solidification and microstructure of metals / orthodontic courses by indian dental academy

Solidification & microstructure of metals

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:

Vacancy

Divacancy

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. Cabe2. Dental Materials – Craig. O’Brien – Powers3. Essentials of Dental Materials – S.H. Soratur4. Material and Metallurgical Science – S.R.J. Shantha Kumar5. Phillips Science of Dental Materials (Eleventh Edition) –

Anusavice6. Restorative Dental Materials (Eleventh Edition) – Robert G.

Craig and John. M. Powers7. Restorative Dental Materials – Floyd. A. Peyton8. J.P.D. April 2002 Volume 87 No.4 Page 351 – 363.

solidification and microstructure of metals / orthodontic courses by indian dental academy

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