Manufacturing Engineering Technology in SI Units, 6th Edition PART I: Fundamental of Materials Their Behavior and Manufacturing Properties Presentation slide for courses, classes, lectures et al. Copyright © 2010 Pearson Education South Asia Pte Ltd

PART I: Fundamental of Materials Their Behavior and Manufacturing Properties Materials were selected because they possess desired properties and characteristics for the intended functions Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

Manufacturing Engineering Technology in SI Units, 6th Edition Chapter 1: The Structure of Metals Presentation slide for courses, classes, lectures et al. Copyright © 2010 Pearson Education South Asia Pte Ltd Copyright © 2010 Pearson Education South Asia Pte Ltd 5

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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) Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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) Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd

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 Copyright © 2010 Pearson Education South Asia Pte Ltd