4- Metal Alloys

Front 1

what do the properties of metals and alloys and their performance depend on?

Back 1

-composition
-structure
-processing history
-heat treatment to which they have been subjected

Front 2

what properties are influenced by alloying and heat treatment?

Back 2

-strength
-ductility
-toughness
-resistance to wear

Front 3

how do you improve the properties of non-heat-treatable alloys?

Back 3

by mechanical working operations (rolling, forging, extrusion)

Front 4

heat treatment

Back 4

modifies microstructures and thereby produces a variety of mechanical properties important in manufacturing (improved formability and machinability, increased strength and hardness)

Front 5

pure metals

Back 5

have atoms all of the same type

Front 6

alloy

Back 6

composition of 2 or more chemical elements, at least 1 of which is a metal

Front 7

alloying

Back 7

process of modifying a pure metal

Front 8

what are the 2 types of alloys?

Back 8

-solid solutions
-intermetallic compounds

Front 9

solute

Back 9

-minor element (salt, sugar)
-composed of solute atoms

Front 10

solvent

Back 10

-major element (water)
-composed of host atoms

Front 11

solid solution

Back 11

when the crystal structure of the solvent is maintained during alloying

Front 12

substitutional solid solution

Back 12

the solute atoms can replace solvent atoms when the size of the solute atoms are similar to that of the solvent atoms

Front 13

conditions for a substitutional solid solution

Back 13

1. the 2 metals must have similar crystal structures
2. the difference in their atomic radii should be less than 15%

Front 14

interstitial solid solutions

Back 14

each solute atom occupies an interstitial position, the size of the solute atom is much smaller than that of the solvent atom

Front 15

conditions for interstitial solid solutions

Back 15

1. solvent atoms must have more than one valence electron
2. atomic radius of solute atom must be less than 59% of that of the solvent atom

Front 16

an important family of interstitial solid solutions (example)

Back 16

-steel, alloy of iron and carbon
-carbon atoms are present in interstitial positions between iron atoms
-carbon atomic radius: .071 nm, iron atomic radius: .124 nm

Front 17

intermetallic compounds

Back 17

-complex structures consisting of 2 metals in which solute atoms are present among solvent atoms in certain proportions
-some have solid solubility
-atomic bonds range from metallic to ionic
-they are strong, hard and brittle
-gas turbine engines
-ex: aluminides of titanium, nickel, and iron

Front 18

two-phase system

Back 18

-alloys with 2 or more solid phases that may be regarded as mechanical mixtures
-2 solid phases
-most alloys

Front 19

phase

Back 19

-a physically distinct and homogeneous portion in a material
-ex: sand and water mixture, ice and water

Front 20

second-phase particles

Back 20

-finely dispersed particles
-alloying with second-phase particles is an important way of strengthening alloys (in two-phase alloys, these particles present obstacles to dislocation)

Front 21

liquidus

Back 21

temperature of molten metal below which solidification begins in alloys

Front 22

solidus

Back 22

temperature at which solidification is complete in alloys

Front 23

phase diagram

Back 23

also equilibrium or constitutional diagram, shows relationship between temp, composition, and phases in an alloy at equilibrium

Front 24

binary phase diagram

Back 24

-2 elements present in system
-ex: copper-nickel alloy

Front 25

lever rule

Back 25

procedure for determining the composition of various phases in phase diagrams

Front 26

eutectic point

Back 26

-point at which the liquid solution decomposes into the components alpha and beta
-"eutektos" = greek for "easily melted"

Front 27

iron-iron-carbide phase diagram

Back 27

see 107

Front 28

alpha ferrite

Back 28

solid solution of bcc iron

Front 29

curie temperature

Back 29

768 degrees C, ferrite is magnetic from room temp to 768 degrees C

Front 30

polymorphic transformation

Back 30

process iron undergoes within a certain temperature range, converting from a bcc to an fcc structure

Front 31

austenite

Back 31

gamma iron, fcc structure, result of polymorphic transformation of iron

Front 32

cementite

Back 32

100% iron carbide

Front 33

carbide

Back 33

cementite

Front 34

eutectoid reaction

Back 34

a single solid phase (austenite) is transformed into 2 other solid phases (ferrite and cementite)

Front 35

pearlite

Back 35

-the structure of eutectoid steel, at low magnifications it resembles mother-of-pearl
-microstructure consists of alternating layers of ferrite and cementite

Front 36

lamellae

Back 36

alternating layers, ex: the alternating layers of ferrite and cementite in pearlite

Front 37

eutectoid ferrite

Back 37

the ferrite in pearlite

Front 38

proeutectoid ferrite

Back 38

-"pro" meaning "before"
-the ferrite phase, forms at temps higher than the eutectoid temp

Front 39

eutectoid cementite

Back 39

cementite in pearlite

Front 40

proeutectoid cementite

Back 40

the cementite phase, forms at temps higher than the eutectoid temp

Front 41

austenite former

Back 41

alloying element such as nickel, since lowering the eutectoid temp means increasing the austenite range
-nickel favors the fcc structure of austenite

Front 42

ferrite stabilizers

Back 42

elements such as chromium and molybdenum, because they have a bcc structure they favor the bcc structure of ferrite

Front 43

cast irons

Back 43

family of ferrous alloys composed of iron, carbon, and silicon

Front 44

classifications of cast irons [according to their solidification morphology from the eutectic temperature]

Back 44

a. gray cast iron
b. ductile cast iron, nodular cast iron, or spherical graphite cast iron
c. white cast iron
d. malleable iron
e. compacted graphite iron

Front 45

classifications of cast irons [according to their structure]

Back 45

ferritic, pearlitic, quenched and tempered, or austempered

Front 46

metastable

Back 46

state of cementite, not completely stable

Front 47

graphitization

Back 47

-formation of graphite
-can be controlled by modifying the composition and the rate of cooling, and by adding silicon

Front 48

gray cast iron

Back 48

-state of graphite where it exists as flakes
-when broken, the fracture path is along the graphite flakes and has gray appearance
-flakes act as stress raisers, causing negligible ductility and weak tension
-strong in compression

Front 49

types of gray cast irons

Back 49

-ferritic (structure: graphite flakes in an alpha-ferrite matrix)
-pearlitic (structure: graphite in a matrix of pearlite)
-martensitic (structure: graphite in a martensite matrix)

Front 50

ductile (nodular) iron

Back 50

-graphite is in nodular or spheroid form (makes ductile and shock resistant)
-graphite flakes become spheres by magnesium/cerium addition to molten metal

Front 51

white cast iron

Back 51

-large amounts of iron carbide instead of graphite (makes hard, wear resistant, brittle)
-obtained by cooling gray iron rapidly
-white crystalline appearance of fracture surface

Front 52

malleable iron

Back 52

-obtained by annealing white cast iron in carbon monoxide and carbon dioxide atmosphere
-cementite decomposes into iron and graphite
-graphite exists as cluster or rosettes in a ferrite or pearlite matrix
-malleable: promotes ductility, strength, shock resistance

Front 53

compacted-graphite iron

Back 53

-graphite in the form of short, thick, interconnected flakes with undulating surfaced and rounded extremities
-physical properties between flake-graphite and nodular-graphite

Front 54

heat-treatment

Back 54

controlling heating and cooling of alloys at various rates

Front 55

phase transformations

Back 55

induced by heat treatments, influence strength, hardness, ductility, toughness, and wear resistance of alloys

Front 56

microstructural changes in the iron-carbon system

Back 56

pearlite, spheroidite, bainite, martensite, tempered martensite

Front 57

fine pearlite

Back 57

ferrite and cementite lamellae in the pearlite structure of eutectoid steel are thin and closely packed

Front 58

coarse pearlite

Back 58

ferrite and cementite lamellae in the pearlite structure of eutectoid steel are thick and widely spaced

Front 59

what does the difference between fine and coarse pearlite depend on?

Back 59

-rate of cooling through the eutectoid temp
-high rate of cooling = fine pearlite, slow rate of cooling = coarse pearlite

Front 60

spheroidite

Back 60

-when pearlite is heated to just below the eutectoid temperature and held there for about a day, cementite lamellae transform to spherical shapes
-less conductive of stress (unlike lamellar shapes of cementite), higher toughness and lower hardness than pearlite, can be cold worked

Front 61

bainite

Back 61

-very fine microstructure of ferrite and cementite, visible only through electron microscopy

Front 62

bainitic steel

Back 62

bainite produced in steels at cooling rates higher than those required for transformation to pearlite, stronger and more ductile than pearlitic steels at the same hardness level

Front 63

matensite

Back 63

-austenite cooled at high rate transforms from fcc to bct structure
-hard, brittle, lacks toughness

Front 64

bct

Back 64

body-centered tetragonal

Front 65

quench cracking

Back 65

caused by rapid cooling during quenching

Front 66

size distortion

Back 66

changes in the dimensions of a part without change in shape

Front 67

distortion

Back 67

irreversible dimensional change of a part during heat treatment

Front 68

shape distortion

Back 68

involves bending, twisting, and similar nonsymmetrical dimensional changes

Front 69

retained austenite

Back 69

-if the temp to which the alloy is quenched is not sufficiently low, only a portion of the structure is transformed to martensite, and the rest is retained austenite
-visible as white areas in the alloy
-can cause dimensional instability and cracking, lowers hardness and strength

Front 70

tempering

Back 70

a heating process by which hardness is reduced and toughness is improved

Front 71

tempered martensite

Back 71

martensite is tempered to improve its mechanical properties, bct martensite is heated to an intermediate temp and decomposes to a 2-phase microstructure of bcc alpha ferrite and particles of cementite

Front 72

isothermal transformation diagrams

Back 72

or TTT (time temp transformation) diagrams, illustrates the transformation to austentite to pearlite

Front 73

hardenability

Back 73

capability of an alloy to be hardened by heat treatment, measure of depth of hardness obtained by heating and quenching

Front 74

factors that affect hardenability

Back 74

carbon content, grain size of the austenite, alloying elements present in the material, and cooling rate

Front 75

jominy test

Back 75

1. a round, 100mm, test bar made from a particular alloy is austenitized
2. it is quenched at one end with water at 24 degrees so that the cooling rate varies throughout the bar
3. the hardness is measures at various lengths
4. results--hardness decreases away from the quenched end of the bar

Front 76

austenitized

Back 76

heated to the proper temperature to form 100% austenite

Front 77

hardenability band

Back 77

rating for jominy test, each alloy has a particular hardenability band

Front 78

how does carbon content affect hardness?

Back 78

hardness increases by increasing carbon content

Front 79

severity of quench

Back 79

the rate of cooling of the alloy, is different for different fluids used for quenching (water, brine (salt water), oils, molten salts, air)

Front 80

cooling capacity of quenching media

Back 80

-brine = 5
-water = 1
-oil =.3
-gas = .1
-air = .02

Front 81

vapor blanket

Back 81

forms along surfaces of metal, water vapor bubbles that form when water boils during rapid cooling using water as quenching media, acts as a heat barrier

Front 82

polymer quenchants

Back 82

-polyvinyl alcohol, polyalkaline oxide, polyvinyl pyrrolidone, polyethyl oxazoline
-used for ferrous and nonferrous alloy quenching
-have cooling characteristics between water and oil
-advantages: better control of hardness results, elimination of fire, reduction of corrosion

Front 83

precipitation hardening

Back 83

-heat treatment/hardening process for nonferrous alloys and stainless steels (because they don't have phase transformations like steels)
-precipitates are uniformly dispersed in the matrix of the original phase causing more precipitates to form

Front 84

precipitates

Back 84

small particles of a different phase, used in precipitation hardening

Front 85

solution treatment

Back 85

1. alloy is heated to within kappa phase (between 500 and 570 degrees)
2. rapidly cooled (ex: quenched in water)
3. result = single phase kappa (alloy with moderate strength and high ductility)

Front 86

precipitation hardening

Back 86

1. alloy is reheated to intermediate temp
2. held at that temp for period of time while precipitation occurs
3. theta phase--submicroscopic precipitates are formed
4. result = strong structure, not very ductile

Front 87

age hardening

Back 87

property improvement that occurs during the precipitation hardening process

Front 88

natural aging

Back 88

process where aluminum alloys are quenched then allowed to harden over time at room temp

Front 89

cryogenic treatment

Back 89

slowing natural aging by refrigerating the quenched alloy

Front 90

overaging

Back 90

-reheated alloy is held at step 2 for and extended period of time
-precipitates become larger but fewer
-result = softer and weaker alloy

Front 91

maraging

Back 91

= martensite age hardening
-precipitation hardening treatment
-steel with 18% Ni
-carried out at 480 degrees C
-results = hardening does not depend on cooling rate

Front 92

case/surface hardening applications

Back 92

gear teeth, cams, shafts, bearings, clutch plates

Front 93

types of case-hardening processes

Back 93

-carburizing (gas, liquid, pack)
-carbonitriding
-cyaniding
-nitriding
-boronizing
-flame hardening
-induction hardening
-laser-beam hardening

Front 94

carburizing

Back 94

1. heat steel (900 degrees) in atmosphere of carbonaceous gases (gas carburizing) or carbon containing solids (pack carburizing)
2. quench
3. result = 55-65 HRC, high-carbon surface, some distortion

Front 95

carbonitriding

Back 95

1. heat steel (750 degrees) in atmosphere of carbonaceous gas and ammonia
2. quench in oil
3. result = 55-62 HRC, mild distortion

Front 96

cyaniding

Back 96

1. heat steel (800 degrees) in molten bath of cyanide solutions and other salts
2. result = 65 HRC, some distortion

Front 97

nitriding

Back 97

1. heat steel (550 degrees) in atmosphere of ammonia gas or mixtures of molten cyanide salts
2. result = 1100 HV

Front 98

boronizing

Back 98

heat by using boron containing gas or solid in contact with part
result = extremely hard, wear resistant

Front 99

flame hardening

Back 99

1. heat with oxyacetylene torch
2. quench with water spray
3. result = 50-60 HRC, little distortion

Front 100

induction hardening

Back 100

1. place metal part in copper induction coils
2. heat using high frequency current
3. quench
4. result = same as flame hardening

Front 101

laser and electron beams

Back 101

-can be effectively used to harden surfaces
-advantages = control of power input, low distortion, reach normally inaccessible areas
-disadvantages = high cost, depth of case hardened layer is small

Front 102

decarburization

Back 102

-undesirable
-phenomenon in which alloys containing carbon lose carbon from their surfaces as a result of heat treatment (caused by oxygen contact)
-affects hardness, strength, fatigue life
-avoid by processing parts in a vacuum

Front 103

annealing

Back 103

term used to describe the restoration of a cold-worked or heat-treated alloy to its original properties (ex: to increase ductility and reduce hardness and strength)
process:
1. heat workpiece to specific temp range in furnace
2. hold at temp for period of time (soaking)
3. cool workpiece (in air or in a furnace)

Front 104

full annealing

Back 104

annealing ferrous alloys

Front 105

normalizing

Back 105

-cooling in still air to avoid excessive softness from the annealing of steels
- done to refine grain structure, homogenize (uniform structure), decrease residual stresses, improve machinability

Front 106

process annealing

Back 106

workpiece is annealed to restore ductility (lost during cold working)

Front 107

stress-relief annealing

Back 107

workpiece is annealing to eliminate residual stresses

Front 108

tempering

Back 108

for steels harden by heat treatment tempering is used to reduce brittleness, increase ductility and toughness, reduce residual stress
1. heat steel to specific temp
2. cool at specific rate

Front 109

drawing

Back 109

= tempering

Front 110

temper embrittlement

Back 110

caused by segregation of impurities along grain boundaries at temperatures between 480 and 590 degrees C

Front 111

austempering

Back 111

-heated steel is quenched from austenitizing temperature rapidly to avoid formation of ferrite or pearlite
1. hold at certain temp until isothermal transformation from austenite to bainite is complete
2. cool at room temp
3. quench using molten salt

Front 112

modified austempering

Back 112

mixed structure of pearlite and bainite it obtained
ex: patenting (high ductility, high strength--ex: patented wire)

Front 113

martempering

Back 113

1. steel or cast iron quenched from austenitizing temp in hot-fluid medium (oil, molten salt)
2. held at temp until temp is uniform throughout
3. cooled at moderate rate (air) to avoid temp gradients
4. tempered
5. result = steels with less tendency to crack, distort, or develop residual stresses

Front 114

modified martempering

Back 114

-quenching temp is lower, thus cooling rate is higher
-suitable for steels with lower hardenability

Front 115

ausforming

Back 115

1. steel is formed into desired shapes within controlled temp and time ranges to avoid formation of nonmartensitic transformation products
2. cooled at various rates to obtain desired microstructures
3. result = superior mechanical properties

Front 116

thermomechanical processing

Back 116

= ausforming

Front 117

types of furnaces

Back 117

batch furnaces, continuous furnaces

Front 118

batch furnaces

Back 118

-types= box furnace, pit furnace, bell furnace, elevator furnace

Front 119

box furnace

Back 119

horizontal rectangular chamber with 1 or 2 access doors

Front 120

car-bottom furnace

Back 120

variation of box furnace, parts are loaded onto flatcar which moves on rails into furnace

Front 121

pit furnace

Back 121

vertical pit below ground level into which parts are lowered, desirable for long parts like rod that are warped in horizontal furnaces

Front 122

bell furnace

Back 122

round or rectangular box furnace without a bottom and is lowered over stacked parts, desirable for coils of wire

Front 123

elevator furnace

Back 123

parts loaded onto car platform and raised into furnace, desirable for alloys that have to be quenched rapidly because quenching tank can be directly under furnace

Front 124

continuous furnace

Back 124

suitable for high production runs

Front 125

salt-bath furnaces

Back 125

-high heating rates, uniformity of temp
-nonferrous strip and wire

Front 126

fluidized beds

Back 126

dry, fine, and loose solid particles (usually aluminum oxide) are heated and suspended in chamber by upward flow of hot gas at various speeds, parts to be heated are placed within the floating particles

Front 127

induction heating

Back 127

part is heated rapidly by the electromagnetic field generated by an induction coil carrying alternating current, which induces eddy currents in the part, coil can be shaped to fit part and is made of copper, coil can be designed to quench part after heating it

Front 128

bluing

Back 128

formation of a thin blue film of oxide on finished part to improve appearance and resistance to oxidation