1Chapter 11 Phase Transformations

Definitions and Basic Concepts

Material properties depend on the ______________.

The microstructure is affected by ____________________.

Concepts

Phase Transformations in Metals

Microstructural and Property Changes in Iron-Carbon Alloys

Isothermal transformation diagrams

Continuous cooling diagram

Chapter 11 Phase Transformations

Definitions and Basic Concepts (cont.)

Mechanical behavior of iron-carbon alloys

Precipitation Hardening

Crystallization, Melting, and Glass Transition Phenomena in Polymers

2Chapter 11 (continued)

Qualitative Questions

60 Explain the shape of the isothermal phase transformation diagram for plain-carbon steel. Include both nucleationand diffusion concepts in your answer.

61 Describe the microstructure and cooling process followed toform the various two-phase mixtures which can be formedby cooling austenite.

63 Describe the mechanism by which martensite is formed.

64 Compare the cooling programs needed to produce pearlite,bainite, and martensite.

Chapter 11 (continued)

Qualitative Questions

65 Explain how alloy steels can be used to make larger partsmade of martensite.

66 Explain how and why the strength of steel varies with carbon content and microstructure.

67 Compare constant cooling transformations and isothermal transformations. Consider both the ease and practical significance of the experiments.

69 Compare the strength and toughness of martensite, tempered martensite and bainite. Explain the reason for this. Consider the location of the carbon and the shape of any cementite particles.

3Chapter 11 (continued)

Qualitative Questions

70 Describe the process of precipitation hardening andexplain why the hardness goes through a maximumvalue as aging time increases.

71 Describe the concept of the glass transition, and how itrelates to the structure of polymers.

72 Describe the factors that affect the melt temperature andglass transition temperature of polymers.

Chapter 11 (continued)

Quantitative Questions

26 Be able to describe how an isothermal transformationdiagram is developed, and identify the phases presentgiven a path or heat treatment

27 Be able to describe how an continuous coolingtransformation (CCT) diagram is developed, and identifythe phases present given a path or heat treatment

4Chapter 11 (continued)

Phase Transformations in Metals

Developing the microstructure of either single-phase or two-phase alloys typically involves a phase transformation.

Definition:phase transformation - An alteration in the _____________

____________ of the phases

Phase transformations usually do not occur _______________ , but they can.

Chapter 11 (continued)

Phase Transformations in Metals (cont.)

Remember from Chapter 5:

Thermodynamics tells us that a phase change will occur.(Phase diagrams deal with equilibrium)

Kinetics tells us the _____ at which this will occur, and the way in which the phases are distributed.

We need to be aware of ______________ and transformation ______ .

5Chapter 11 (continued)

Phase Transformations in Metals (cont.)

We are interested in three types of phase transformations:

1 ____________________________ with _________ in the number or composition of the phases.(ex. - solidification, recrystallization, allotropic changes)

2 diffusion-dependent transformations with _________ in number and/or the composition of the phases.(ex. - the eutectoid reaction)

3 ______________________ where a metastable phase is produced.(ex. - martensite formation)

Chapter 11 (continued)

Phase Transformations in Metals (cont.)

Remember the two steps are involved:

___________ of a new phase

The formation of small particles of the new phase(maybe several hundred atoms)(remember homogeneous and heterogeneous nucleation mechanisms)

____________ of the new phase. This step requires ________________ to the new phase to occur.

6Chapter 11 (continued)

Phase Transformations in Metals

Temperature effect on nucleationrate.

Note that heterogeneous nucleation _______________ (ie. with less undercooling)

Nucleation rates go througha peak.

Figure 11.7

Chapter 11 (continued)

Phase Transformations in Metals (cont.)

__________ has a huge effect on both nucleation and growth.

The transformation rate is the _____________________.

At high temperature _____________________________ because diffusionis fast

At low temperature ______________ (Chapter 6)

7Chapter 11 (continued)

Phase Transformations in Metals (cont.)

Note the strongpeak in the total curve.

Figure 11.8

Chapter 11 (continued)

Phase Transformations in Metals (cont.)

The common plot is temperature vs. transformation time.

Figure 11.9

8Chapter 11 (continued)

Microstructural and Property Changes in Iron-Carbon Alloys

Isothermal transformation diagrams

The concept is the sudden cooling of a sample from above tobelow 727 C, where it is then _____________________.

Remember the eutectoid reaction

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

At any fixed temperature, the transformation proceedsas follows.

There is a delay in the start for nucleation.

Figure 11.10

9Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

This transformation plot is completed at a number of different temperatures.

The time at which the transformation ____________ are identified.

These are all plotted together as temperature vs. log time.

_________________________ are created at different transformation temperatures.

The various microstructures are identified on the graph.

Chapter 11 (continued)

Isothermal transformation diagrams

____________ mimics equilibrium cooling. Remember the figures in Chapter 10 were based on slow equilibrium cooling.

Slow cooling occurs when the isothermal transformation is completed at a ____________________.

When slow cooling is done __________ is formed.

At other cooling rates the microstructure changes.

10

Chapter 11 (continued)

Microstructural and Property Changes in Iron-Carbon Alloys

Isothermal transformation diagrams (cont.)

Pearlite

At T between 540 C and 727 C, pearlite is formed.

Two parallel curves

start of formation

end of formation

Figure 11.14

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

At 540 C the change begins the soonest __________(remember the total rate curve)

The layers in the pearlite are ____ due to a higher nucleation rate and slower diffusion rate.

The transformation is much slower at higher T because nucleation is difficult.

The layers of pearlite are ________, diffusion is easier.

11

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

Pearlite structure

(a) coarseformed at higher T

(b) fineformed at lower T

Figure 11.15

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

Bainite

As lower temperatures are used for the transformation, bainite is formed. You are further from equilibrium cooling.

Below 540 C you form bainite, a __________________.

Transformations below 540 C is slower because ________ ___________ is difficult.

12

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

Bainite

Electron microscopy required to see the structure.

Figure 11.17

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

Bainite

It is important to note that both pearlite and bainite are _______________ of ferrite and cementite.

Once any portion of the material transforms to either pearlite or bainite, _______________ to the other without heating to reform ____________.

13

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

Bainite

The bottom part of the curve is now included.

Figure 11.18

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

Spheroidite

If either pearlite or bainite are held at a high temperature (below 727 C) for an extended time period (18 to 24 hrs) diffusion occurs.

The cementite regions ________________.

___________________ means lower energy.

14

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

Spheroidite

Pearlite starting to transform into spheroidite.

Figure 11.20

Chapter 11 (continued)

Isothermal transformation diagrams (cont.)

Spheroidite

Completed transformationinto spheroidite.

Figure 11.19

15

Chapter 11 (continued)

Martensite

Martensite is not on the iron-iron carbide phase diagram. It is not an _____________________.

When austenite is very rapidly cooled (quenched) to reasonably low temperatures (< 200 C).

Doesnt allow time for diffusion of carbon to occur. A _________________ is formed. This is the ___________ transformation mentioned earlier.

Since this is a diffusionless transformation, the change is essentially _______________ .

Chapter 11 (continued)

Martensite (cont.)

The FCC structure transforms to a ____________________ (BCT) structure.

BCT is basically a distorted BCC lattice.

16

Chapter 11 (continued)

Martensite (cont.)

FCC BCC BCT

austenite ferrite martensite

The carbon is still an interstitial impurity. It doesn't want to be in the BCC ferrite structure, but is stuck.

Chapter 11 (continued)

Martensite (cont.)

The dark regions are pearlite.

The light regions are austenite yet to transform.

Figure 11.22

17

Chapter 11 (continued)

Martensite (cont.)

Because of the instantaneous change, martensite appears differently on the ITT diagram.

It shows up as a ___________ .

Figure 11.23

Chapter 11 (continued)

Tempered martensite

There is one other two-phase mixture that can be created, called ___________________ .

Created when martensite is heated to between 250 and 650 C.

___________________ form within the continuous ferrite matrix.

18

Chapter 11 (continued)

Tempered martensite

Structurally similar to spheroidite, but the cementiteregions are _______________.

Martensite is not an equilibrium phase, but tempered martensiteis, because it is a two-phase structure of ferrite and cementiteas given on the equilibrium phase diagram.

Figure 11.34

Chapter 11 (continued)

Summary of formationprocess foriron-iron carbidestructures

Figure 11.37 Spheroidite

( + Fe3C phases)

extended heating

extended heating

19

Chapter 11 (continued)

Microstructural and Property Changes in Iron-Carbon Alloys

A non-eutectoid steel will show an ______________ on the ITT diagram.

This is due to the formation of the _____________ phase which precedes the eutectoid reaction

Could be ferrite or cementite depending on whether it is a hypo- or hypereutectoid steel

Remember this forms above 727 C

Chapter 11 (continued)

Formation of a hyper-eutectoidsteel.

Figure 11.16

start of formation of primary cementite

20

Chapter 11 (continued)

Microstructural and Property Changes in Iron-Carbon Alloys

An alloy steel can show several effects on the transformation diagram.

One of the most important is that alloying can cause a ______________ in the onset of the transformation.

This is important as this means it is easier to form martensite.

Chapter 11 (continued)

Transformation diagram for a 4340 alloy steel.

Note the large time delay in the changes.

Figure 11.24

21

Chapter 11 (continued)

Example Problem 11.2

Chapter 11 (continued)

Example Problem 11.2