Nickel Base Super Alloys For Aerospace Materials Engineering Essay
A super alloy, high performance alloy is an alloy which exhibits excellent mechanical strength and creep resistance and also have good corrosion and oxidation resistance. Super alloy typical have a matrix with face centered cubic crystal structure. A Super alloy base alloying element is nickel, cobalt, or nickel-iron. In Earlier 1950's stainless steel is used as a super alloy. Super alloy growth that has rapidly improved both on chemical and processing of that which led to rapid growth in aerospace, industrial gas turbine and marine turbine industry. In which nickel base super alloy are used in the preparation of the turbine blades which can withstand long time at the elevated temperatures for the improvement of the performance of the turbine.
Desirable characteristics of high temperature super alloys:
1. Nickel base alloy should have ability to withstand loading at an operating temperatures close to the melting point.
2. It has substantial resistance to mechanical degradation over the extended period of time (resistant to creep).
3. It should tolerate severe operating environments (resistant to corrosive atmosphere).
Chemical composition and Microstructure of Nickel base super alloys.
Earlier Ni-based super alloys was derived from alloys containing additions of cr, cu, co and fe.With properties superior to that of stainless steels as these single phase Ni-base alloy exhibits good high temperature strength and corrosion resistance. As stringent demands of the rapidly developing aerospace industry. To improve the above desirable characteristics of the super alloys aluminum additions to be added to single phase nickel base super alloys to produce two phase microstructure to form ordered γ1 matrix distributed within a disordered γ matrix. In the phase diagram as the Al levels typically at approximately 18 atomic % and remaining 70-80% as nickel.
As the large stresses are required for the dislocation of the γ1precipitates additionally shearing of the precipitates the ordering of the γ1 precipitates requires the formation of the anti phase boundary. As the large stress required for orowan bowing and the shearing of the precipitates. So, these cuboidal crystals of the secondary phase are extremely effective in inhibiting the mobility of the dislocations. And also improves the creep properties at the temperatures from 700-10000c.As the high temperature properties of Ni-base super alloys are:Ni has a face centered cubic crystal structure with high melting temperature which makes it ductile and tough.Ni is stable in Fcc crystal structure from room temperature to its melting point. so there is no phase change and diffusion rates in Ni are low which improves microstructure stability at elevated temperatures. As the misfit between the γ and γ1 precipitates will change the microstructure under the influence of the stress as this is controlled by altering the chemical compositions and processing conditions.
As in the Ni base γ - γ1 precipitates the strength and creep resistance has been increased by developing some engineering solutions to overcome the limitations. The major alloying elements that should be added to Ni base alloys are Al, Ni, Ti and Nb as the addition of these tend to partition of preferentially to intermetallic γ1 precipitates. Co addition provides solid solution strengthening but is mainly added to modify the γ1 solvus temperatures. And other alloying additions are Re, W, Mo, V, Cr and platinum group metals are added to strengthen to both solid γ and γ1 precipitates at elevated temperatures. As the al and Cr both are added to strengthen the Ni-base alloys but depending upon the properties required should be added for one particular set of properties.
The minor alloying elements such as B, C, Zr and Hf, were added resulting in the formation of the carbides and occasionally borides at the grain boundary. As the carbon atoms exhibit high affinity for the elements such as Hf, Zr, Ta and Ti, Nb, w, Mo, V and Cr, the majority of carbides in Ni-base alloys is metal atom carbides may precipitate from liquid during solidification. As the carbides may affect the fatigue properties of the material the presence of the discrete carbide at the grain boundary inhibits the sliding and damage during the creep.
Creep behaviour of Ni-based alloys:
Creep is the deformation under the influence of stresses at elevated temperature. Creep is the time dependent, inelastic and irrerecoverable deformation. Creep is more severe in materials subjected to heat for a long periods. As creep always increases with temperature. The rate of deformation of the material depends upon the material properties, time, temperature and applied stress. Consider the blades of the turbines as the creep of the blade is to contact the casing, resulting in the failure of the blade. Creep does not occur suddenly like brittle materials as it is a time dependent deformation.
The stages of the creep are in the primary stage strain rate is relatively high, but slows with increasing strain this is due to work hardening, in the secondary stage the strain rate eventually reaches minimum and becomes constant and in the final stage strain increases rapidly because of the necking phenomena.
General creep equations:
dε/dt = Cσm/db e-q/kt
Where ε is the creep strain, C is constant dependent on the material, σ is the applied stress, m and b are the exponents depend on creep is the grain size,k is Boltzmann's constant.
There are 3 types of creep:
Dislocation creep of the material is the movement of the dislocations through the crystal lattice. It causes plastic deformation of the individual crystals at the end of the material.
dε/dt = Aσne-q/kt
Nabarro-herring creep is a form of diffusion creep in which atoms migrate within the grain boundary to elongate grain along the stress axis. At higher temperatures the diffusivity increases due to the direct temperature dependence of the equation, the increase in the vacancy through defect formation, an increase in the average energy of the atoms in the material.
Coble creep is also one of the diffusion controlled creep as the atoms diffuse along the grain boundary which produces a net flow of the material and a sliding of the grain boundaries.
Defects in crystals:
a - interstitial impurity atom in the crystal lattice
b - Edge dislocation in the crystal
c - Self interstitial atom of the material
d - Vacancy in the lattice structure
e - Precipitate of impurity atoms
f - Vacancy dislocation of the loop
g - Interstitial dislocation in the loop
h - Substitution impurity atom in the material
Dislocation is a 1-D defect as the lattice is only disturbed along the dislocation line. The dislocation of the crystal may be generated due to some vacancies, point defects, interstitial impurities in the crystal lattice.
The movement of the dislocation moves the crystal from one side relative to the other. In the figure below the left figure shows the closing of the dislocation crystal.
And the right figure shows same chain of base vectors in a perfect reference lattice and the circuit does not closes the vector which closes the circuit is called burgers vector which represents the dislocation of the crystal.
The atomic representation of the screw dislocation is complicated and still burgers vector is possible to represent the dislocation. If we move on the circuit of the dislocation it will moves in a circular like a screw. So, this is called as screw dislocation as the burgers vector does not changes in both the dislocations but there is a change in the sign convention depending upon the clockwise and anti clock rotation of the vectors along the circuit.
Defects in the gamma prime phase:
The defects in the gamma prime phase undergo 3 types of defects they are
1. Planar defects
2. Line defects
3. Point defects
As the defects the Ni and Al atoms, when bonded together an interface boundary known as the anti-phase boundary separates Ni-Ni and Al-Al bonds as the number of Ni-Al bonds near the APB is substantially reduced. In the line defects the phase dissociate into partial dislocations. In the point defects as the compositional range of Al is 23 to 27%.Thus, only small deviations causes the point defects.
Strengthening in nickel base alloys:
The mechanical properties of the Ni based alloys depend on the state of microstructure, chemical composition and processing conditions. As the dislocations of the material is reduced by some strengthening mechanisms to increase the hardness and strength.
Solid solution strengthening/alloying:
In this mechanism the solute atoms of one element are added to another, resulting in either substantial or interstitial point defects. The solute atoms cause lattice distortions that impede dislocation motion.
The stress required to move dislocations in the material is:
Δτ = Gbc1/2ε3/2
Where c is the solute concentration and ε is the strain on the material caused by solute
In most alloys, second phase can be precipitated from matrix in solid state. The particles that compose the second phase precipitates act as pinning points in a similar manner to solutes. The dislocations in a material can interact with the precipitate atoms in one of two ways. If the precipitate atoms are small, the dislocations would cut through them. If larger precipitate particles, looping or bowing of the dislocations would occur.
For particle looping/bowing
Δτ = Gb/L-2r
For particle cutting
Δτ = γπr/bL
Grain boundary strengthening:
In metals grain size has tremendous influence on the mechanical properties. Because grains usually have varying crystallographic orientations, grain boundary arises. The stress required to move a dislocation from one grain to another in order to plastically deform a material depends on the grain size. The average number of dislocations per grain decreases with average grain size.
Processing of single crystal Nickel based alloys:
Equations for growth:
Solidification is a physical change from liquid state to solid state of the material. As the heat transfer from the system to the surroundings. In general, the composition of solid should be different from that of the liquid with an impure material will also require transport of solute.
The governing equations for the diffusion of heat and solute is
i = S for solid and L for liquid,Ci =solute concentration in phase i,Ti = temperature in phase i,Di= solute diffusion coefficient in phase I,α = Thermal diffusion coefficient,
At the interface between solid and liquid
Ts1 = TL1 = TI
TI = Temperature of solid/liquid interface.
CsI = kCL1
K = distribution coefficient
Solute under cooling:
This is the equation under cooling due to the presence of the solute in the material. The composition of the liquid at the interface, CL1 will in general will be different from the bulk composition. If the material is pure (ΔTs = 0)
Curvature under cooling, ΔTγ
ΔTγ = Ð“ (1/R1+1/R2)
Ð“ = Gibbs Thomson coefficient.
R1, R2 = principle radii of curvature
As the Gibbs Thomson effect arise due to the excess energy associated with the formation of a solid/liquid interface.
Kinetic under cooling ΔTk:
Growth of phase is a non equilibrium process during the process atoms gain energy between liquid and solid and a net transfer of atom will only occur. As the driving force through the under cooling is known as kinetic under cooling.
The interface structure is dependent on the solid/liquid interface bonding. The growth of the interface may be of two types.
1. Faceted growth:
In faceted growth the crystals are bonded by angular surfaces growing to crystallographic plane. As the substances exhibit complex crystal structure and direction bonding.
2. Non-Faceted growth:
There is similarity between structures, density and bonding in the solid and liquid interface. The kinetics is independent of crystal orientation and the interface between the two phases will be more gradual and it becomes automatically rough.
Solidification of pure materials:
For pure materials the above solute equations are not significant. As in the under cooling for the pure materials ΔTs = 0.There stability of the solid/liquid interface for pure materials will be dependent on the conditions of growth. There are two methods
1. Columnar (or) directional solidification:
In the directional solidification heat is extracted through the solid in the opposite direction to the growth direction.
2. Equiaxed solidification:
The heat extracted through the under cooled liquid into which the free crystals are growing. As the perturbation in the solidification forms spherical interface. Heat rejected is more the spherical interface will always be unstable.
Solidification of Binary alloys:
Directional solidification of binary alloys for planar interface:
The growth produced by slowly moving liquid specimen from a furnace. It is known as directional solidification. For planar interface, ΔTr =0 and the equilibrium at the interface ΔTk =0.Therfore the under cooling and compositions at the interface will be given equilibrium. For the solidification of the alloy three cases are to be considered.
A Typical phase diagram of a two component alloy.
1. Complete mixing in liquid, none in solid:
This is practically only possible when either the specimen length is very small or if a convective mixing in the liquid. The solute rejected by each small volume of the solid to form distributed evenly throughout the remaining liquid.
2. No convection diffusion in liquid, none in solid:
This situation occurs in thin specimen, gravity stabilised or space experiment. Solidification begins with an initial transient during which an enriched solute boundary layer builds up ahead of the solid/liquid interface.
3. Partial mixing by convection in liquid:
In this method assume a stagnant boundary layer of width ahead of the solid/liquid interface in which transport occurs by diffusion only. Outside this layer there is a complete mixing in the liquid.
Columnar and Equiaxed grain structure:
Equiaxed grain structure: The heat is extracted through the under cooling liquid. The temperature at the tip of the dendrite is negative. This is also known as isolated growth or unconstrained growth. The controlling parameter for the growth velocity is simply the under cooling
Columnar grain structure: The heat is extracted through the solid in the opposite direction to the growth direction. The temperature at the tip of the dendrites is positive. It has constrained growth because the velocity is fixed.
Investment casting for single crystal turbine blades:
Investment casting is also known as low wax casting. This process is one of the oldest manufacturing processes. It can be used to make the parts that cannot be produced by normal manufacturing technique such as turbine blades and high temperature aerospace materials.
The mold is made up of pattern using wax or some other material that can be melted away. This wax pattern is dipped in the refractory slurry, which coats the wax. This is dried and the process of dipping in the slurry and drying is repeated until a robust thickness is achieved. After the entire pattern is placed in the oven and the wax is melted away. The material used for the slurry consisting of binder and a mixture of alumina, ziricon and silica followed by stuccoing. The mold thus produced can be used directly for the light casting.
A schematic diagram of investment casting process
Grain boundary selector:
Growth of single crystal alloy using grain selector:
The blades of single grain structure is achieved by directional solidification combined with a spiral grain selector with cylindrical base seed as in casting foundries, the block is placed at the bottom of the mould. The mould is withdrawn from the furnace to grow the blade. Several grains nucleated with the starter block can grow into a spiral passage way and most of them will be eliminated and only one grain survives during the growth. If the solidification in the body starts from a single crystal. As the grain orientation optimization and a spiral grain selector facilitating dendrite branching to ensure that only single grain eventually survives at the top of the seed
Directional solidification in investment casting:
In the directional solidification to grow columnar grains the heat should be extracted through the solid, in the opposite direction to the growth direction. For Ni-base alloys, the most rapid dendrite growth direction is selected as the long axis for the blades.
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