Nickel Base super-alloy in Aerospace turbine blades
The following essay will discuss the topic of Nickel base alloys in turbine blades production where aircraft‟s engines are considered the back bone of this industry. Through a full examination of the properties, micro structure and production process this paper will be divided into 4 main topics. The first will be a brief descriptive introduction, followed with an explanation about the chemistry of Nickel base alloy used for turbine blades to determine the phases within and the complementary role they share. The third part of this essay will describe the production operation which a turbine blade undergo and examine the microstructure of the alloy. At the end, a couple of processing techniques are described and an introduction to freckle chain defect will be discussed.
The aviation industry is rapidly developing due to the global demands of commercial and military uses. Along these demands, safety is a world – wide concern that imposes itself on the manufacturers who are responsible for achieving simultaneously, efficient and safe designs. The race was to look for materials which are capable to face serious challenges in new complex generation of sophisticated airplanes. The jet engine of an aircraft, for example, contains parts that are exposed to extreme conditions (temperature 1500 Cₒ & centrifugal forces) while functioning, because of the heat generation in the combustion chamber and the fast revolutions of blades in the compressor and turbine. The Turbine blades are exposed to higher temperatures than the compressor and to corrosive gases. This is due to the fact that the combustion occurs behind the compressor and so the combusted Fuel-air mixtures are passed through the turbine blades, therefore exposing them to higher temperatures compared to other parts within the jet engine. Once the fuel air mixtures are passed through the blades, they are ejected down to the nozzle and then expand in the atmosphere.
Figure -1- Turbine blade of an aircraft engine.
For these kind of applications, a special material is used to withstand these enormous stresses and high temperatures. They are called super alloys. An example of such an alloy would be Nickel base alloy that is composed of two phases. The first phase is made out of Nickel or aluminium (in case of using aluminium in the alloy). Nickel, or Ni, has an atomic number of 28. It‟s electronic configuration is 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d8, and is not very abundant in its raw state on the earth‟s surface. However, it can be found combined with Iron, in more plentiful amounts.
The second phase is an ordered inter metallic Nickel and Aluminum compound (NI3AL). Nickel based alloy take the form of a faced centered cube of 𝛾 matrix (Ni or Al) and γ‟ f.c.c which is the ordered compound phase (NI3AL) ( 1 ) . The stability of Nickel in the crystal structure makes it ductile and tough, and in addition to that this kind of super-alloy gain its strength from the difficulty of a dislocation to pass through the γ‟ cuboids.
Creep resistance of turbine blades
Materials have certain tolerances toward deformation under high stresses due to mechanical work or exposure to extreme heat for long periods of time. The deformation rate depends on the chemical properties of the material itself, the time of exposure to temperatures and load, in the case of a airplane turbine such a deformation can results in a contact between the blades and the casing which can be catastrophic. It is normal for alloys to undergoes some high stresses because of the nature of its functions but the stresses must always be a certain degree lower at which fracturing can take place at room temperature regardless to the duration of mechanical work. Creep occur in different stages where the first one is represented by an initial extension of the material, in the second stage the creep occur in a constant rate and a third stage and when the extension rate becomes more rapid and lead to fracture.
Figure-3- Creep curve
The existence of a stable dislocation substructure gives the material a resistivity toward the movement of these dislocations. In the case of Nickel base alloy the substructure becomes rough with the straining creep and as more rough it becomes the creep‟s strength is reduced ( 4 ). Particle stability and a dispersion of precipitates ensure the stability of the substructure. Using Larson –Miller parameter it is possible to calculate the time to rupture if the temperature at which the alloy is functioning and the stress load that is exposed to by using a graph ( stress over Larson –Miller parameter).
T : temperature in Kelvin
tr : time to rupture
P : Larson-Miller parameter.
The change of phase from liquid to solid is a physical transformation of matter called solidification which requires a heat transfer from the matter outward to the surrounding. The equations of heat diffusion and solute diffusion are reported as follow:
Ti = phase temperature
𝛼 = thermal diffusion coefficient
Ci = Solute concentration in phase i
t = time
Directional and single crystal solidification is a technique used to process the metals in investment castings by preheating the mold to a high temperature which is closely equal to the melting temperature and the bottom of the mold is connected to a chill plate which is a water cooling system. A system of heat baffles surrounds the mold and the melted metal is poured and it begins to crystallize in the cold region where the chill plate is attached. The mold is withdrawn and small crystals start to grow at the chill plate and grains grow in a columnar manner normal to the plate and at the end the result would be made up of parallel columns of grains approximately of the same orientation.
Some grain defects may occur and the blades production would be rejected, one of these defects is called freckle chain.
Freckle chains appear away from the chill plate where the thermal gradient is at its minimum and the growth of the mushy zone decrease. Freckles are long chains of equiaxed grains that are situated parallel to the initial dendrite arm, they can be located on the surface. If the casting increase in diameter it becomes more likely to find more freckle chains. It has been noticed that the freckle chains appears when the critical Rayleigh number is exceeded.
By reducing the height of the mushy zone through increasing the thermal gradient the defect can be avoided.
Single crystal castings with no grain boundaries can be produced in similar way where the molten metal is poured into a ceramic mold that has pig tale shape selector between the chill plate and the top part of the mold. Columnar grains starts to grow but the selector is narrow to let more than one crystal grow through and when the mold get larger in the top region the diameter of the singular crystal increase.
The absence of grain boundaries improves the creep resistivity of the turbine blades.
This document briefly presented an introduction and simple explanation for the technology used in the production of aeronautical turbine blades through taking advantage of Nickel based alloys and the knowledge possessed in investment castings. The futuristic technology is focusing on producing alloys of a light weight, hard and anti – corrosive.
Some laboratories like Sandia national in the United States are working on new super alloys through Nano – particle synthesis which give some hope of improving the properties of the alloys used in industry.