1. The document discusses the constitution of alloys and phase diagrams. It describes different types of solid solutions like substitutional and interstitial solutions and classifies phase diagrams as unary, binary, and ternary. 2. The iron-iron carbide equilibrium diagram is examined in detail. It identifies the various phases involved like ferrite, austenite, and cementite. Critical temperatures like A1, A2, A3 are defined. 3. The microstructure and properties of steels and cast irons are determined by their position in the iron-carbon phase diagram and the phases present at room temperature. Hypoeutectoid steels contain ferrite and pearlite while hyp

1. ENGINEERING MATERIALS AND
METALLURGY
1
By
Prof.S.Rajendiran
Mechanical Department
Ashoka Institute of Engineering and Technology

2. Constitution of alloys – Solid solutions, substitutional and
interstitial – Phase diagrams, isomorphous, eutectoid, eutectic,
peritectic, and peritectroid reactions – Iron – Iron carbide
equilibrium diagram – Classification of steel and cast iron,
microstructure, properties and applications.
UNIT – I CONSTITUTION OF ALLOYS
AND PHASE DIAGRAMS
2

3. CRYSTALLIZATION
1. Crystallization is the transition from the liquid to the solid state or
transformation of liquid phase to solid crystalline phase.
2. It occurs in two stages,
1. Nucleus formation - Nucleation is a process of formation of
stable crystallization centers of a new phase.
2. Crystal growth - atoms attaching themselves in certain preferred
directions, usually along the axes of a crystal.
3

4. 1. Nucleation may occur by either homogeneous or heterogeneous
mechanism.
2. Presence of foreign particles or other foreign substance in the liquid
alloy allows to initiate crystallization at minor value of under cooling
(few degrees below the freezing point). This is heterogeneous
nucleation.
3. If there is no solid substance present, under cooling of a hundred
degrees is required in order to form stable nuclei or “seeds” crystals,
providing homogeneous nucleation.
NUCLEATION
4

5. CRYSTAL GROWTH
1. Number of stable nuclei per unit volume of crystallizing alloy
determines the grain size.
2. The difference in potential energy between the liquid and solid states
is known as the latent heat of fusion.
3. When the temperature of the liquid metal has dropped sufficiently
below its freezing point, stable aggregates or nuclei appear
spontaneously at various points in the liquid.
4. These nuclei, which have now solidified, act as centers for further
5

6. CRYSTAL GROWTH
5. As cooling continues, more atoms tend to freeze, and attach
themselves to already existing nuclei or form new nuclei.
6. Each nucleus grows by the attraction of atoms from the liquid into its
space lattice.
7. Crystal growth continues in three dimensions, the atoms attaching
themselves in certain preferred directions, usually along the axes of a
crystal.
8. This gives rise to a characteristic treelike structure which is called
6

7. 7

8. MECHANISM OF SOLIDIFICATION
NUCLEUS
FORMATION
GROWTH OF CRYSTALLITES
GRAIN
BOUNDARIES
8

9. PROCESS OF CRYSTALLIZATION AND
DENDRITIC GROWTH
9

10. 10

11. 1. Since each nucleus is formed by chance, the crystal axes are pointed
at random and the dendrites will grow in different directions in each
crystal.
2. Finally, as the amount of liquid decreases, the gaps between the arms
of the dendrite will be filled and the growth of the dendrite will be
mutually obstructed by that of its neighbors.
3. This leads to a very irregular external shape called as grains.
4. The area along which crystals meet, known as the grain boundary,
is a region of mismatch.
GRAIN AND GRAIN
BOUNDARY
11

12. GRAIN BOUNDARY
FORMATION OF DENDRITES
IN MOLTEN METAL
12

13. POLYMORPHISM AND ALLOTROPY OF
METALS
1. Polymorphism is a physical phenomenon where a material may
have more than one crystal structure.
2. A material that shows polymorphism exists in more than one type of
space lattice in the solid state.
3. If the change in structure is reversible, then the polymorphic change
is known as allotropy. The prevailing crystal structure depends on
both the temperature and the external pressure.
13

14. 4. Polymorphism example is found in carbon: Graphite is the stable
polymorph at ambient conditions, whereas Diamond is formed at
extremely high pressures.
5. The best known example for allotropy is iron. When iron crystallizes
at 2800 o
F it is B.C.C. (δ -iron), at 2554 o
F the structure changes to
F.C.C. (γ -iron or austenite), and at 1670 o
F it again becomes
B.C.C. (α -iron or ferrite).
POLYMORPHISM AND ALLOTROPY OF
METALS
14

15. ALLOTROPIC BEHAVIOR OF
PURE IRON
15

16. CRYSTAL IMPERFECTIONS
(DEFECTS)
1. The perfectly regular crystal structure are called as IDEAL
CRYSTALS in which atoms are arranged in regular way.
2. In actual crystals, imperfections or defects are always present, which
affect the properties of crystals.
3. The crystallographic defects are classified as,
1. Point defects or Zero dimensional defects.
2. Line defects or One dimensional defects.
3. Surface defects or Plane defects or Two dimensional defects. 16

17. POINT DEFECTS
1. Vacancy – missing atom at a certain crystal lattice position.
2. Interstitial impurity atom – extra impurity atom in an interstitial
position.
3. Self-interstitial atom – extra atom in an interstitial position
4. Substitution impurity atom – impurity atom, substituting an atom
in crystal lattice.
5. Frenkel defect – extra self-interstitial atom, responsible for the
vacancy nearby 17

18. 18

19. LINE DEFECTS
Linear crystal defects are edge and screw dislocations.
1. Edge dislocation is an extra half plane of atoms “inserted” into the
crystal lattice. Due to the edge dislocations metals possess high
plasticity characteristics: ductility and malleability.
2. Screw dislocation forms when one part of crystal lattice is shifted
(through shear) relative to the other crystal part. It is called screw as
atomic planes form a spiral surface around the dislocation line.
19

20. LINE DEFECTS
EDGE DISLOCATION SCREW DISLOCATION
20

21. 1. Planar defect is an imperfection in form of a plane between uniform
parts of the material.
2. Important planar defect is a Grain boundary. Formation of a
boundary between two grains may be imagined as a result of rotation
of crystal lattice of one of them about a specific axis.
3. Tilt boundary – rotation axis is parallel to the boundary plane;
4. Twist boundary - rotation axis is perpendicular to the boundary
plane.
SURFACE DEFECTS
21

22. 5. Diffusion along grain boundaries is much faster, than throughout the
grains.
6. Grain boundaries accumulate crystal lattice defects (vacancies,
dislocations) and other imperfections, therefore they effect on the
metallurgical processes, occurring in alloys and their properties.
SURFACE DEFECTS
22

23. 1. A solid solution is simply a solution in the solid state and consists of
two kinds of atoms combined in one type of space lattice.
2. Any solution is composed of two parts a solute and a solvent.
3. The solute is the minor part which is dissolved and the solvent is the
major portion of the solution.
4. The amount of solute that may be dissolved by the solvent is
generally a function of temperature, which usually increases with
increasing temperature.
SOLID SOLUTIONS
23

24. There are three possible conditions of a solution,
1. Unsaturated – The solvent is dissolving less of the solute than it
could dissolve at a given temperature and pressure.
2. Saturated – The solvent is dissolving the limiting amount of solute.
3. Supersaturated – The solvent dissolves more than the solute than it
should under equilibrium conditions.
SOLID SOLUTIONS
24

25. SUBSTITUTIONAL SOLID
SOLUTION
If the atoms of the solvent metal and solute element are of similar
sizes (not more, than 15% difference), they form substitution solid
solution, where part of the solvent atoms are substituted by atoms of
the alloying. Example – Cu-Ni
TYPES
1.Ordered
2. Disordered
Cu
Ni
25NSK - AAMEC

26. INTERSTITIAL SOLID SOLUTION
1. If the atoms of the alloying elements are considerably smaller, than
the atoms of the matrix metal, interstitial solid solution forms,
where the matrix solute atoms are located in the spaces between
large solvent atoms.
Smaller Atoms
Hydrogen, Carbon,
Boron and Nitrogen

27. INTERSTITIAL SOLID SOLUTION
2. The interstitial solution of carbon in iron constitutes the basis of
steel hardening.
3. Very small amount of hydrogen introduced into steels during acid
picking (cleaning), plating or welding operations cause a sharp
decrease in ductility known as Hydrogen embrittlement.
27

28. PHASE DIAGRAM
1. Phase Diagram or Equilibrium Diagram or Constitutional Diagrams
indicate the structural changes due to variation of temperature and
composition.
2. The diagram is essentially a graphical representation of an alloy
system.
3. The phase diagram will show the phase relationships under
equilibrium conditions.
4. Phase diagrams are plotted with temperature in ordinate and alloy
28

29. F = CF = C −− P + 2P + 2For a system in equilibrium
FF −− C + P = 2C + P = 2
or
F – Degrees of Freedom
C – Number of Components
P – Number of Phases
= What you can control What the system controls−
F = C + 2 P−
Can control the no. of
components added and P
& T
System decided how many
phases to produce given
the conditions
Degrees of Freedom
GIBBS PHASE RULE
2 = Temperature and
Pressure
29

30. CLASSIFICATION OF PHASE
DIAGRAM
1. UNARY - One component phase diagram.
2. BINARY - Two component phase diagram.
3. TERNARY - Three component phase diagram.
30

31. UNARY PHASE DIAGRAM
The simplest case-Water.
Also known as a P-T diagram
Sign of [dP/dT] for:
Solid-Liquid
Liquid-Gas
Gas-Solid equilibria
31

32. BINARY PHASE DIAGRAM
Copper-Nickel equilibrium diagram
32

33. TERNARY PHASE DIAGRAM
33

34. PHASE DIAGRAM
1. SYSTEM – A system is a substance so isolated from its
surroundings that it is unaffected by these and is subjected to
changes in overall composition, temperature, pressure.
2. COMPONENT – A component is a unit of the composition variable
of the system. A system that has one component (Unary), two
(Binary), three (Ternary) and four (Quaternary).
3. PHASE – A phase is a physically and chemically homogeneous
portion of the system, separated from the other portions by a surface,
the interface.
34

35. CLASSIFICATION OF PHASE DIAGRAM
Phase diagrams are classified according to the relation of the components in
the liquid and solid states.
1. Components completely soluble in the liquid state,
1. And also completely soluble in solid state. (Isomorphous reaction)
2. But partly soluble in the solid state. (Eutectic reaction).
3. But insoluble in the solid state. (Eutectic reaction)
2. Components completely partially soluble in liquid state,
1. But completely soluble in the solid state.
2. And partly soluble in the solid state.
3. Components completely insoluble in the liquid state and completely insoluble in
the solid state. 35

36. 36

37. IRON – IRON CARBIDE
EQUILIBRIUM DIAGRAM
1. The following phases are involved in the transformation, occurring
with iron-carbon alloys:
1. L - Liquid solution of carbon in iron;
2. δ-ferrite – Solid solution of carbon in iron.
2. Maximum concentration of carbon in δ-ferrite is 0.09% at
2719 ºF (1493ºC) – temperature of the peritectic transformation.
3. The crystal structure of δ-ferrite is BCC (cubic body centered).
37

38. 4. Austenite – interstitial solid solution of carbon in γ-iron.
5. Austenite has FCC crystal structure, permitting high solubility of
carbon – up to 2.06% at 2097 ºF (1147 ºC).
6. Austenite does not exist below 1333 ºF (733ºC) and maximum
carbon concentration at this temperature is 0.83%.
7. α-ferrite – solid solution of carbon in α-iron.
8. α-ferrite has BCC crystal structure and low solubility of carbon – up
to 0.25% at 1333 ºF (733ºC).
9. α-ferrite exists at room temperature.
10. Cementite – iron carbide, intermetallic compound, having fixed
composition Fe3C. It is hard and brittle. 38

39. The following phase transformations occur with iron-carbon
alloys:
1. Alloys, containing up to 0.51% of carbon, start solidification with
formation of crystals of δ-ferrite. Carbon content in δ-ferrite
increases up to 0.09% in course solidification, and at 2719 ºF
(1493ºC) remaining liquid phase and δ-ferrite perform peritectic
transformation, resulting in formation of austenite.
2. Alloys, containing carbon more than 0.51%, but less than 2.06%,
form primary austenite crystals in the beginning of solidification and
when the temperature reaches the curve ACM primary cementite
stars to form.
39

40. 3. Iron-carbon alloys, containing up to 2.06% of carbon, are called
steels.
4. Alloys, containing from 2.06 to 6.67% of carbon, experience eutectic
transformation at 2097 ºF (1147 ºC). The eutectic concentration of
carbon is 4.3%.
5. In practice only hypoeutectic alloys are used. These alloys (carbon
content from 2.06% to 4.3%) are called cast irons. When
temperature of an alloy from this range reaches 2097 ºF (1147 ºC), it
contains primary austenite crystals and some amount of the liquid
phase. The latter decomposes by eutectic mechanism to a fine
mixture of austenite and cementite, called ledeburite.
40

41. 6. All iron-carbon alloys (steels and cast irons) experience eutectoid
transformation at 1333 ºF (733ºC). The eutectoid concentration of
carbon is 0.83%.
7. When the temperature of an alloy reaches 1333 ºF (733ºC),
austenite transforms to pearlite (fine ferrite-cementite structure,
forming as a result of decomposition of austenite at slow cooling
conditions).
41

42. CRITICAL TEMPERATURES
1. Upper critical temperature (point) A3 is the temperature, below
which ferrite starts to form as a result of ejection from austenite in
the hypoeutectoid alloys.
2. Upper critical temperature (point) ACM is the temperature, below
which cementite starts to form as a result of ejection from austenite
in the hypereutectoid alloys.
3. Lower critical temperature (point) A1 is the temperature ofthe
austenite-to-pearlite eutectoid transformation. Below this
temperature austenite does not exist.
4. Magnetic transformation temperature A2 is the temperature below42

43. Phase compositions of the iron-carbon alloys
at room temperature
1. Hypoeutectoid steels (carbon content from 0 to 0.83%) consist of
primary (proeutectoid) ferrite and pearlite.
2. Eutectoid steel (carbon content 0.83%) entirely consists of pearlite.
3. Hypereutectoid steels (carbon content from 0.83 to 2.06%) consist
of primary (proeutectoid)cementite and pearlite.
4. Cast irons (carbon content from 2.06% to 4.3%) consist of
proeutectoid cementite C2 ejected from austenite according to the
curve ACM , pearlite and transformed ledeburite (ledeburite in which
austenite transformed to pearlite).
43

44. CLASSIFICATION OF
STEEL
Classification of steels by composition
Carbon steels
1. Low carbon steels (C < 0.25%);
2. Medium carbon steels (C =0.25% to 0.55%);
3. High carbon steels (C > 0.55%).
44

45. DESIGNATION OF STEEL
American Iron and Steel Institute (AISI) together with Society of
Automotive Engineers (SAE) have established four-digit (with
additional letter prefixes) designation system:
SAE 1XXX
1. First digit 1 indicates carbon steel (2-9 are used for alloy steels);
2. Second digit indicates modification of the steel.
3. 0 - Plain carbon, non-modified
4. 1 - Resulfurized
5. 2 - Resulfurized and rephosphorized
6. 5 - Non-resulfurized, Mn over 1.0%
7. Last two digits indicate carbon concentration in 0.01%.
45

46. A letter prefix before the four-digit number indicates the steel
making technology:
1. A - Alloy, basic open hearth
2. B - Carbon, acid Bessemer
3. C - Carbon, basic open hearth
4. D - Carbon, acid open hearth
5. E - Electric furnace
Example: AISI B1020 means non modified carbon steel,
produced in acid Bessemer and containing 0.20% of carbon.
46

47. 1. Low alloy steels (alloying elements < 8%);
2. High alloy steels (alloying elements > 8%).
3. According to the four-digit classification SAE-AISI system:
4. First digit indicates the class of the alloy steel:
5. 2- Nickel steels;
6. 3- Nickel-chromium steels;
7. 4- Molybdenum steels;
8. 5- Chromium steels;
9. 6- Chromium-vanadium steels;
10. 7- Tungsten-chromium steels;
11. 9- Silicon-manganese steels.
12. Second digit indicates concentration of the major element in percents (1 means
1%).
13. Last two digits indicate carbon concentration in 0,01%.
Example: SAE 5130 means alloy chromium steel, containing 1% of chromium and
0.30% of carbon.
47

48. CLASSIFICATION OF STEELS
BY APPLICATION
Stainless steels
AISI has established three-digit system for the stainless steels:
1. 2XX series – chromium-nickel-manganese austenitic stainless steels;
2. 3XX series – chromium-nickel austenitic stainless steels;
3. 4XX series – chromium martensitic stainless steels or ferritic stainless
steels;
4. 5XX series – low chromium martensitic stainless steels.
48

49. TOOL AND DIE STEELS
Designation system of one-letter in combination with a number is accepted for tool
steels. The letter means:
1. W - Water hardened plain carbon tool steels;
2. O - Oil hardening cold work alloy steels;
3. A - Air hardening cold work alloy steels;
4. D -Diffused hardening cold work alloy steels;
5. S – Shock resistant low carbon tool steels;
6. T – High speed tungsten tool steels;
7. M - High speed molybdenum tool steels;
49

50. 50