Materials for Civil and Construction Engineers CHAPTER 2. Nature of Materials

Transcription

1 Materials for Civil and Construction Engineers CHAPTER 2 Nature of Materials

2 Bonds 1. Primary Bond: forms when atoms interchange or share electrons in order to fill the outer (valence) shells like noble gases. Types: a) Ionic b) Covalent c) Metallic 2. Secondary Bond: forms from an imbalanced electric charge among atomic arrangements. 2

3 Secondary Bonds Dipolar electrostatic attraction and are much weaker than primary bonds. Dipolar molecules (e.g., H 2 O) are asymmetric and have one side positive while the other pole is negative. van der Waals force. Hydrogen bonds are a stronger type of secondary bond because hydrogen atoms easily form dipoles and can bond this way in chains with many other atoms.

4 Materials Classification by Bond Type Metals metallic bonds between atoms with 1, 2, or valence electrons steel, iron, aluminum, etc. Inorganic Solids covalent and ionic bonds between atoms with 5, 6, or 7 valence electrons Ceramics Portland cement concrete, bricks, diamond, glass, aggregates (rock) Organic Solids long molecules of covalent hydrogen-carbon molecules with secondary bonds between chains hydrocarbons asphalt, plastics, wood 4

5 Metallic Materials Crystal Lattice Structure Lattice repeating pattern of atoms -D geometric pattern Unit Cell smallest repeating unit Grain Structure collection of unit cells 5

6 -D Lattice Structures BCC 14 possible -D lattice structures Three common ones: body center cubic (BCC) FCC face center cubic (FCC) hexagonal close pack (HCP) HCP 6

7 Body Centered Cubic Face Centered Cubic each corner each corner center of lattice center of faces 9 atoms 14 atoms Hexagonal Close Pack each corner center top and bottom face center plane 17 atoms 7

8 Equivalent Number of Atoms in Unit Cell Nine atoms but corner atoms are shared BCC Number of equivalent atoms Center atom 1 Corner atoms 8x(1/8) 1 Total eq. atoms 2 Corner atoms shared with seven other cells Each corner atom contributes 1/8 to the equivalent atom count Number of equivalent atoms BCC 2 FCC 4 HCP 6 8

9 Atomic Packing Factor Volume of unit cell occupied by the atoms APF Vol FCC radius of atom: r 4r of Vol atoms in of unit unit cell cell Volume of atoms in unit cell, V a 4 r V Vsphere Va a n V sphere Volume of unit cell, V c a 2 2 r V c 2 2 r n 4 r Length of side, a No. eq. atoms, FCC n=4 APF n r r

10 Density na V c N A Where, = density n = number of equivalent atoms in unit cell A = atomic mass (gram/mole) V c = volume of unit cell N A = Avogadro s number (6.02 x 10 2 atoms/mole) 10

11 Imperfect World Perfect lattice structures only exist under ideal conditions and small quantities of material. Defects Point Line Area Volume 11

12 Point Defects in Crystalline Structure Self interstitial Interstitial impurity atom Substitutional impurity atom Vacancy 12

13 Line Defects 1

14 Plastic Deformations Along a Slip Plane Shear stresses Shear stresses 14

15 Grain Development As molten metal cools atoms loose energy and form together into lattice structures. Multiple nuclei develop creating grains. Grain Boundaries 1. Perfect grain growth 2. Grain starts at a new nuclei. Grains grow together with perfect alignment (coherent boundary) 4. Grains grow together with imperfect alignment (coherent strain boundary) 5. Grains grow together with imperfect alignment (semicoherent boundary) 6. Grains grow together with skewed alignment (incoherent boundary) 15

16 Alloys Alloys have one or more compounds dissolved in a metal Steel is an alloy of iron and carbon but frequently contains chromium, copper, nickel, phosphorous, etc. This is only possible if the different materials have compatible crystal structures Interstitial atoms fit between the metal atoms Must have an atomic radius less than 60% of the host metal Can dissolve only about 6% into the host Substitutional atoms take the place of host atoms in the lattice If the atoms are similar enough, the compounds can mix easily 16