CONTENTS:
|
1. Introduction
2. Metals
3. History of Metals
4. Properties of Metals
5. Classification of
Metals
6. Inter Atomic Bonds
7. Microscopic
Structure of Metals
8. Space Lattices
9. Lattice Imperfection
10.
Heat Treatment
11.
Strengthening of Metals
12.
References
|
INTRODUCTION:
Metals and alloys play an important
role in dentistry. These form one of the
four possible groups of materials used in dentistry which include ceramics,
composites and polymers. These are used
in almost all the aspects of dentistry including the dental laboratory, direct
and indirect dental restorations and instruments used to prepare and manipulate
teeth. Although the latest trend is
towards the “metal free” dentistry, the metals remain the only clinically
proven material for long term dental applications..
METALS:
Chemical elements in
general can be classified as 1. Metals
2.
Non-metals
3. Metalloids
Metalloids
are those elements on the border line showing both metallic and non metallic
properties, e.g. carbon and silica. They
do not form free positive ions but their conductive and electronic properties
make them important.
Metals
constitute about 2/3rd of the periodic table published by DMITRI MEDELEYEV in 1868. Of the 103
elements which are categorized in the periodic table according to the chemical
properties, 81 are metals.
According
to the metals hand book, they can be defined as “AN OPAQUE LUSTROUS CHEMICAL
SUBSTANCE, THAT IS A GOOD CONDUCTOR OF HEAT AND ELECTRICITY AND WHEN POLISHED
IS A GOOD REFLECTOR OF LIGHT”
HISTORY
OF METALS:
Metals
have been used by man ever since he first discovered them. In ancient and pre-historic times, only a few
metals were known and accordingly these periods were called as “COPPER AGE”, “BRONZE AGE” and “IRON AGE”. Today more than 80 metallic elements and a
large number of alloys have been developed.
Ore is a mineral containing one or more metals in a free or combined
state.
PROPERTIES
OF METALS:
All
metals are solids except for mercury and gallium which are liquid at room
temperature and hydrogen which is a gas.
The properties of metals can be listed out as follows:
1.
They
have a metallic luster and mirror like surface
2.
They
make a metallic sound when struck
3.
Are
hard, strong and dense
4.
Ductile
and malleable
5.
Conduct
heat and electricity
6.
Have
specific melting and boiling points
7.
Form
positive ions in solution and get deposited at the cathode during
electrolysis. E.g. copper in copper plating.
The outer most
electrons of the atom are known as valence electrons. These are readily given up and are
responsible for most of the properties.
Metals are tough and
this is due to the fact that the atoms of the metals are held together by means
of metallic bonds.
The chemical
properties of metals are based upon the electromotive series which is a table
of metals arranged in decreasing order of their tendency to lose
electrons. The higher an element is in
the series, the more metallic it is.
This tendency of metals of lose electrons is known as oxidation
potential.
CLASSIFICATION
OF METALS:
They can be done in many ways like:
1.
Pure
metal and mixture of metals (alloys)
2.
Noble
metals and base metals :
Noble metal is one whose compounds
are decomposable by heat alone, at a temperature not exceeding that of
redness. E.g. Au, Ag, and Pd.
Base metal
is one whose compounds with oxygen are not decomposable by heat Alone,
retaining oxygen at high temperature. E.g. Zn, Fe, and Al
3.
Case
metal and wrought metal
Cast metal is any metal that is
melted and poured into the mould
Wrought
metal is a cast metal which has been worked upon in cold condition
4.
Light
metal e.g. Al and heavy metal e.g. Fe
5.
High
melting metal e.g. chromium and low melting metal e.g. tin
6.
Highly
malleable and ductile metal e.g. gold and silver
INTER
ATOMIC BONDS:
The atoms are held together in place
by atomic bonds or forces. They may be
1.
Primary
2.
Secondary
Primary
bonds or inter atomic bonds:
These are very strong bonds
and may be of either type:
a. Ionic - These are seen in ceramics
b. Covalent - They
are seen in organic compounds
c.
Metallic bonds - They are seen in metals and are non
directional
Secondary
bonds or inter molecular bonds:
These are weak forces
and are otherwise known as Vander waal’s forces. The various types are:
a.
Hydrogen
bonds
b.
Dipole
bonds
c.
Dispersion
bonds
Of all these, the most
important one is the metallic bond which was explained for the first time by
LORENTZ, a Dutch scientist in 1916. It
can be explained by using the atomic and sub atomic structures.
The sub – atomic
structures
1.
Protons
– positive charge
2.
Neutrons
– neutral charge
3.
Electrons
- negative charge
The center or the nucleus of an
atom consists of proton and neutrons and are therefore positively charged. This is balanced by the revolving electrons
which are negatively charged and arranged in concentric shells with
progressively increasing energy. The
electrons in the outer most shell are known as VALENCE ELECTRONS.
These are loosely
bound and are therefore readily given up by the atom to form positive ions. The cations thus formed behave like hard
spheres and the electron cloud formed by the freed valence electrons roam about
freely in the interstices formed by the arrangement of the solid spheres. The electrons act like glue to hold all atoms
together and are known as INTER ATOMIC CEMENT.
Because of this, the metals are strong, hard, malleable, ductile and
good conductors of heat and electricity.
MICROSCOPIC
STRUCTURE OF METALS:
In the solid state,
most metals have crystalline structure in which atoms are held together by
metallic bonds. This crystalline array
extends for many repetitions in 3 dimensions.
In this array, the atomic centers are occupied by nuclei and core
electrons. The ionisable electrons float
freely among the atomic positions.
The space lattice is a
3 dimensional pattern of points in space and hence called as point
lattice. In this the simplest repeating
unit is called as the UNIT CELL. The size and shape of the unit cell are
described by three vectors. They are
a,b,c, and known as crystallographic axes. The length and angle between them
are known as LATTICE CONSTANTS AND
LATTICE PARAMETERS.
When a molten metal is
cooled the solicitation process is one of crystallization. These are initiated at specific sites called
nuclei. These in the molten metal are
present as numerous unstable atomic aggregates or clusters that tend to form
crystal nuclei. These temporary nuclei are known as EMBRYOS. These are generally
formed from impurities within the molten metal.
In the case of pure metals, the crystals grow as dendrites which can be
defined as a three dimensional network which is branched like a tree. The critical radius is the minimal radius of
the embryo at which the first permanent solid space lattice is formed.
The crystals are
otherwise known as grains since they seldom exhibit the customary geometric
forms due to interference from adjacent crystals during the change of
state. The grains meet at grain
boundaries which are regions of transition between differently oriented
crystals. These are regions of
importance as they are sites of:
1.
Less
resistance to corrosion
2.
High
internal energy and non crystalline
3.
Collection
of impurities
4.
Barriers
for dislocations
The nuclei can be
homogeneous or heterogenous based upon whether they are developed from the
molten liquid or formed as a result of foreign bodies incorporated into the
molten metal. When the crystals meet at
the grain boundaries they stop growing further.
The grain boundaries are about 1-2 atomic distances thick. Grain boundaries can be high angles
(>10-15 degrees) or low angled (< 10 degree).
The grain structure
can be fine where in, it contains numerous nuclei as obtained during the rapid
cooling process (quenching) or refined when foreign bodies are added to obtain
the fine grain structure.
EQUALIXED
GRAINS
When
cooling occurs and grains are formed, the grains start growing from the nuclei
peripherally. This takes the shape of a sphere and are equalized in structure
meaning that they have the same dimensions in any direction.
COLUMNAR
AND RADIAL GRAINS
In
a square mould, crystals grow from the edges towards the centre to form
columnar grains whereas in the cylindrical mould the grains grow perpendicular
to the wall surface and form radial grains.
Columnar grains are weak due to interferences in the converging
grains. Sharp margins have columnar
grains.
GRAIN SIZE:
The
grain size can be altered by heating.
When the metal is heated above the solidus temperature to the molten
state and rapidly quenched, small grains are formed whereas, when they are
allowed to cool slowly to room temperature the grains tend to grow due to
atomic diffusion and this results in an increased grain size and subsequent
decrease in the number. The more fine the grain structure, the more uniform and
better are the properties.
ANISOTROPHY:
Alloys
with uniform properties due to the presence of fine grain structure are said to
be anisotropic.
METHODS
OF FABRICATION OF METALS AND ALLOYS
1.
CASTING: It is the best and most popular method.
2.
WORKING ON THE METAL: They can be worked in the hot or cold
conditions. They are known as wrought
metals. They can be pressed, rolled,
forged or hammered.
3.
EXTRUSION: A process in which a metal is forced through a
die to form metal tubing.
4.
POWDER METALLURGY: It involves the
pressing of the powdered metal into the mould of desirable shape and heating it
to a high temperature to cause a solid mass.
SPACE
LATTICES:
The
structure of the crystal can be determined using the BRAGG’S LAW OF X-RAY DIFFRACTION.
There are 14 lattices known as BRAVIS
LATTICES and these are grouped under six families. These vary depending upon the
crystallographic axes and lattice constants which are the length of the
vertices and the angle between them. The
six families are:
1.
Cubic
Simple
Body
centered
Face
centered
2.
Triclinic
3.
Tetragonal
Simple
Body
centered
Rhombohedral
4.
Orthorombic
5.
Hexagonal
Simple
Body
centered
Face
centered
Base
centered
6.
Monoclinic
Simple
Base
centered
The arrangement of atoms in the
crystal lattice depends on the atomic radius and charge distribution of atoms.
The most commonly used metals in
dentistry have one of the following space lattices: body centered cubic, face centered cubic or
hexagonal lattice.
SIMPLE
CUBIC LATTICE SYSTEM
LATTICE
IMPERFECTIONS AND DISLOCATIONS
Crystallization
from the nucleus does not occur in a regular fashion, lattice plane by lattice
plane. Instead, the growth is likely to
be more random with some lattice positions left vacant and others overcrowded with
atoms being deposited interstitially.
These are called defects and can be classified as:
A. POINT DEFECTS OR ZERO DIMENSIONAL DEFECTS
1. Vacancies
or equilibrium defects:
Absence
of an atom from its position. This can
be:
Ø Vacancy
Ø Divacancy
Ø Trivacancy
2.
Interstitialcies:
Presence
of extra atoms in the interstitial spaces.
3. Impurities
4. Electronic defects
Point defects are responsible for
increased hardness, increased tensile strength, electrical conductance, and
phase transformations.
B. LINE DEFECTS OR SINGLE DIMENSIONAL DEFECTS:
These can be
1. Edge dislocation
2. Screw dislocation
The planes along which a dislocation
moves is called as slip planes and when this occurs in groups it is called as
slip bands. The crystallographic
direction in which the atomic planes move is called as the slip direction and
the combination of slip plane and slip direction is called as slip system.
These are responsible
for ductility, malleability, strain hardening, fatigue, creep and brittle
fracture.
The face centered
cubic consists of large number of slip systems and therefore is very
ductile. This is seen in gold.
The hexagonal closely
packed system seen in zinc possesses relatively few slip systems and is
therefore very brittle.
In between these is
the body centered cubic with intermediate properties.
The strain required to initiate
movement is the elastic limit. The
method of hardening of metals and alloys is based on the impedance to the
movement of dislocations.
C.
SURFACE DEFECTS OR PLANE DEFECTS OR TWO
DIMENSIONAL
DEFECTS:
1. Grain
boundaries
2. Twin
boundaries:
These
are seen in the NiTi wires responsible for transformation between the
austenitic and martensitic phases. These
are important for the deformation of the α titanium alloys. The atoms have a mirror relationship.
3. Stacking
fault
4. Tilt boundaries
D. VOLUME DEFECTS
These include cracks
ALLOTROPHY
AND ISOMORPHOUS STATE:
ALLOTROPHY
This
ability to exist in more than one stable crystalline form is called as
allotrophy. The various forms have the
same composition but different crystal structure.
ISOMORPHOUS
STATE
The
ability to exist as a single crystal at any atomic composition of binary alloys
is known as iomorphous state e.g. Au-Ag, Au-Cu.
HEAT
TREATMENT OR SOLID STATE REACTIONS:
Heat treatment of meals
(non-melting) in the solid state is known as solid state reactions. This is a method to cause diffusion of atoms
of the alloy by heating a solid metal to a certain temperature and for a certain
period of time. This will result in
changes in the microscopic structure and physical properties.
Important criteria are:
1.
Composition
of the alloy
2.
Temperature
to which it is heated
3.
Time
of heating
4.
Method
of cooling slowly or quenching.
The purpose of heat treatment is:
1.
Shaping
and working on the appliance in the laboratory is made easy when the alloy is
soft. This is the first stage and called
as softening heat treatment.
2.
To
harden the alloy to withstand high oral stresses, it is again heated and this
is called hardening heat treatment.
i. ANNEALING OR SOFTENING HEAT TREATMENT
This
is done for structures that are cold worked.
These cold worked structures are characterized by:
1.
Low
ductility
2.
Distorted
and fibrous grains
When cold work is continued
in these, they will eventually fracture.
This may be:
1.
Transgranular
– through the crystals and occur at room temperature
2.
Intergranular
– in between the crystals and occurs at elevated temperature
These can be reversed by
annealing. The various phase are:
1. Recovery
2. Recrystallization and
3. Grain growth
Technique:
The
alloy is placed in an electric furnace at a temperature of 700° C for 10mins
and then rapidly quenched. Annealing
temperature should be half that necessary to melt the metal in degrees Kelvin.
Recovery
During this phase, the
cold work properties begin to disappear.
There is a slight decrease in tensile strength and no change in
ductility. The tendency for warping
decreases in this stage.
Recrystallization
There is a radical
change in the microstructure. The old
grains are replaced by a set of new strain free grains. These nucleate in the most severely cold
worked regions in the metal. The
temperature at which this occurs is the recrystallization temperature. During this the metal gets back to the
original soft and ductile nature.
Grain growth
If
the fine grain structure in a crystallized alloy is further heated, the grains
begin to grow. This is essentially a
process in which the larger grains consume the smaller grains. This process minimizes the grain boundary
energy. This does not progress until the
formation of a coarse grain structure.
Properties
of an annealed metal:
1.
There
is an increase in ductility
2.
Makes
the metal tougher and less brittle
Stress relief annealing is a process which is
done after cold working a metal to eliminate the residual stress. This is done at relatively low temperatures
with no change in the mechanical properties.
ii.
HARDENING HEAT TREATMENT
This
is done for cast removable partial dentures, saddles, bridges but not for
inlays. This is done for clasps after
the try in stage so that adjustments can be carried out during the try in when
the metal is soft.
Technique
The
appliance is heat soaked at a temperature between 200-450° C for 15-30 minutes
and then rapidly quenched. The result is:
1. Increased strength
2. Increased hardness
3. Increased proportional
limit
4. Decreased ductility
Microscopic changes
Diffusion
and rearrangement of atoms occur to form an ordered space lattice. Therefore this is called as order hardening
or precipitations hardening.
iii.
SOLUTION HEAT TREATMENT OR SOLUTION HARDENING
When
the alloy is soaked at 700°C for 10 minutes and then rapidly quenched like that
for a softening treatment, any precipitation formed during the earlier heat
treatment will become soluble in the solvent metal.
iv.
AGE HARDENING
This
is a process in which following solution heat treatment; the alloy is once
again heated to bring about further precipitation as a finally dispersed phase. This causes hardening of the alloy and it is
known as age hardening because the alloy will maintain the quality for many
years. E.g. Duralium.
METHODS OF STRENGTHENING METALS AND ALLOYS :
All
metals possess an inherent barrier to dislocations. This is relatively small and known as pearls stress. This is imposed by the bonds associated with
the arrangement of atoms in a given crystal structure. Thus to improve the mechanical properties,
other methods of hardening are used.
These are:
1.
GRAIN BOUNDARY HARDENING OR GRAIN REFINEMENT HARDENING
A
poly crystalline metal contains numerous grains or crystals. These meet at the grain boundaries. The grain boundary is non –crystalline and
contains impurities. These act as barriers to dislocations as it moves by slip
planes from one grain to another.
Finely
grained structure contains large grain boundaries and hence the obstacle to
motion of dislocations is higher. Therefore
dislocation density rises rapidly due to plastic deformation. These dislocations at the grain boundaries
increase and therefore the stress necessary to continue the plastic deformation
also increases. Therefore, there is an
increase in the yield strength and ultimate tensile strength. The yield strength varies inversely with the square
root of grain size (hall petch
equation).
Grain refinement can be done by:
1.
Heat
treatment
2.
Addition
of grain refiners which act as nucleating agents.
Grains refiners are
metals or foreign bodies of high melting temperature. They crystallize out at high temperature and
act as nuclei or seeds for further solidification. e.g. iridium, rhodium.
The best method to
improve properties of alloys and metals is by the addition of grain
refiners. Finely reined grains structure
contain grain size >70µm.
2. SOLUTION HARDENING OR SOLID SOLUTION
STRENGTHENING
An
alloy is a solid solution; it has a solute and a solvent. The atomic diameter of a solute and solvent
will never be the same.
The
principle of solid solution hardening is by introducing either tensile or
compressive strain depending on whether the solute atom is smaller or larger
than the solvent respectively and finally distorting the grain structure. This solute can be either:
-
Substitutional
-
Interstitial
3. PRECIPITATION HARDENING
Another
method of strengthening alloys is by means of this technique. In this, the alloy is heated so that
precipitates are formed as a second phase which blocks the movement of
dislocations. The effectiveness is
greater if the precipitate is part of the normal crystal lattice which is known
as coherent precipitation.
4. DISPERSION STRENGTHENING
It
is a means of strengthening a metal by adding finely divided hard insoluble
particles in the soft metal matrix as a result of which, the resistance to
dislocations is increased. This
increases hardness and tensile strength.
The
ideal properties are seen when the particles range from 2-15% by volume with
spacing at 0.1 – 1.0µm intervals and particle size from 0.01 – 0.1µ.
The
ideal shape of the dispersed particle is a needle like LAMELLAR SHAPE which can intersect with the slip planes. Powdered metallurgy makes use of this method
for strengthening.
5.
STRAIN HARDENING OR WORK HARDENING
This
is seen in wrought metals. The metals
are worked after casting to improve their mechanical properties. They may be forged, hammered, drawn as wires,
etc. All this is done below the
re-crystallization temperatures. This
working causes vast number of deformations within the alloys or metals. These interact with each other mutually,
impeding the movements. The increased
stress required for further dislocation movement to achieve permanent
deformation provides the basis for work hardening. This result is distorted grain structure with
the grains being fibrous.
REFERENCES:
1.
Anderson’s Applied
Dental Materials – John F.Mc. Cabe
2.
Dental Materials –
Craig. O’Brien – Powers
3.
Essentials of
Dental Materials – S.H. Soratur
4.
Material and
Metallurgical Science – S.R.J. Shantha Kumar
5.
Phillips Science of
Dental Materials (Eleventh Edition) –
Anusavice
6.
Restorative Dental
Materials (Eleventh Edition) – Robert G. Craig and John. M. Powers
7.
Restorative Dental
Materials – Floyd. A. Peyton
8.
J.P.D. April 2002
Volume 87 No.4 Page 351 – 363.
No comments:
Post a Comment