1. Manufacturing Engineering Technology in SI Units, 6th Edition PART I: Fundamental of Materials Their Behavior and Manufacturing Properties
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2. PART I: Fundamental of Materials Their Behavior and Manufacturing Properties
Materials were selected because they possess desired properties and characteristics for the intended functions
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3. PART I: Fundamental of Materials Their Behavior and Manufacturing Properties
Task of engineers becomes very challenging when ever- increasing variety of materials are now available
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4. PART I: Fundamental of Materials Their Behavior and Manufacturing Properties
Material behavior, properties and characteristics will help the engineer understand their relevance to the manufacturing processes
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5. Manufacturing Engineering Technology in SI Units, 6th Edition Chapter 1: The Structure of Metals
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6. Chapter Outline Introduction Types of Atomic Bonds
The Crystal Structure of Metals
Deformation and Strength of Single Crystals
Grains and Grain Boundaries
Plastic Deformation of Polycrystalline Metals
Recovery, Recrystallization, and Grain Growth
Cold, Warm, and Hot Working
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7. Introduction
Different metals behave differently under different situations
We would need to study the atomic structure of metals, which is the arrangement of the atoms within the metals
This allows us to predict and evaluate properties, thus we can make appropriate selections of metals
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8. Types of Atomic Bonds
All matter is made up of atoms containing a nucleus of protons and neutrons and surrounding orbits of electrons
Atom is called an ion
Excess of electrons results in a negatively charged atom, called an anion
Too few electrons results in a positively charged atom, called a cation
Multiple atoms combine to form molecules
Molecules held together by bonds through electron interaction
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9. Types of Atomic Bonds
Basic types of atomic attraction are primary or strong bonds
Ionic Bond
When one or more electrons are transferred from one material to another, a strong attractive force develops between the two ions
Example is sodium (Na) and chlorine (Cl) in table salt
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10. Types of Atomic Bonds Covalent Bond
Electrons in outer orbits are shared by atoms to form molecules
Examples are water (H2O) and nitrogen gas (N2)
Metallic bonds
Metals have few electrons in their outer orbits, thus cannot complete the outer shell of other self-mated atoms
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11. The Crystal Structure of Metals
Metals solidify from a molten state and the atoms arrange themselves into crystals
Atomic arrangement is called crystal structure or crystalline structure
Smallest group of atoms showing the lattice structure is known as a unit cell
3 basic atomic arrangements in metals:
Body-centered cubic (bcc)
Face-centered cubic (fcc)
Hexagonal close-packed (hcp)
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12. The Crystal Structure of Metals
Distance between the atoms is on the order of 0.1 nm
Models shown are known as hard-ball or hard-sphere models
Single crystal with many unit cells
Hard-ball model
Unit cell
BCC Structure
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13. The Crystal Structure of Metals
FCC Structure
Single crystal with many unit cells
Hard-ball model
Unit cell
HCP Structure
Single crystal with many unit cells
Unit cell
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14. The Crystal Structure of Metals
HCP crystals have the most densely packed configurations, followed by fcc and bcc
Arrangements can be modified by adding atoms of other metals known as alloying
BCC Structure
Single crystal with many unit cells
unit cell
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15. Deformation and Strength of Single Crystals
When a single crystal is subjected to an external force, it returns to its original shape when the force is removed (elastic deformation)
When force increased, the crystal does not return to its original shape when the force is removed (plastic deformation or permanent deformation)
2 basic mechanisms:
Slipping
Twinning
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16. Deformation and Strength of Single Crystals
Slipping is where one plane of atoms slide over an adjacent plane (slip plane) under a shear stress
A single crystal exhibits different properties when tested in different directions is called anisotropy
Twinning is where a portion of the crystal forms a mirror image of itself across the plane of twinning
Slip System
Combination of a slip plane and its direction of slip is known as a slip system
Metals with 5 or more slip systems are ductile
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17. Deformation and Strength of Single Crystals
Slip System
Bcc crystal has 48 slip systems, high b/a ratio, high shear stress, good strength and moderate ductility
Fcc crystal has 12 slip systems, low b/a ratio, low shear stress, moderate strength and good ductility
Hcp crystal has 3 slip systems, low slip, brittle at room temperature
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18. Deformation and Strength of Single Crystals: Imperfections in the Crystal Structure of Metals
Actual strength of metals is one to two orders of magnitude lower than theoretical calculations
Discrepancy is due to defects and imperfections in the crystal structure
They are categorized as:
Point defects
Linear defects
Planar imperfections
Volume imperfections
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19. Deformation and Strength of Single Crystals: Imperfections in the Crystal Structure of Metals
Types of defects in a single-crystal lattice
Types of dislocations in a single crystal
Edge dislocation
Screw dislocation
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20. Deformation and Strength of Single Crystals: Imperfections in the Crystal Structure of Metals
Dislocations
Dislocations are defects in the orderly arrangement of a metal’s atomic structure
2 types of dislocations: edge and screw
Edge dislocation is the progress of an earthworm
Screw dislocations are due to atomic planes forming a spiral ramp
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21. Deformation and Strength of Single Crystals: Work Hardening (Strain Hardening)
Dislocations can:
Become entangled and interfere with each other
Be impeded by barriers
Increase in the strength and the hardness of the metal is known as work hardening or strain hardening
Work hardening is used extensively for strengthening in metalworking processes at ambient temperatures
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22. Grains and Grain Boundaries
When molten metal solidify, crystals begin to form at various locations and have random orientations
These crystals then grows into a crystalline structure or grain
Number and size of the grains depends on the rate at which nucleation takes place
Surfaces that separate individual grains are called grain boundaries
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23. Grains and Grain Boundaries: Grain Size
Grain size influences the mechanical properties of metals
Grain size number, n, is related by
N = number of grains
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24. Grains and Grain Boundaries: Grain Size
Example 1.1
Assume that the ball of a ballpoint pen is 1 mm in diameter and has an ASTM grain size of 10. Calculate the number of grains in the ball.
Solution
The volume of the 1-mm-diameter ball is
Total number of grains is
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25. Grains and Grain Boundaries: Influence of Grain Boundaries
Grain boundaries influence the strength and ductility of metals as they interfere with the movement of dislocations
Depend on temperature, deformation rate, and the type and amount of impurities present along the grain boundaries
Creep is the elongation under stress over time, usually at elevated temperatures
Results from grain-boundary sliding
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26. Grains and Grain Boundaries: Influence of Grain Boundaries
Grain-boundary embrittlement: When exposed to certain low-melting-point metals
Liquid-metal embrittlement:
Embrittling element is in a liquid state
Solid-metal embrittlement: Embrittlement occur at temperatures below the melting point of the embrittling element
Hot shortness is caused by local melting of a constituent or of an impurity in the grain boundary at a temperature below the melting point of the metal itself
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27. Plastic Deformation of Polycrystalline Metals
When a polycrystalline metal with uniform equiaxed grains is subjected to plastic deformation at room temperature, the grains become deformed and elongated
During plastic deformation, the grain boundaries remain intact and mass continuity is maintained
Increase in strength depends on the degree of deformation (strain)
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28. Plastic Deformation of Polycrystalline Metals
Anisotropy (Texture)
A result of plastic deformation
Grains have elongated in one direction and contracted in the other
Metal has become anisotropic, where properties in the vertical direction are different from those in the horizontal direction
Anisotropy influences both mechanical and physical properties of metals
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29. Plastic Deformation of Polycrystalline Metals
Preferred Orientation
Also called crystallographic anisotropy
When metal is subjected to tension, the sliding blocks rotate toward the direction of the tensile force
Slip planes and bands tend to align themselves with the general direction of deformation
Mechanical Fibering
Results from the alignment of inclusions (stringers), impurities, and voids in the metal during deformation
Impurities will weaken the grain boundaries and become less ductile when tested in the vertical direction
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30. Recovery, Recrystallization, and Grain Growth
Properties of the metal can be recovered by heating the metal to a specific temperature range for a given period of time
A process called annealing
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31. Recovery, Recrystallization, and Grain Growth
3 events take place consecutively during the heating process:
Recovery: Occurs below recrystallization temperature, stresses in the highly deformed regions are relieved
Recrystallization: Within a certain temperature range, new equiaxed and strain-free grains are formed to replace older grains
Grain growth: Grains begin to grow in size and exceed the original grain size when temperature is raised further
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32. Cold, Warm, and Hot Working
Cold working is plastic deformation that is carried out at room temperature
Hot working is when deformation occurs above the recrystallization temperature
Warm working is carried out at intermediate temperatures
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