MATERIALS ENGINEERING
MT30001
3‐0‐0
Offered by:
Metallurgical & Materials Engineering Dept.
Instructors:
Prof. Siddhartha Roy
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Instructor’s contact information
Prof. Siddhartha Roy
Email: siddhartha@metal.iitkgp.ernet.in
Office: First floor of Metallurgical & Materials
Engineering Department, IIT Kharagpur
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Concept of alloying
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Content of this course
Definition of an alloy
Alloy classification
Order‐disorder transformation
Textbooks referred to:
Physical Foundations of Materials Science – G. Gottstein
Introduction to Physical Metallurgy – S. H. Avner
Majority of the images in this course have been collected
from different textbooks and scientific documents available
in internet. They are not from my own research and have
been used solely here for teaching purpose
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Definition of an alloy
An alloy is a material that has metallic characteristics and is
composed of at least two or more chemical elements, of
which one is a metal.
An alloy system made up of only two elements is known
as binary alloy.
Alloy system made up of three elements is known as
ternary alloy.
Frequently an alloy consists several elements
Examples of technologically important alloys
Steel is primarily an alloy of iron and carbon
Brass is primarily an alloy of copper and zinc
Bronze is primarily an alloy of copper and tin
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Why do alloying
Pure metals are often very soft. Alloying enhances the
strength and hardness of metals.
Melting point of pure metals is high. Alloying decreases
the melting poing so that metals become more easily
fusible for processing purposes.
Many metals are very reactive in pure form and hence
corrodes very easily. Alloying can enhance the corrosion
resistance of the metal, thereby enhanching life of the
component.
Alloying can sometimes alter the colour of the metal
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Alloy classification
Alloy
Homogeneous Mixture
Any
Solid solution Intermediate combination of
alloy phase solid phases
Substitutional Intermetallic
Interstitial Interstitial
Electron
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Alloy classification
Alloy
Homogeneous Mixture
Any
Solid solution Intermediate combination of
alloy phase solid phases
Substitutional Intermetallic
Interstitial Interstitial
Electron
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Alloy classification
Homogeneous alloy
A homogeneous alloy consists of a single phase
An alloy can be homogeneous only if it is a solid solution
or an intermediate alloy phase
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Alloy classification
Mixture
An alloy in mixture form is composed of any combination
of solid phases each having distinct composition and
properties; e.g.:
Mixture of two pure metals
Mixture of two solid solutions
Mixture of two compounds
Mixture of a metal and a solid solution
The properties of a heterogeneous mixture are sensitive
to the way it is processed.
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Alloy classification
Alloy
Homogeneous Mixture
Any
Solid solution Intermediate combination of
alloy phase solid phases
Substitutional Intermetallic
Interstitial Interstitial
Electron
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Intermediate alloy phase
Intermediate alloy phases are phases whose chemical
compositions are intermediate between the two pure
phases and generally have crystal structures different from
the pure elements.
Stoichiometric
They have a fixed composition and are represented by a
vertical line in phase diagram
Nonstoichiometric
They have a range of compositions and are represented by
an area in the phase diagram
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Stoichiometric compound
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Nonstoichiometric compound
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Intermetallic compounds
Generally formed between chemically dissimilar metals
following the rules of chemical valence.
They generally have ionic and/or covalent bonding and
their properties are non‐metallic
They are typically very hard and brittle
Examples of intermetallic compounds
Mg2Pb, Mg2Sn, Cu2Se etc.
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Interstitial compounds
These are formed between the metallic elements (like Ti,
Ta, W, Fe etc.) and five relatively small elements (C, H, O,
N and B).
The compounds are metallic, very high melting points,
extremely hard and brittle
Examples of intermetallic compounds
TiC, TaC, Fe3C, W2C etc.
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Electron compounds
They exist at definite ratios of valence electrons to atoms
Many electron compunds have high ductility and low
hardness
Examples of electron compounds:
Ratio of valence electrons to atoms Examples
3:2 AgCd, Cu3Al, AgZn etc.
7:4 AgCd3, Cu3Si etc.
21:13 Cu9Al4, Au5Zn8 etc.
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Alloy classification
Alloy
Homogeneous Mixture
Any
Solid solution Intermediate combination of
alloy phase solid phases
Substitutional Intermetallic
Interstitial Interstitial
Electron
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Solid solution
A solid solution is formed when as the solute atoms are
added to the host material (i.e. solvent), the crystal
structure is maintained and no new structures are formed
Characteristics
The allowable amount of solute dissolvable in the
solvent is a function of temperature and generally
increases with increase in temperature.
The solute is generally more soluble in the liquid state
than in the solid state.
Most solid solutions solidify over a range of
temperatures
Solid solutions can be substitutional or interstitial
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Solid solution
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Substitutional solid solution
In this type of solution the atoms of the solute substitute
for the atoms of the solvent in the lattice structure of the
solvent.
Example: Gold‐silver alloy
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Factors affecting solid solubility
William Hume‐Rothery proposed a set of basic factors
(widely known as Hume‐Rothery rules) that control the
range of solubility in alloy systems
a) Crystal structure factor
Complete solid solubility of two elements is never attained
unless the elements have the same type of crystal lattice
structure
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Factors affecting solid solubility
William Hume‐Rothery proposed a set of basic factors
(widely known as Hume‐Rothery rules) that control the
range of solubility in alloy systems
b) Relative size factor
Appreciable quantities of a solute may be accomodated to
form substitutional solid solution only when the difference
in atomic radii between the two atom types is less than
15%.
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Factors affecting solid solubility
William Hume‐Rothery proposed a set of basic factors
(widely known as Hume‐Rothery rules) that control the
range of solubility in alloy systems
c) Electronegativity factor
The greater the difference in electronegativities of the two
metals, the more restricted will be the formation of solid
solutions and the greater is the tendency for a compound
formation.
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Factors affecting solid solubility
William Hume‐Rothery proposed a set of basic factors
(widely known as Hume‐Rothery rules) that control the
range of solubility in alloy systems
d) Valency factor
Complete solubility occurs when the solvent and solute
have the same valency. A metal of higher valency is more
likely to dissolve in a metal of lower valency.
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Interstitial solid solution
Interstitial solid solutions are formed, when the atoms of
small atomic radii (these are C, N, H, O, B) occupy the
spaces or interstices of the lattice structure of the larger
solvent atoms.
Example: Steel is an alloy of carbon (interstitials) in iron
(solvent)
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Order‐disorder transformation
Ordinarily in substitutional solid solutions the solute and
solvent atoms are distributed randomly in the lattice
structure Disordered structure
Some random solutions if cooled slowly undergo an
atomic rearrangement, whereby solute and solvent
atoms occupy specific positions in the lattice structure
Ordered structure or superlattice
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Order‐disorder transformation
Usually the maximum amount of ordering occurs at a
simple atomic ratio of two metals ordered phase is
usually given a chemical formula e.g. AuCu3
Thermal motion of atoms tend to destroy the ordering
degree of ordering as temperature
Temperature at which the ordered state completely
changes to a disordered state is called critical
temperature.
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Effect of ordering on properties
If the ordered phase has the same lattice structure as
the disordered phase, the effect on mechanical phase is
negligible
Formation of ordered
structure drastically
reduces the electrical
resistivity, as in an
ordered structure the
lattice perturbations
reducing conductivity
are minimal.
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