Chapter 5 Flashcards
1
Q
define alloy
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.
2
Q
define alloy system
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.
3
Q
define binary metal alloy
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?)
4
Q
define coring
A
A microstructure in which a composition gradient exists between the center and the surface of cast dendrites, grains, or particles.
5
Q
define dendritic microsctructure
A
A cast alloy structure of highly elongated crystals with a branched morphology.
6
Q
define equiaxed grain microstructure
A
A cast alloy microstructure with crystal (grain) dimensions that are similar along all crystal axes.
7
Q
define grain
A
A single crystal in the microstructure of a metal.
8
Q
define grain boundary
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.
9
Q
define heterogenous nucleation
A
Formation of solid nuclei on the mold walls or on particles within a solidifying molten metal.
10
Q
define homogenous nucleation
A
Formation of nuclei that occur at random locations within a supercooled molten metal in a clean, inert container.
11
Q
define metal
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.
12
Q
define microstructure
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.
13
Q
define nucleus
A
A stable cluster of atoms in a new phase that forms within a parent matrix phase during the solidification of a microstructure.
14
Q
define phase
A
A homogeneous, physically distinct, and mechanically separable portion of a metal microstructure.
15
Q
define phase/constituitive diagram
A
A graph of equilibrium phases and solubility limits for an alloy system as a function of composition and temperature.
16
Q
define quaternary alloy
A
An alloy that contains four elements at least one of which is a metal/ or if all four are metals??
17
Q
define solid sol’n (metallic)
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.
18
Q
define tarnish
A
Superficial discoloration or dulling of a metal surface that is often caused by a reaction with oxygen or sulfur
19
Q
define ternary alloy
A
An alloy that contains three elements at least one of which is a metal.
20
Q
CQ: why is the term metal difficult to define?
A
no single characteristic defines a metal completely (some produce a ringing sound when struck, some have lustre, magnetism, etc)
21
Q
give examples of how pure metal can differ from alloy
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)
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
A
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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.
A
ye
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).
A
ehhhh
25
what is Kic?
fracture toughness--metals have a higher fracture toughness than ceramics, polymers, and composites; in MPa x m^1/2 (=MPa/m^2??)
26
define fracture toughness
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?
27
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.
ye
28
which metals are magnetic but can also be produced in a non-mag state?
iron, nickel, and cobalt— can be magnetic, but they can also be produced in a nonmag- netic state
29
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?
ye
30
why might Fe, Sn, or Indium be added to a high noble/noble metal alloys?
the first three form oxide layers--this promotes bonding of metal to ceramic veneer
31
critical question: Why are the general physical and mechanical properties of metallic dental materials different from those of ceramic and polymeric dental materials?
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
32
The boundary between metals and nonmetals is indistinct, and the elements near the boundary exhibit characteristics of both metals and nonmetals.
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33
describe metallic bonding; why is it not covalent?
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
34
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
ye
35
CQ: why are most pure metals not useful for most dental applications?
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)
36
how does Cr impart corrosion-resistance?
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
37
why might Cu be added to a pure metal?
increase strength and decrease perm. def (creep)
38
how are cast dental alloys classified?
(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).
39
define eutectic
eutectic alloy is 2 or more metals/metal+elements that melt and solidify at the same temp
40
define peritectic
ehhhh coming soon
41
define the simplest alloy
all components are mutually soluble, the crystals (e.g. FCC or BCC) are homogenous-->one phase is produced-->MICROSTRUCTURE resembles a pure metal
42
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,
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43
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.
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44
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?
coming soon
45
why might gold be used in an alloy?
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 (?)
46
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.
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47
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.
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48
why add Pt to an alloy?
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.
49
why is silver added to a dental cast alloy?
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,
50
why is Cu added to alloys?
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.
51
why is Zn added to alloys?
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.
52
why is indium added to alloys?
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.
53
purpose of Fe in alloys?
Iron is used primarily to strengthen Au-Pt alloys for PFM applications. Like Sn and In, it also forms a bonding oxide.
54
what is the purpose of grain boundary modification and what metals affect it?
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?
55
CQ: Why do dental alloys begin freezing by heterogeneous nucle- ation rather than by homogeneous nucleation?
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
56
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.
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.
57
describe supercooling of metal
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.
58
why is fusion temp and soldification behaviour important to dentists?
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.
59
describe nucleus formation
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.
60
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?
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)
61
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;
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62
what is critical nucleus size?
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.
63
how is supercooling related to ro?
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.
64
decreased radii-->decrease surface energy
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65
describe homogenous nucleation
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;
66
what is heterongenous nuc.
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.
67
CQ: Why does the grain size of a cast dental alloy significantly decrease when the rates of cooling and solidification increase?
coming soon
68
describe the liquid-solid crystallization
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)
69
why are dendritic microstructures bad for cast dental alloys?
the interdendritic regions serve for crack formation and propagation
70
what are hot tears and how do we avoid them?
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.
71
Although predominantly base metal (PB) alloys for dental prostheses typically solidify with a dendritic microstructure, most high noble (HN) and noble (N) metal casting alloys solidify with an equiaxed polycrystalline microstructure (grains)
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72
equiaxed--the three dimensions of the grains of equiaxed polycrystalline microstructure are similar, as oppsoed to dendritic microstructure
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73
describe equiaxed polycryst. microstructure formation
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,
74
how to visualize grain boundaries?
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)
75
what is strain/work hardening?
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.
76
what occurs at grain boundaries?
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.
77
CQ: how does grain size affect properties of dental materials?
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)
78
CQ: Why should a dentist or lab technician consider both the weight percent and the atomic percent of potentially toxic elements when selecting an alloy for dental prostheses?
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%
79
what is the liquidus temp?
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.
80
what is the solidus temp?
metal at the liquidus temp -->fully converts to solid
81
alloy-->temp range for melting/solidifying, bc alloys will melt and solidify at diff temps
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82
e concentration of each element can be expressed as a weight percentage (wt%) or as an atomic percentage (at%). As an example, the AuCu3 phase, which can form during slow cooling of molten Au-Cu alloys in a speci c composition range, contains 51 wt% Au and 49 wt% Cu, but on an atomic basis, it contains 25 at% Au and 75 at% Cu.
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83
describe how a uniform microstructure of the Ag-Pd alloy is achieved
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
84
what is the solvent in an alloy?
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.
85
define solute
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
86
pg 80 says AuCu3 is FCC but then says its simple cubic??
ask dr. h
87
how can two solids be mutually soluble?
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.
88
solid sol'ns more likely to form if the constits. have the same valence
When the valences are dif- ferent, the electron/atom ratio for the alloy is di erent
89
If elemental species have a high degree of chemical affinity, they tend to form an intermetallic compound during solidi- cation rather than a solid solution. Metals such as Au and Cu, which have some chemical affinity for each other, form ordered structures at lower temperatures.
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90
the di erence between the atomic diameters of Ag and Sn is approximately 4%. However, these two metals di er in valence and have di erent crystal struc- tures, so there is limited solid solubility for Sn in Ag.
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91
how to increase strength, hardness, and proportional limit in an alloy?
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.
92
Solid solution strengthening of an alloy increases with greater concentrations of the solute atoms and with increasingly dissimilar sizes of the solvent and solute atoms up to the solid solubility limit
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93
CQ: Why is the study of binary-phase diagrams important, even though dental alloys contain more than three elements and conditions during solidification and cooling after casting are far removed from equilibrium?
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.
94
what are eq. phase diagrams and why are they needed?
useful to identify the phases present in an alloy system for di erent compositions and temperatures.
95
CQ: Why is the development of microsegregation in cast alloys more pronounced when there is a greater difference between the liqui- dus and solidus temperatures?
yes
96
A homogenization heat treatment, which pro- motes atomic di usion, can eliminate as-cast compositional di erence and produce equiaxed grains
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97
An inhomogeneous dental gold alloy is more subject to tarnish and corrosion than the same alloy a er homogeniza- tion. is is also important for Ag-Pd alloys, since silver-rich phases tend to tarnish readily in the oral environment
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98
Alloys with heterogeneous microstructures have greater resistance to permanent deformation than alloys with homogeneous microstructures. Consequently the ductility of an alloy usually increases with a corresponding decrease in brittleness a er a homogenization heat treatment.
????????
99
CQ: What are the advantages and disadvantages of eutectic alloys? How do alloy compositions of binary eutectic systems affect their properties?
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100
the temperature at which the eutectic composition melts (779 °C, corresponding to line BEG) is lower than the fusion temperature of 100% silver or 100% copper (eutectic means “lowest melting”) and is the lowest temperature at which any alloy of silver and copper is entirely liquid.
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101
describe eutectic alloys
has a lower fusion temp than its constits; even if there are multiple phases, they solidfy at a single temp instead of a range
102
Decrease melting range as in lowers it from say 1000-1500 degree to 900-1400 degree, or makes it smaller like 1000-1300 degree?
Idk m8
