SOLIDIFICATION OF SOLIDIFICATION OF METALSMETALS

DR.SHAGUN AGARWALDR.SHAGUN AGARWALDEPARTMENT OF PEDIATRICS DEPARTMENT OF PEDIATRICS AND PREVENTIVE DENTISTRYAND PREVENTIVE DENTISTRY

CONTENTSCONTENTS

• IntroductionIntroduction• MetalsMetals• History of metalsHistory of metals• Properties of metalProperties of metal• Classification of metalsClassification of metals• Inter atomic bondsInter atomic bonds• Solidification of metalsSolidification of metals• Nucleation and growth of crystalsNucleation and growth of crystals

INTRODUCTIONINTRODUCTION

• Metals and alloys play an important role in dentistry.Metals and alloys play an important role in dentistry.

• These form one of the four possible groups of materials used These form one of the four possible groups of materials used in dentistry which include Ceramics, Composites and Polymers.in dentistry which include Ceramics, Composites and Polymers.

• These are used in almost all the aspects of dentistry including These are used in almost all the aspects of dentistry including the dental laboratory , direct and indirect dental restorations the dental laboratory , direct and indirect dental restorations and instruments used to prepare and manipulate teeth.and instruments used to prepare and manipulate teeth.

• Although the latest trend is towards the “metal free” dentistry, Although the latest trend is towards the “metal free” dentistry, the metals remain the only clinically proven material for long the metals remain the only clinically proven material for long term dental applications.term dental applications.

METALSMETALS

• Chemical elements in general can be classified asChemical elements in general can be classified as

• MetalsMetals

• Non- metalsNon- metals

• MetalloidsMetalloids

• Metalloids are those elements on the border line showing both Metalloids are those elements on the border line showing both metallic and non metallic properties, example: carbon and silica. metallic and non metallic properties, example: carbon and silica.

• They do not form free positive ions but their conductive and They do not form free positive ions but their conductive and electronic properties make them important.electronic properties make them important.

• Metals constitute about 2/3Metals constitute about 2/3rdrd of the periodic table published by of the periodic table published by DMITRI MEDELEYEV in 1868.DMITRI MEDELEYEV in 1868.

• Of the 103 elements which are categorized in the periodic table Of the 103 elements which are categorized in the periodic table according to chemical properties , 81 are metals.according to chemical properties , 81 are metals.

• According to metals handbook, they can be defined as “ AN According to metals handbook, they can be defined as “ AN OPAQUE LUSTUROUS CHEMICAL SUBSTANCE , THAT IS GOOD OPAQUE LUSTUROUS CHEMICAL SUBSTANCE , THAT IS GOOD CONDUCTOR OF HEAT AND ELECTRICITY AND WHEN POLISHED CONDUCTOR OF HEAT AND ELECTRICITY AND WHEN POLISHED IS A GOOD REFLECTOR OF LIGHT.”IS A GOOD REFLECTOR OF LIGHT.”

HISTORY OF METALSHISTORY OF METALS

• Metals have been used by man ever since he first Metals have been used by man ever since he first discovered them.discovered them.

• In ancient and prehistoric times , only a few metals were In ancient and prehistoric times , only a few metals were known and accordingly these periods were called as : known and accordingly these periods were called as : “COPPER AGE”, “BRONZE AGE”, AND “ IRON AGE”.“COPPER AGE”, “BRONZE AGE”, AND “ IRON AGE”.

• Today more than 80 metallic elements and a large Today more than 80 metallic elements and a large number of alloys have been developed.number of alloys have been developed.

• Ore is a mineral containing one or more metals in a free Ore is a mineral containing one or more metals in a free or combined state.or combined state.

PROPERTIES OF METALSPROPERTIES OF METALS

• All metals are solid expect for mercury and gallium All metals are solid expect for mercury and gallium which are liquid at room temperature and hydrogen which are liquid at room temperature and hydrogen which is a gas.which is a gas.

• They have a metallic luster and mirror like surface.They have a metallic luster and mirror like surface.• They make a metallic sound when struck.They make a metallic sound when struck.• Are strong, hard and dense.Are strong, hard and dense.• Ductile and Malleable.Ductile and Malleable.• Conduct heat and electricity.Conduct heat and electricity.

• Have specific melting and boiling point.Have specific melting and boiling point.• Form positive ions in solution and get deposited at the Form positive ions in solution and get deposited at the

cathode during electrolysis.cathode during electrolysis.• Example- copper in copper plating.Example- copper in copper plating.• Metals are tough and this is due to the fact that the atoms Metals are tough and this is due to the fact that the atoms

of metals are held together by means of metallic bonds.of metals are held together by means of metallic bonds.• The chemical properties are based upon the electromotive The chemical properties are based upon the electromotive

series.series.• The higher the element in the series, the more metallic it The higher the element in the series, the more metallic it

is.is.• This tendency of metals to lose electrons is known as This tendency of metals to lose electrons is known as

oxidation potential. oxidation potential.

CLASSIFICATION OF METALSCLASSIFICATION OF METALS

• Pure metals and Mixture of metalsPure metals and Mixture of metals• Noble metals and Base metalsNoble metals and Base metals• Cast metal and Wrought metalCast metal and Wrought metal• Light metal and Heavy metalLight metal and Heavy metal• High melting metal and Low melting metalHigh melting metal and Low melting metal• Highly malleable and ductile metals Highly malleable and ductile metals

SOLIDIFICATION OF METALSSOLIDIFICATION OF METALS

• During solidification ,the liquid changes in to solid During solidification ,the liquid changes in to solid during cooling.during cooling.

• The energy of liquid is less than that of the solid The energy of liquid is less than that of the solid above the melting point. Hence liquid is stable above above the melting point. Hence liquid is stable above the melting point.the melting point.

• Below the melting point, the energy of liquid Below the melting point, the energy of liquid becomes more than that of the solid.becomes more than that of the solid.

• Hence below the melting point, the solid becomes Hence below the melting point, the solid becomes more stable than than the liquid.more stable than than the liquid.

• Therefore at the melting point, liquid gets converted Therefore at the melting point, liquid gets converted into solid during cooling.into solid during cooling.

• This transformation of liquid into solid below melting This transformation of liquid into solid below melting point is known as solidification.point is known as solidification.

SOLIDIFICATION OF PURE SOLIDIFICATION OF PURE METALSMETALS

Figure :Temperature as a function of time for the solidification of pure metals. Note that the freezing takes place at a constant temperature. (b) Density as a function of time

• Thermodynamically , both liquid and solid have equal energy Thermodynamically , both liquid and solid have equal energy at melting point and therefore both are equally stable at at melting point and therefore both are equally stable at melting point.melting point.

• Therefore ,no solidification or melting will take place at the Therefore ,no solidification or melting will take place at the melting pointmelting point

• Liquid will remain liquid and solid will remain solid.Liquid will remain liquid and solid will remain solid.

• Some under-cooling will be essential for solidification.Some under-cooling will be essential for solidification.

• This transformation occurs by nucleation and growth.This transformation occurs by nucleation and growth.

SOLIDIFIED SKIN ON A STEEL SOLIDIFIED SKIN ON A STEEL CASTINGCASTING

Figure 10.11 Solidified skin on a steel casting. The remaining molten metal is poured out at the times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which is based on this principle. Source: After H. F. Taylor, J. Wulff, and M. C. Flemings

SOLIDIFICATION OF IRON AND SOLIDIFICATION OF IRON AND CARBON STEELSCARBON STEELS

Figure 10.5 (a) Solidification patterns for gray cast iron in a 180-mm (7-in.) square casting. Note that after 11 minutes of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes about two hours for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal) molds. Note the difference in solidification patterns as the carbon content increases. Source: After H. F. Bishop and W. S. Pellini

NUCLEATION AND GROWTH OF NUCLEATION AND GROWTH OF CRYSTALSCRYSTALS

• At the solidification temperature, atoms from the At the solidification temperature, atoms from the liquid, such as molten metal, begin to bond together liquid, such as molten metal, begin to bond together and start to form crystals.and start to form crystals.

• The moment a crystal begins to grow is know as The moment a crystal begins to grow is know as nucleus and the point where it occurs is the nucleus and the point where it occurs is the nucleation point.nucleation point.

• When a metal begins to solidify, multiple crystals When a metal begins to solidify, multiple crystals begin to grow in the liquid.begin to grow in the liquid.

• The final sizes of the individual crystals depend on The final sizes of the individual crystals depend on the number of nucleation points.the number of nucleation points.

• The crystals increase in size by the progressive The crystals increase in size by the progressive addition of atoms and grow until they impinge upon addition of atoms and grow until they impinge upon adjacent growing crystaladjacent growing crystal

SegregationThe non-uniform distribution of impurities or alloying elements. The degree of segregation depends not only on the chemical composition of the alloy, but also on the rate of cooling, both of the ingot as a whole, and of each individual point within the mass. For example, near the surface, where the rate of cooling is rapid, the segregated impurities are trapped in the rapidly growing crystals. Fur- their inside the ingot, where the cooling is slower, the segregates will collect together and produce the so-called ghosts, or they may tend to rise to the surface and collect in the scrapped ingot head. In normal segregation, the constituents with the lowest melting points concentrate in the last portions to solidify, but in inverse segregation this is reversed. The segregation tends to form in bands sloping inwards to the top of the ingot (A segregate) and at the same time, due to shrinkage, it takes a V shape (V segregate) along the upper part of the ingot axis.

MICROSEGREGATIONMICROSEGREGATION

CAST STRUCTURES OF SOLIDIFIED CAST STRUCTURES OF SOLIDIFIED METALSMETALS

Figure Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals; (b) solid-solution alloys; and (c) structure obtained by using nucleating agents. Source: After G. W. Form, J. F. Wallace, J. L. Walker, and A. Cibula

Figure Development of a preferred texture at a cool mold wall. Note that only favorably oriented grains grow away from the surface of the mold

SOLIDIFICATION CONTRACTION OR SOLIDIFICATION CONTRACTION OR EXPANSIONEXPANSION

REFERENCES

DENTAL MATERIALS CLINICAL APPLICATIONS

Pankaj Datta

BASIC DENTAL MATERIALS John J.Manappallil

PHILLIPS’ SCIENCE OF DENTAL MATERIALS

Kenneth J. Anusavice