Chapter 5 Flashcards

Question 1

Q

define alloy

Answer

A

A crystalline solid with metallic properties that is composed of two or more chemical elements at least one of which is a metal and all of which are mutually soluble in the molten state.

Question 2

Q

define alloy system

Answer

A

All possible alloyed combinations of two or more elements at least one of which is a metal. For example, the binary gold-silver system includes all possible alloys of gold and silver, varying from 100% gold and 0% silver to 100% silver and 0% gold.

Question 3

Q

define binary metal alloy

Answer

A

An alloy that contains two chemical elements at least one of which is a metal/if both are metals?? or if there are only 2 metals in a lloy (and the rest are non-metals?)

Question 4

Q

define coring

Answer

A

A microstructure in which a composition gradient exists between the center and the surface of cast dendrites, grains, or particles.

Question 5

Q

define dendritic microsctructure

Answer

A

A cast alloy structure of highly elongated crystals with a branched morphology.

Question 6

Q

define equiaxed grain microstructure

Answer

A

A cast alloy microstructure with crystal (grain) dimensions that are similar along all crystal axes.

Question 7

Q

define grain

Answer

A

A single crystal in the microstructure of a metal.

Question 8

Q

define grain boundary

Answer

A

The interface between adjacent grains in a polycrystalline metal. Dental alloys are polycrystalline solids consisting of many individual grains (crystals) separated by grain boundaries.

Question 9

Q

define heterogenous nucleation

Answer

A

Formation of solid nuclei on the mold walls or on particles within a solidifying molten metal.

Question 10

Q

define homogenous nucleation

Answer

A

Formation of nuclei that occur at random locations within a supercooled molten metal in a clean, inert container.

Question 11

Q

define metal

Answer

A

(1) An element or alloy whose atoms readily lose electrons to form positively charged ions. (2) A metallic material composed of one or more chemical elements that is opaque, ductile, relatively malleable, a good conductor of electricity, a good thermal conductor, and usually lustrous when light is reflected from its polished surface.

Question 12

Q

define microstructure

Answer

A

Structural features of a metal, including grains, grain boundaries, phases, and defects such as porosity, revealed by microscopic imaging of the chemically or electrolytically etched surface of a flat, polished specimen.

Question 13

Q

define nucleus

Answer

A

A stable cluster of atoms in a new phase that forms within a parent matrix phase during the solidification of a microstructure.

Question 14

Q

define phase

Answer

A

A homogeneous, physically distinct, and mechanically separable portion of a metal microstructure.

Question 15

Q

define phase/constituitive diagram

Answer

A

A graph of equilibrium phases and solubility limits for an alloy system as a function of composition and temperature.

Question 16

Q

define quaternary alloy

Answer

A

An alloy that contains four elements at least one of which is a metal/ or if all four are metals??

Question 17

Q

define solid sol’n (metallic)

Answer

A

A solid crystalline phase containing two or more elements at least one
of which is a metal and whose atoms share the same crystal lattice.

Question 18

Q

define tarnish

Answer

A

Superficial discoloration or dulling of a metal surface that is often caused by a reaction with oxygen or sulfur

Question 19

Q

define ternary alloy

Answer

A

An alloy that contains three elements at least one of which is a metal.

Question 20

Q

CQ: why is the term metal difficult to define?

Answer

A

no single characteristic defines a metal completely (some produce a ringing sound when struck, some have lustre, magnetism, etc)

Question 21

Q

give examples of how pure metal can differ from alloy

Answer

A

iron becomes much stronger and harder when complexed with small bits of carbon (Fe+C = steel); when chromium is alloyed with steel we get stainless steel because an adherent chromium oxide (Cr2O3) forms; can electroplate Cr onto steel instruments to make them corrosion resistant (e.g. Fe is very corrosive–>not when you add Cr)

Question 22

Q

all metal dental appliances are ALLOYS except pure gold foil, pure titanium, & endodontic silver points–however, the commercial pure Ti may be considered an alloy because a certain amount of impurities is allowed in each grade

Question 23

Q

All pure metals and alloys used as restorative materials in den- tistry are crystalline solids when used to produce their func- tional prosthesis or restoration forms. Because the metals are crystalline, the microstructural changes that occur during processing or heat treatment control desired properties for dental applications.

Question 24

Q

Dental casting alloys are associated with the fol- lowing groups:

Dental amalgams, which contain mercury (Hg), silver (Ag), tin (Sn), and copper (Cu).
High noble (HN) alloys, which contains at least 40 wt% gold (Au) and 60 wt% of noble metals.
Noble (N) metal alloys, which are typically based on pal- ladium (Pd) as the main noble metal with a total noble metal content of at least 25 weight percent. Noble metal alloys may also contain gold, silver, copper, gallium (Ga), indium (In), platinum (Pt), and tin (Sn).
Predominantly base (PB) metal alloys, which contain less than 25 wt% of noble metals, are most commonly com- posed of one of the following groups: nickel and chromium (Ni-Cr); cobalt and chromium (Co-Cr); iron, carbon, and chromium (Fe-C-Cr); commercially pure titanium (CP- Ti); and titanium-aluminum and vanadium (Ti-Al-V).

Question 25

Q

what is Kic?

Answer

A

fracture toughness–metals have a higher fracture toughness than ceramics, polymers, and composites; in MPa x m^1/2 (=MPa/m^2??)

Question 26

Q

define fracture toughness

Answer

A

a measure of the resis- tance of a material to crack propagation when a microcrack exists in its structure–don’t understand, crystalline solid = propagates fracture?

Question 27

Q

Generally metal alloys are stronger and more dense than nonmetallic structures. Most metals are also far more ductile and malleable than nonmetals, which are generally brittle.

Question 28

Q

which metals are magnetic but can also be produced in a non-mag state?

Answer

A

iron, nickel, and cobalt— can be magnetic, but they can also be produced in a nonmag- netic state

Question 29

Q

the noble metals are resistant to corrosion bc they are inert and do not need to form oxides to reduce their energy state (?)–reduce internal energy?

Question 30

Q

why might Fe, Sn, or Indium be added to a high noble/noble metal alloys?

Answer

A

the first three form oxide layers–this promotes bonding of metal to ceramic veneer

Question 31

Q

critical question: Why are the general physical and mechanical properties of metallic dental materials different from those of ceramic and polymeric dental materials?

Answer

A

metallic bond (free cloud of e-s) responsible for: thermal conductivity, electrical conductivity, lustre of metal, ability to undergo significant plastic deformation (with forces above elastic limit)–>associated with malleability and ductility; these characteristcs are not seen with ceramics and polymers, which atoms are covalently or properly ionically bonded

SO BECAUSE OF THE ELECTRON CLOUD

Question 32

Q

The boundary between metals and nonmetals is indistinct, and the elements near the boundary exhibit characteristics of both metals and nonmetals.

Question 33

Q

describe metallic bonding; why is it not covalent?

Answer

A

atoms in solid metals held together by metallic bonding–a “sea” or “cloud” of free valence electrons–>shared by multiple atoms so the bonding is non-directional; no exact sharing of atoms bw two adjacent metal atoms–>therefore more like ionic

Question 34

Q

metallic bond (free cloud of e-s) responsible for: thermal conductivity, electrical conductivity, lustre of metal, ability to undergo significant plastic deformation (with forces above elastic limit)–>associated with malleability and ductility; these characteristcs are not seen with ceramics and polymers, which atoms are covalently or properly ionically bonded

Question 35

Q

CQ: why are most pure metals not useful for most dental applications?

Answer

A

they are too soft and some may corrode excessively (with the exception of the noble metals gold, iridium, ruthenium, titanium, osmium, platinum, rhodium, and palladium)

Question 36

Q

how does Cr impart corrosion-resistance?

Answer

A

Chromium provides this corrosion resis- tance by forming a very thin, adherent surface oxide (Cr2O3) that prevents the di usion of oxygen or other corroding species to the underlying metal.

added to Fe + C –> stainless steal
also added to cobalt and nickel alloys

Question 37

Q

why might Cu be added to a pure metal?

Answer

A

increase strength and decrease perm. def (creep)

Question 38

Q

how are cast dental alloys classified?

Answer

A

(1) use (all-metal inlays, crowns and bridges, metal-ceramic prostheses, posts and cores, removable partial dentures, and implants); (2) major elements (gold-based, palladium-based, silver-based, nickel-based, cobalt-based, and titanium-based); (3) nobility (high noble, noble, and pre- dominantly base metal); (4) three principal elements (such as Au-Pd-Ag, Pd-Ag-Sn, Ni-Cr-Be, Co-Cr-Mo, Ti-Al-V, and Fe-Ni-Cr); and (5) dominant phase system (single phase, eutectic, peritectic, and intermetallic types).

Question 39

Q

define eutectic

Answer

A

eutectic alloy is 2 or more metals/metal+elements that melt and solidify at the same temp

Question 40

Q

define peritectic

Answer

A

ehhhh coming soon

Question 41

Q

define the simplest alloy

Answer

A

all components are mutually soluble, the crystals (e.g. FCC or BCC) are homogenous–>one phase is produced–>MICROSTRUCTURE resembles a pure metal

Question 42

Q

When two metals are not completely soluble in each other, the solid state is a mixture of two or more phases. Important examples are the eutectic alloys and peritectic alloys,

Question 43

Q

Alloys can have inter- mediate phases, which have a range of compositions di erent from the solid solutions formed by the nearly pure metals. In some alloy systems, intermetallic compounds with a xed composition can also be formed.

Question 44

Q

CQ: How do gold, platinum, palladium, and silver concentrations affect the color of gold alloys and the thermal expansion coefficient of high noble and noble metal alloys? Which of these elements strengthen and harden noble metal alloys? Which three alloy ele- ments promote bonding to porcelain?

Answer

A

coming soon

Question 45

Q

why might gold be used in an alloy?

Answer

A

A high gold concentration provides a warm, esthetically attractive hue, superb tarnish and corrosion resistance, excel- lent ductility, minimal abrasiveness, and superb wear resis- tance for its alloys; raises thermal expansion coeff. for palladium alloys (Ag can also do this)–>doesn’t expand as much (?)

Question 46

Q

palladium whitens alloys; raises the melting range of gold alloys as well as their elastic modulus, strength, and hardness; lowers the density of gold alloys; On the other hand, palladium is used to lower the TEC of gold-based PFM (porcelain fused metal) alloys.

Question 47

Q

gold and palladium are common in PFM bc they offset each other’s limitations; Gallium is used primarily in Pd-based PFM alloys. Gallium strengthens these alloys and decreases their melting range.

Question 48

Q

why add Pt to an alloy?

Answer

A

used primarily in yellow-gold PFM alloys to increase the melting range, hardness, strength, and elastic modulus. Like palladium (Pd), it also decreases the TEC of gold alloys. Additions of Pt affect the properties of gold alloys to a lesser extent compared with Pd. However, Pt has less of an e ect on changes in the color of alloys with high gold content.

Question 49

Q

why is silver added to a dental cast alloy?

Answer

A

Silver is added to Au-Ag-Cu casting alloys to o set the reddish hue contributed by Cu. However, Ag-rich Au-Ag-Cu alloys tend to have a slightly greenish hue. In Pd-based PFM alloys, Ag is used primarily to raise the thermal expansion coe cient (TEC). Silver decreases the melting range of both Pd and Au alloys. It also tends to improve the ow of casting alloys and solders. However, Ag has been reported to cause a greenish-yellow discoloration of some dental porcelains,

Question 50

Q

why is Cu added to alloys?

Answer

A

Copper strengthens and reddens Au-Ag-Cu crown and bridge alloys. However, it is not used in alloys with high gold contents because, like Ag, it also tends to discolor porcelain. It is added to Pd-based PFM alloys to increase their TECs. However, higher Cu contents produce dark-colored oxide layers that may adversely a ect the esthetics of metal-ceramic (PFM) restorations. Copper does not seem to cause porcelain discoloration if it is alloyed primarily with Pd. Cobalt has been used as an alternative to Cu in Pd-based PFM alloys. However, like Cu, it also forms dark-colored oxides.

Question 51

Q

why is Zn added to alloys?

Answer

A

Zinc is added to crown and bridge alloys as an oxygen scavenger, thereby reducing gas porosity in castings. For PFM alloys, zinc can be added also to strengthen and harden the alloys and/or to increase the TEC. It also decreases the melting range.

Question 52

Q

why is indium added to alloys?

Answer

A

Indium (In) is used in some Au-Ag-Cu casting alloys to improve their castability. In Au- and Pd-based alloys, it strengthens and hardens the alloys, increases their TECs, and decreases their melting temperature range. It also contributes to the formation of a bonding oxide in PFM alloys. Tin also contributes to the formation of a bonding oxide and it strengthens and hardens Au- and Pd-based PFM alloys. It also decreases the melting range of Au-based and Pd-based alloys and increases the TECs of these alloys.

Question 53

Q

purpose of Fe in alloys?

Answer

A

Iron is used primarily to strengthen Au-Pt alloys for PFM applications. Like Sn and In, it also forms a bonding oxide.

Question 54

Q

what is the purpose of grain boundary modification and what metals affect it?

Answer

A

Three noble elements that are used to refine the grain structure of alloys are iridium (Ir), rhenium (Re), and ruthe- nium (Ru). Grain re nement restricts the growth of grains during solidi cation. Smaller grains block dislocation move- ment from grain to grain, resulting in increased yield strength.

Also, does it prevent plastic deformation?

Question 55

Q

CQ: Why do dental alloys begin freezing by heterogeneous nucle- ation rather than by homogeneous nucleation?

Answer

A

they are not pure metals…the components may freeze at different temperatures so net nucleation of one component will begin when net nucleation cannot for another

Question 56

Q

During melting, the temperature remains constant. During freezing or solidi cation, heat is released as the metal changes from a high-energy liquid to a low-energy solid. is energy di erence is the latent heat of solidi cation and is equal to the heat of fusion.

Answer

A

It is defined as the number of calories of heat liberated from 1 g of a substance when it transforms from a liquid to a solid.

Question 57

Q

describe supercooling of metal

Answer

A

metal initial cools from point B’ to Tf–>supercooling; crystallization of a pure metal begins during SCing, and once the crystals begin to form the release of latent heat of fusion causes temp to right to Tf, where the temp remains until crystallization has completed at point C;
KEEP IN MIND: Supercooling of pure metals occurs only in clean, inert containers under cir- cumstances in which heterogeneous nucleation of metal crystals is not possible.

Question 58

Q

why is fusion temp and soldification behaviour important to dentists?

Answer

A

Typically, an exact wax or plastic replica of the prosthesis form is prepared initially. Using a highly accurate dental investment, an expanded mold is prepared from the pattern, into which the molten alloy is cast under pressure. When the alloy solidifies, it shrinks and the original pattern is precisely reproduced as a cast metal structure.

Question 59

Q

describe nucleus formation

Answer

A

forms from embryos…

Liquid metals do not differ in structure from one another as much as do solid metals, and these relationships change–solidification begins with the formation of small embryos in molten metal (small clusters of atoms that form nuclei of crystallization that have the same arrangement as the long-range atomic order found in the solidified metal.)

At temperatures above the fusion tem- perature (Tf), these embryos will also form spontaneously in the molten metal, but they are unstable, since the liquid state has a lower free energy than the solid state.

Question 60

Q

CQ: What are the contributions of the surface free energy and the volume free energy to the overall free energy of these embryos as a function of the embryo size?

Answer

A

SFE: surface energy higher than internal energy when embryos are formed bc of the unequal forces on the surface ‘cules–>draws these atoms together and surface tension increases; so work is required to create an embryo;

Fv (vol free energy) contribution: diff. bw the free energy of the solid and liquid states; solid state has the lowest free energy when temp is below Tf (so free energy becomes more negative and energetically fav. as temp of supercooled liquid is decreased below Tf)

Question 61

Q

At a speci c temperature of the supercooled liquid metal, the overall, resultant (R) free energy of an embryo as a func- tion of its radius is the sum of the surface free energy (positive) and the volume free energy change (negative) con- tributions;

Question 62

Q

what is critical nucleus size?

Answer

A

a radius ro at which point any addition of another atom the overall free energy (R) continues to decrease

corre- sponds to the maximum point in total free energy of the embryos as a function of the radius.

Question 63

Q

how is supercooling related to ro?

Answer

A

The greater the amount of supercooling, or the greater the rate of cooling below Tf,, the smaller is the critical radius ro because the value of FV for a given embryo size becomes increasingly negative.

value of FS per unit area is not greatly a ected by the amount of supercooling.

Question 64

Q

decreased radii–>decrease surface energy

Answer 52

A

small values of ro that occur during the solidification of metals. Hence, an increasing number of embryos become stable as supercooling increases, and these embryos have reduced surface energy because of their decreased radii. If the molten metal is cooled so rapidly that solidi cation occurs at a much lower tempera- ture than Tf, there is a tendency for many small, stable nuclei of solidi cation to form.

a random process, having an equal probability of occurring at any point in the molten metal; surface energy of embryos must be reduced so atoms can contact the wet surface of areas in molten metal or on the mold surface; when embryo surface makes contact with molten metal/mold surface, SE is reduced, and a stable embryo is formed;

Answer 53

A

is mechanism is controlled by seeding the nuclei with impurities on the mold wall. When pure gold solidi es, ne particles of gold di use into the molten metal and cause nucleation. In this way, imperfections in the mold walls, particles of dust, and other impurities in the molten metal can produce heterogeneous nucleation of crystals.

Answer 54

A

coming soon

Answer 55

A

spontaneous embryos form; atoms diffuse from molten metal onto nuclei–>random patterns of crystallization–>continue having this branched cryst. into cooler regions of the mold (mold walls cool fastest = that’s why cryst. happens there)

Answer 56

A

the interdendritic regions serve for crack formation and propagation

Answer 57

A

Microcracks, called “hot tears,” form at elevated temperatures in thin cast areas of these alloys. is process occurs when there is insufficient metal thickness to resist the stresses caused by the casting invest- ment during solidification. To avoid hot tears, castings must have adequate thickness, and alloys should have equiaxed grains in the as-cast condition rather than a dendritic struc- ture.

A lower burnout temperature (what is burnout temp?) can also minimize hot tearing because the alloy will have greater strength at lower mold temperatures.

Answer 58

A

Solidi cation starts from isolated nuclei in the molten metal, and these crystals gradually grow by the clus- tering of atoms and the crystals extend toward each other. When the adjacent crystals eventually contact one another, their growth stops,

Answer 59

A

The grain boundaries in a solidi ed metal are revealed by polishing and etching of the cast metal surface by a specific solution for a given time–the chemical or electrolytic etching solution preferentially removes atoms and creates grooves at the grain boundaries, because these atoms have a higher energy com- pared with atoms in the interiors of grains (must view in SEM and not light microscope)

Answer 60

A

As is true also for wrought metals, the movement of disloca- tions along their atomic slip planes during permanent defor- mation of ductile dental alloys continues until the dislocations meet a grain boundary. Since dislocations cannot progress along atom planes into adjacent grains because of their mis- alignment of the planes with those of adjacent grains, they will subsequently accumulate at the grain boundaries–>further dislocation requires greater greater stress–>strain/work hardening occurs–>leads to increases strength/decreased ductility

Pure Au is not useful for low-stress areas of restorations unless it is strain-hardened, as occurs when direct- lling gold is formed into a prepared tooth preparation.

Answer 61

A

strength/work hardening, final sites at which a molten metal solidi- es during the formation of an equiaxed grain structure. the grain boundaries also accumulate low-melting phases, precipitates, and porosity.

Answer 62

A

grain size of noble and high noble metal casting alloys controls the yield strength, (YS = 1/sqrt*grain size)

smaller grain size = rapid solidification; also, more uniformity and and corrosion resistance because there is less opportunity for microsegregation (happens with large grain)

Answer 63

A

a toxic element may be present in a low wt%, but it may be present at a high at%–>and the toxicity is usually based on at% rather than wt%

Answer 64

A

a low temp at which a liquid convers to liquid + solid phase

the liquidus temperature is the lowest temperature at which all components of an alloy can be in a liquid state.

Answer 65

A

metal at the liquidus temp –>fully converts to solid

Answer 66

A

when alloy is solidifed slowly, the Ag atoms can diffuse throughout the Pd atoms–>get uniform solid with no separate phases; but if it cooled rapidly, Ag atoms don’t have time to diffuse–>may create separate, segregateable phases–>must heat treat at high temps to get the uniformity

Answer 67

A

that metal whose crystal structure is retained over the composition range of interest.

In palladium-silver (Pd-Ag) alloys, the two metals are completely soluble in all propor- tions and the same type of crystal structure occurs through- out the alloy system (i.e., all compositions of the two elements). In such a case, the solvent is defined as the metal whose atoms occupy the majority of the total number of positions in the crystal structure.

Answer 68

A

solute metal–>atoms occupy the spaces of the solvent atoms

usually or must be smaller than solvent atoms

also must be present in small amounts

are said to be dissolved interstitially

Answer 69

A

ask dr. h

Answer 70

A

several conditions must be satis ed, including atom size difference, valence, type of crystal structure, and the potential for solvent atoms to become ordered.

The average distance between solvent atoms in a substitutional solid solution changes in response to the diameters of the solute atoms. If the sizes of two metallic atoms di er by less than approxi- mately 15%, they possess a favorable size factor for solid solu- bility. If the size is greater than 15%, multiple phases appear during solidi cation.

Answer 71

A

When the valences are dif- ferent, the electron/atom ratio for the alloy is di erent

Answer 72

A

substitute solvent atom for larger solute atoms–>localized distortion occurs and dislocation movement is impeded;

ductility is also decreased

effects of this solid-solution strengthening are similar to those achieved by cold working.

Answer 73

A

three elements–>3D phase diagram–>very complex

binary-phase diagrams are useful for understanding the structure of dental alloys and they can provide microstructural predictions when cast dental alloys are subjected to heat treatment.

Answer 74

A

useful to identify the phases present in an alloy system for di erent compositions and temperatures.

Answer 75

A

has a lower fusion temp than its constits; even if there are multiple phases, they solidfy at a single temp instead of a range

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Chapter 5 Flashcards

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