Lecture 2: Relationships between Atomic Structure and Engineering Properties (read Chapters 2 and 3 in Young et al.) CE 3020 – Georgia Tech Spring 2007 Roberto Leon

From micro to macro behavior

From D. R. Askeland, The Science and Engineering of Materials, 3rd Ed., PWS, 1994

Overview Primary Bonds (strong) • Ionic • Covalent • Metallic Secondary Bonds (weak) • Van der Waals

Complete study of these interactions is beyond our scope → quantum mechanics

Material Characterization

From D. R. Askeland, The Science and Engineering of Materials, 3rd Ed., PWS, 1994

• Strength Limit State: yield/ultimate strength, elongation • Service: stiffness Slope of line is the stiffness Ultimate Mild steel

Yield

Elongation (%) = strain x 100 Gaylord, Gaylord and Stallmeyer, Design of Steel Structures, McGraw-Hill, 1992

Atomic Structure (Bohr Atomic Model) (electron sub-shells)

s=2 p =6 d = 10 f = 14

1s = -13.6eV 2s,p = -3.4 ev 3s,p = -1.5 eV

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5p, 6p, 7s, 5f, 6d .. Sr = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 Atomic number = 38 = [1s2 2s2 2p6 3s2 3p6 3d10 ] 4s2 4p6 5s2

Atomic Models • The simplified Bohr atomic model (a) assumes a fixed location of the electrons and a fixed energy level • To explain a number of atomic phenomena, a wavemechanical model with a given probabilistic distribution is now accepted (b) •A wave-mechanical model implies both wave-like and particle-like characteristics

From The Science and Design of Engineering Materials by Schaffer, Saxena, Antolovich, Sanders and Warner, McGraw-Hill, 1999

Same number of valance electrons (number of electrons in outer shell)

Elements in Groups VII and VIII readily gain electrons to form anions (-ve charge)

Elements in Groups I and II readily lose electrons to form cations (+ve charge)

Ionic Bonding

Na (11)

Cl (17)

Ionic Bonding NaCl

(+)

(-)

Cation (+)

Anion (-)

Covalent Bonding

+17

+17

Cl

Cl

“sharing of electrons”

Metallic Bonding

+2

+12

Mg

Mg

Metallic Bond (electron cloud) The mutual attraction to the cloud holds the metal ions together.

+2

+2

Good conductors. +2

+2

+2

Secondary or Weak Bonding (Van der Waals Bonds) • Permanent Dipole Bonds – Weak intermolecular bonds are formed between molecules which possess permanent dipoles. – A dipole exists in a molecule if there is asymmetry in its electron density distribution. • Fluctuating Dipole Bonds – Weak electric dipole bonding can take place among atoms due to an instantaneous asymmetrical distribution of electron densities around their nuclei. – This type of bonding is termed fluctuation since the electron density is continuously changing.

Secondary or Weak Bonding (Van der Waals Bonds)

Secondary or Weak Bonding (Hydrogen Bonds)

From The Science and Design of Engineering Materials by Schaffer, Saxena, Antolovich, Sanders and Warner, McGraw-Hill, 1999

Effect of Bond Type

Ionic bonding = brittle behavior

Metallic bonding = ductile behavior

Crystal formation

• • • •

Nucleation Growth: Grain formation Grain size Grain boundaries

Atomic Packing - Metals

Two possibilities for third layer (solid circle = ABCABC, open = ABAB) From Henkel and Pense, Structure and Properties of Engineering Materials, McGraw-Hill, 2002

Face-centered cubic (FCC)

• Each atom is in contact with 12 atoms surrounding it • Four atoms per unit cell = ½ of each face-centered atom plus 1/8 of each corner atom From Henkel and Pense, Structure and Properties of Engineering Materials, McGraw-Hill, 2002

Hexagonal close packed (HCP)

• Each atom is in contact with 12 atoms surrounding it From Henkel and Pense, Structure and Properties of Engineering Materials, McGraw-Hill, 2002

Body centered cube (HCP)

• Less densely packed, but still dense • Each atom is in contact with 8 atoms surrounding it From Henkel and Pense, Structure and Properties of Engineering Materials, McGraw-Hill, 2002

Crystal Structure • Atomic size – distance between atoms in a unit cell (if the side of • • • •

a fcc is “a”, the atomic size is “ √2a”) Coordination number = number of atoms in contact (fcc, hcp=12, bcc=8) Lattice – array of atoms repeated regularly throughout space Lattice number indicates planes of symmetry Some metals exist in more than one form and are called allotropic. Iron is bcc up to 910ºC, but becomes fcc from 910ºC to 1400ºC before returning to bcc

Lattice planes

(two unit cells of a cubic lattice, showing only corner atoms) From Henkel and Pense, Structure and Properties of Engineering Materials, McGraw-Hill, 2002

Ionic Structures

Based on fcc, but two atoms each centered at ½ a From Henkel and Pense, Structure and Properties of Engineering Materials, McGraw-Hill, 2002

Covalent Structures

Diamond = Four bonds per atom From Henkel and Pense, Structure and Properties of Engineering Materials, McGraw-Hill, 2002

Covalent Structures

Olivene = Mg2SiO4 From Henkel and Pense, Structure and Properties of Engineering Materials, McGraw-Hill, 2002

Grain Structure • Metals are almost always crystalline, but

polycrystalline • Grouped in structures having 103 to 108 atomic diameters • The axes of different crystals area aligned at random directions • Since they do not “fit” together neatly, a transition structure must exist at boundaries – highly disorganized atomic structures with high entropy = surface tension

Grain Structure

Tetrakaidecahedra = Angles at 120º and 109.28º From Henkel and Pense, Structure and Properties of Engineering Materials, McGraw-Hill, 2002

Grain Structure

From The Science and Design of Engineering Materials, by Schaffer, Saxena, Antolovich, Sanders and Warner, McGraw-Hill, 1999

Effect of Atomic Structure

*J. F. Shackelford, Introduction to materials science for engineers, 1985

Effect of Atomic Structure

*J. F. Shackelford, Introduction to materials science for engineers, 1985

Effect of Atomic Structure The ductility (or deformation capacity is directly related to the type and strength of bonds

*J. F. Shackelford, Introduction to materials science for engineers, 1985