materials science manual chapter 7

7/29/2019 Materials Science Manual Chapter 7

1/14

Chapter 7. Non-equilibrium solid phase transformations

Chapter 7.NON-EQUILIBRIUM SOLID PHASE TRANSFORMATIONS

7.1 Introduction

The purpose for strengthening metallic materials is to extend the elastic range and raise their yield

and ultimate strength. The underlying mechanism responsible is the generation of more

dislocation while restricting their movement. However, complete pile up of dislocations must be

prevented, because it may result into brittle fracture. Six techniques are recognised to give rise to

strengthening in metallic materials.

(i) Workhardening: Strengthening due to creation of dislocations during cold working, while

distorting the lattice.

(ii) Solid solution hardening: Introduction of substitutional or interstitial solid solute atoms that

result in lattice distortion which restricts dislocation movement, thus strengthening the metal.

(iii) Grain size: Fine grain size raises the grain boundary area hence the dislocation density.

Hence fine grain size is associated with high strength and density.

(iv) Dispersion hardening: When particles of a second phase are dispersed in the matrix of a

metallic phase, they may form a coherent matrix lattice with some degree of lattice distortion.

Such a situation inhibits dislocation movement, thus strengthening the material.

(v) Transformation hardening: The transformation of the crystalline structure during heat

treatment may result into formation of a structure with fewer slip planes and distortions.

Martensitic transformation in steels is an example of such mechanism which results into

considerable hardening of steels.

(vi) Irradiation hardening: X-ray or Gamma radiations can produce vacancies or lattice

defects. However this technique is not commercially applied.

In this chapter, we cover two basic strengthening mechanisms resulting from non-equilibrium

phase transformations in metallic materials, namely precipitation hardening and martensitic

transformations.

129


2/14


7.2 Dispersion (Precipitation) Hardening

PRECIPITATION is the decomposition of a solid solution into two solid phases of different

composition, the precipitate and the solid solution.

' + (7.1)

PRECIPITATION HARDENING is the process whereby hardening of an alloy is caused by the

precipitation of a constituent from a supersaturated solid solution by heating to some elevated

temperature.

AGE HARDENING is a form of precipitation hardening in which there is a spontaneous increase of

hardness at room temperature with lapse of time, on a supersaturated solid solution.

The phenomena of precipitation hardening can occur ONLY in those alloys in which there is a

decrease of solid solubility with decreasing temperature resulting in formation of a supersaturated

solid solution upon fast cooling of the alloy from above the solvus line. This phenomena occurs in

certain types of aluminium alloys (e.g. Al-Ag, Al-Mg) and in Cu-Be alloys.

The process of precipitation hardening has three important steps as illustrated in Fig. 7.1:

(a) Solution treatment: The alloy is first heated to a temperature above the solvus temperature

and held until a homogeneous solid solution is produced. This step dissolves the

precipitate and reduces any segregation present in the original alloy.

(b) Quench: The solid solution which contains the solid solution only, is then rapidly cooled or

quenched. The atoms do not have enough time to diffuse to potential nucleation sites and

allow the phase to form. After the quench, the structure contains , the supersaturated

solid solution of.

(c) Age: Finally, the supersaturated is heated to a temperature below the solvus

temperature. At this aging temperature the atoms are able to diffuse short distances.

Because the supersaturated solid is unstable, extra solute atoms diffuse to numerous

nucleation sites and a precipitate forms and grows. If we were to hold the alloy for a

sufficient time at the aging temperature, the equilibrium and structure is produced.

The first stage in the precipitation reaction is the formation of nuclei of the precipitating phase,followed by the growth of the nuclei to larger particles. This results into lattice distortion which is

130


3/14


responsible for the hardening of the alloys. The actual cause of age hardening however, is the

obstruction to the motion of dislocations set up by the fine transition precipitate particles and the

strains produced by the mismatch of the transition products with the matrix.

If an alloy is held for a long period of time at the treatment temperature, coagulation of the

particles is observed and this intermediate phase becomes stable. This phenomena is called

overaging.

Other changes that are observed to accompany precipitation are:

1. Increase in electrical conductivity (except in Al-Cu alloys)

2. Increase in hardness, passing through a max. and then decreasing.

3. Increase in strength with hardness, decrease in ductility.

L

+L600

700

500

400

300

200

0 2 4 6 8

Weight percent Copper

1

2

3

5.65

548

'

''

Quench

Age

Fig 7.1: Steps in the heat treatment of an Al-Cu precipitation hardening alloy

During the aging of Al-Cu alloys, a series of precipitates form before the equilibrium is produced.

At the start of aging, the copper atoms concentrate on {100} planes in the matrix and produce

very thin clusters of copper atoms called GUINIER-PRESTON, or GP-I, zones. As aging

continues, more copper atoms diffuse to the precipitate and the GP-I zones grow into thin disks, or

GP-II zones. Later, the GP-II zones dissolve and , which is similar to the stable , forms.

Finally, dissolves and the stable phase precipitates.

131


4/14


The non-equilibrium precipitates - GP-I, GP-II and the - are coherent precipitates (Fig. 7.2). The

strength of the alloy increases with aging time as these coherent phases grow in size during the

initial stages of heat treatment. When these coherent precipitates are present, the alloy is in the

aged condition.

Fig. 7.2: A coherent precipitate

Fig. 7.3: A non coherent precipitate phase

When the stable non-coherent phase precipitates, the strength of the alloy decreases. Now the

alloy is in the overaged condition, as illustrated by Fig. 7.3.

Aging at room temperature is called natural aging. Aging at higher temperatures is called artificial

aging, because the alloy is heated to produce precipitation.

132


5/14


REQUIREMENTS FOR AGE HARDENING

Four conditions must be satisfied:

1 The phase diagram 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, rapidly enough to suppress formation of the second phase.

4 The precipitate that forms must be coherent with the matrix structure in order to develop the

maximum strength and hardness.

7.3 Non-equilibrium solid phase transformations of Austenite

7.3.1 Introduction

When steel is quenched rapidly from high temperatures, there is no time for the austenite (-Fe) to

transform to the lamellar structure called pearlite.

Pearlite consists of alternate plates of a-iron (ferrite) and iron carbide (cementite, Fe3C). During

fast cooling, the separate ferrite and cementite particles cannot form.

When the - phase is quenched below a certain temperature called Ms (Martensite starts

temperature), a new phase called martensite begins to form. Martensite has a body centred

tetragonal structure. The c/a ratio is proportional to the carbon content of the steel. On the other

hand, martensite can be regarded as a BCC crystal structure which has been distorted along one

axis as a result of the presence of carbon atoms. The atoms of martensite are less densely

packed than austenite, hence an expansion occurs during the transformation giving rise to

evolution of internal stresses.

The martensitic transformation is diffusionless and usually results in the formation of platelets

which form by a shearing mechanism at velocities above one third the velocity of sound in the

material.

133


6/14


Below a temperature called Mf(martensite finish temp.) no more austenite is transformed to

martensite. There is usually some retained austenite. Ms and Mfvary with carbon content in

carbon steel, as illustrated in Fig.7.4. The temperature Ms is further affected by the alloy content:

Ms = 561-474 (%C)-33(%Mn) - 17 (%Ni)-17 (%Cr)-21 (%Mo)

Mf is about 215 C below Ms

The structure martensite is very hard and strong. Dislocations move only with great difficulty.

The strength and hardness of martensite increase with carbon content, as shown in Fig. 7.5.

-200

-100

0

100

200

300

400

500

600

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Carbon wt %

Temperature,C

Ms C

Mf C

Fig. 7.4. Effect of carbon content on Ms and Mf Temperatures.

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8

Carbon, wt %

Hardness,HRC

50% Martensite

100% Martensite

Fig. 7.5. The effect of carbon content on Martensite hardness

134


7/14


7.3.2. Decomposition of Austenite

In alloys, the structural changes are preferably documented in equilibrium phase diagrams.Equilibrium means that they have been established for conditions where the properties of thesystem do not change with time and infinitum. In reality, equilibrium conditions are rarelyencountered; i.e. the cooling (or heating) rates are not slow enough to allow the continual phaseadjustments (diffusion) to occur. Under these conditions, data from equilibrium diagrams are nolonger directly applicable but serve only as an estimate.

Under equilibrium conditions, austenite, a solid solution of carbon in iron, is decomposed topearlite after precipitating ferrite in hypo-eutectoid steels, or cementite in hyper-eutectoid steels, asshown in Fig. 7.6.

0.035 0.8

723 C

910 C

Carbon, wt %

Austenite

()

Pearlite + Ferrite -Fe

+

P

Pearlite + Fe 3C

+ Fe 3C

P

Fe 3C

Fig. 7.6. The steel part of the iron carbon phase diagram

Mechanism of Pearlite Formation

The pearlite transformation of supercooled austenite is a diffusion mechanism. Austenitedecomposes with the formation of ferrite (almost pure iron) and cementite containing 6.67 %

carbon. The carbide appears first as its nuclei are formed at the boundaries of the austenitegrains. (Fig. 7.7). As a result, the adjacent volumes of austenite Are depleted of carbon, becomeless stable, and undergo the allotropic transformation from austenite to ferrite. Thus small crystalsof ferrite are formed adjacent to the cementite (iron carbide). Subsequent diffusion leads toformation of more platelets of iron carbide with simultaneous formation of ferrite plates, to form amixture or colony, called pearlite. The structure of pearlite is characteristically lamellar. Thegreater the degree of supercooling, the finer the ferrite-cementite structure obtained.

135


8/14


Fe 3C

Grain

Boundary

(a) (b)(c)

Fig. 7.7. Development of pearlite structure

Transient coolingThe effect of cooling rate on the decomposition of Austenite in a eutectoid steel is illustrated in Fig.7.8.

Cooling rate, v

Lower c riticalcooling rate

Upper criticacooling rate

A+T

Arz

M s

M f

F

+

Fe 3C

A r1 Beginning of decompositionA to F+Fe3C (Pearlite)

End of decomposition to P

T+M+A M+ A ret

M

Fig. 7.8. Effect of cooling rate on the transformation of austenite

As we increase the cooling rate, the temperature at which the austenite transforms to Pearlite islowered. The higher the cooling rate, the lower the decomposition temperature, the moredispersed (finer) the ferrite - cementite structure obtained. At higher cooling rates, the pearliteoccurs as a very fine structure often called Sorbite or Troostite (T) . At some high cooling rates,below the temperature Arz, a new modified structure called Bainite occurs. Bainite consists of

ferrite with finely dispersed cementite globules. When the cooling rate exceeds a critical coolingrate, at a lower temperature, Ms, martensite begins to form. This transformation continues until a

136


9/14


temperature Mfis reached. Any untransformed austenite remains in the structure as retained

austenite (Aret).

7.3.3. Kinetics of Martensitic Transformation

Baines and Davenproof studied the isothermal decomposition of austenite. Small specimen wereheated above the upper critical temperature, Ac3, to form austenite. Subsequently they werequenched in a suitable bath at a certain temperature. After holding at that temperature for differentperiods of time, the specimen were withdrawn and quenched in water. By so doing, themicrostructure transformed so far is kept and the remaining austenite is converted to martensite.From the microsection, the amount of transformed austenite can be assessed. If the procedure isexecuted for different halting temperatures a series of s-shaped curves is obtained. These canthen be summarized on a Time-Temperature-Transformation (TTT) diagram.

1 2 3 4

100

80

60

40

20

0

Holding Time (s)

Thalt

time

Temp

Thalt

A31

to P at GB

2

3

4

100% Pearlite

P

Fig. 7.9. Isothermal transformation of austenite

Fig. 7.9 shows how the S-curve is produced, and Fig. 7.10 shows how the TTT diagram isproduced.

Three ranges are distinct on the TTT diagram:

(a) PEARLITIC: Down to temperatures of about 550 C, pearlite is formed. At highertemperatures the pearlite is coarse, and at lower temperatures it is fine.

137


10/14


(b) BAINITE: Between 550 and 350 C, Bainite is formed. Bainite consists of cementite, finelydispersed particles in ferrite. It is hard but reasonably ductile. Upper Bainite is coarse, andits microstructure can be resolved in an optical microscope. Lower Bainite is very fine, andcan only be resolved by an electron microscope.

(c) MARTENSITE: This range exists at temperatures below Ms. The formation of martensite isdiffusionless and is therefore independent of time.

100

50

0

800

700

600

500

400

300

200

100

0

-100

Thalt(C)

650 500 350

A3

A1

Coarse

Fine

PEARLITE

Coarse

Fine

PEARLITE

M

Fig. 7.10. Development of the TTT diagram from the isothermal transformation of Austenite.

7.3.4 TTT Diagrams for Continuous Cooling

In the foregoing section, the kinetics of austenite transformation were established for the case ofisothermal cooling. A more realistic approach is achieved by studying the behaviour while coolingcontinuously.

Consider the TTT diagram shown in Fig. 7.11, showing various cooling curves super - posed onthe diagram.

138


11/14


800

700

600

500

400

300

200

100 I II IIIIV

Log (time)

A3

A1

B

P

1

2

3

4

5

5

Transformations

1. Ferrite starts

2. Ferrite ends, Pearlite starts

3. Pearlite ends

4. Bainite starts5. Bainite ends

Composition of microstructure

I : Pure Martens ite

II: Fine pearlite, coarse bainite

III: Ferrite, Fine pear lite, Bainite

IV: Ferrite, Pearlite

Fig. 7.11: TTT diagram of a carbon steel for continuous cooling (CCT)

Alloying of steels alters the form and shape of the TTT diagram.

7.3.5 Hardenability Curves

CCT and TTT diagrams are not available for all steels, and it is not easy to accurately determine

the cooling rates. Instead, a JOMINY TEST is used to cmpare the hardenability of steels. A bar100 mm long and 25 mm in diameter is austenitized, placed in afixture , and sprayed at one endwith water. This procedure produces a wide range of cooling curves - very fast at the quenchedend, and almost air cooling at the opposite end. Fig. 7.12 illustrates the set up of the Jominy test.

139


12/14


Support

Water

splash Hardness

50 HRC

Hardenability

index

Fig. 7.12 The Jominy Test

After the quenching, hardness measurements are made along the length of the specimen andplotted as shown to produce a hardenability curve. The distance from the quenched end whichgives a minimum hardness of 50 HRC is called the hardenability index. This represents theregion that full underwent martensitic transformation. The hardenability index therefore representsthe depth to which a particular steel can be hardened, and increases accordingly with alloyadditions. Refer to notes in the Practical handbook.

7.3.6. Tempering of Martensite

Martensite is extremely brittle, and its transformation give rise to high internal stresses which maycause cracking. A subsequent heat treatment, called tempering, may relieve the stresses and/oralter the microstructure to produce a better material. For stages of martensite tempering aredistinguished:

80-150 C The specimen contracts, C atoms can move to a certain extent and the tetragonaldistortion is reduced. Slowly the so called cubic martensite is formed. Fine

carbide precipitates are precipitated (-carbide).

150-290 C Specimen expands (0-1) % elongation, the tetragonal lattice is transformed to thecubic lattice, fine carbides are precipitated, retained austenite transformed intocubic martensite.

290-400 C Specimen contracts again, all carbon is precipitated

Above 400

Carbide coagulate, globules can be seen in an optical microscope.

TABLE 7.1 EFFECT OF TEMPERING TEMPERATURE ON THE HARDNESS OF A CARBONSTEEL, C=1.3%

Tempering Temp.(C)

20 100 200 300 400 500 600 700

HRC 63 63 59 55 48 41 34 25

140


13/14


TABLE 7.2 EFFECT OF TEMPERING TEMPERATURE ON THE MECHANICAL PROPERTIESOF A CARBON STEEL, C= 0.45 %

TemperingTemp. (C)

HardnessHBN

Tensilestrength(MPa)

Yield strength(MPa)

Elongation (%) Reduction inArea(%)

300 320 1050 750 10 30400 285 1000 700 15 40

500 250 900 620 20 50

600 220 800 520 25 55

700 200 700 430 30 60

141


14/14

Chapter 7. Non-equilibrium solid phase transformations 142

materials science manual chapter 7

Documents