Structure of materials Marerials Science and Metallurgy

1. Prf Balbirsingh Guron
balbirguronrcert@gmail.com
Materials Science & Metallurgy
BTMEC302 ESC11
Mechanical III Semester
DBATU
RCERT
Text Book
Engineering Metallurgy : Vinod S. Gorantiwar

2. Unit 1
Structure of Materials
Crystal structures
Indexing of lattice planes
Indexing of lattice directions
Imperfections in crystals -
point defects, line defects, surface &bulk defects
Mechanism of plastic deformation
Deformation of single crystal by slip
Plastic deformation of polycrystalline materials

3. Structure of Crystal
BONDS IN SOLIDS
Primary Bonds
Chemical Bonds
Secondary Bonds
Van Der Waals bonds
Strong inter-atomic bonds Weaker molecular bonds-
TYPES
Covalent or Homopolar
Electrovalent or Ionic
Metallic
TYPES
Dispersion
Dipole
Hydrogen
Materials Inter Atomic / Molecular
Forces
SOLIDS Very Strong
LIQUIDS Moderate
GASES Negligible

4. .
• Alloys & Metals have primary metallic bonds
• Binding Energy –
The force needed
to break the inter-atomic bond resistance,
so that atoms are infinitely separated

5. .
SOLIDS
Crysatlline Materials Amorphous Materials
Periodic arrangements of atoms
& have unit cells
Random arrangement of atoms
& no unit cells
Different physical properties in different
directions ( Anisotropy)
Same physical properties in all directions
( Isotropy)
Sharp melting point Not exact melting point
Atoms are closely packed, so are denser Less dense
Thermal, Mechanical, Electrical, Magnetic
properties can be changed as suitable by
changing the crystal structure , because
they are related
Physical properties cannot be changed
thus
Eg.
All metals, some ceramics, some plastics,
most minerals
Fe Au NaCl etc.
Eg.
Glass, Some ceramics, Polymers etc.

6. Crystalline Solids
Lattice - an interlaced structure or pattern

7. Unit Cell
Smallest regular shape in space,
basic building block,
When repeated,generates the whole crystal lattice
3 vectors a,b,c & 3angles α, β, ᵧ opposite to them

8. Metals crystallize into
BCC FCC or HCP

9. BCC Body Centered Cubic Structure
8 Atoms at all 8 corners, shared by 8 cubes,
All touch additional 1 atom at centre
Shape is cubic
Eg. α-Fe, δ-Fe, Cr, W, V, Mo, Na etc.

11. Calculation of (Rank)No.of atoms per unit cell for BCC
Rank = No. of atoms per unit cell for BCC = 2

12. Calculation of packing density for BCC
Packing Density = Volume of atoms / Volume of unit cell
= 0.68 or 68 %
It is the volume occupied by atoms out of volume of unit cell

13. FCC Face Centered Cubic Structure
8 Atoms at all 8corners, shared by 8 cubes,
additional 6 atom at each 6 faces, touches corner atoms
Eg. ϒ-Fe, Cu, Al, Au, Ag etc.

14. Calculation of(Rank)No.of atoms per unit cell for FCC
Rank = No. of atoms per unit cell for FCC = 4

15. Calculation of packing density for FCC
PackingDensity=Volume of atoms/Volume of unit cell
= 0.74 or 74%
It is the volume occupied by atoms out of volume of
unit cell

16. HCP Hexagonal Close Packed Structure

17. .
12 Atoms at all 12 corners
2 Atoms - one each at top &
bottom faces
3 additional Atoms at middle
layer touching all atoms
Eg. Cd, Zn, Mg, Co, Zr, Ti, Be…

18. Calculation of (Rank)No.of atoms per unit cell for HCP
Rank = No. of atoms per unit cell for FCC = 6

19. Calculation of packing density for HCP
PackingDensity=Volume of atoms/Volume of unit cell
= 0.74 or 74%
It is the volume occupied by atoms out of volume of
unit cell

20. MILLER INDICES
Indexing of lattice planes
• Crystal has various lattice / crystal planes
• Planes are parallel or intersecting
• Planes are designated by 3 INTIGER numbers in
bracket (hkl) – called Miller Indices
• Miller Indices denote orientation & direction of
an atomic plane in a crystal lattice
• Miller Indices are a symbolic vector
representation
• Defined as the
reciprocals (to get intigers) of the fractional
intercepts,
which the plane makes with the crystallographic
axes
Miller indices are represented by a set of 3 integer numbers

21. .

22. Example of the (111) plane

23. Example of the (101) plane
The plane
intercepts the a axis at one unit length
intercepts the c axis at one unit length
never intersects the b axis
The intercept to the b axis is infinity
The intercepts are 1, infinity, 1
The reciprocals are 1/1, 1/infinity, 1/1
The indices become (101)

24. Example of the (102) plane

25. Example of the (-102) plane

26. Examples of the (102) and (201) planes

27. .

28. MILLER INDICES
Indexing of crystal lattice directions

29. .
Polymorphism
Material exists in more than one type of space lattice,
in solid state, under different conditions
Allotropy
If this change is reversible , then it is called Allotropy
Changes in about 15 metals; & alloys are allotropic

30. Imperfections in crystals
Structural flaws, defects
Due to
• Impurity
• Alloying elements added to enhance properties
• Uneven cooling in casting
• Working(Forging,Turning,Shaping,Cutting,extrusion)

31. Point defect
Zero dimension defect
It is related to single atom
Occurs during atomic diffusion
TYPES
Vacancy ( Schottky )defect : Missing atom due to
*atom vibration in heat *Rapid cooling
*Plastic deformation
Interstitial defect : Smaller foreign atom of other
element occupies an interstitial site
Interstitialcy (Frenkel) defect : Lattice atom displaces
to an intertitial site
Substitutional defect : Same size foreign atom
occupies the lattice atom site

32. Point defects

33. Line Defects / Dislocations
(It is Lattice displacement of, dislocation or imperfection )
It is one dimension defect
It is Plastic ( Parmanent ) Mechanical deformation
Quantitative description is Burger’s vector ‘b’
Dislocations move by slip, under applied stress,
& terminate at the end of the crystal
Slip is easier on high atomic density planes & directions
TYPES
Edge Dislocation
The defect runs along the edge of the extra row of atoms
The Burger vector for edge location is perpendicular to the dislocation
line
Screw Dislocation
Spiral stacking of crystal planes around the dislocation line
The Burger vector for edge location is parallel to the dislocation line

34. Line Defects

35. Surface Defect
Occurs in
solidification, mechanical & thermal treatment
It is 2 dimensional defect,
along grain, tilt & twin boundaries, stacking fault,
surface of precipitate, external surface etc.
Along grain boundary,
atoms are bonded less regularly, resulting in defects
Grain boundary defects reduce upon heating,
as the grain size increases
while no. of grains decreases

36. Bulk ( volume ) defect
It is 3 dimensional defect
Pores cracks foreign inclusions & other phases
Occur in processing & fabrication
They are large & affect more on material properties

37. Elastic vs Plastic Deformation

38. Mechanism of
Plastic ( parmanent) deformation
In metals this is via the motion of linear defects at
the atomistic scale,
called dislocations
Motion of dislocations causes
irreversible shifting of atomic planes
called slip
Use of plastic deformation
For shaping different types of metal
by rolling, forging, drawing & extrusion

39. .
• Plastic deformation is a process in which the
size and shape of the object changes
due to external load or forces
• It is is a permanent deformation
which does not change after removal of
external load or forces
• It is is an irreversible process of deformation
• It causes a strain hardening effect
• It takes place due to the slip and twinning
mechanism

40. .

41. Deformation of single crystal by slip
Slip (plastic deformation ) is prominent in metals
Slip occurs when applied shear stress exceeds a critical value
Under tensile stress, the single crystal elongates slightly
A step appears on the surface
showing displacement of one part of the crystal i.e. slip
changing the geometry ( form ) of the sample i.e. deformation
Sections of the crystal slide relative to one another,
along parallel crystallographic planes, called slip planes
The slip direction lies in the slip plane
Slip plane has greatest atomic density
It is like a deck of cards when it is pushed from one end

42. .
• In this mechanism, one plane slides over
another plane
This does not affect the crystal structure of the
metal
No change in the arrangement of atoms
• Slip takes place in one particular slip direction
& in one particular slip plane

43. .
Slip or glide
An external force makes parts of the crystal lattice glide along,
relative to each other,
changing the material's geometry
along crystallographic planes and directions
Slip occurs by the passage of dislocations on slip planes
The glide planes have the greatest number of atoms per area
in close-packed directions (most atoms per length)
A slip system is
the set of symmetrically identical slip planes and associated slip directions
In these, dislocation is easy & causes plastic deformation
The magnitude and direction of slip are represented by the Burgers vector
A critical resolved shear stress is required to initiate a slip

44. Plastic deformation
of polycrystalline materials
• Slip (deformation) is complex in these
• Due to random crystallographic orientations
of the numerous grains
& the effect of neighboring atoms
the direction of slip
varies from one grain to another
• These materials are made up of a number of
small crystals or grains
• For each crystal
slip occurs along the slip system
that has the most favorable orientation
• Polycrystalline materials are stronger
& resist deformation

45. Assignment 1
• Explain Unit Cell, BCC, FCC, HCP with rank,
packing density
• Detail the system of Miller Indices with
examples
• What is polymorphism & allotropy
• Summarize the defects in crystal structures
• Explain Plastic deformation by Slip in single
crystalline materials
• How is slip different in polycrystalline
materials