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1. METALS AND
ALLOYS
Dr. Shoaib Khan
Associate Professor
Head of the Department
Dept of Science of Dental Materials
Ziauddin College of Dentistry

2. USES IN DENTISTRY
 Steel and Stainless steel are used to make
instruments and wires and orthodontic
appliances.
 Gold and gold alloys are used for filling as
well as for crown & bridge purposes.
 Base metal alloys are used for crown &
bridge , and as denture base alloys.
 Dental amalgam is used as filling material.

3. METALS
 Hard , Lustrous , Dense and Opaque
 Metals are said to be having
following properties
 They are ductile and malleable.
 They are good conductors of heat and
electricity

4. SHAPING AND FORMING
 Casting
 Cold working
 Amalgamation

5. NATURE OF BONDING
 A solid metal may be considered to
consist of Positive ions, held
together by a cloud of free
electrons because metals can easily
lose their outer most electrons. The
ability of metals to conduct heat
and electricity is due to these free
electrons.

6. STRUCTURE OF METALS
 Generally metals
are arranged in
specific crystal
lattices.
 There are many
different types of
crystal lattices,
those important
from dental point if
view are:

7. STRUCTURE OF METALS
 1) BODY CENTERED CUBIC: Occurs
in some metals like Iron below 910 -
C degree centigrade. The unit cell
contains 1 atom at each corner of
the cubic+1 atom in the center of
the cubic.

8.  2) FACE CENTERED CUBIC: It occurs in
Iron above 910-C. The unit cell contain 1
atom at each corner and 1 atom at each
face of the cubic.

9.  3) HEXAGONAL CLOSED PACK
STRUCTURE: It occurs in some
metals like Zinc & Magnesium etc.
It contains 1 atom at each corner of
the hexagon.

10. CRYSTALLIZATION
 In general solidification occurs by
“CRYSTALLIZATION" with crystals growing
at nuclei of crystallization.

11. How are grains formed?
 Molten metal begins to solidify
 Crystals begin to form independently of each
other
 Each crystal has random unrelated orientation to
the other
 Each of these crystals grows into a crystalline
structure (a grain)

12. CRYSTALLIZATION
 Crystals grow as dendrites from these
nuclei, the dendrite can be described as a
3 dimensional branched structure starting
from a central point i:e. nucleus.
 Crystal growth continues until all the
metal has solidified and all different
dendrites are in contact. Each crystal is
known as a "GRAIN"

13. What difference does Grain Size
make?
 Significantly influences mechanical properties of the metal
 Large grain size is generally associated with
- Low strength
- Low hardness
- Low ductility (extent of deformation before fracture)
 Small grain size is generally the opposite

14. CRYSTALLIZATION
 After crystallization the grains are
neither spherical nor cubic nor do
they conform to any other
geometrical form they are said to
have an equiaxed structure, but
even then the atoms are arranged
in there specific arrangement

15. Equiaxed grains
•Equiaxed grains
•Having equal
dimensions in all
Directions

16. Controlling grain size
In general:
 Rapid cooling produces smaller grains.
 Slow cooling produces larger grain
 The smaller the grain size,
the stronger the metal

17. PROPERTIES
 Fine grain structure can
be obtained by rapid
cooling of metal from its
melting point;
QUENCHING.
 Slow cooling results in a
courser grain structure
with poor mechanical
properties.

18. Cold Work
 Altering the shape or
size of a metal by
plastic deformation. it
is carried out usually at
room temperature
 Below recrystalization
point

19. Effects of cold work
 Increased Strength
 Increased Hardness
 Decreased
Malleability
 Decreased Ductility

20. Hot Working
 The rolling, forging
or extruding of a
metal at a
temperature above
its recrystallisation
point.

21. Effects of Hot Work
 Undesirable surface
finish
 Less force required
than cold work
 Decreased yield
strength

22. ALLOYS
 An alloy can be
defined as," A mixture
of two or more metals
or a metal with a non
metal provided that
the mixture exhibits
the properties of a
metal".

23.  The properties of metals depends
upon the thermal and mechanical
treatments given to them, similar is
the case of alloys but the properties
of an alloy can be different from
the metals from that of its
constituents.

24. MANUFACTURING
 When two molten metals are mixed they
usually forms a solution on cooling such a
solution there may be one of the
following possibilities
 Either they will form a solid solution
 There may be partial solubility of the
metals with each other
 Or there may be complete insolubility
of the metals i:e they will remains as
two different phases

25. PHASE
 A phase can be defined as any
physically distinct, homogeneous,
mechanically separable part of a
system.

26. SOLID SOLUTION
 A solid solution is that when two solid
substances are homogeneously mixed and
there is only one phase present. There
are two types of solid solutions
 Substitutional solid solution
 Interstitial solid solution

27. SUBSTITUTIONAL SOLID
SOLUTION
 It is formed when two different types of
atoms occur in different position in the
same crystal lattice . This type of solid
solution is formed between two metals if:
 Their atomic sizes differ by less than
15%
 They have the same type of crystal
lattices i:e face centered or body
centered cubic.
 They have the same chemical valency

28. TYPES OF SSS
 Substitutional solid solutions are of two
types
 Ordered S.S.S.: Where atoms
substitute each other in a specific
order
 Random S.S.S.: Where atoms substitute
each other randomly.

29. INTERSTITIAL SOLID
SOLUTION
 It is formed where smaller atoms occupy
spaces in the interstices of the crystal
lattices of larger atoms.
 INTER METALLIC COMPOUND: Metals with
chemical affinity for each other can form
inter metallic compounds.

30. STRUCTURE OF ALLOY ON
COOLING
 If we start cooling an alloy rapidly
from molten state all the crystals
will not be homogenized and will
result in coring in the cooled alloy.

31.  Coring means the composition of each
grain of the alloy will not be the same.
However, if the alloy is cooled down
slowly there will be more time for
crystallization and will result in more
homogenization.

32. Definitions
 Quenching: Rapid cooling from a high
temperature by immersion in a liquid bath of oil
or water. Used to harden metal.
 Hardness: is the characteristic of a solid
material expressing its resistance to permanent
deformation
 Ductility: is the physical property of being
capable of sustaining large plastic deformation
without fracture
 Tensile strength: measures the force required to
pull something such as rope, wire, or a structural
beam to the point where it breaks
 Plasticity: is the ability of a material to d
eform permanently without breaking or rupturing
 Brittleness: is the opposite of the property of
plasticity. A brittle metal is one that breaks or
shatters before it deforms
 Malleability: is the property
that enables a material to
deform by compressive forces without
developing defects
 Annealing: is the process by which the
distorted cold worked lattice structure is
changed back to one which is strain free
through the application of heat.