Summary: Processing Options

Austenite (γ)

Bainite (α + Fe3C plates/needles)

Pearlite (α + Fe3C layers + a

proeutectoid phase)

Martensite (BCT phase diffusionless

transformation)

Tempered Martensite (α + very fine

Fe3C particles)

slow cool

moderate cool

rapid quench

reheat

Str

en

gth

Du

cti

lity

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends

Adapted from Fig. 10.27, Callister 6e.

Chapter 11: Metal AlloysApplications and Processing

ISSUES TO ADDRESS...

• How are metal alloys classified and how are they used?

• What are some of the common fabrication techniques?

• How do properties vary throughout a piece of materialthat has been quenched, for example?

• How can properties be modified by post heat treatment?

Taxonomy of Metals

Adapted from Fig. 9.21,Callister 6e. (Fig. 9.21 adapted from Binary Alloy Phase Diagrams, 2nd ed.,Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

Adapted from Fig. 11.1, Callister 6e.

Fe3C

cementite

Metal Alloys

Steels

Ferrous Nonferrous

Cast Irons Cu Al Mg Ti<1.4wt%C 3-4.5wt%C

1600

1400

1200

1000

800

600

4000 1 2 3 4 5 6 6.7

L

γaustenite

γ+L

γ+Fe3Cα

ferriteα+Fe3C

α+γ

L+Fe3C

δ

(Fe) Co, wt% C

Eutectic:

Eutectoid:0.77

4.30

727°C

1148°C

T(°C)

Steels<1.4wt%C

Cast Irons3-4.5wt%C

microstructure: ferrite, graphite cementite

Steels

Low Alloy High Alloy

low carbon <0.25wt%C

med carbon 0.25-0.6wt%C

high carbon 0.6-1.4wt%C

Uses auto struc. sheet

bridges towers press. vessels

crank shafts bolts hammers blades

pistons gears wear applic.

wear applic.

drills saws dies

high T applic. turbines furnaces V. corros. resistant

Example 1010 4310 1040 4340 1095 4190 304

Additions noneCr,V Ni, Mo

noneCr, Ni Mo

noneCr, V, Mo, W

Cr, Ni, Mo

plain HSLA plainheat

treatableplain tool

austentitic stainless

Name

Hardenability 0 + + ++ ++ +++ 0TS - 0 + ++ + ++ 0EL + + 0 - - -- ++

increasing strength, cost, decreasing ductilityBased on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister 6e.

Basic Ideas in Alloying Steels:1. Phase Partitioning at Austenite/Pearlite Interface

Carbide FormersV, Ti, Nb

Ferrite Formers (solid solution)Ni, Si, Mn

Paritioning at the Austenite/Pearlite InterfaceSlows transformationAllows Bainite or Martensite to form on cooling

Basic Ideas in Alloying Steels:2. Alloying to Control the Eutectoid Transformation

AlloyingControl ‘Nose’ in TTT DiagramControl Eutectoid Temperature and C Composition

Cr: added (~8 – 12wt %) to make steel ‘stainless’Ni: High concentrations to stabilize austenite – austenitic steels

Hardenability of Steels

• Ability to form martensite• Jominy end quench test to measure hardenability.

• Hardness versus distance from the quenched end.

24°C water

specimen (heated to γ phase field)

flat ground

4”

1”

Ha

rdn

ess

, HR

C

Distance from quenched end

Adapted from Fig. 11.10, Callister 6e. (Fig. 11.10 adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill Book Company, New York, 1978.)

Adapted from Fig. 11.11, Callister 6e.

Why Hardness Changes with Position

• The cooling rate varies with position.

Adapted from Fig. 11.12, Callister 6e. (Fig. 11.12 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 376.)

distance from quenched end (in)Ha

rdn

ess

, HR

C

20

40

60

0 1 2 3

600

400

200A → M

A → P

Martensite

Martensite + Pearlite

Fine Pearlite

Pearlite

0.1 1 10 100 1000

T(°C)

M(start)

Time (s)

0

0%100%

M(finish)

Hardenability vs. Alloy Content

• Jominy end quenchresults, C = 0.4wt%C

• "Alloy Steels"(4140, 4340, 5140, 8640)--contain Ni, Cr, Mo

(0.2 to 2wt%)--these elements shift

the "nose".--martensite is easier

to form.

Adapted from Fig. 11.13, Callister 6e. (Fig. 11.13 adapted from figure furnished courtesy Republic Steel Corporation.)

T(°C)

10-1 10 103 1050

200

400

600

800

Time (s)

M(start)M(90%)

TE

A Bshift from A to B due to alloying

Cooling rate (°C/s)

Ha

rdn

ess

, HR

C

20

40

60

100 20 30 40 50Distance from quenched end (mm)

210100 3

4140

8640

5140

1040

50

80

100

%M4340

Quenching Medium and Geometry

• Effect of quenching medium:

Mediumairoil

water

Hardnesssmall

moderatelarge

Severity of Quenchsmall

moderatelarge

• Effect of geometry:When surface-to-volume ratio increases:

--cooling rate increases--hardness increases

Positioncenter

surface

Cooling ratesmalllarge

Hardnesssmalllarge

Nonferrous Alloys

NonFerrous Alloys

• Cu AlloysBrass: Zn is subst. impurity (costume jewelry, coins, corrosion resistant)Bronze: Sn, Al, Si, Ni are subst. impurity (bushings, landing gear)Cu-Be: precip. hardened for strength

• Al Alloys-lower ρ: 2.7g/cm3 -Cu, Mg, Si, Mn, Zn additions -solid sol. or precip. strengthened (struct.

aircraft parts & packaging)

• Mg Alloys-very low ρ: 1.7g/cm3 -ignites easily -aircraft, missles

• Refractory metals-high melting T -Nb, Mo, W, Ta• Noble metals

-Ag, Au, Pt -oxid./corr. resistant

• Ti Alloys-lower ρ: 4.5g/cm3

vs 7.9 for steel -reactive at high T -space applic.

Based on discussion and data provided in Section 11.3, Callister 6e.

Metal Fabrication Methods (1)

CASTING JOININGFORMING

Ao Ad

force

dieblank

force

• Forging(wrenches, crankshafts)

• Rolling(I-beams, rails)

• Extrusion(rods, tubing)

Adapted from Fig. 11.7, Callister 6e.

ram billet

container

containerforce

die holder

die

Ao

Adextrusion

roll

AoAd

roll

• Drawing(rods, wire, tubing)

often atelev. T

tensile force

AoAddie

die

Forming Temperature

• Hot working--recrystallization--less energy to deform--oxidation: poor finish--lower strength

• Cold working--recrystallization--less energy to deform--oxidation: poor finish--lower strength

• Cold worked microstructures--generally are very anisotropic!--Forged --Fracture resistant!

Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.), John Wiley and Sons, Inc., 1996. (a) Fig. 10.5, p. 410 (micrograph courtesy of G. Vander Voort, Car Tech Corp.); (b) Fig. 10.6(b), p. 411 (Orig. source: J.F. Peck and D.A. Thomas, Trans. Metall. Soc. AIME, 1961, p. 1240); (c) Fig. 10.10, p. 415 (Orig. source: A.J. McEvily, Jr.and R.H. Bush, Trans. ASM 55, 1962, p. 654.)

(a) (b) (c)

--Swaged

Metal Fabrication Methods (2)

plasterdie formedaround waxprototype

FORMING JOININGCASTING• Sand Casting

(large parts, e.g.,auto engine blocks)

Sand Sand

molten metal

• Investment Casting(low volume, complex shapese.g., jewelry, turbine blades)

wax

• Die Casting(high volume, low T alloys)

• Continuous Casting(simple slab shapes)

molten

solidified

Metal Fabrication Methods (3)

CASTINGFORMING JOINING• Powder Processing

(materials w/low ductility)

pressure

heat

point contact at low T

densification by diffusion at higher T

area contact

densify

• Welding(when one large part isimpractical)

• Heat affected zone:(region in which themicrostructure has beenchanged).

Adapted from Fig. 11.8, Callister 6e.(Fig. 11.8 from Iron Castings Handbook, C.F. Walton and T.J. Opar (Ed.), 1981.)

piece 1 piece 2

fused base metal

filler metal (melted)base metal (melted)

unaffectedunaffectedheat affected zone

Thermal Processing of Metals

Annealing: Heat to Tanneal, then cool slowly.

Types of Annealing

• Process Anneal: Negate effect of cold working by (recovery/ recrystallization)

• Stress Relief: Reduce stress caused by:

-plastic deformation -nonuniform cooling -phase transform.

• Normalize (steels): Deform steel with large grains, then normalize to make grains small.

• Full Anneal (steels): Make soft steels for good forming by heating to get γ, then cool in furnace to get coarse P.

• Spheroidize (steels): Make very soft steels for good machining. Heat just below TE & hold for

15-25h.

Based on discussion in Section 11.7, Callister 6e.

Precipitation Hardening

• Particles impede dislocations.• Ex: Al-Cu system• Procedure:

--Pt A: solution heat treat(get α solid solution)

--Pt B: quench to room temp.--Pt C: reheat to nucleate

small θ crystals withinα crystals.

• Other precipitationsystems:• Cu-Be• Cu-Sn• Mg-Al

Pt A (sol’n heat treat)

Pt B

Pt C (precipitate θ)

Temp.

Time

Adapted from Fig. 11.22, Callister 6e. (Fig. 11.22 adapted from J.L. Murray, International Metals Review 30, p.5, 1985.)

Adapted from Fig. 11.20, Callister 6e.

300

400

500

600

700

0 10 20 30 40 50wt%Cu(Al)

Lα+Lα

α+θθ

θ+L

T(°C)

A

B

C

composition range needed for precipitation hardening

CuAl2

Precipitate Effect on TS, %El

• 2014 Al Alloy:

• TS peaks withprecipitation time.

• Increasing T acceleratesprocess.

• %EL reaches minimumwith precipitation time.

precipitation heat treat time (h)

ten

sile

str

en

gth

(M

Pa

)

300

400

500

2001min 1h 1day 1mo 1yr

204°C

149°C

non-

equi

l. so

lid s

olut

ion

man

y sm

all

prec

ipita

tes

“ag

ed”

fe

wer

larg

e

pre

cipi

tate

s

“ove

rage

d”%

EL

(2

in s

am

ple

)10

20

30

0 1min 1h 1day 1mo 1yr

204°C 149°C

precipitation heat treat time (h)

Adapted from Fig. 11.25 (a) and (b), Callister 6e. (Fig. 11.25 adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, 1979. p. 41.)

Summary

• Steels: increase TS, Hardness (and cost) by adding--C (low alloy steels)--Cr, V, Ni, Mo, W (high alloy steels)--ductility usually decreases w/additions.

• Non-ferrous:--Cu, Al, Ti, Mg, Refractory, and noble metals.

• Fabrication techniques:--forming, casting, joining.

• Hardenability--increases with alloy content.

• Precipitation hardening--effective means to increase strength in

Al, Cu, and Mg alloys.

Chapter 8: Mechanical Failure

ISSUES TO ADDRESS...• How do flaws in a material initiate failure?• How is fracture resistance quantified; how do different

material classes compare?• How do we estimate the stress to fracture?• How do loading rate, loading history, and temperature

affect the failure stress?

Ship-cyclic loadingfrom waves.

Computer chip-cyclicthermal loading.

Hip implant-cyclicloading from walking.

Adapted from Fig. 8.0, Callister 6e.(Fig. 8.0 is by Neil Boenzi, The New York Times.)

Adapted from Fig. 18.11W(b), Callister 6e. (Fig. 18.11W(b) is courtesy of National Semiconductor Corporation.)

Adapted from Fig. 17.19(b), Callister 6e.

Ductile vs. Brittle Failure

Highly Ductile Moderately Ductile Brittle

Ductile:Warning before

fracture

Brittle:Little

warning

Example: Pipe Failure

• Ductile failure:--one piece--large deformation

• Brittle failure:--many pieces--small deformation

Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., 1987. Used with permission.

Moderately Ductile Failure

• Evolution to failure:

necking void nucleation

void growth and linkage

shearing at surface

fracture

σ

• Resultingfracturesurfaces(steel)

50 µm

particlesserve as voidnucleationsites.

50 µm

100 µm

From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures(2nd ed.), Fig. 11.28, p. 294, John Wiley and Sons, Inc., 1987. (Orig. source: P. Thornton, J. Mater. Sci., Vol. 6, 1971, pp. 347-56.)

Fracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission.

“Cup-and-Cone” Fracture and Brittle Fracture

Cup-and-cone fracture in Al

Dimpling (Center) Dimpling (Edge)

Brittle Fracture Surfaces

• Intergranular(between grains)

• Intragranular(within grains)

Al Oxide(ceramic)

Reprinted w/ permission from

"Failure Analysis of Brittle Materials", p. 78.

Copyright 1990, The American Ceramic

Society, Westerville, OH. (Micrograph by R.M. Gruver and H.

Kirchner.)

316 S. Steel (metal)

Reprinted w/ permission from

"Metals Handbook", 9th ed, Fig. 650, p. 357.

Copyright 1985, ASM International, Materials Park, OH. (Micrograph

by D.R. Diercks, Argonne National Lab.)

304 S. Steel (metal)Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p. 650. Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge National Lab.)

Polypropylene(polymer)Reprinted w/ permission from R.W. Hertzberg, "Defor-mation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.35(d), p. 303, John Wiley and Sons, Inc., 1996.

3µm

4 mm160µm

1 mm(Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p. 1119.)

Brittle Fracture Surfaces

Transgranular fracture(ductile cast iron)

Intergranular fracture

Fractography – study of fracture surfaces (usually SEM)

Brittle Fracture Surfaces

Brittle fracture surfaces

Arrows indicate origin

Fracture surface structure:Crack OriginCrack path

Ideal vs. Real Materials

• Stress-strain behavior (Room T):

σ

ε

E/10

E/100

0.1

perfect mat’l-no flaws

carefully produced glass fiber

typical ceramic typical strengthened metaltypical polymer

TS << TSengineeringmaterials

perfectmaterials

• DaVinci (500 yrs ago!) observed...--the longer the wire, the

smaller the load to fail it.• Reasons:

--flaws cause premature failure.--Larger samples are more flawed!

Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.4. John Wiley and Sons, Inc., 1996.

Flaws are Stress Concentrators!

• Elliptical hole ina plate:

• Stress distrib. in front of a hole:

• Stress conc. factor:

BAD! Kt>>3NOT SO BAD

Kt=3

σmax

ρt

≈ 2σo

a + 1⎛ ⎝ ⎜

⎞ ⎠ ⎟

ρtσ

σo

2a

Kt = σmax /σo

• Large Kt promotes failure:

σo = average stressρT = radius of crack tip2a = crack length

Engineering Fracture Design

• Avoid sharp corners!

r/h

sharper fillet radius

increasing w/h

0 0.5 1.01.0

1.5

2.0

2.5

Stress Conc. Factor, Ktσmax

σo=

Adapted from Fig. 8.2W(c), Callister 6e.(Fig. 8.2W(c) is from G.H. Neugebauer, Prod. Eng. (NY), Vol. 14, pp. 82-87 1943.)

r , fillet

radius

w

h

σo

σmax

When Does a Crack Propagate?

• ρt at a cracktip is verysmall!

σ

• Result: crack tipstress is very large.

• Crack propagates when:the tip stress is largeenough to make:

distance, x, from crack tip

σtip = K2π x

σtip

increasing K

K ≥ Kc

Geometry, Load, & Material

• Condition for crack propagation:

• Values of K for some standard loads & geometries:σ

2a2a

σ

aa

K = σ πa K = 1.1σ πa

K ≥ KcStress Intensity Factor:--Depends on load &

geometry.

Fracture Toughness:--Depends on the material,

temperature, environment, &rate of loading.

units of K :MPa mor ksi in

Adapted from Fig. 8.8, Callister 6e.

Fracture Toughness

Kc = Y σc (πa)½

Kc Fracture toughnessY Geometric factor (crack geometry)σc Critical stress for crack propagationa Crack length (depends on crack geometry)

Mode I (Tensile)

Mode II (Sliding)

Mode III (Tearing)

Crack Mode Designations

Graphite/ Ceramics/ Semicond

Metals/ Alloys

Composites/ fibersPolymers

5

KIc

(MP

a ·

m0

.5)

1

Mg alloysAl alloys

Ti alloys

Steels

Si crystalGlass-soda

Concrete

Si carbide

PC

Glass6

0.5

0.7

2

4

3

10

20

30

<100>

<111>

Diamond

PVC

PP

Polyester

PS

PET

C-C(|| fibers)1

0.6

67

40506070

100

Al oxideSi nitride

C/C( fibers)1

Al/Al oxide(sf)2

Al oxid/SiC(w)3

Al oxid/ZrO2(p)4Si nitr/SiC(w)5

Glass/SiC(w)6

Y2O3/ZrO2(p)4

Kcmetals

Kccomp

Kccer≈ Kc

poly

inc

rea

sin

g

Based on data in Table B5,Callister 6e.Composite reinforcement geometry is: f = fibers; sf = short fibers; w = whiskers; p = particles. Addition data as noted (vol. fraction of reinforcement):1. (55vol%) ASM Handbook, Vol. 21, ASM Int., Materials Park, OH (2001) p. 606.2. (55 vol%) Courtesy J. Cornie, MMC, Inc., Waltham, MA.3. (30 vol%) P.F. Becher et al., Fracture Mechanics of Ceramics, Vol. 7, Plenum Press (1986). pp. 61-73.4. Courtesy CoorsTek, Golden, CO.5. (30 vol%) S.T. Buljan et al., "Development of Ceramic Matrix Composites for Application in Technology for Advanced Engines Program", ORNL/Sub/85-22011/2, ORNL, 1992.6. (20vol%) F.D. Gace et al., Ceram. Eng. Sci. Proc., Vol. 7 (1986) pp. 978-82.

Fracture Toughness