1. The Structure of Metals
Chapter 1 Manufacturing and Engineering Technology

2. Atomic structure-arrangement of atoms within the metals
All matter is made up of atoms containing a nucleus of protons and neutrons and surrounding clouds, or orbits, of electrons.
Atoms can transfer or share electrons; in doing so, multiple atoms combine to form molecules. Molecules are held together by attractive forces called bonds
Atomic structure-arrangement of atoms within the metals

3. FIGURE 1.1 An outline of the topics described in Chapter 1.

4. Types of Atomic Bonds
Ionic bonds-one or more electrons from an outer orbit are transferred from one material to another (example Na+ and Cl- form salt)
Covalent bonds- electrons in outer orbits are shared by atoms to form molecules (H20 water). Typically low conductivity and high hardness
Metallic bonds-available electrons are shared by all atoms in contact. The resultant electron cloud provides attractive forces to hold the atoms together and results in generally high thermal and electrical conductivity.
Van Der Waals forces are weak attractions occurring between molecules.

5. The crystal structure of metals- when metals solidify from a molten state, the atoms arrange themselves into various orderly configurations called CRYSTALS.
Body-centered cubic (BCC) least dense
Face-centered cubic (FCC) more dense
Hexagonal close-packet (HCP) most dense
CRYSTAL STRUCTURE

6. FIGURE The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells.

7. FIGURE The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells.

8. FIGURE The hexagonal close-packed (hcp) crystal structure: (a) unit cell; and (b) single crystal with many unit cells.

9. At different temperatures the same metal may form different structures
The reason that metals form different crystal structures is to minimize the energy required to fill space
At different temperatures the same metal may form different structures

10. Allotropism or polymorphism (MEANING MANY SHAPES)
- the appearance of more than one type of crystal structure

11. Deformation & Strength of Single Crystals
Elastic deformation- a single crystal is subject to an external force, but returns to its original shape when the force is removed
Plastic deformation-a permanent deformation when the crystal does not return to its original shape
Deformation & Strength of Single Crystals

12. Two Basic Mechanisms for Plastic Deformations
Slipping of one plane of atoms over another adjacent plane (slip plane) under shear stress
Twinning- the second and less common mechanism of plastic deformation where a portion of the crystal forms a mirror image of itself across the plane of twinning
Definition: Anisotropy-a single crystal exhibits different properties when tested in different directions (ex. Woven cloth, plywood)
Two Basic Mechanisms for Plastic Deformations

13. FIGURE Permanent deformation of a single crystal under a tensile load. The highlighted grid of atoms emphasizes the motion that occurs within the lattice. (a) Deformation by slip. The b/a ratio influences the magnitude of the shear stress required to cause slip. (b) Deformation by twinning, involving the generation of a “twin” around a line of symmetry subjected to shear. Note that the tensile load results in a shear stress in the plane illustrated.

14. Point defects-vacancy, missing atoms, interstitial atom extra atom in the lattice or impurity foreign atom that has replaced the atom of pure metal
Linear defections called dislocations
Planar imperfections such as grain boundaries and phase boundaries
Volume or bulk imperfections-voids, inclusions, other phases, cracks
Imperfections in the crystal structure of metals explains why actual strength levels are one or two orders of magnitude lower than the theoretical calculations

15. FIGURE Schematic illustration of types of defects in a single-crystal lattice: selfinterstitial, vacancy, interstitial, and substitutional.

16. Dislocations-defects in the orderly arrangement of a metal’s atomic structure. Because a slip plane containing a dislocation requires less shear stress to allow slip than does a plane in a perfect lattice, dislocations are the most significant defects that explain the discrepancy between the actual and theoretical strengths of metals.

17. FIGURE Types of dislocations in a single crystal: (a) edge dislocation; and (b) screw dislocation.

18. FIGURE Movement of an edge dislocation across the crystal lattice under a shear stress. Dislocations help explain why the actual strength of metals is much lower than that predicted by theory.

19. Work Hardening (Strain Hardening)
Dislocations can become entangled and interfere with each other and be impeded by barriers such as grain boundaries, impurities, and inclusions in the material. The increased shear stress required to overcome entanglements and impediments results in an increase in overall strength and hardness of the metal and is known as work hardening or strain hardening. (Ex. Cold rolling, forging, drawing)
Work Hardening (Strain Hardening)

20. Grains and Grain Boundaries
When molten metal solidifies, crystals begin for form independently of each other. They have random and unrelated orientations. Each of these crystals grows into a crystalline structure or GRAIN.
The number and size of the grains developed in a unit volume of the metal depends on the rate at which NUCLEATION (the initial stage of crystal formation) takes place
Is this what I mean by grain?
Grains and Grain Boundaries

21. FIGURE Schematic illustration of the stages during the solidification of molten metal; each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other.

22. Rapid cooling – smaller grains
Slow cooling – larger grains
Grain boundaries – the surfaces that separate individual grains
Grain size- at room temperature a large grain size is generally associated with low strength, low hardness, and low ductility (ductility is a solid material's ability to deform under tensile stress)
Grain size is measured by counting the number of grains in a given area or by counting the number of grains that intersect a length of line randomly drawn on an enlarged photograph of the grains

23. TABLE Grain Sizes

24. Plastic deformation of polycrystalline metals
Cold working – a polycrystalline metal with uniform equiaxed grains is subject to plastic deformation at room temperature.
The grains become deformed and elongated.
The deformed metal exhibits higher strength because of the entanglement of dislocations with grain boundaries and with each other.
The higher the deformation, the stronger the metal becomes.
Strength is higher for metals with small grains because they have larger grain-boundary surface area per unit volume of metal hence more entanglements of dislocations
Plastic deformation of polycrystalline metals

25. FIGURE Plastic deformation of idealized (equiaxed) grains in a specimen subjected to compression (such as occurs in the forging or rolling of metals): (a) before deformation; and (b) after deformation. Note the alignment of grain boundaries along a horizontal direction; this effect is known as preferred orientation.

26. Metal properties are different in the vertical direction from those in the horizontal direction
It influences both mechanical and physical properties of metals
ANISOTROPY (texture)

27. FIGURE (a) Schematic illustration of a crack in sheet metal that has been subjected to bulging (caused, for example, by pushing a steel ball against the sheet). Note the orientation of the crack with respect to the rolling direction of the sheet; this sheet is anisotropic. (b) Aluminum sheet with a crack (vertical dark line at the center) developed in a bulge test; the rolling direction of the sheet was vertical. Courtesy: J.S. Kallend, Illinois Institute of Technology.

28. Annealing – heating metal to a specific temperature range for a given period of time
Recovery- stresses in the highly deformed regions of the metal piece are relieved. Subgrain boundaries begin to form

29. New equiaxed and strain-free grains are formed replacing the older grains. Between .3Tm and .5Tm where Tm is melting point of the metal on the absolute scale. Recrystallization temperature is defined as the temperature at which complete recrystallization occurs in approximately one hour.
Decrease density of dislocations
Lowers strength
Raises ductility
Recrystallization

30. temperature of metal increases further, the grain size grows and the size may exceed the original grain size
We grow lots of grain in Indiana, but this is not what is meant by grain growth
Grain growth

31. FIGURE Schematic illustration of the effects of recovery, recrystallization, and grain growth on mechanical properties and on the shape and size of grains. Note the formation of small new grains during recrystallization. Source: After G. Sachs.

32. TABLE 1.2 Homologous Temperature Ranges for Various Processes

33. Cold working- plastic deformation at room temperature
Hot working – deformation occurs above the recrystallization temperature
Warm working is carried out at intermediate temperatures, thus it is a compromise between cold working and hot working
Note: Deforming lead at room temperature is hot working since the recrystallization temperature of lead is about room temperature