AE 1354 - HIGH TEMPERATURE MATERIALS

1. AE 1354 - HIGH TEMPERATURE MATERIALS UNIT 1

2. UNIT 1 CREEP

4. CREEP Definition

5. 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.

6. Creep

7. 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

8. 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.

10. Figure Showing slip of an edge dislocation

11. 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.

12. Evidence for grain boundary sliding

15. Mechanisms of Creep

16. Classical creep curve

19. 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

20. 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

21. 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

22. 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

23. In steam turbine power plants, steam pipes carry superheated vapour under high temperature (1050�F/565.5�C) and high pressure often at 3500 psi (24.131 MPa) or greater. In a jet engine temperatures may reach to 1000�C, 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.

24. 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.

26. 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

40. Unit III Fracture

59. Unit IV Oxidation and corrosion

60. 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).

62. 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.

63. 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.

64. 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

65. 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:

66. 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

67. 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

68. 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

69. 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.

70. 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

74. 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

75. Combat of hot corrosion

76. Unit V Super alloys and other Material

77. Introduction

83. 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

87. Alloy designations for some common superalloys

90. 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

91. 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

92. 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.

93. TCP phase

94. 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

96. 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