alloys lec 5

ALLOYS

Metal containing 2 or more elements, at least one of which is a metal, and all of which are mutually soluble in the molten state

Carbon steel

An alloy of Iron and Carbon (Fe+C) that easily rusts

Stainless steel

An alloy of Iron, Carbon, and at least 10.5% Chromium that forms a passive oxide layer

Amalgam

An alloy where Hg (mercury) is used in its composition

Noble metals

Metals highly resistant to corrosion and oxidation in the oral environment

Passivation

The formation of a protective oxide film by a reactive substance

High Noble (HN) Alloys

Alloys that must contain at least 40 wt% Gold (Au) and a total of 60 wt% noble metals

Noble (N) Alloys

Alloys that must contain at least 25 wt% noble metal content

Predominantly Base Metal (PBM) Alloys

Alloys that contain less than 25 wt% of noble metals

White

The color of High Noble (HN) alloys with a gold content of approximately 40%

84%

The minimum gold content typically required in PFM alloys to achieve a rich yellow color

EUTECTIC ALLOYS

Homogeneous mixtures that melt or solidify at a single temperature lower than the melting point of any constituent metal

PERITECTIC ALLOYS

A reaction where a solid phase and a liquid phase will together form a second solid phase at a particular temperature and composition

Solid Solution

The simplest alloy where atoms of two metals are mutually completely soluble and dispersed in the same crystal structure

Layered or lamellar structure

The typical microstructure formed upon solidification of Eutectic alloys

Silver-Copper (Ag−Cu)

A system that forms a eutectic alloy, often used for dental solders

Ag−Sn system

The system in amalgam that is an example of a peritectic alloy

Palladium (Pd)

An alloying element that whitens gold alloys and raises their melting range, strength, and hardness

Zinc (Zn)

An element added to crown and bridge alloys as an oxygen scavenger to reduce gas porosity in castings

Iridium, Rhenium, Ruthenium (Ir,Re,Ru)

The three noble elements used as grain refiners to improve the grain structure of alloys

Copper (Cu)

An alloying element that strengthens and reddens Au−Ag−Cu crown and bridge alloys

Silver (Ag)

An alloying element added to Au−Ag−Cu casting alloys to offset the reddish hue contributed by Cu

Platinum (Pt)

An alloying element primarily used in yellow-gold PFM alloys to increase the melting range, hardness, and strength

Iron (Fe)

An element primarily used to strengthen Au−Pt alloys for PFM applications

Indium (In)

An element used in some Au−Ag−Cu casting alloys to improve their castability

Hot tears

Microcracks that form at elevated temperatures in thin cast areas due to stress from the casting investment

Dendrites

Tree-branch patterns of crystallization that occur during solidification of alloys because of constitutional supercooling

Base metal (PB) alloys

Alloys that typically have a dendritic microstructure

High Noble (HN) and Noble (N) alloys

Alloys that typically have an equiaxed polycrystalline microstructure

Supercooling

The initial cooling of the liquid metal below the solidification temperature (Tf​) before freezing begins

Latent heat of solidification

The heat released by solidifying metal as it changes from a high-energy liquid to a low-energy solid

Nucleation

The formation of a stable center around which a metal crystal can grow

Eutectic Solidification Reaction

Liquid → Solid solution α + Solid solution β

Peritectic Solidification Reaction

Liquid + Solid solution β→ Solid solution α

Type I Gold Alloys

Casting gold alloys classified as Soft

Type IV Gold Alloys

Casting gold alloys classified as Extra hard

11−16%

The range of Copper (Cu) content in Type IV (Extra hard) gold alloys

60−70%

The range of Gold (Au) content in Type IV (Extra hard) gold alloys

Biocompatibility

A desirable characteristic of dental casting alloys (along with ease of melting and casting)

Little solidification shrinkage

A desirable characteristic of dental casting alloys during manufacturing (along with good wear resistance)

Osmium (Os)

A noble metal listed in the group of elements resistant to corrosion (along with Au,Pd,Pt,Rh,Ir,Ru)

Binary alloys

Alloys composed of exactly two metals

Coring

The susceptibility of peritectic alloys to form inhomogeneous structures during rapid cooling

Lowers the density

The effect that Palladium has on gold alloys

Zinc and Iridium (Zn,Ir)

Elements present in amounts less than 1% in some dental gold casting alloys

Au-Cu alloys

The best-known example of ordering that strengthens some dental alloys via solid-state reactions

Taggert

The person who introduced a method for producing precise cast metal crowns and bridge forms in 1907

Endodontic posts and cores

A specific dental use for which pure noble metals must be alloyed for sufficient resistance to deformation

Nickel and Chromium (Ni−Cr)

One of the groups of alloys that predominantly base metals are composed of

Tin (Sn)

An element that contributes to the formation of a bonding oxide and strengthens Au- and Pd-based PFM alloys

POLY

Greek root meaning 'many'

MER

Greek root meaning 'unit/part' or the repeating structural unit of a polymer

Polymer

A large organic molecule ("macromolecule") formed by the union of many smaller repeating units (mers)

Monomers

Molecules that can react with other molecules to form very large molecules, or polymers

Polymerization

Chemical reaction converting low molecular weight monomers into high molecular weight polymers

Thermoplastic Resins

Resins that soften on heating and application of pressure (a reversible change)

Thermosetting Resins

Resins that become permanently hard when heated above the polymerization temperature (an irreversible change)

Resins

Compositions of monomers or polymers blended with other components to provide a useful set of properties

Vulcanized rubber

The material used for denture bases in 1853

PMMA

Polymethylmethacrylate, introduced in 1936 as a heat-processed thermosetting material

Gutta percha

Material used for temporary crowns/fillings in the 1890s, containing trans-polyisoprene

Macromolecule

The term for the large organic molecule that makes up a polymer

Chain-like molecular structure

The feature that allows polymers limitless configurations and conformations

Molecular weight

A factor that, when increased, increases polymer rigidity, strength, and melting temperature

Polydispersity

The ratio Mw​/Mn​, a measure of the range and distribution of chain sizes

Crosslinking

A molecular structure feature that dramatically increases molecular weight and prevents dissolution

Glass Transition Temperature (Tg​)

The temperature at which macromolecule molecular motion begins to force the polymer chains apart

Copolymers

Polymers with two or more chemically different monomers combined to yield specific physical properties

Plastic strain

Irreversible deformation that results in a new, permanent shape, caused by slippage (flow) among chains

Elastic strain

Reversible deformation that is quickly and completely recovered, caused by chains uncoiling and recoiling

Viscoelastic strain

A combination of both elastic and plastic deformation where only the elastic portion is recovered (not instantaneously)

Plasticization

The lubricating effect when absorbed molecules spread polymer chains apart and facilitate slippage

Plasticizers

Compounds often added to resins (like soft liners) to reduce their softening or melting temperatures

Addition Polymerization

Polymerization that starts from an active center and adds one monomer at a time to rapidly form a chain

Condensation Polymerization

Polymerization that proceeds by stepwise intermolecular condensation, often forming by-products like water

Vinyl groups

The characteristic unit in many addition polymerizations, featuring carbon-carbon double bonds

Heat Activation

Polymerization initiation typically used in denture base resins to produce free radicals from an initiator

Chemical Activation

Polymerization initiation involving the mixing of at least two reactants (e.g., tertiary amine + benzoyl peroxide)

Light Activation

Polymerization initiation where photons activate the initiator (e.g., Camphorquinone)

Benzoyl peroxide

The most common chemical initiator used in polymerization

Propagation

The stage in polymerization where the free radical monomer acts as a new free radical center to grow the chain

Termination

The stage in polymerization that occurs by direct coupling of two free radical chains or hydrogen exchange

Acrylic resins

Polymers derived from acrylic acid or methacrylic acid containing a vinyl group

18−20 KHN

The typical Knoop hardness number (KHN) for acrylic resins

1.19 g/cm3

The density of acrylic resins

125∘C

The softening temperature of PMMA, above which it depolymerizes

Imbibition

The tendency of acrylic resins to absorb water

Methyl Methacrylate (MMA)

The transparent liquid monomer mixed with the polymer powder to form a plastic doughlike material

100.8∘C

The boiling point of methyl methacrylate monomer

0.945 g/mL at 20∘C

The density of methyl methacrylate monomer

12.9 kcal/mol

The heat of polymerization for methyl methacrylate

Prosthodontics

One of the areas of dentistry where polymers are used (along with Orthodontics and Endodontics)

Extremely stable, does not discolor

A key property of PMMA regarding UV light and aging

60 MPa

The Tensile strength of acrylic resins

2400 MPa

The Modulus of elasticity of acrylic resins

Not very technique-sensitive

How the mixing of liquid monomer and polymer is characterized

Organic solvent

What liquid monomer methyl methacrylate is characterized as (it is an excellent one)

48∘C

The melting point of methyl methacrylate monomer

Evaporates

What happens to the liquid monomer methyl methacrylate at room temperature due to its high vapor pressure