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The Science and Engineering of Materials, 4th edDonald R. Askeland – Pradeep P. Phulé

Chapter 2 – Dispersion Strengthening by Phase Transformations and Heat Treatment

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Chapter Outline 2.1 Principles of Dispersion Strengthening

2.2 Alloys Strengthened by Exceeding the Solubility Limit

2.3 Age or Precipitation Hardening

2.4 Requirements for Age Hardening

2.5 The Eutectoid Reaction

2.6 Controlling the Eutectoid Reaction

2.7 The Martensitic Reaction and Tempering

2.8 The Shape-Memory Alloys (SMAs)

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Dispersion strengthening - Increasing the strength of a

material by forming more than one phase.

Matrix - The continuous solid phase in a complex

microstructure.

Precipitate - A solid phase that forms from the original

matrix phase when the solubility limit is exceeded.

Section 2.1

Principles of Dispersion Strengthening

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The precipitate phase should be hard and discontinuous, while the matrix should

be continuous and soft.

The dispersed phase particles should be small and numerous.

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Larger amounts of dispersed phase increase strengthening.

The dispersed phase particles should be round rather than needle-like.

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Coherent precipitate - A precipitate whose crystal structure and atomic

arrangement have a continuous relationship with the matrix from which

the precipitate is formed.

Section 2.2

Alloys Strengthened by Exceeding the

Solubility Limit

A noncoherent precipitate has no relationship

with the crystal structure of the surrounding

matrix.

A coherent precipitate forms so that there is a

definite relationship between the precipitate’s

and the matrix’s crystal structure.

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The A1-4% Cu alloy is 100% α above 500°C. The α phase is a solid solution of

aluminum containing copper up to 5.65 wt%.

On cooling below the solvus temperature, a second phase, θ, precipitates. The θ

phase, which is the hard, brittle intermetallic compound CuAl2, provides

dispersion strengthening.

At 200°C and below, in a 4% Cu alloy, only about 7.5% of the final structure is θ.

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Age hardening - A special dispersion-strengthening heat treatment. By

solution treatment, quenching, and aging, a coherent precipitate forms

that provides a substantial strengthening effect. Also known as

precipitation hardening, it is a form of dispersion strengthening.

Section 2.3

Age or Precipitation Hardening

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Step 1: Solution Treatment - The alloy is first heated above the solvus

temperature and held until a homogeneous solid solution α is produced.

This step dissolves the θ phase precipitate and reduces any microchemical

segregation present in the original alloy.

Step 2: Quench - The alloy, which contains only α in its structure, is rapidly

cooled, or quenched. The atoms do not have time to diffuse to potential

nucleation sites, so the θ does not form. After the quench, the structure is a

supersaturated solid solution αss containing excess copper, and it is not an

equilibrium structure. It is a metastable structure.

Age Hardening Steps

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Step 3: Age - The supersaturated α is heated at a temperature below the

solvus temperature. At this aging temperature, atoms diffuse only short

distances. Because the supersaturated α is metastable, the extra copper

atoms diffuse to numerous nucleation sites and precipitates grow.

Eventually, if we hold the alloy for a sufficient time at the aging

temperature, the equilibrium structure is produced.

Note that even though the structure that is formed has two equilibrium

phases (i.e., α+ θ), the morphology of the phases is different from the

structure that would have been obtained by the slow cooling of this alloy.

When we go through the three steps described previously, we produce the θ

phase in the form of ultra-fine uniformly dispersed second-phase precipitate

particles. This is what we need for effective precipitation strengthening.

Age Hardening Steps

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The aluminum-rich end of the aluminum-copper phase diagram showing the three steps in

the age-hardening heat 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 under equilibrium conditions with that when

the alloy is quenched.

Example 2.1 Composition of Al-4% Cu Alloy Phases

Example 2.1 SOLUTION

A tie line can be drawn at room

temperature. The composition

of the α determined from the

tie line is about 0.02% Cu.

However, the composition of

the α after quenching is still

4% Cu. Since α contains more

than the equilibrium copper

content, the α is supersaturated

with copper.

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1. The alloy system must display decreasing solid solubility with

decreasing temperature.

2. The matrix should be relatively soft and ductile, and the precipitate

should be hard and brittle.

3. The alloy must be quenchable.

4. A coherent precipitate must form.

A number of important alloys, including certain stainless steels and alloys

based on aluminum, magnesium, titanium, nickel, chromium, iron, and copper,

meet these conditions and are age hardenable.

We cannot select an age-hardened alloy for use at high temperatures.

Section 2.4

Requirements for Age Hardening

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Austenite - The name given to the FCC crystal structure of iron.

Ferrite - The name given to the BCC crystal structure of iron that can occur as α or δ.

Cementite - The hard, brittle ceramic-like compound Fe3C that, when properly

dispersed, provides the strengthening in steels.

Martensite - A metastable phase formed in steel and other materials by a

diffusionless, athermal transformation.

Pearlite - A two-phase lamellar microconstituent, containing ferrite and cementite,

that forms in steels cooled in a normal fashion or isothermally transformed at

relatively high temperatures.

Bainite - A two-phase microconstituent, containing ferrite and cementite, that forms

in steels that are isothermally transformed at relatively low temperatures. The

cementite is more rounded than in pearlite.

Tempered martensite - The microconstituent of ferrite and cementite (a mixture of

very fine and nearly round cementite in ferrite) formed when martensite is tempered.

Section 2.5

The Eutectoid Reaction

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Electron micrographs of (a) pearlite, (b) bainite, and (c)

tempered martensite, illustrating the differences in cementite

size and shape among these three microconstituents ( 7500).

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Eutectoid reaction - A three-phase invariant reaction in which one solid

phase transforms to two different solid phases.

Eutectic reaction - A three-phase invariant reaction in which one liquid

phase solidifies to produce two solid phases.

The eutectic reaction that occurs in Fe-C alloys at 1140°C is

The eutectic reaction that occurs in Fe-Fe3C system at 1146°C is

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The Fe-Fe3C phase diagram ( a portion of the Fe-C diagram). The vertical line at 6.67%

C is the stoichiometric compound Fe3C.

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The A3 shows the temperature at

which ferrite starts to form on

cooling.

The Acm shows the temperature at

which cementite starts to form.

The A1 is the eutectoid

temperature.

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Growth and structure of pearlite: (a) redistribution of carbon and iron, and (b)

photomicrograph of the pearlite lamellae (2000).

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Calculate the amounts of ferrite and cementite present in pearlite.

Example 2.2 Phases and Composition of Pearlite

%3.111000218.067.60218.077.0%

%7.881000218.067.677.067.6%

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CFe

Example 2.2 SOLUTION

Since pearlite must contain 0.77% C, using the

lever rule:

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The evolution of the microstructure of hypoeutectoid and hypereutectoid steels during

cooling, in relationship to the Fe-Fe3C phase diagram.

Hypoeutectoid steels contain less than 0.77% C, and hypereutectoid steels contain more

than 0.77% C.

Ferrite is the primary or proeutectoid microconstituent in hypoeutectoid alloys, and

cementite is the primary or proeutectoid microconstituent in hypereutectoid alloys.

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A hypereutectoid steel showing primary

Fe3C surrounding pearlite ( 800).

A hypoeutectoid steel showing primary

α (white) and pearlite ( 400).

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Calculate the amounts and compositions of phases and microconstituents in a

Fe-0.60% C alloy at 726oC.

Example 2.3 SOLUTION

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

Example 2.3 Phases in Hypoeutectoid Plain Carbon Steel

%7.81000218.067.60218.060.0)%%67.6(

%3.911000218.067.660.067.6)%%0218.0(

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CFeCCFe

C

All of the austenite at 727oC will have eutectoid composition

(i.e., it will contain 0.77% C) and will transform to pearlite;

all of the proeutectoid ferrite will remain as primary ferrite.

Primary α: 0.0218%C %primary α = [0.77−0.60

0.77−0.0218]100

= 22.7%

%Pearlite = [0.60−0.0218

0.77−0.0218]100

= 77.3%

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Controlling the Amount of the Eutectoid - By changing the composition of

the alloy, we change the amount of the hard second phase.

Controlling the Austenite Grain Size - We can increase the number of

pearlite colonies by reducing the prior austenite grain size, usually by using low

temperatures to produce the austenite.

Controlling the Cooling Rate - By increasing the cooling rate during the

eutectoid reaction, we reduce the distance that the atoms are able to diffuse.

Consequently, the lamellae produced during the reaction are finer or more

closely spaced. By producing fine pearlite, we increase the strength of the alloy.

Controlling the Transformation Temperature - The solidstate eutectoid

reaction is rather slow, and the steel may cool below the equilibrium eutectoid

temperature before the transformation begins (i.e., the austenite phase can be

undercooled). Lower transformation temperatures give a finer, stronger

structure, influence the time required for transformation, and even alter the

arrangement of the two phases.

Section 2.6

Controlling the Eutectoid Reaction

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TTT diagram - The time-temperature-transformation diagram describes

the time required at any temperature for a phase transformation to begin

and end.

Isothermal transformation - When the amount of a transformation at a

particular temperature depends on the time permitted for the

transformation.

TTT diagram, also called the isothermal transformation (IT) diagram or

the C-curve, permits us to predict the structure, properties, and heat

treatment required in steels.

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The time-temperature-transformation (TTT) diagram for an eutectoid steel.

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The sigmoidal curve is related to the start and finish times on the TTT diagram for

steel. In this case, austenite is transforming to pearlite.

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Lower bainite (dark needles) ( 400).

Upper bainite (gray, feathery plates) ( 600).

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The effect of transformation temperature on the properties of an eutectoid steel.

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Excellent combinations of hardness, strength, and toughness are obtained from

bainite. One heat treatment facility austenitized an eutectoid steel at 750oC,

quenched and held the steel at 250oC for 15 min, and finally permitted the steel

to cool to room temperature. Was the required bainitic structure produced?

Example 2.4 Heat Treatment to Generate Bainite Microstructure

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Example 2.4 SOLUTION

After heating at 750oC, the

microstructure is 100% γ. After

quenching to 250oC, unstable

austenite remains for slightly

more than 100 s, when fine

bainite begins to grow. After 15

min, or 900 s, about 50% fine

bainite has formed and the

remainder of the steel still

contains unstable austenite.

The heat treatment was not

successful! The heat treatment

facility should have held the

steel at 250oC for at least 104 s,

or about 3 h.

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Tempering - A low-temperature heat treatment used to reduce the

hardness of martensite by permitting the martensite to begin to

decompose to the equilibrium phases.

Section 2.7

The Martensitic Reaction and Tempering

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(a) The unit cell of BCT martensite is related to the FCC austenite unit cell. (b) As the

percentage of carbon increases, more interstitial sites are filled by the carbon atoms and

the tetragonal structure of the martensite becomes more pronounced.

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Unusual combinations of properties can be obtained by producing a steel whose

microstructure contains 50% ferrite and 50% martensite; the martensite provides

strength and the ferrite provides ductility and toughness. Design a heat treatment

to produce a dual phase steel in which the composition of the martensite is

0.60% C.

Example 2.5 Design of a Heat Treatment for

a Dual Phase (DP) Steel

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Example 2.5 SOLUTION

The heat treatment temperature is fixed by the requirement that the martensite

contain 0.60% C. From the solubility line between the γ and the α + γ regions, we

find that 0.60% C is obtained in austenite when the temperature is about 750oC. To

produce 50% martensite, we need to select a steel that gives 50% austenite when

the steel is held at 750oC. If the carbon content of the steel is x, then:

C 0.31% or x 5010002.060.0(

0.02) (x %

Our final design is:

1. Select a hypoeutectoid steel containing 0.31% C.

2. Heat the steel to 750oC and hold (perhaps for 1

h, depending on the thickness of the part) to

produce a structure containing 50% ferrite and 50%

austenite, with 0.60% C in the austenite.

3. Quench the steel to room temperature. The

austenite transforms to martensite, also containing

0.60% C.

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The effect of carbon content on the hardness of martensite in steels.

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Plate martensite in high-carbon steel ( 400).

Lath martensite in low-carbon steel ( 80).

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Effect of tempering temperature on the properties of a eutectoid steel.

If the steel is tempered just below the eutectoid temperature, the Fe3C becomes

very coarse, and the dispersion-strengthening effect is greatly reduced.

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Shape-memory effect - The ability of certain materials to develop

microstructures that, after being deformed, can return the material to its

initial shape when heated (e.g. Ni-Ti alloys).

Smart materials - Materials that can sense an external stimulus (e.g.,

stress, pressure, temperature change, magnetic field, etc.) and initiate a

response. Passively smart materials can sense external stimulus, actively

smart materials have sensing and actuation capabilities.

Section 2.8

The Shape-Memory Alloys (SMAs)