1. CHAPTER 2 : STRUCTURE OF METALS

2. TOPIC  1) ATOMIC ARRANGEMENT  2) ATOMIC STRUCTURE  3) BONDING BETWEEN ATOMS  4) LATTICE STRUCTURE  5) CRYSTAL SYSTEM  6) SOLIDIFICATION OF METAL  7) STRUCTURE OF CASTING & INGOTS

3. ATOMIC ARRANGEMENT  3 Chemical elements are : 1) Solid 2) Liquids depends on temperature & pressure 3) gases Solid -Atom arranged closer to each other = solidification -Atom movement becomes difficult –only vibrate -Fix position -Low diffusion rate -Strong attraction Liquid -atom in contact with each other to form bonds= condensation -Limited atom movement -Still no orderly arrangement -Potential energy released Gaseous -particle in constant motion -no orderly arrangement -particles impact on walls gives rise to potential energy

4.  2 types of solid :  1) crystalline  2) amorphous crystalline -Atomic arrangement in orderly manner -Slow cooling during solidification -Small atomic distance -Posses elasticity -Ex : metals & their alloys,some polymers,some ceramics. amorphous-atomic arrangement in disorderly manner -Rapid cooling during solidification -Large atomic structure -Posses plasticity -Ex : glasses, plastic, wood, rubber

5. ATOMIC STRUCTURE  Atomic number (Z) = no of proton on nucleus  Atomic weight (A) = no of proton + no of neutron  No of proton= no of electron  EX : atomic no = 13 atomic weight = 26.98 ~ 27 no of neutron = 27 – 13 = 14 13 Al 26.98

6.  Nucleus is surround by orbital electron(s)  First orbit – 2 electrons  Second orbit – 8 electrons  Third orbit – 8 electrons  Valence electrons  Electrons that occupy the outermost orbit of an atom  Participate in bonding between atoms  Determine many physical & chemical properties of solids.

7. BONDING BETWEEN ATOMS  2 types of bonds : 1) primary bonds 2) secondary bonds  Primary bonds  An interatomic bond hold the atoms together  Strong and more stable  Secondary bond  An intermolecular bond in which weak forces hold the molecules together  Known as Van der Waals  Weak and less stable

8.  3 types of primary bonds :  1) ionic bonds  2) covalent bonds  3) metallic bonds

9. Ionic bonds covalent bondsmetallic bonds -One or more electrons are transferred from one atom to other -Produce from electrostatics force between ions of opposite charge (+, -) -An atom which loses the electron becomes positively charged (cation) -An atom which gains the electron becomes negatively charged (anion) - occur between metallic & non metallic elements. -Ex : Na & Cl atomsits valence electrons to Cl atom. - + -produce from sharing of electrons between adjacent atoms. -At least one atom donating one electron to be shared with adjacent atom -Bonding force produced from attraction between shared electrons. - Ex : in CH 4 molecule C + 4 [H] -Atom C has 4 valence electrons -Atom H has 1 valence electron -1 atom H can only accept 1 valence electron from atom C to complete its orbit -formed in metals & their alloys -atoms arranged closer to each other -each atoms supplies one or more electrons that drift throughout the entire metal. -valence electrons are not tied to any particular atoms. -free movement of valence electron forms. -act as a sea electrons – glue to hold ion cores together. -free valence electrons in metal – good conductors of electricity & heat. Na Cl Na + Cl H H C H H

10. LATTICE STRUCTURE  -regularity of crystalline solid – due to arrangement of structural unit  Formed by stacking up together identical small portion of crystal  Unit cell – smallest repeating unit of crystal  Atom arranged in pattern that repeats itself in 3 dimension to form solid that has crystal structure.  Lattice points of lines intersection

11. CRYSTAL SYSTEM  There are 7 basic shape possible for unit cell – 7 crystal system used to classify crystal  Each unit cell shape characterized by angles between edges & length of edges of unit cell  There are crystal systems that have more than 1 type of unit cell.  Ex : crystal system of cubic has 3 types of unit cell ( simple cubic, body-centered cubic and face- centered cubic)

12.  Different in arrangement of atom Simple cubicBody-centered cubicFace-centered cubic Atoms at each corner of unit cell -Atom at the center of unit cell -atom at each corner of unit cell -atoms at each surface of unit cell -atoms at each corner of unit cell

13.  Crystal structure of metals  Metals usually have 3 crystal structures: Crystal structure Face-centered cubic (FCC) Body-centered cubic (BCC) Hexagonal closed packed (HCP) Example Cu, Al,Ag, Au, γ-Fe Cr, W, α-Fe Mg, Ti, Zn, Cd Number of atoms in a unit cell Corner atom = 1 Face-centered atom = 3 Center atom = 0 Total of atom = 4 atoms Corner atom = 1 Face-centered atom = o Center atom = 1 Total of atom = 2 atoms Corner atom = 2 Face-centered atom = 1 Center atom = 3 Total of atom = 6 atoms

14. Atomic Packing Factor  Important characteristics of crystal structure  Show fraction of unit cell filled with atom APF = volume of atoms in a unit cell Volume of unit cell  APF for FCC (assume that an atom is a solid sphere) R = atomic radius A = cube edge length Volume of atoms in a unit cell : V = 4/3 π R 3

15.  FCC has 4 atoms V = (4/3 π R 3 ) V =4( 4/3 π R 3 ) V = (16/3 π R 3 ) a a 4R a a a a a 2 + a 2 = (4R) 2 2a 2 = 16R 2 a 2 = 8R 2 a = 2R √2 Volume per unit cell = a 3 =(2R √2) 3 = 8R 3 (2 √2) = 16R 3 √2

16. = APF = = 0.74 (16/3 π R 3 ) 16R 3 √2

17.  Calculation of density  Where : n = number of atoms in unit cell A =atomic weight V c = volume of unit cell N A =Avogadro's number (6.023 x 10 23 atom/mol) ρ = nA V c N A

18. example Cuprum (Cu) has: R = 0.128 nm FCC structure = 4 atoms in unit cell) A = 63.5 g/mol ρ ?ρ ? ρ = nA V c N A Nm cm 0.128 nm = 1.28 X 10 -8 cm ρ = (4) (63.5) 16 √2 ( 1.28 X 10 -8 ) 3 (6.023 x 10 23 ) ρ = 8.89 g/cm 3 Vc = 16R 3 √2

19. SOLIDIFICATION OF METAL  Liquid metal experiences hardening or cooling process at a fixed temperature called freezing temperature.  The graph below shows this behavior : Temp Freezing temp. under cooling B A Cooling curve for metal time

20. Graph AGraph B -Horizontal line of the curve shows the release of the latent heat at the freezing temperature. -Happens when there are impurities in the melt -Difficult for liquid metal to start the formation of crystal without impurity -first nucleus start to form at 0.1 – 100 ᵒc below freezing temperature. -Followed by secondary growth & temp rises back to freezing temp.

21. STRUCTURE OF CASTING & INGOTS  When liquid metal poured into mould it will become solid – called castings  Casting are fabricated by rolling, extrusion,forging – called ingots or blanks (ex : bar, rod)  Important to have a uniform fine grains  Coarse grains – weakness of materials  Shape & size of grains depends on : Number & distribution of nucleus Growth rate & direction of grains

22.  Pure metal – produce bigger grains  Both factors depends on :  Casting temperature  Thermal conductivity  Mould materials  Composition of liquid metal  Slow cooling cause :  Formation of a few nucleus  Resultant grains will be large

23.  Rapid cooling:  Formation of many nucleus  Resultant grains will be small  Ex : grain size of pressure die casting vs sand casting  In large ingots grains size varies from outside surface to the center – due to temp gradient.  During pouring,liquid in contact with cold mould wall – rapidly cooled.  Cold mould have chilling effect many nucleus form on mould wall Begin to growth in all direction Grains touching each other – chill crystal

24.  Mould warms-up - reduce chilling effect  Formation of nucleus retarded  Grains with direction close to direction of heat flow will grow faster (perpendicular to mould walls)  Lead to formation of elongated grain moving toward center of ingots – columnar crystal  Equiaxed crystal  Almost oriented in the center of ingots  Almost same dimension  Stopping growth of columnar crystal  Formed when ; Pouring temp is low Existence of impurities

25.  If pouring temp is high :  Columnar crystal will grow to the center of ingots  If pouring temp is too low ;  Produces an entirely equiaxed crystal  Liquid will be rapidly cooled

26. Tutorial 1  Refer to e-learning  www.elearning.kl.utm.my  Submit before 12/7/2012  Hand-writing