6 PROYECCIONES DE DEMANDA
8.4 Gas natural licuado
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EQUILIBRIUM PHASES IN CAST ALLOYS
is any homogeneous, physically distinct, and mechanically separable portion of theALLOYS
crown and bridges, ceramic prostheses, posts and cores,
(gold based, palladium based, [isomorphous], eutectic, peritectic and
two metals are present three metals are present
four metals are present
atoms of two metals are located in the same crystal structure,
limited solid solubility
PHASE DIAGRAMS
• A phase is a state of matter that is distinct some way from the matter around it.
• In a mixture of ice and water there are two phases because although ice and water are the same chemically, they each have distinct arrangements of atoms. Ice has the crystalline arrangement of a solid whereas water h random atomic arrangement
• A solid dental alloy may also if the composition of the alloy is homogeneous throughout.
• If the alloy has areas where compositions are different, it is called a multiple
• Phase diagrams are "maps" of the phases that occur when metals are mixed together.
• If there are two metals in an alloy, a binary diagrams describe only binary alloys.
• A typical phase diagram for a theoretical binary alloy AB is shown in
x-axis describes the composition of the elements in either weight percent or atomic percent. The y-axis is the temperature of the alloy system. It is important to remember that a phase diagram shows the composition and types of phases at a given temperatur equilibrium. Every phase diagram divides an alloy system into at least three areas: the phase, the liquid + solid phases,
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is a state of matter that is distinct in way from the matter around it.
ice and water there are two although ice and water are the they each have distinct Ice has the crystalline whereas water has the random atomic arrangement of a liquid.
A solid dental alloy may also have one phase if the composition of the alloy is essentially homogeneous throughout.
has areas where compositions are multiple-phase alloy.
Phase diagrams are "maps" of the phases that occur when metals are mixed together.
are two metals in an alloy, a binary is used. If three metals are in ternary phase diagram may be
describing alloys with more metals are not used because they the vast majority of phase only binary alloys.
A typical phase diagram for a theoretical alloy AB is shown in above figure. The describes the composition of the either weight percent or atomic axis is the temperature of the alloy system. It is important to remember that phase diagram shows the composition and of phases at a given temperature and at Every phase diagram divides an into at least three areas: the liquid liquid + solid phases, and the solid
© BRIHASPATHI ACADEMY these two lines contains some liquid and some solid. At 0% B, a single phase exists (100% A) that has a single melting point (800°
100% B, a single phase exists melting point (210°C). At any between these extremes, the melting defined as the temperature difference the liquidus (ACB) and solidus (ADB).
• When metals are mixed together in the molten state, then cooled to the solid state, there are several outcomes depending on the solubility of the metals in each other. If the metals remain soluble in one another, the result is a solid solution. If the metals are not soluble in the solid state, then a eutectic
Sometimes, the elements react to form a specific compound, called an
compound.
SOLID SOLUTIONS
• By far the greatest number of alloys that are useful for dental restorations ar
solid solutions Types
1. Substitutional solid solution
The atoms of the solute metal occupy the positions in the crystal structu
normally occupied by the solvent atoms in the pure metal
For a palladium silver alloy in which palladium is the solvent metal, the silver atoms replace the palladium atoms randomly in the crystal structure
Another example include gold alloy which is a disorded solid solution high temperatures
© BRIHASPATHI ACADEMY ׀ SUBSCRIBER’S COPY ׀ NOT FOR SALE example, everything above the
shown by line ACB is liquid.
e called the Liquidus.
Everything below line ADB is solid. This line The area between these two lines contains some liquid and some a single phase exists (100% A) melting point (800°C). At that has a composition between these extremes, the melting range is defined as the temperature difference between the liquidus (ACB) and solidus (ADB).
When metals are mixed together in the molten e, then cooled to the solid state, there are several outcomes depending on the solubility the metals in each other. If the metals soluble in one another, the result is a If the metals are not soluble in may form.
elements react to form a called an intermetallic
r of alloys that are ul for dental restorations are based on
he atoms of the solute metal occupy the cture that are ed by the solvent atoms
For a palladium silver alloy in which palladium is the solvent metal, the silver atoms replace the palladium atoms randomly in the crystal structure
Another example include gold-copper disorded solid solution at
2. Interstitial solid solution
The solute atoms are present
3. Super lattice/ordered structure
If an alloy containing
43.8 wt% copper is allowed to cool slowly below 400°C (752°F), the AuCu
forms, in which the gold atoms are located at the corners
cubic (fcc) unit cells and the coppe are located at the centers of the faces
This unit cell is equ since two adjacent uni
the six copper atoms at the centers of each of the six faces of the cube and eight unit cells share each of the e
solution ordinarily hat the solute atoms should be diameter than the solvent atoms, and these solid solutions limited to relatively small
of solute
Examples are carbon in iron i.e. carbon steel and commercially pure (CP)
Super lattice/ordered structure
ing 50.2wt% gold and lowed to cool slowly the AuCu3 structure the gold atoms are of the face-centered (fcc) unit cells and the copper atoms
the centers of the faces.
equivalent to AuCu3, adjacent unit cells share each of atoms at the centers of each s of the cube and eight unit he eight gold corner ordered structure is called super
Ordered solution impart higher hardness and strengths to alloys
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• Above graph shows a series of binary Ag-Cu alloys. In this phase diagram, the liquidus and solidus meet at a mid-range composition and the solidus is lower (at 779.4°C) than either pure Ag (960.5°C) or Cu (1083°C). This liquidus-solidus configuration is characteristic of an eutectic alloy system. The Ag-Cu system is especially important in high-Cu dental amalgam, but is also important in the formulation of some dental casting alloys.
• The eutectic system shown in above figure contains a pure solid solution below 400°C only at either extreme of composition (4% Ag or 4 % Cu) because the Ag and Cu are essentially insoluble in one another in the solid state. At all other equilibrium conditions below the solidus, a mixture of a solid solution (either α or β) and the eutectic composition occurs. The eutectic composition is 28.1% Cu (and 71.9% Ag) and has a layered appearance under a light microscope. If the alloy is at the eutectic composition, all of the alloy will be in the eutectic phase at room temperature (see the dotted line in figure). If the composition is other than this, then the alloy will be some combination of the solid solution and eutectic phases. The exact proportions of eutectic and solid solution can also be determined from the phase diagram.
• A pure eutectic composition has a melting point (vs, a melting range) that is substantially
lower than either of the pure components. The eutectic system of Pb-Sn uses the eutectic alloy composition as plumber's solder because of its low melting point.
• Hypo-eutectic alloys: If an alloy composition C is lower, than eutectic composition CE, solidification of the alloy starts from formation of the primary crystals of α-phase according to the left branch of the liquidus curve. These alloys are called hypo-eutectic.
• Hyper-eutectic alloys: If an alloy composition C is higher, than eutectic composition CE, solidification of the alloy starts from formation of the primary crystals of β-phase according to the right branch of the liquidus curve. These alloys are called hyper-eutectic.
PERITECTIC ALLOYS
• Sometimes a solid solution phase, which has already been formed, and the residual liquid phase react and form another solid solution phase or intermetallic compound, having a composition between the compositions of the liquid and the first solid. This is peritectic transformation (peritectic reaction).
INTERMETALLIC COMPOUNDS
• If two metals react to form a new compound with a specific composition, the phase diagram reflects an intermetallic compound.
• Ag3Sn is a fundamentally important intermetallic compound in dental amalgam.
TERNARY PHASE DIAGRAMS
• These phase diagrams are three-dimensional.
• Two dimensional representations in the shape of an equilateral triangle are also used to represent the three-dimensional structure.
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• The vertical axis represents increasing temperature (T). The AB, AC, and BC axes reflect concentrations of the components. The three-dimensional surface that is visible represents the liquidus for this system. Other features of the phase diagram below the liquidus require a two-dimensional cross section parallel to the ABC plane. The shape of the liquidus indicates that this is a eutectic system.
ALLOY STRENGTHENING MECHANISMS
• Precipitation hardening is used to strengthen dental alloys. By heating some cast alloys carefully, a second phase can be made to appear in the body of the alloy. The new phase blocks the movement of dislocations, thereby increasing strength and hardness. The effectiveness of precipitation hardening is greater if the precipitate is still part of the normal crystal lattice. This type of precipitation is called coherent precipitation.
Overheating may reduce alloy properties by allowing the second phase to grow outside of the original lattice structure.
• Grain refiners such as Ir, Rh, and Ru improve the strength of alloys by several times.
Moreover, strength and hardness are generally improved without sacrificing ductility.
• Cold working an alloy will significantly strengthen it. Cold working works out the dislocations, thereby making further deformation more difficult.
BASIC PRINCIPLES OF HEAT TREATMENT
• Heat treatment of a metal or alloy is a technological procedure, including controlled heating and cooling operations, conducted for the purpose of changing the alloy microstructure and resulting in achieving required properties.
• There are two general objectives of heat treatment: hardening and annealing.
Hardening
• Hardening is a process of increasing the metal hardness, strength, toughness, fatigue resistance.
Strain hardening (work hardening) – strengthening by cold-work (cold plastic deformation) Cold plastic deformation causes increase of concentration of dislocations, which mutually entangle one another, making further dislocation motion difficult and therefore resisting the deformation or increasing the metal strength.
Grain size strengthening (hardening) – strengthening by grain refining. Grain boundaries serve as barriers to dislocations, raising the stress required to cause plastic deformation.
Solid solution hardening – strengthening by dissolving an alloying element. Atoms of solute element distort the crystal lattice, resisting the dislocations motion.
Interstitial elements are more effective in solid solution hardening, than substitution elements.
Dispersion strengthening – strengthening by addition of second phase into metal matrix. The second phase boundaries resist the dislocations motions, increasing the material strength. The strengthening effect may be significant if fine hard particles are added to a soft ductile matrix (composite materials).
Precipitation hardening (age hardening) – strengthening by precipitation of fine particles of a second phase from a supersaturated solid solution. The second phase boundaries resist the dislocations motions, increasing the material strength.
Annealing
• Annealing is a heat treatment procedure involving heating the alloy and holding it at a certain temperature (annealing temperature), followed by controlled cooling.
• Annealing results in relief of internal stresses, softening, chemical homogenizing and
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© BRIHASPATHI ACADEMY ׀ SUBSCRIBER’S COPY ׀ NOT FOR SALE transformation of the grain structure into
more stable state.
• It takes place in three successive stages i.e.
recovery, recrystallization and grain growth