal excitations. However Ni based superalloys containing Gamma prime () having formula is hoghly resistant to temperature and thus suitable for the protecting against hot corrosion.
Turbine blades usually exposed to corrosive environment. Some external factors that can enhance corrosion on turbine blades include atmospheric contaminants like volcanic ash activity which can produce high level of sulphur and when these contents mix with the combustion fuel which already has vanadium, lead or bromine at high temperature attack nickel base alloy and thus a protective coating is applied to protect the turbine blade material from corrosion. Turbine blade coatings usually include chromium, aluminum, yttrium and platinum . These coatings will prevent the turbine blade from very highly contaminated environment and at high temperature. Currently the most advanced material used for turbine blade is single crystal Ni-base superalloys. In general the advancements in the material of gas turbine are due to thermo mechanical treatments to reduce the stresses that occurred at high temperature.