AE 1354 - HIGH TEMPERATURE MATERIALS

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AE 1354 - HIGH TEMPERATURE MATERIALS

• UNIT 1

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UNIT 1

• CREEP

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CREEP

• Definition

Creep is the
tendency of a solid material to slowly move or
deform permanently under the influence of
stresses. 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 subjected to heat
for long periods, and near the melting point.
Creep always increases with temperature
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Creep

• The rate of this deformation is a function of the
  material properties, exposure time, exposure
  temperature and the applied structural load.
  Depending on the magnitude of the applied stress
  and its duration, the deformation may become so
  large that a component can no longer perform its
  function for example creep of a turbine blade
  will cause the blade to contact the casing,
  resulting in the failure of the blade.

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Creep
Stress
Strain
Time
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Mechanisms of Creep

• High rates of diffusion permit reshaping of
  crystals to relieve stress
• Diffusion significant at both grain boundaries
  and in the bulk
• High energy and weak bonds allow dislocations to
  climb around structures that pin them at lower
  temperature

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Mechanisms of Creep in metals

• Mechanisms of Creep in metals There are three
  basic mechanisms that can
• contribute to creep in metals, namely
• (i) Dislocation slip and climb.
• (ii) Grain boundary sliding.
• (iii) Diffusional flow.

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Figure Showing slip of an edge dislocation
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Grain Boundary sliding

• Grain Boundary sliding The onset of
  tertiary creep is a sign that structural damage
  has occurred in an alloy. Rounded and wedge
  shaped voids are seen mainly at the grain
  boundaries and when these coalesce creep rupture
  occurs. The mechanism of void formation involves
  grain boundary sliding which occurs under the
  action of shear stresses acting on the
  boundaries.

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Evidence for grain boundary sliding
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Mechanisms of Creep
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Classical creep curve
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• Creep deformation is important not only in
  systems where high temperatures are endured such
  as nuclear power plants, jet engines and heat
  exchangers, but also in the design of many
  everyday objects

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• Generally, the minimum temperature required for
  creep deformation to occur is 30-40 of the
  melting point for metals and 40-50 of melting
  point for ceramics
• creep can be seen at relatively low temperatures
  for some materials. Plastics and
  low-melting-temperature metals, including many
  solders, creep at room temperature as can be seen
  marked in lead and zinc

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Practical example

• An example of an application involving creep
  deformation is the design of tungsten lightbulb
  filaments. Sagging of the filament coil between
  its supports increases with time due to creep
  deformation caused by the weight of the filament
  itself. If too much deformation occurs, the
  adjacent turns of the coil touch one another,
  causing an electrical short and local
  overheating, which quickly leads to failure of
  the filament

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Other some examples

• Other examples
• Though mostly due to the reduced yield stress at
  higher temperatures, the Collapse of the World
  Trade Center was due in part to creep from
  increased temperature operation.
• The creep rate of hot pressure-loaded components
  in a nuclear reactor at power can be a
  significant design-constraint, since the creep
  rate is enhanced by the flux of energetic
  particles.
• Creep was blamed for the Big Dig tunnel ceiling
  collapse in Boston, Massachusetts that occurred
  in July 2006

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• In steam turbine power plants, steam pipes carry
  superheated vapour under high temperature
  (1050F/565.5C) and high pressure often at 3500
  psi (24.131 MPa) or greater.
• In a jet engine temperatures may reach to 1000C,
  which may initiate creep deformation in a weak
  zone.
• For these reasons, it is crucial for public and
  operational safety to understand creep
  deformation behavior of engineering materials.

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Stages of creep

• In the initial stage, known as primary creep, the
  strain rate is relatively high, but slows with
  increasing strain.
• The strain rate eventually reaches a minimum and
  becomes near-constant. This is known as secondary
  or steady-state creep. This stage is the most
  understood. The characterized "creep strain
  rate", typically refers to the rate in this
  secondary stage. The stress dependence of this
  rate depends on the creep mechanism.

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Mechanisms of creep The mechanism of creep
depends on temperature and stress. The various
methods are Thermally activated glide - e.g.,
via cross-slip Climb assisted glide - here the
climb is an enabling mechanism, allowing
dislocations to get around obstacles Climb -
here the strain is actually accomplished by climb
Grain boundary diffusion
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• General creep equation
• where e is the creep strain, C is a constant
  dependent on the material and the particular
  creep mechanism, m and b are exponents dependent
  on the creep mechanism, Q is the activation
  energy of the creep mechanism, s is the applied
  stress, d is the grain size of the material, k is
  Boltzmann's constant, and T is the absolute
  temperature

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Dislocation creep At high stresses (relative to
the shear modulus), creep is controlled by the
movement of dislocations. When a stress is
applied to a material, plastic deformation
occurs due to the movement of dislocations in
the slip plane. Materials contain a variety of
defects, for example solute atoms, that act as
obstacles to dislocation motion.
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Creep arises from this because of the phenomenon
of dislocation climb. At high temperatures
vacancies in the crystal can diffuse to the
location of a dislocation and cause the
dislocation to move to an adjacent slip plane.
By climbing to adjacent slip planes dislocations
can get around obstacles to their motion,
allowing further deformation to occur. Because
it takes time for vacancies to diffuse to the
location of a dislocation this results in time
dependent strain, or creep.
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Effect of stress on creep curves at constant
temperature
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Unit III

• Fracture

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Unit IV

• Oxidation and corrosion

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What is Oxidation ?

• Oxidation means the loss of electrons. The
  oxidation of a metal occurs when the metal loses
  one or more electrons, so that the atoms of the
  metal go from the neutral state and become a
  positively charge ion. This commonly results in
  the formation of a metal oxide (in the case of
  iron, that is known as rust).

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Pilling-Bedworth ratio

• In their 1923 paper "The oxidation of metals in
  high temperature" presented to the Institute of
  Metals, N. B. Pilling and R. E. Bedworth first
  correlated the porosity of a metal oxide with the
  specific density1. The Pilling-Bedworth ratio,
  (P-B ratio) R, of a metal oxide is defined as the
  ratio of the volume of the metal oxide, which is
  produced by the reaction of metal and oxygen, to
  the consumed metal volume
• M and D are the molecular weight and density of
  the metal oxide whose composition is
  (Metal)a(oxygen)b m, and d are the atomic weight
  and density of the metal.

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continued

• Pilling and Bedworth realized that, when R is
  less than 1, a metal oxide tends to be porous and
  non-protective because it cannot cover the whole
  metal surface. Later researchers found that, for
  excessively large R, large compressive stresses
  are likely to exist in metal oxide, leading to
  buckling and spalling. In addition to R, factors
  such as the relative coefficients of thermal
  expansion and the adherence between metal oxide
  and metal should also be favorable in order to
  produce a protective oxide.

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Kinetic laws of corrosion

• Three basic kinetic laws have been used to
  characterize the oxidation rates of pure metals.
  It is important to bear in mind that these laws
  are based on relatively simple oxidation models.
  Practical oxidation problems usually involve
  alloys and considerably more complicated
  oxidation mechanisms and scale properties than
  considered in these simple analyses

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• The parabolic rate law assumes that the diffusion
  of metal cations or oxygen anions is the rate
  controlling step and is derived from Fick's first
  law of diffusion. The concentrations of diffusing
  species at the oxide-metal and oxide-gas
  interfaces are assumed to be constant. The
  diffusivity of the oxide layer is also assumed to
  be invariant. This assumption implies that the
  oxide layer has to be uniform, continuous and of
  the single phase type. Strictly speaking, even
  for pure metals, this assumption is rarely valid.
  The rate constant, kp, changes with temperature
  according to an Arrhenius type relationship

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• where
• x is the oxide film thickness (or the mass gain
  due to oxidation, which is proportional to the
  oxide film thickness)
• t is time
• kp is the rate constant, directly proportional to
  the diffusivity of the ionic species that is rate
  controlling
• x0 is a constant

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• The logarithmic rate law is an empirical
  relationship with no fundamental underlying
  mechanism. This law is mainly applicable to thin
  oxide layers formed at relatively low
  temperatures, and therefore rarely applicable to
  high temperature engineering problems
• where
• ke is the rate constant
• c and b are constants

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Third law

• The linear rate law is also an empirical
  relationship that is applicable to the formation
  and build-up of a non-protective oxide layer.
• where
• kL is the rate constant

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Introduction to High Temperature Corrosion

• High temperature corrosion is a form of corrosion
  that does not require the presence of a liquid
  electrolyte. Sometimes, this type of damage is
  called "dry corrosion" or "scaling". The term
  oxidation is ambivalent since it can either refer
  to the formation of oxides or to the mechanism of
  oxidation of a metal, i.e. its change to a higher
  valence than the metallic state. Strictly
  speaking, high temperature oxidation is only one
  type of high temperature corrosion. In fact,
  oxidation is the most important high temperature
  corrosion reaction.

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continued

• In most corrosive high temperature environments,
  oxidation often participates in the high
  temperature corrosion reactions, regardless of
  the predominant mode of corrosion. Alloys often
  rely upon the oxidation reaction to develop a
  protective scale to resist corrosion attack such
  as sulfidation, carburization and other forms of
  high temperature attack. In general, the names of
  the corrosion mechanisms are determined by the
  most abundant dominant corrosion products

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

• power generation (nuclear and fossil fuel)
• aerospace and gas turbine
• heat treating
• mineral and metallurgical processing
• chemical processing
• refining and petrochemical
• automotive
• pulp and paper
• waste incineration

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Combat of hot corrosion
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Unit V

• Super alloys and other Material

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Introduction
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superalloy

• A superalloy is a metallic alloy which can be
  used at high temperatures, often in excess of 0.7
  of the absolute melting temperature. Creep and
  oxidation resistance are the prime design
  criteria. Superalloys can be based on iron,
  cobalt or nickel, the latter being best suited
  for aeroengine applications

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Alloy designations for some common superalloys
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Solid solution strengthening

• Solid solution strengthening is a type of
  alloying that can be used to improve the strength
  of a pure metal. The technique works by adding
  atoms of one element (the alloying element) to
  the crystalline lattice another element (the base
  metal). The alloying element diffuses into the
  matrix, forming a solid solution. In most binary
  systems, when alloyed above a certain
  concentration, a second phase will form. When
  this increases the strength of the material, the
  process is known as precipitation strengthening

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Precipitation hardening

• Precipitation hardening, also called age
  hardening, is a heat treatment technique used to
  strengthen malleable materials, including most
  structural alloys of aluminum, magnesium, nickel
  and titanium, and some stainless steels. It
  relies on changes in solid solubility with
  temperature to produce fine particles of an
  impurity phase, which impede the movement of
  dislocations, or defects in a crystal's lattice.
  Since dislocations are often the dominant
  carriers of plasticity, this serves to harden the
  material. The impurities play the same role as
  the particle substances in particle-reinforced
  composite materials

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Grain boundary strengthening

• Grain boundary strengthening (or Hall-Petch
  strengthening) is a method of strengthening
  materials by changing their average crystallite
  (grain) size. It is based on the observation that
  grain boundaries impede dislocation movement and
  that the number of dislocations within a grain
  have an effect on how easily dislocations can
  traverse grain boundaries and travel from grain
  to grain. So, by changing grain size one can
  influence dislocation movement and yield
  strength.

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TCP phase
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Embrittlement

• Embrittlement is a loss of ductility of a
  material, making it brittle. Various materials
  have different mechanisms of embrittlement.
• Hydrogen embrittlement is the effect of hydrogen
  absorption on some metals and alloys.
• Sulfide stress cracking is the embrittlement
  caused by absorption of hydrogen sulfide.
• Liquid metal embrittlement (LME) is the
  embrittlement caused by liquid metals.
• (MIE) is the embrittlement caused by diffusion
  of atoms of metal, either solid or liquid, into
  the material.
• Neutron radiation causes embrittlement of some
  materials, neutron-induced swelling, and buildup
  of Wigner energy. This is a process especially
  important for neutron moderators and nuclear
  reactor vessels

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Intermetallics

• Intermetallics are compounds that form when
  certain combinations of two or more metals are
  mixed together in certain proportions and react
  to produce a solid phase that is distinctively
  different from the constituent elements

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