BY

Dr. Bibin Bhaskaran

Solidification And Microstructure Of

Metals

The history of dental restorations and implants dates back to the ancient Egyptians who used bone and bands of gold wire to replace missing teeth.

The modern era of dental restorations just began after the turn of the 20th century with the use of number of precious metals as well as some attempts to use zinc,steel,copper and even brass

Introduction

Definition Classification Physical properties Chemical properties Metallic bonds Solidification Nucleus formation Crystallization Dendrite formation Grain size Prosthodontic considerations Summary References

Index

Definition Any strong and relatively ductile substance

that provides electropositive ions to a corrosive environment and that can be polished to a high luster. (G.P.T-8).

Metals

Metals may be classified into two basic groups :-

a) Ferrous

b) Non ferrous

Classification

Metals are sometimes described as a lattice of positive ions surrounded by a cloud of delocalized electrons.

• On the periodic table, a diagonal line drawn from boron (B) to polonium (Po) separates the metals from the non-metals.

In dentistry ,metals represent one of the four major classes of materials used for the reconstruction of decayed, damaged or missing teeth.

Although metals are easy to distinguish from ceramics,polymers,and composites,they are not easy to define.

Metals are usually inclined to form cations through electron loss.

They react with oxygen in the air to form oxides

Iron rusts over years, while potassium burns in seconds.

Chemical properties

The transition metals (such as iron, copper, zinc, and nickel) take much longer to oxidize.

Others, like palladium, platinum and gold, do not react with the atmosphere at all.

Some metals form a barrier layer of oxide on their surface which cannot be penetrated by further oxygen molecules and thus retain their shiny appearance and good conductivity for many decades (like aluminium, some steels, and titanium).

The oxides of metals are generally basic (as opposed to those of non-metals, which are acidic.

Metals in general have high electric, thermal conductivity, lustre and density, and the ability to be deformed under stress without cleaving.

Physical Properties

Metals have a high fracture toughness when compared to ceramic, polymers and composites.

The fracture toughness of most metals vary between 25 and 60 Mpa compared with a range of 0.75 to 2.5 Mpa.

Conductivity

One of the chief characteristics of a metal is its ability to conduct heat and electricity.

Such energy conduction is associated with the mobility of free electrons present in metals.

Density

The high density of most metals is due to the tightly packed crystal lattice of the metallic structure.

Gallium and Mercury elements that are commonly used as alloying elements in dental alloys are liquid at room temperature.

Metals are generally resistant to chemical attack but some metals require alloying elements to resist tarnish and corrosion.

Metallic bonding is the electromagnetic interaction between delocalized electrons, called conduction electrons, and the metallic nuclei within the metals.

Understood as the sharing of "free" electrons among a lattice of positively-charged ions.

Metallic Bonds

Metallic bonding accounts for many physical properties of metals, such as strength, malleability, ductility, thermal and electrical conductivity, opacity, and luster.

The combination of two phenomena gives rise to metallic bonding:

Delocalization of electrons and the availability of a larger number of delocalized energy states than of delocalized electrons.

The latter could be called electron deficiency.

Nature Of Metallic Bonding

Metallic bonding is found in metals like zinc

Definition :

Alloys are materials made up of more than one chemical element, at least one of which must be a metal.

ALLOYS

For example a small amount of carbon is added to iron to form steel.

A certain amount of chromium is added to iron and carbon to form stainless steel, an alloy that is highly corrosion resistant.

Although pure gold is highly corrosion resistance, copper is added to it to enhance its strength .

A pure metal solidifies at one fixed temperature, a fact which can be checked by plotting a cooling curve.

A cooling curve may be obtained  by melting a small amount of a metal and recording the temperature drop at suitable time intervals as this metal solidifies (the metal must be allowed to cool very slowly i.e. under equilibrium conditions) .

Solidification of metals

We can then plot a graph of temperature against time to give us the cooling curve for that particular metal.

As the metal is cooled, clusters of atoms come together from the liquid to form solid crystal nuclei.

Nucleus formation

This nuclei will be stable and grow into crystallites or grains.

Nucleation can occur by two processes –

1) Homogenous nucleation2) Heterogeneous nucleation

Crystallization is controlled by atomic diffusion from melt to the nuclei.

Characteristically a pure metal may crystallize in a tree branch pattern from a nucleus. Such formations are called dendrites.

Mechanism of crystallization

Microstructure of copper-tin alloy showing branch like dendritic formations

Microstructure of brass alloy showing branch like dendritic formations

In crystallization growth starts from the centre of the nuclei and crystals grow towards each other.

When two or more crystals collide their growth is stopped.

Finally the entire space is filled with crystals.

Stages in the formation of metallic grains during the solidification of a molten metal

The metal is therefore made up of thousands of tiny crystals. Such a metal is said to be polycrystalline.

Each crystal in the structure is known as a grain.

Factors affecting grain size :– Number and location of the nuclei at the time

of solidification Shape of the mould in which the metal

solidifies Rate of crystallisation Rate of cooling Cold working Nucleating agents

Grain size

In polycrystalline metal shape of the grains is influenced by the shape of the mold.

Smaller the grain size of the metal ,the better its physical properties.

Can be controlled to an extent by super cooling and rate of cooling.

Control of grain size

The latent heat given up by initial solidification raises the temperature in the vicinity of the solidification front and this condition becomes favorable for dendrite growth resulting in columnar grains.

If the mold had been cylindrical grains would have grown perpendicular. Such grains are called radial grains.

Decreasing the grain size can have a number of beneficial effects on the cast alloy structure of a crown or removable partial denture.

The finer grain size can raise the yield stress, increase the ductility and raise the ultimate strength.

The change in the grain size is related to the process of plastic deformation and fracture.

The grain boundary is assumed to be a region of transition between differently oriented crystal lattices of two neighboring grains.

Structure is more nearly non crystalline, particularly towards the central region of the grain boundary.

Impurities in the metal may be found in greater concentration at the grain boundaries.

Also this region is readily attacked by chemicals.

Grain boundaries

Microstructure of gold casting

It has been observed that the position of the neighboring atoms surrounding every atom of a crystal lattice is identical in a pure crystalline metal.

When the property of identical periodic points in space was explored mathematically it was discovered there are 14 ways to arrange points in space.

Body centered cubic Simple cubic

Face centered cubic

Face centered orthorhombic

Body centered orthorhombic

Simple triclinic

Simple monoclinic Base centered monoclinic

ELEMENT UNIT CELL MELTING TEMPERATUREIn degree celesius

Mercury Rhombohedral -39

Gallium Orthorhombic 30

Indium Tetragonal 156

Tin Face-centred cubic 419

Aluminum Face-centred cubic 660

Silver Face-centred cubic 960

Gold Face-centred cubic 1,063

Copper Face-centred cubic 1,083

Manganese Cubic 1,244

Beryllium Hexagonal close pack 1,284

Nickel Face-centred cubic 1,452

Cobalt Body-centred cubic 1,493

Iron Body-centred cubic 1,535

Palladium Face-centred cubic 1,552

Titanium Hexagonal close pack 1,668

Platinum Face-centred cubic 1,769

Chromium Body-centred cubic 2,610

Cobalt- chromium & titanium alloys are used in prosthetic dentistry for fabrication of implants

Nickel chromium alloys are used for porcelain fused to metal restorations

Nickel chromium alloys were introduced for crowns bridges and partial denture frame works

Prosthodontic Considerations:-

A variety of metal alloys are used in dental specialties. In general they are very strong and stiff and have excellent tarnish and corrosion and are biocompatible.

As the grain size becomes smaller the better will be its physical properties.

Summary:

Givan.D.A, Precious metals in dentistry, Dental Clinics of North America, July, 2007, pg: 591-602.

Roach.M, Base metal alloys used for dental restorations and implants, Dental Clinics of North America, July, 2007, pg: 603-628.

Wataha.J.C, Alloys for prosthodontic restorations, journal of prosthetic dentistry, 2002, 87 (4), pg 351-363.

O’Brien.W.J, Dental materials and their selection, 3rd edition, Quintessence publications.

References:-

Ferracane.J.L, Materials in Dentistry, 2nd edition, Lippincott Williams and Wilkins.

Williams.D.F, Cunnigham.J, Materials in Clinical Dentistry, 1st edition, oxford medical publication.

Craig.R.G, Dental Materials, 8th edition, Elsevier publications.

Anusavice, Phillip’s Science of Dental Materials, 11th edition, Saunders publications. Alloys for prosthodontic restorations

The Journal of Prosthetic Dentistry, Volume 87, Issue 4, Pages 351-363

Thank you