application marten last lect

8/4/2019 Application Marten Last Lect

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The Science and Engineering

of Materials, 4th edDonald R. Askeland Pradeep P. Phul

Chapter 11 Dispersion Strengthening byPhase Transformations and Heat Treatment


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Objectives of Chapter 11

Discuss dispersion strengthening bystudying a variety of solid-statetransformation processes including

precipitation or age hardening and theeutectoid reaction.

Examine how nonequilibrium phasetransformationsin particular, themartensitic reactioncan providestrengthening.


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Chapter Outline

11.1 Nucleation and Growth in Solid-StateReactions

11.2 Alloys Strengthened by Exceeding the

Solubility Limit 11.3 Age or Precipitation Hardening

11.4 Applications of Age-Hardened Alloys

11.5 Microstructural Evolution in Age or

Precipitation Hardening 11.6 Effects of Aging Temperature and

Time


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11.7 Requirements for Age Hardening 11.8 Use of Age-Hardenable Alloys at

High Temperatures

11.9 The Eutectoid Reaction

11.10 Controlling the EutectoidReaction

11.11 The Martensitic Reaction andTempering

11.12 The Shape-Memory Alloys(SMAs)

Chapter Outline (Continued)


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Strain energy - The energy required to permit aprecipitate to fit into the surrounding matrix duringnucleation and growth of the precipitate.

Avrami relationship - Describes the fraction of atransformation that occurs as a function of time. Thisdescribes most solid-state transformations that involvediffusion, thus martensitic transformations are notdescribed.

Section 11.1Nucleation and Growth in

Solid-State Reactions


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Figure 11.1 Sigmoidal curve showing the rate oftransformation of FCC iron at a constant temperature. The

incubation time t0 and the time for the 50%

transformation are also shown.


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Figure 11.2 The effect of temperature on recrystallizationof cold-worked copper.


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Figure 11.3 (a) The effect of temperature on the rate of a

phase transformation is the product of the growth rate andnucleation rate contributions, giving a maximumtransformation rate at a critical temperature. (b)Consequently, there is a minimum time (tmin) required for thetransformation, given by the C-curve.


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Determine the activation energy for the recrystallization ofcopper from the sigmoidal curves in Figure 11.2.

Example 11.1Activation Energy for the Recrystallization

of Copper

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herein under license.

Figure 11.2The effect oftemperatureonrecrystallization of cold-

workedcopper.


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Figure 11.4Arrhenius plot oftransformation rateversus reciprocal

temperature forrecrystallization ofcopper (for Example11.1.


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Example 11.1 SOLUTION

From Figure 11.2, the times required for 50%transformation at several different temperatures can be

calculated:

The rate of transformation is an Arrhenius equation, soa plot of ln (rate) versus 1/T (Figure 11.4 and

Equation 11-4) allows us to calculate the constants inthe equation. Taking natural log of both sides ofEquation 11-4:

ln(Growth rate) = ln A (Q/RT)


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Example 11.1 SOLUTION (Continued)

Thus, if we plot ln(Growth rate) as a function of 1/T, weexpect a straight line that has a slope of - Q/R.


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Widmansttten structure - The precipitation of a secondphase from the matrix when there is a fixedcrystallographic relationship between the precipitate andmatrix crystal structures.

Interfacial energy - The energy associated with theboundary between two phases.

Dihedral angle - The angle that defines the shape of a

precipitate particle in the matrix. Coherent precipitate - A precipitate whose crystal

structure and atomic arrangement have a continuousrelationship with the matrix from which the precipitate isformed.

Section 11.2Alloys Strengthened by

Exceeding the Solubility Limit


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Figure 11.5 The aluminum-copper phase diagram and themicrostructures that may develop curing cooling of an Al-4%Cu alloy.


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Figure 11.6 (a) Widmansttten needles in a Cu-Ti alloy( 420). (From ASM Handbook, Vol. 9, Metallography

and Microstructure (1985), ASM International,Materials Park, OH 44073.) (b) Continuous precipitatein an Al-4% Cu alloy, caused by slow cooling ( 500).(c) Precipitates of lead at grain boundaries in copper( 500).


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Figure 11.7 The effect of surface energy and the dihedralangle on the shape of a precipitate.


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Figure 11.8 (a) A noncoherent precipitate has norelationship with the crystal structure of the surroundingmatrix. (b) A coherent precipitate forms so that there is adefinite relationship between the precipitates and thematrixs crystal structure.


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Age hardening - A special dispersion-strengthening heattreatment. By solution treatment, quenching, and aging,a coherent precipitate forms that provides a substantialstrengthening effect. Also known as precipitation

hardening, it is a form of dispersion strengthening.

Section 11.3Age or Precipitation Hardening


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Section 11.4Applications of Age-Hardened

Alloys

Figure 11.9 (a) Astress-strain curveshowing theincrease instrength of a bake-hardenable steel asa result of strainhardening andprecipitationhardening.(Source: U.S. SteelCorporation,Pittsburgh, PA.)


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Figure 11.9 (b) A graph showing the increase in the yieldstrength of a bake hardenable steel (Source: BethlehemSteel, PA.) (c) A TEM micrograph of a steel containingniobium (Nb) and manganese (Mn). The niobium react withcarbon (C) and forms NbC precipitates that lead tostrengthening. (Courtesy of Dr. A.J. Deardo, Dr. I. Garcia,Dr. M. Hua, University of Pittsburgh.)


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Step 1: Solution Treatment

Step 2: Quench

Step 3: Age Guinier-Preston (GP) zones - Tiny clusters of atoms that

precipitate from the matrix in the early stages of the age-hardening process.

Section 11.5Microstructural Evolution in

Age or Precipitation Hardening


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Figure 11.10 The aluminum-rich end of the aluminum-copperphase diagram showing the three steps in the age-hardeningheat treatment and the microstructures that are produced.


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Compare the composition of the a solid solution in the Al-4%Cu alloy at room temperature when the alloy cools underequilibrium conditions with that when the alloy is quenched.

Example 11.2Composition of Al-4% Cu Alloy Phases

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under license.

Figure 11.5The

aluminum-copper phasediagram andthemicrostructures that maydevelop curingcooling of anAl-4% Cualloy.


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From Figure 11.5, a tie line can be drawn at roomtemperature. The composition of the determined fromthe tie line is about 0.02% Cu. However, the compositionof the after quenching is still 4% Cu. Since containsmore than the equilibrium copper content, the issupersaturated with copper.


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Step 1: Solution-treat at a temperature between thesolvus and the eutectic to avoid hot shortness. Thus,heat between 340oC and 451oC.

Step 2: Quench to room temperature fast enough toprevent the precipitate phase from forming.

Step 3: Age at a temperature below the solvus, that is,below 340oC, to form a fine dispersion of phase.


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Figure 11.12 An electron micrographof aged Al-15% Ag showing

coherent g0 plates and round GPzones (40,000). (Courtesy of J.B.Clark.)


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Section 11.6Effects of Aging Temperature

and Time

Figure 11.13 Theeffect of agingtemperature andtime on the yieldstrength of an Al-4% Cu alloy.

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herein under license.


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The operator of a furnace left for his hour lunch break withoutremoving the Al-4% Cu alloy from the furnace used for theaging treatment. Compare the effect on the yield strength ofthe extra hour of aging for the aging temperatures of 190oC

and 260

o

C.

Example 11.4Effect of Aging Heat Treatment Time on

the Strength of Aluminum Alloys

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Learning is a trademark used herein under license.

Figure 11.13 Theeffect of agingtemperature and

time on the yieldstrength of an Al-4% Cu alloy.


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At 190oC, the peak strength of 400 MPa (60,000 psi)

occurs at 2 h (Figure 11.13). After 3 h, the strength isessentially the same.

At 260oC, the peak strength of 340 MPa (50,000psi) occurs at 0.06 h. However, after 1 h, the strengthdecreases to 250 MPa (40,000 psi).

Thus, the higher aging temperature gives lowerpeak strength and makes the strength more sensitive toaging time.


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The alloy system must display decreasing solid solubilitywith decreasing temperature.

The matrix should be relatively soft and ductile, and theprecipitate should be hard and brittle.

The alloy must be quenchable.

A coherent precipitate must form.

Section 11.7Requirements for Age

Hardening


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Section 11.8Use of Age-Hardenable Alloys

at High Temperatures


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Austenite - The name given to the FCC crystal structureof iron.

Ferrite - The name given to the BCC crystal structure ofiron that can occur as or .

Cementite - The hard, brittle ceramic-like compoundFe3C that, when properly dispersed, provides thestrengthening in steels.

Pearlite - A two-phase lamellar microconstituent,containing ferrite and cementite, that forms in steels

cooled in a normal fashion or isothermally transformedat relatively high temperatures.

Section 11.9The Eutectoid Reaction


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Figure 11.15 The Fe-Fe3C phase diagram ( aportion of the Fe-C

diagram). The verticalline at 6.67% C is thestoichiometriccompound Fe3C.


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Figure 11.16 Growth and structure of pearlite: (a)redistribution of carbon and iron, and (b)photomicrograph of the pearlite lamellae (2000).(From ASM Handbook, Vol. 7, (1972), ASM

International, Materials Park, OH 44073.)


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Tungsten carbide-cobalt composites, known as cementedcarbides or carbides, are used as bits for cutting tools anddrills (Chapter 1). What features are similar between thesecemented carbides and pearlite, a microconstituent insteels? What are some of the major differences?


In both materials, we take advantage of the toughness of onephase (ferrite or cobalt metal, in the case of pearlite in steeland WC-Co, respectively) and the hard ceramic like phase (WC

and Fe3C, in WC-Co and steel, respectively). The metallicphase helps with ductility and the hard phase helps withstrength.

Example 11.6Tungsten Carbide (WC)-Cobalt (Co)

Composite and Pearlite


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Figure 11.18 (a) A hypoeutectoid steel showing primary

(white) and pearlite ( 400). (b) A hypereutectoidsteel showing primary Fe3C surrounding pearlite (800). (From ASM Handbook, Vol. 7, (1972), ASM


E l 11 7


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Calculate the amounts and compositions of phases andmicroconstituents in a Fe-0.60% C alloy at 726oC.


The phases are ferrite and cementite. Using a tie line andworking the lever law at 726oC, we find:

Example 11.7Phases in Hypoeutectoid

Plain Carbon Steel

%7.81000218.067.60218.060.0)%%67.6(

%3.911000218.067.660.067.6)%%0218.0(

33

CFeCCFe

C


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Figure 11.23 (a) Upper bainite (gray, feathery plates)( 600). (b) Lower bainite (dark needles) ( 400).(From ASM Handbook, Vol. 8, (1973), ASM



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Figure 11.24 The effect of transformation temperature onthe properties of an eutectoid steel.


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Design a heat treatment to produce the pearlite structureshown in Figure 11.16(b).

Example 11.8Design of a Heat Treatment to Generate

Pearlite Microstructure

Figure 11.16 Growth andstructure of pearlite: (b)photomicrograph of thepearlite lamellae ( 2000).(From ASM Handbook, Vol. 7,

(1972), ASM International,Materials Park, OH 44073.)


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Excellent combinations of hardness, strength, and toughnessare obtained from bainite. One heat treatment facilityaustenitized an eutectoid steel at 750oC, quenched and heldthe steel at 250oC for 15 min, and finally permitted the steel

to cool to room temperature. Was the required bainiticstructure produced?

Example 11.9Heat Treatment to Generate Bainite

Microstructure


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Martensite- A metastable phase formed in steel andother materials by a diffusionless, athermaltransformation.

Displacive transformation - A phase transformation thatoccurs via small displacements of atoms or ions andwithout diffusion. Same as athermal or martensitictransformation.

Tempering - A low-temperature heat treatment used to

reduce the hardness of martensite by permitting themartensite to begin to decompose to the equilibriumphases.

Section 11.11The Martensitic Reaction and

Tempering


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is a t rademark used herein under license.

Figure 11.26 The effect of carbon content on the hardness ofmartensite in steels.


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Figure 11.27 (a) Lath martensite in low-carbon steel( 80). (b) Plate martensite in high-carbon steel( 400). (From ASM Handbook, Vol. 8, (1973), ASM



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Figure 11.28 Effect of

tempering temperatureon the properties ofand eutectoid steel.


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Close to a half million people in the world have coronarystents. These are mostly made from 316 stainless steel, butsome are made from platinum. How would you go aboutdesigning a material for making a cardiovascular stent?[15]

A conventional stent is essentially a slotted tube that isinserted into an artery. This procedure is typically done bydoctors after a procedure known as an angioplasty isconducted.

Example 11.12Selection of Material for a Self-Expandable

Cardiovascular Stent


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Figure 11.33 TheZr02-Ca0 phasediagram. Apolymorphic phasetransformationoccurs for pureZr02. Adding 16 to26% Ca0 producesa single cubiczirconia phase at alltemperatures (for

Problem 11.62).


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Figure 11.36 The effect of temperature on the crystallizationof polypropylene (for Problems 11.4 and 11.106).

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