HIGH TEMPERATURE HIGH TEMPERATURE MATERIALSMATERIALS

BYBY

M. SANTHOSHM. SANTHOSH,,Lecturer,Lecturer,

Department of Aeronautical EngineeringDepartment of Aeronautical Engineering

Contact:Contact:mkkksans@yahoo.co.inmkkksans@yahoo.co.in

MATERIALMATERIAL Material is synonymous with substance, and is anything made of matter – Material is synonymous with substance, and is anything made of matter –

hydrogen, air and water are all examples of materialshydrogen, air and water are all examples of materials

The basis of materials science involves relating the desired properties and The basis of materials science involves relating the desired properties and relative performance of a material in a certain application to the structure relative performance of a material in a certain application to the structure of the atoms and phases in that material through characterization. of the atoms and phases in that material through characterization.

The major determinants of the structure of a material and thus of its The major determinants of the structure of a material and thus of its properties are its constituent chemical elements and the way in which it has properties are its constituent chemical elements and the way in which it has been processed into its final form.been processed into its final form.

The manufacture of a perfect crystal of a material is currently physically The manufacture of a perfect crystal of a material is currently physically impossible. Instead materials scientists manipulate the defects in crystalline impossible. Instead materials scientists manipulate the defects in crystalline materials such as precipitates, grain boundaries, interstitial atoms, materials such as precipitates, grain boundaries, interstitial atoms, vacancies or substitutional atoms, to create materials with the desired vacancies or substitutional atoms, to create materials with the desired propertiesproperties..

CRYSTALLOGRAPHYCRYSTALLOGRAPHY

What is a crystal?What is a crystal? In materials science a In materials science a crystalcrystal is a solid  substance in which the atoms,  molecules  is a solid  substance in which the atoms,  molecules 

or ions are arranged in an orderly repeating pattern extending in all three spatial or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions - length, width and height.dimensions - length, width and height.

The process of forming a crystalline structure from a fluid or from materials The process of forming a crystalline structure from a fluid or from materials dissolved in the fluid is often referred to as dissolved in the fluid is often referred to as crystallizationcrystallization..

Various types of crystal structures in interestVarious types of crystal structures in interest

1.1. SCSC

2.2. BCCBCC

3.3. FCCFCC

4.4. HCPHCP

• Cubic unit cell is 3D repeat unit • Rare (only Po has this structure)• Close-packed directions (directions along which atoms touch each other) are cube edges.

(Courtesy P.M. Anderson)

SIMPLE CUBIC STRUCTURE (SC)SIMPLE CUBIC STRUCTURE (SC)

• Coordination # = 8

Adapted from Fig. 3.2, Callister 6e. (Courtesy P.M. Anderson)

• Close packed directions are cube diagonals.--Note: All atoms are identical; the center atom is shaded differently only for ease of viewing.

BODY CENTERED CUBIC BODY CENTERED CUBIC STRUCTURE (BCC)STRUCTURE (BCC)

• Coordination # = 12

Adapted from Fig. 3.1(a), Callister 6e. (Courtesy P.M. Anderson)

• Close packed directions are face diagonals.--Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing.

FACEFACE CENTEREDCENTERED CUBICCUBIC STRUCTURESTRUCTURE (FCC) (FCC)

HEXAGONAL CLOSE-PACKED HEXAGONAL CLOSE-PACKED STRUCTURE (HCP)STRUCTURE (HCP)

Ideally, c/a = 1.633 for close packingHowever, in most metals, c/a ratio deviates from this value

Some metals & their crystal structuresSome metals & their crystal structures

CRYSTAL DEFECTSCRYSTAL DEFECTS A perfect crystal, with every atom of the same type in the A perfect crystal, with every atom of the same type in the

correct position, does not exist. All crystals have some defects. correct position, does not exist. All crystals have some defects. Defects contribute to the mechanical properties of metals Defects contribute to the mechanical properties of metals

There are basic classes of crystal defects:There are basic classes of crystal defects: Point defectsPoint defects, which are places where an atom is missing or irregularly , which are places where an atom is missing or irregularly

placed in the lattice structure. Point defects include lattice vacancies, placed in the lattice structure. Point defects include lattice vacancies, self-interstitial atoms, substitution impurity atoms, and interstitial self-interstitial atoms, substitution impurity atoms, and interstitial impurity atoms impurity atoms

Linear defectsLinear defects, which are groups of atoms in irregular positions. , which are groups of atoms in irregular positions. Linear defects are commonly called Linear defects are commonly called dislocationsdislocations. .

Planar defectsPlanar defects, which are interfaces between homogeneous regions of , which are interfaces between homogeneous regions of the material. Planar defects include grain boundaries, stacking faults the material. Planar defects include grain boundaries, stacking faults and external surfaces. and external surfaces.

POINT DEFECTSPOINT DEFECTS

A self interstitial atom is an extra atom that has crowded its way into an A self interstitial atom is an extra atom that has crowded its way into an interstitial void in the crystal structure. interstitial void in the crystal structure.

A substitutional impurity atom is an atom of a different type than the bulk atoms, A substitutional impurity atom is an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms in the lattice. Substitutional impurity which has replaced one of the bulk atoms in the lattice. Substitutional impurity atoms are usually close in size (within approximately 15%) to the bulk atom. atoms are usually close in size (within approximately 15%) to the bulk atom.

An example of substitutional impurity atoms is the zinc atoms in brass. In brass, An example of substitutional impurity atoms is the zinc atoms in brass. In brass, zinc atoms with a radius of 0.133 nm have replaced some of the copper atoms, zinc atoms with a radius of 0.133 nm have replaced some of the copper atoms, which have a radius of 0.128 nm.which have a radius of 0.128 nm.

Interstitial impurity atoms are much smaller than the atoms in the bulk matrix. Interstitial impurity atoms are much smaller than the atoms in the bulk matrix. Interstitial impurity atoms fit into the open space between the bulk atoms of the Interstitial impurity atoms fit into the open space between the bulk atoms of the lattice structure. An example of interstitial impurity atoms is the carbon atoms lattice structure. An example of interstitial impurity atoms is the carbon atoms that are added to iron to make steel. Carbon atoms, with a radius of 0.071 nm, fit that are added to iron to make steel. Carbon atoms, with a radius of 0.071 nm, fit nicely in the open spaces between the larger (0.124 nm) iron atoms.nicely in the open spaces between the larger (0.124 nm) iron atoms.

Vacancies are empty spaces where an atom should be, but is missing. They are Vacancies are empty spaces where an atom should be, but is missing. They are common, especially at high temperatures when atoms are frequently and common, especially at high temperatures when atoms are frequently and randomly change their positions leaving behind empty lattice sites. In most cases randomly change their positions leaving behind empty lattice sites. In most cases diffusion (mass transport by atomic motion) can only occur because of vacanciesdiffusion (mass transport by atomic motion) can only occur because of vacancies

LINEAR DEFECTSLINEAR DEFECTS Dislocations are another type of defect in crystals. Dislocations are areas were the atoms are Dislocations are another type of defect in crystals. Dislocations are areas were the atoms are

out of position in the crystal structure. Dislocations are generated and move when a stress is out of position in the crystal structure. Dislocations are generated and move when a stress is applied. The motion of dislocations allows slip – plastic deformation to occur. applied. The motion of dislocations allows slip – plastic deformation to occur.

In the early 1900’s scientists estimated that metals undergo plastic deformation at forces In the early 1900’s scientists estimated that metals undergo plastic deformation at forces much smaller than the theoretical strength of the forces that are holding the metal atoms much smaller than the theoretical strength of the forces that are holding the metal atoms together. together.

There are two basic types of dislocations,There are two basic types of dislocations, the edge dislocation the edge dislocation the screw dislocation. the screw dislocation.

EDGE DISLOCATIONSEDGE DISLOCATIONS

The edge defect can be easily visualized as an extra half-plane of atoms in aThe edge defect can be easily visualized as an extra half-plane of atoms in alattice. lattice.

The dislocation is called a line defect because the locus of defective pointsThe dislocation is called a line defect because the locus of defective pointsproduced in the lattice by the dislocation lie along a line.produced in the lattice by the dislocation lie along a line.

This line runs along the top of the extra half-plane. This line runs along the top of the extra half-plane. The inter-atomic bonds are significantly distorted only in the immediate vicinity of the The inter-atomic bonds are significantly distorted only in the immediate vicinity of the

dislocation line.dislocation line. Dislocation motion is analogous to movement of a caterpillarDislocation motion is analogous to movement of a caterpillar

SCREW DISLOCATIONSSCREW DISLOCATIONS

The motion of a screw dislocation is The motion of a screw dislocation is also a result of shear stress, but the also a result of shear stress, but the defect line movement is defect line movement is perpendicular to direction of the perpendicular to direction of the stress and the atom displacement, stress and the atom displacement, rather than parallel.rather than parallel.

The image aside shows the screw The image aside shows the screw dislocationdislocation

16

Planar Defects in SolidsPlanar Defects in Solids One case is a One case is a twin boundary (plane)twin boundary (plane)

Essentially a reflection of atom positions across Essentially a reflection of atom positions across the the twin planetwin plane..

Stacking faultsStacking faults For FCC metals an error in ABCABC packing For FCC metals an error in ABCABC packing

sequencesequence Ex: ABCABABCEx: ABCABABC

Adapted from Fig. 5.14, Callister & Rethwisch 3e.

GRAIN BOUNDARY CONCEPTGRAIN BOUNDARY CONCEPT If you were to take a small section of a common metal and examine it under a microscope, you If you were to take a small section of a common metal and examine it under a microscope, you

would see a structure similar to that shown in figure.would see a structure similar to that shown in figure. Each of the light areas is called a grain, or crystal, which is the region of space occupied by a Each of the light areas is called a grain, or crystal, which is the region of space occupied by a

continuous crystal lattice.continuous crystal lattice. The dark lines surrounding the grains are grain boundaries.  The grain structure refers to the The dark lines surrounding the grains are grain boundaries.  The grain structure refers to the

arrangement of the grains in a metal, with a grain having a particular crystal structure. arrangement of the grains in a metal, with a grain having a particular crystal structure. The grain boundary refers to the outside area of a grain that separates it from the other grains. The grain boundary refers to the outside area of a grain that separates it from the other grains. The grain boundary is a region of misfit between the grains and is usually one to three atom The grain boundary is a region of misfit between the grains and is usually one to three atom

diameters    wide. diameters    wide. A  very  important  feature  of  a  metal  is  the  average  size  of  the  grain.    The  size  of  the A  very  important  feature  of  a  metal  is  the  average  size  of  the  grain.    The  size  of  the

 grain determines the properties of the metal.  For example, smaller grain size increases tensile  grain determines the properties of the metal.  For example, smaller grain size increases tensile strength and tends to increase ductility.   A larger grain size is preferred for improved high-strength and tends to increase ductility.   A larger grain size is preferred for improved high-temperature creep properties.  temperature creep properties.  

Some of the more important physical and chemical propertiesSome of the more important physical and chemical propertiesfrom an engineering material standpoint will be discussed in thefrom an engineering material standpoint will be discussed in thefollowing sections.following sections.

Phase Transformation Temperatures Phase Transformation Temperatures Density Density Specific Gravity Specific Gravity Thermal Conductivity Thermal Conductivity Linear Coefficient of Thermal Expansion Linear Coefficient of Thermal Expansion Electrical Conductivity and Resistivity Electrical Conductivity and Resistivity Magnetic Permeability Magnetic Permeability Corrosion ResistanceCorrosion Resistance

You should be familiar with the following terms You should be familiar with the following terms which you would have studied in lower classes.which you would have studied in lower classes.

1.1. Engineering stressEngineering stress

2.2. Engineering strainEngineering strain

3.3. True stressTrue stress

4.4. True strainTrue strain

5.5. Yield strengthYield strength

6.6. Yield pointYield point

7.7. Ultimate pointUltimate point

And some basic definitions related to strength of materialsAnd some basic definitions related to strength of materials

What is meant by loading a material?What is meant by loading a material? The application of a force to an object is known as loading. Materials can be subjected to The application of a force to an object is known as loading. Materials can be subjected to

many different loading scenarios and a material’s performance is dependant on the loading many different loading scenarios and a material’s performance is dependant on the loading conditions. conditions.

There are five fundamental loading conditions; tension, compression, bending, shear, and There are five fundamental loading conditions; tension, compression, bending, shear, and torsion.torsion.

Tension is the type of loading in which the two sections of material on either side of a Tension is the type of loading in which the two sections of material on either side of a plane tend to be pulled apart or elongated. plane tend to be pulled apart or elongated.

Compression is the reverse of tensile loading and involves pressing the material together.  Compression is the reverse of tensile loading and involves pressing the material together.  Loading by bending involves applying a load in a manner that causes a material to curve Loading by bending involves applying a load in a manner that causes a material to curve

and results in compressing the material on one side and stretching it on the other.  and results in compressing the material on one side and stretching it on the other.  Shear involves applying a load parallel to a plane which caused the material on one side of Shear involves applying a load parallel to a plane which caused the material on one side of

the plane to want to slide across the material on the other side of the plane. the plane to want to slide across the material on the other side of the plane. Torsion is the application of a force that causes twisting in a material.Torsion is the application of a force that causes twisting in a material. If a material is subjected to a constant force, it is called static loading. If the loading of the If a material is subjected to a constant force, it is called static loading. If the loading of the

material is not constant but instead fluctuates, it is called dynamic or cyclic loading. The material is not constant but instead fluctuates, it is called dynamic or cyclic loading. The way a material is loaded greatly affects its mechanical properties and largely determines way a material is loaded greatly affects its mechanical properties and largely determines how, or if, a component will fail; and whether it will show warning signs before failure how, or if, a component will fail; and whether it will show warning signs before failure actually occurs.actually occurs.

PROBLEMS FACED BY PROBLEMS FACED BY MATERIALS OPERATED MATERIALS OPERATED

AT AT

ELEVATED ELEVATED TEMPERATURESTEMPERATURES

The mechanical strength of metals decreases with increasing temperature The mechanical strength of metals decreases with increasing temperature and the properties become much more time dependent. and the properties become much more time dependent.

In the past the operating temperatures in applications like steam power In the past the operating temperatures in applications like steam power plant, chemical plant and oil refineries seldom exceeded 500plant, chemical plant and oil refineries seldom exceeded 500ooC, but since C, but since the development of the gas turbine in the 1940's successive designs have the development of the gas turbine in the 1940's successive designs have pushed this temperature up to typically 1000pushed this temperature up to typically 1000 o oC. C.

Developments in high temperature alloys with improved high temperature Developments in high temperature alloys with improved high temperature strength and oxidation resistance have had to keep pace with these strength and oxidation resistance have had to keep pace with these demands, and applications like rocket engines present greater problems.demands, and applications like rocket engines present greater problems.

At At homologous temperatures homologous temperatures of more than 0.5, creep is of engineering of more than 0.5, creep is of engineering significancesignificance

HIGH TEMPERATURE HIGH TEMPERATURE >0.3TM>0.3TM

CreepCreepHigh temperature fractureHigh temperature fractureCorrosionCorrosionFatigueFatigueEmbrittlementEmbrittlementThese are the factors affecting the functional These are the factors affecting the functional

or service life of components at elevated or service life of components at elevated temperaturestemperatures

CREEPCREEP

CREEPCREEP Creep is the tendency of a solid material to slowly move or Creep is the tendency of a solid material to slowly move or

deform permanently under the influence of stressesdeform permanently under the influence of stresses . . It occurs as a result of long term exposure to levels of stress that are below the It occurs as a result of long term exposure to levels of stress that are below the

yield strength of the material. Creep is more severe in materials that are yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. subjected to heat for long periods, and near the melting point.

Creep always increases with temperature.Creep always increases with temperature. The rate of this deformation is a function of the material properties, exposure The rate of this deformation is a function of the material properties, exposure

time, exposure temperature and the applied structural load.time, exposure temperature and the applied structural load. The temperature range in which creep deformation may occur differs in The temperature range in which creep deformation may occur differs in

various materials.various materials. the effects of creep deformation generally become noticeable at approximately the effects of creep deformation generally become noticeable at approximately

30% of the melting point for metals and 40–50% of melting point for ceramic30% of the melting point for metals and 40–50% of melting point for ceramic Creep deformation is important not only in systems where high temperatures Creep deformation is important not only in systems where high temperatures

are endured such as nuclear power plants, jet engines and heat exchangersare endured such as nuclear power plants, jet engines and heat exchangers In steam turbine power plants, pipes carry steam at high temperatures (566 °C In steam turbine power plants, pipes carry steam at high temperatures (566 °C

or 1050 °F) and pressures (above 24.1 MPa or 3500 psi). In jet engines, or 1050 °F) and pressures (above 24.1 MPa or 3500 psi). In jet engines, temperatures can reach up to 1400 °C (2550 °F) and initiate creep deformation temperatures can reach up to 1400 °C (2550 °F) and initiate creep deformation in even advanced-coated turbine blades. Hence, it is crucial for correct in even advanced-coated turbine blades. Hence, it is crucial for correct functionality to understand the creep deformation behavior of materials.functionality to understand the creep deformation behavior of materials.

Creep data for general design use are usually obtained under conditions of Creep data for general design use are usually obtained under conditions of constant uniaxial loading and constant temperature. Results of tests are constant uniaxial loading and constant temperature. Results of tests are usually plotted as strain versus time up to rupture. usually plotted as strain versus time up to rupture.

As indicated in the image, creep often takes place in three stages.  In the As indicated in the image, creep often takes place in three stages.  In the initial stageinitial stage, strain occurs at a relatively rapid rate but the rate gradually , strain occurs at a relatively rapid rate but the rate gradually decreases until it becomes approximately constant during the second stage. decreases until it becomes approximately constant during the second stage. 

This constant creep rate is called the This constant creep rate is called the minimum creep rate or steady-stateminimum creep rate or steady-state creep ratecreep rate since it is the slowest creep rate during the test. In the since it is the slowest creep rate during the test. In the third stagethird stage, , the strain rate increases until failure occurs.  the strain rate increases until failure occurs. 

Creep in service is usually affected by changing conditions of loading and Creep in service is usually affected by changing conditions of loading and temperature and the number of possible stress-temperature-time temperature and the number of possible stress-temperature-time combinations is infinite.  While most materials are subject to creep, the combinations is infinite.  While most materials are subject to creep, the creep mechanisms is often different between metals, plastics, rubber, creep mechanisms is often different between metals, plastics, rubber, concrete.concrete.

High homologous temperatures (Tservice/Tmelting)High homologous temperatures (Tservice/Tmelting) Unlike brittle fracture, creep deformation does not occur suddenly upon the Unlike brittle fracture, creep deformation does not occur suddenly upon the

application of stress. Instead, application of stress. Instead, strain accumulates as a result of long-term accumulates as a result of long-term stress. Creep deformation is "time-dependent" deformationstress. Creep deformation is "time-dependent" deformation ..

MECHANISMS OF CREEP IN METALSMECHANISMS OF CREEP IN METALS

There are three basic mechanisms that canThere are three basic mechanisms that can

contribute to creep in metals, namely:contribute to creep in metals, namely:

(i) Dislocation slip and climb.(i) Dislocation slip and climb.

(ii) Grain boundary sliding.(ii) Grain boundary sliding.

(iii) Diffusional flow(iii) Diffusional flow

DISLOCATION CREEPDISLOCATION CREEP

Dislocations slip is hindered by obstacles suchDislocations slip is hindered by obstacles such (i) grain boundaries, (i) grain boundaries, (ii) impurity particles, (ii) impurity particles, (iii) the stress field around solute atoms in (iii) the stress field around solute atoms in

solution orsolution or(iv) the strain fields of other dislocations.(iv) the strain fields of other dislocations.

DIFFUSIONAL CREEPDIFFUSIONAL CREEP

GRAIN BOUNDARY SLIDINGGRAIN BOUNDARY SLIDING

The onset of tertiary creep is a sign that structural damage has The onset of tertiary creep is a sign that structural damage has occurred in an alloy.occurred in an alloy.

Rounded and wedge shaped voids are seen mainly at the grain Rounded and wedge shaped voids are seen mainly at the grain boundaries and when these coalesce creep rupture occurs.boundaries and when these coalesce creep rupture occurs.

The mechanism of void formation involves grain boundary The mechanism of void formation involves grain boundary sliding which occurs under the action of shear stresses acting sliding which occurs under the action of shear stresses acting on the boundarieson the boundaries

Voids in creep ruptured Nimonic 80A. Showing scratch lines displaced across a grain boundary in Aluminium.

A model for the formation of cracks due to grain boundary sliding

The formation of wedge cracksduring grain boundary sliding.

DATA EXTRAPOLATION METHODSDATA EXTRAPOLATION METHODS

LARSEN MILLER PARAMETERLARSEN MILLER PARAMETER

AN EXAMPLEAN EXAMPLE

STRAIN HARDENINGSTRAIN HARDENING

Strain hardening (also called work-hardening or cold-working) is the Strain hardening (also called work-hardening or cold-working) is the process of making a metal harder and stronger through plastic process of making a metal harder and stronger through plastic deformation. deformation.

When a metal is plastically deformed, dislocations move and additional When a metal is plastically deformed, dislocations move and additional dislocations are generated. The more dislocations within a material, the dislocations are generated. The more dislocations within a material, the more they will interact and become pinned or tangled. This will result in more they will interact and become pinned or tangled. This will result in a decrease in the mobility of the dislocations and a strengthening of the a decrease in the mobility of the dislocations and a strengthening of the material. This type of strengthening is commonly called cold-working. It material. This type of strengthening is commonly called cold-working. It is called cold-working because the plastic deformation must occurs at a is called cold-working because the plastic deformation must occurs at a temperature low enough that atoms cannot rearrange themselves. temperature low enough that atoms cannot rearrange themselves.

When a metal is worked at higher temperatures (hot-working) the When a metal is worked at higher temperatures (hot-working) the dislocations can rearrange and little strengthening is achieved. dislocations can rearrange and little strengthening is achieved.

Strain hardening can be easily demonstrated with piece of wire or a Strain hardening can be easily demonstrated with piece of wire or a paper clip. Bend a straight section back and forth several times. It paper clip. Bend a straight section back and forth several times. It increases the strength of the wire. This is said to be strain hardened wire.increases the strength of the wire. This is said to be strain hardened wire.

Effects of Elevated Temperature on Strain Effects of Elevated Temperature on Strain Hardened MaterialsHardened Materials

When strain hardened materials are exposed to elevated temperatures, the When strain hardened materials are exposed to elevated temperatures, the strengthening that resulted from the plastic deformation can be lost. This strengthening that resulted from the plastic deformation can be lost. This can be a bad thing if the strengthening is needed to support a load. can be a bad thing if the strengthening is needed to support a load.

Heat treatment can be used to remove the effects of strain hardening. Three Heat treatment can be used to remove the effects of strain hardening. Three things can occur during heat treatment things can occur during heat treatment

Recovery Recovery Recrystallization Recrystallization Grain growth Grain growth

RECOVERYRECOVERY

When a stain hardened material is held at an elevated temperature an When a stain hardened material is held at an elevated temperature an increase in atomic diffusion occurs that relieves some of the internal strain increase in atomic diffusion occurs that relieves some of the internal strain energy. energy.

Remember that atoms are not fixed in position but can move around when Remember that atoms are not fixed in position but can move around when they have enough energy to break their bonds. they have enough energy to break their bonds.

Diffusion increases rapidly with rising temperature and this allows atoms Diffusion increases rapidly with rising temperature and this allows atoms in severely strained regions to move to unstrained positions. In other in severely strained regions to move to unstrained positions. In other words, atoms are freer to move around and recover a normal position in the words, atoms are freer to move around and recover a normal position in the lattice structure. This is known as the recovery phase and it results in an lattice structure. This is known as the recovery phase and it results in an adjustment of strain on a microscopic scale.adjustment of strain on a microscopic scale.

Internal residual stresses are lowered due to a reduction in the dislocation Internal residual stresses are lowered due to a reduction in the dislocation density and a movement of dislocation to lower-energy positions. The density and a movement of dislocation to lower-energy positions. The tangles of dislocations condense into sharp two-dimensional boundaries tangles of dislocations condense into sharp two-dimensional boundaries and the dislocation density within these areas decrease. These areas are and the dislocation density within these areas decrease. These areas are called sub grains. There is no appreciable reduction in the strength and called sub grains. There is no appreciable reduction in the strength and hardness of the material but corrosion resistance often improves.hardness of the material but corrosion resistance often improves.

RECRYSTALLIZATIONRECRYSTALLIZATION

At a higher temperature, new, strain-free grains nucleate and grow inside At a higher temperature, new, strain-free grains nucleate and grow inside the old distorted grains and at the grain boundaries. These new grains grow the old distorted grains and at the grain boundaries. These new grains grow to replace the deformed grains produced by the strain hardening. With to replace the deformed grains produced by the strain hardening. With recrystallization, the mechanical properties return to their original weaker recrystallization, the mechanical properties return to their original weaker and more ductile states. and more ductile states.

Recrystallization depends on the temperature, the amount of time at this Recrystallization depends on the temperature, the amount of time at this temperature and also the amount of strain hardening that the material temperature and also the amount of strain hardening that the material experienced. experienced.

The more strain hardening, the lower the temperature will be at which The more strain hardening, the lower the temperature will be at which recrystallization occurs. Also, a minimum amount (typically 2-20%) of recrystallization occurs. Also, a minimum amount (typically 2-20%) of cold work is necessary for any amount of recrystallization to occur. The cold work is necessary for any amount of recrystallization to occur. The size the new grains is also partially dependant on the amount of strain size the new grains is also partially dependant on the amount of strain hardening. The greater the stain hardening, the more nuclei for the new hardening. The greater the stain hardening, the more nuclei for the new grains, and the resulting grain size will be smaller (at least initially). grains, and the resulting grain size will be smaller (at least initially).

GRAIN GROWTHGRAIN GROWTH

If a specimen is left at the high temperature beyond the time needed for If a specimen is left at the high temperature beyond the time needed for complete recrystallization, the grains begin to grow in size. This occurs complete recrystallization, the grains begin to grow in size. This occurs because diffusion occurs across the grain boundaries and larger grains because diffusion occurs across the grain boundaries and larger grains have less grain boundary surface area per unit of volume. have less grain boundary surface area per unit of volume.

Therefore, the larger grains lose fewer atoms and grow at the expense of Therefore, the larger grains lose fewer atoms and grow at the expense of the smaller grains. Larger grains will reduce the strength and toughness of the smaller grains. Larger grains will reduce the strength and toughness of the material. the material.

SOLID SOLUTION STRENGTHENINGSOLID SOLUTION STRENGTHENING Solid solution strengtheningSolid solution strengthening is a type of alloying that can be used to is a type of alloying that can be used to

improve the strength of a pure metal. The technique works by adding improve the strength of a pure metal. The technique works by adding atoms of one element (the alloying element) to the crystalline lattice atoms of one element (the alloying element) to the crystalline lattice another element (the base metal). The alloying element diffuses into the another element (the base metal). The alloying element diffuses into the matrix, forming a solid solutionmatrix, forming a solid solution

Depending on the size of the alloying element, Substitutional solid solution or Intersitial solid solution

Substitutional solid solution strengthening occurs when the solute atom is large enough that it can replace solvent atoms in their lattice positions. According to the Hume-Rothery rules, solvent and solute atoms must differ in atomic size by less than 15% in order to form this type of solution.

When the solute atom is much smaller than the solvent atoms, an interstitial solid solution forms. This typically occurs when the solute atoms are less than half as small as the solvent atoms. The smaller solute atom essentially "crowds" into the spacings within the lattice structure, causing defects in the material. Elements commonly used to form interstitial solid solutions include H, N, C, and O. Carbon in iron (steel) is one example of interstitial diffusion.

FRACTUREFRACTURE

FRACTUREFRACTURE

A separation of an object into two or more pieces in response A separation of an object into two or more pieces in response to active stresses below the melting temperature of the to active stresses below the melting temperature of the material.material.

Two steps in the process of fracture:Two steps in the process of fracture: Crack initiationCrack initiation Crack propagationCrack propagation

Types of Failure in Materials

FIGURE 3.20 Schematic illustration of types of failure in materials: (a) necking and fracture of ductile materials; (b) buckling of ductile materials under a compressive load; (c) fracture of brittle materials in compression; (d) cracking on the barreled surface of ductile materials in compression. (See also Fig. 6.1b)

FIGURE 3.21 Schematic illustration of the types of fracture in tension: (a) brittle fracture in polycrystalline metals; (b) shear fracture in ductile single crystals (see also Fig. 3.4a); (c) ductile cup-and-cone fracture in polycrystalline metals (see also Fig. 2.2 ); (d) complete ductile fracture in polycrystalline metals, with 100% reduction of area.

DUCTILE AND BRITTLE FRACTUREDUCTILE AND BRITTLE FRACTURE

Ductile FractureDuctile Fracture

TRANSGRANULAR CREEP TRANSGRANULAR CREEP FRACTUREFRACTURE

INTERGRANULAR CREEP INTERGRANULAR CREEP FRACTUREFRACTURE

PURE DIFFUSIONAL FRACTUREPURE DIFFUSIONAL FRACTURE

RUPTURERUPTURE

HIGH TEMPERATURE HIGH TEMPERATURE CORROSIONCORROSION

BASICSBASICS CORROSION is the deterioration of a material by its reaction with the CORROSION is the deterioration of a material by its reaction with the

surroundings. It adversely affects those properties that are to be preserved. surroundings. It adversely affects those properties that are to be preserved. At higher temperature, this mode of degradation is known as oxidation or At higher temperature, this mode of degradation is known as oxidation or

dry corrosion or scaling. Metals and alloys sometimes experience dry corrosion or scaling. Metals and alloys sometimes experience accelerated oxidation when their surfaces are covered with a thin film of accelerated oxidation when their surfaces are covered with a thin film of fused salt in an oxidizing atmosphere at elevated temperatures. This mode fused salt in an oxidizing atmosphere at elevated temperatures. This mode of attack is called ‘hot corrosion’, where a porous, non-protective oxide of attack is called ‘hot corrosion’, where a porous, non-protective oxide scale is formed at the surface and sulphides in the substratescale is formed at the surface and sulphides in the substrate

High temperature corrosion is a form of corrosion that does not require the High temperature corrosion is a form of corrosion that does not require the presence of a liquid electrolyte.presence of a liquid electrolyte.

In general, the names of the corrosion mechanisms are determined by the In general, the names of the corrosion mechanisms are determined by the most abundant dominant corrosion products. For example:most abundant dominant corrosion products. For example: Oxidation implies oxides,Oxidation implies oxides, Sulfidation implies sulfides,Sulfidation implies sulfides, Sulfidation/oxidation implies sulfides plus oxides, andSulfidation/oxidation implies sulfides plus oxides, and Carburization implies carbidesCarburization implies carbides

High temperature corrosion is a widespread High temperature corrosion is a widespread problem in various industries such as:problem in various industries such as:

power generation (nuclear and fossil fuel)power generation (nuclear and fossil fuel)

aerospace and gas turbineaerospace and gas turbine

heat treatingheat treating

mineral and metallurgical processingmineral and metallurgical processing

chemical processingchemical processing

refining and petrochemicalrefining and petrochemical

automotiveautomotive

pulp and paperpulp and paper

waste incinerationwaste incineration

During operation, blades and vanes of gas During operation, blades and vanes of gas turbines are subjected to high thermal stresses turbines are subjected to high thermal stresses and mechanical loads.and mechanical loads.

In addition, they are also attacked chemically by In addition, they are also attacked chemically by oxidation and/or high-temperature corrosion. oxidation and/or high-temperature corrosion. Only composite materials are able to meet such a Only composite materials are able to meet such a demanding spectrum of requirements; the base demanding spectrum of requirements; the base material provides the necessary mechanical material provides the necessary mechanical properties and coatings provide protection against properties and coatings provide protection against oxidation and corrosionoxidation and corrosion

Hot corrosionHot corrosion Hot corrosion may be defined as an accelerated corrosion, resulting Hot corrosion may be defined as an accelerated corrosion, resulting

from the presence of salt contaminants such as Na2SO4, NaCl, and from the presence of salt contaminants such as Na2SO4, NaCl, and V2O5 that combine to form molten deposits, which damage the V2O5 that combine to form molten deposits, which damage the protective surface oxidesprotective surface oxides

Hot corrosion occurs when metals are heated in the temperature Hot corrosion occurs when metals are heated in the temperature range 700–900°C in the presence of sulphate deposits formed as a range 700–900°C in the presence of sulphate deposits formed as a result of the reaction between sodium chloride and sulphur result of the reaction between sodium chloride and sulphur compounds in the gas phase surrounding the metals.compounds in the gas phase surrounding the metals.

At higher temperatures, deposits of Na2SO4 are molten (m.p. At higher temperatures, deposits of Na2SO4 are molten (m.p. 884°C) and can cause accelerated attack on Ni- and Co-based 884°C) and can cause accelerated attack on Ni- and Co-based superalloys. This type of attack is commonly called ‘hot corrosion’. superalloys. This type of attack is commonly called ‘hot corrosion’. Accelerated corrosion can also be caused by other salts, viz. Accelerated corrosion can also be caused by other salts, viz. vanadates or sulphates– vanadate mixtures and in the presence of vanadates or sulphates– vanadate mixtures and in the presence of solid or gaseous salts such as chloridessolid or gaseous salts such as chlorides

CHARACTERISTICS AND MECHANISM OF CHARACTERISTICS AND MECHANISM OF HOT CORROSIONHOT CORROSION

Hot corrosion can occur at high temperatures, Hot corrosion can occur at high temperatures, where the deposit is in the liquid state right where the deposit is in the liquid state right from the beginning, or the solid deposit turns from the beginning, or the solid deposit turns into liquid during exposure as a result of into liquid during exposure as a result of reaction with the environment.reaction with the environment.

These two types of hot corrosion processes are These two types of hot corrosion processes are termed astermed as

High Temperature Hot Corrosion (HTHC) or Type IHigh Temperature Hot Corrosion (HTHC) or Type I Low Temperature Hot Corrosion (LTHC) or Type II Low Temperature Hot Corrosion (LTHC) or Type II

respectivelyrespectively

TYPES OF OXIDE LAYERSTYPES OF OXIDE LAYERS High temperature oxidation usually results in formation of an High temperature oxidation usually results in formation of an

oxide layer on the surface of the oxidizing metal. Thin oxide oxide layer on the surface of the oxidizing metal. Thin oxide layers (commonly thinner than 3000 Å) are called layers (commonly thinner than 3000 Å) are called filmsfilms. . Thicker oxide layers (above 3000 Å) are called Thicker oxide layers (above 3000 Å) are called scalesscales.(1.0 Å = .(1.0 Å = 1010-10-10 m) Oxide scale may be composed of several layers of m) Oxide scale may be composed of several layers of different oxides. At the temperatures above 1050ºF (566ºC) different oxides. At the temperatures above 1050ºF (566ºC) iron scale consists of three layers: FeO (the layer adjacent to iron scale consists of three layers: FeO (the layer adjacent to iron), Feiron), Fe33OO44 (middle layer) and Fe (middle layer) and Fe22OO33 (surface layer). (surface layer).

Depending on their structures, the scales may be categorized Depending on their structures, the scales may be categorized into two groups: protective scales and non-protective scales.into two groups: protective scales and non-protective scales.

Protective scaleProtective scale prevents access of oxygen to the metal surface prevents access of oxygen to the metal surface due to non-porous continuous structure of the oxide layer.due to non-porous continuous structure of the oxide layer.

Non-protective scaleNon-protective scale has loose porous structure providing free has loose porous structure providing free access of oxygen to the underlaying metal.access of oxygen to the underlaying metal.

The scales type may be determined by the The scales type may be determined by the Pilling- Bedworth rulePilling- Bedworth rule::

The scale is protective (adherent and non-porous) if the volume of the The scale is protective (adherent and non-porous) if the volume of the

oxide is not less than the volume of metal, from which the oxide was oxide is not less than the volume of metal, from which the oxide was

formed.formed.

The scale is non-protective (porous) if the volume of the oxide is less than The scale is non-protective (porous) if the volume of the oxide is less than

the volume of metal, from which the oxide was formed.the volume of metal, from which the oxide was formed.

Oxides with volume much greater (twice and more) than the volume of Oxides with volume much greater (twice and more) than the volume of metal, from which the oxide was formed cause developing compressive metal, from which the oxide was formed cause developing compressive stresses. The stresses may lead to cracking and spalling of the scale, which stresses. The stresses may lead to cracking and spalling of the scale, which result in faster penetration of oxygen to the metal surface.result in faster penetration of oxygen to the metal surface.

Protective oxidesNon protective

oxides

Be  1.59 K  0.45

Cu  1.68 Ag  1.59

Al  1.28 Cd  1.21

Cr  1.99 Ti  1.95

Mn  1.79 Mo  3.40

Fe  1.77 Hf  2.61

Co  1.99 Sb  2.35

Ni  1.52 W  3.40

Pd  1.60 Ta  2.33

Pb  1.40 U  3.05

Ce  1.16 V  3.18

Effect of oxide structure on oxidationEffect of oxide structure on oxidation

Most of oxides are not ideal. Their compositions Most of oxides are not ideal. Their compositions differ from the stoichiometric ratios. The oxide differ from the stoichiometric ratios. The oxide structure may be divided into two groups:structure may be divided into two groups: n-type oxidesn-type oxides with anion deficiency (ZnO, ZrO with anion deficiency (ZnO, ZrO22, MgO, , MgO,

AlAl22OO33, SiO, SiO22, SnO, SnO22, PbO, PbO22). ). Anion: negatively charged ion Anion: negatively charged ion

(oxygen).(oxygen). p-type oxidesp-type oxides with cation deficiency (NiO, CoO, FeO, with cation deficiency (NiO, CoO, FeO,

PbO, MnO, CuPbO, MnO, Cu22O). O). Cation: positively charged ion (metal).Cation: positively charged ion (metal).

Additions of alloying elements having a valencies, which differ from the Additions of alloying elements having a valencies, which differ from the valency of the base metal may effect on the oxidation rate if it is controlled by valency of the base metal may effect on the oxidation rate if it is controlled by diffusion:diffusion:

In In n-type oxidesn-type oxides.. Addition of higher valency cation (eg. addition of Al to Zn) results in Addition of higher valency cation (eg. addition of Al to Zn) results in

lowering the oxidation rate.lowering the oxidation rate. Addition of lower valency cation (eg. addition of Li to Zn) results in Addition of lower valency cation (eg. addition of Li to Zn) results in

increasing the oxidation rate.increasing the oxidation rate. In In p-type oxidesp-type oxides..

Addition of higher valency cation (eg. addition of Cr to Ni) results in Addition of higher valency cation (eg. addition of Cr to Ni) results in increasing the oxidation rate.increasing the oxidation rate.

Addition of lower valency cation (eg. addition of Ni to Cr) results in Addition of lower valency cation (eg. addition of Ni to Cr) results in lowering the oxidation rate. lowering the oxidation rate.

Kinetics laws of oxidationKinetics laws of oxidation

Three basic kinetic laws have been used to Three basic kinetic laws have been used to characterize the oxidation rates of pure metals.characterize the oxidation rates of pure metals.

Parabolic rate lawParabolic rate law Logarithmic rate lawLogarithmic rate law Linear rate lawLinear rate law

SUPER ALLOYSSUPER ALLOYS

The different materials used in a Rolls-Royce The different materials used in a Rolls-Royce jet engine. In blue, titanium is ideal for its jet engine. In blue, titanium is ideal for its strength and density, but not at high strength and density, but not at high temperatures, where it is replaced by nickel-temperatures, where it is replaced by nickel-based superalloys (red). In orange: steel used based superalloys (red). In orange: steel used for the static parts of the compressor. for the static parts of the compressor.

SINGLE CRYSTAL SOLIDIFICATIONSINGLE CRYSTAL SOLIDIFICATION