AIRCRAFT MATERIALS AND PROCESSES

1. AIRCRAFT MATERIALS AND PROCESSES AERONAUTICAL ENGINEERING

2. MATERIAL • Material is synonymous with substance, and is anything made of matter – hydrogen, air and water are all examples of materials • The basis of materials science involves relating the desired properties and relative performance of a material in a certain application to the structure of the atoms and phases in that material through characterization. • 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 been processed into its final form. • The manufacture of a perfect crystal of a material is currently physically impossible. Instead materials scientists manipulate the defects in crystalline materials such as precipitates, grain boundaries, interstitial atoms, vacancies or substitutional atoms, to create materials with the desired properties.

3. CRYSTALLOGRAPHY What is a crystal? In materials science a crystal is a solid  substance in which the atoms,  molecules  or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions - length, width and height. The process of forming a crystalline structure from a fluid or from materials dissolved in the fluid is often referred to as crystallization. Various types of crystal structures in interest SC BCC FCC HCP

4. SIMPLE CUBIC STRUCTURE (SC) • • 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)

5. BODY CENTERED CUBIC STRUCTURE (BCC) • 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.

6. FACECENTEREDCUBICSTRUCTURE (FCC) • 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.

7. HEXAGONAL CLOSE-PACKED STRUCTURE (HCP) Ideally, c/a = 1.633 for close packing However, in most metals, c/a ratio deviates from this value

9. Some metals & their crystal structures

10. CRYSTAL DEFECTS A perfect crystal, with every atom of the same type in the correct position, does not exist. All crystals have some defects. Defects contribute to the mechanical properties of metals There are basic classes of crystal defects: Point defects, which are places where an atom is missing or irregularly placed in the lattice structure. Point defects include lattice vacancies, self-interstitial atoms, substitution impurity atoms, and interstitial impurity atoms Linear defects, which are groups of atoms in irregular positions. Linear defects are commonly called dislocations. Planar defects, which are interfaces between homogeneous regions of the material. Planar defects include grain boundaries, stacking faults and external surfaces.

11. POINT DEFECTS A self interstitial atom is an extra atom that has crowded its way into an interstitial void in the crystal structure. 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 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, zinc atoms with a radius of 0.133 nm have replaced some of the copper atoms, which have a radius of 0.128 nm. 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 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 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 common, especially at high temperatures when atoms are frequently and randomly change their positions leaving behind empty lattice sites. In most cases diffusion (mass transport by atomic motion) can only occur because of vacancies

13. LINEAR DEFECTS 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 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 much smaller than the theoretical strength of the forces that are holding the metal atoms together. There are two basic types of dislocations, the edge dislocation the screw dislocation. EDGE DISLOCATIONS The edge defect can be easily visualized as an extra half-plane of atoms in a lattice. The dislocation is called a line defect because the locus of defective points produced in the lattice by the dislocation lie along a line. 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 dislocation line. Dislocation motion is analogous to movement of a caterpillar

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

16. Planar Defects in Solids • One case is a twin boundary (plane) • Essentially a reflection of atom positions across the twin plane. • Stacking faults • For FCC metals an error in ABCABC packing sequence • Ex: ABCABABC Adapted from Fig. 5.14, Callister & Rethwisch 3e. 16

22. GRAIN BOUNDARY CONCEPT 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. Each of the light areas is called a grain, or crystal, which is the region of space occupied by a continuous crystal lattice. 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. 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 diameters    wide. 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 strength and tends to increase ductility.   A larger grain size is preferred for improved high-temperature creep properties.  

23. Some of the more important physical and chemical properties from an engineering material standpoint will be discussed in the following sections. Phase Transformation Temperatures Density Specific Gravity Thermal Conductivity Linear Coefficient of Thermal Expansion Electrical Conductivity and Resistivity Magnetic Permeability Corrosion Resistance

24. You should be familiar with the following terms which you would have studied in lower classes. • Engineering stress • Engineering strain • True stress • True strain • Yield strength • Yield point • Ultimate point And some basic definitions related to strength of materials

25. What is meant by loading a material? • 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 conditions. • There are five fundamental loading conditions; tension, compression, bending, shear, and torsion. • 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. • 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 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 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. • 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 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 actually occurs.

27. PROBLEMS FACED BY MATERIALS OPERATED AT ELEVATED TEMPERATURES

28. The mechanical strength of metals decreases with increasing temperature and the properties become much more time dependent. • In the past the operating temperatures in applications like steam power plant, chemical plant and oil refineries seldom exceeded 500oC, but since the development of the gas turbine in the 1940's successive designs have pushed this temperature up to typically 1000 oC. • Developments in high temperature alloys with improved high temperature strength and oxidation resistance have had to keep pace with these demands, and applications like rocket engines present greater problems. • At homologous temperatures of more than 0.5, creep is of engineering significance

29. HIGH TEMPERATURE >0.3TM • Creep • High temperature fracture • Corrosion • Fatigue • Embrittlement These are the factors affecting the functional or service life of components at elevated temperatures

32. CREEP

33. CREEP 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. The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. The temperature range in which creep deformation may occur differs in various materials. the effects of creep deformation generally become noticeable at approximately 30% 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 are endured such as nuclear power plants, jet engines and heat exchangers 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, 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 functionality to understand the creep deformation behavior of materials.

34. Creep data for general design use are usually obtained under conditions of constant uniaxial loading and constant temperature. Results of tests are usually plotted as strain versus time up to rupture. As indicated in the image, creep often takes place in three stages.  In the initial stage, strain occurs at a relatively rapid rate but the rate gradually decreases until it becomes approximately constant during the second stage.  This constant creep rate is called the minimum creep rate or steady-state creep rate since it is the slowest creep rate during the test. In the third stage, the strain rate increases until failure occurs.  Creep in service is usually affected by changing conditions of loading and temperature and the number of possible stress-temperature-time combinations is infinite.  While most materials are subject to creep, the creep mechanisms is often different between metals, plastics, rubber, concrete. High homologous temperatures (Tservice/Tmelting) Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Creep deformation is "time-dependent" deformation.

38. 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

39. DISLOCATION CREEP • Dislocations slip is hindered by obstacles such • (i) grain boundaries, • (ii) impurity particles, • (iii) the stress field around solute atoms in solution or • (iv) the strain fields of other dislocations.

41. DIFFUSIONAL CREEP

46. 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

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

48. A model for the formation of cracks due to grain boundary sliding The formation of wedge cracks during grain boundary sliding.