GRAIN BOUNDARY ANALYSIS OF HT9 STEEL

UNDER CREEP TEST

By

ZHE LENG

A thesis submitted in partial fulfillment of

the requirements for the degree of

Master of Material Science and Engineering

WASHINGTON STATE UNIVERSITY

School of Mechanical and Materials Engineering

AUGUST 2010

II

To the Faculty of Washington State University:

The members of the Committee appointed to examine the thesis of ZHE

LENG find it satisfactory and recommend that it be accepted.

_____________________________

David P Field, Ph.D., Chair

_____________________________

Hussein M. Zbib, Ph.D.

_____________________________

Jow-Lian Ding, Ph.D.

III

Acknowledgements

First and foremost, I would like to express my deepest gratitude to my supervisor, Dr.

David Field, a respectable, responsible and resourceful professor, who has provided

me with valuable guidance in every stage of the writing of this thesis. Without his

enlightening instruction, impressive kindness and patience, I could not have

completed my thesis. His keen and vigorous academic observation enlightens me not

only in this thesis but also in my future study.

Secondly, I would like to give my thanks to Dr. Ding, for the access to the testing

frame.

Finally, my thanks go to Dr. Alankar and my colleagues at WSU for their valuable

suggestions.

IV

GRAIN BOUNDARY ANALYSIS OF HT9 STEEL

UNDER CREEP TEST

Abstract

By Zhe Leng, M. S.

Washington State University

August 2010

Chair: David P. Field

Ferritic/martensitic steels are attractive materials for use as components in nuclear

reactors because of their high strength and good swelling resistance. Grain boundary

specific phenomena (such as segregation, voiding, cracking, etc) are prevalent in these

materials so grain boundary character is of primary importance. Certain types of

boundaries are more susceptible to thermal creep damage whereas others tend to resist

damage. If more damage resistant boundaries can be introduced into the structures,

this will result in steel that is more resistant to the processes of degradation that

prevail in high-temperature environments. In this study, the grain boundary structure

in HT9 steel was characterized by electron backscatter diffraction to identify

boundaries that are resistant or susceptible to damage in extreme environments. It is

found that intergranular damage is mitigated by a high fraction of low angle

boundaries, and certain kinds of grain boundaries, such as the 3 boundary, are more

favored by intergranular cracks.

V

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ......................................................................................... III

ABSTRACT ................................................................................................................. IV

LIST OF FIGURES ................................................................................................... VII

CHAPTER

1. INTRODUCTION ........................................................................................... 1

1.1 General ................................................................................................... 1

1.2 Objective ................................................................................................ 3

2. BACKGROUND AND LITERATURE REVIEW .......................................... 4

2.1 Review of creep phenomenon ................................................................ 4

2.2 Creep mechanism ................................................................................... 6

2.3 Grain boundary analysis ........................................................................ 8

2.4 EBSD observation ................................................................................ 11

2.4.1 Crystal orientation map ............................................................. 11

2.4.2 Image quality (IQ) map ............................................................. 12

2.4.3 Kernel average misorientation map .......................................... 13

3. MATERIAL AND EXPERIMENT PROCEDURE ..................................... 15

3.1 Material and microstructure ............................................................... 15

3.2 Creep test ........................................................................................... 16

4. EXPERIMENT RESULTS .......................................................................... 19

4.1 Microstructures of material before creep testing ............................... 19

VI

4.2 Microstructures of materials

after short term creep testing............................................................ 22

4.3 Microstructures of materials

after long term creep testing ............................................................ 30

4.4 The distribution of grain boundary planes

for ∑3 boundary ............................................................................... 36

5. DISCUSSION ............................................................................................ 39

6. SUMMARY AND CONCLUSION ........................................................... 43

7. FUTURE WORK ....................................................................................... 44

8. REFERENCE ............................................................................................. 45

VII

LIST OF FIGURES

Fig.1 The illustration of Ashby map ..................................................................... 2

Fig.2 Wedge crack formed at the triple junction in association with grain

boundary sliding......................................................................................... 3

Fig.3 Schematic creep curves for constant tensile load ........................................ 5

Fig.4 Effect of Applied Stress on a Creep Curve at Constant Temperature .......... 6

Fig.5 Schematic illustration of dislocation creep involving both climb and glide

of dislocations ............................................................................................ 7

Fig.6 Illustration of diffusion creep ...................................................................... 8

Fig.7 Illustration of different oriented grain boundary plane segments ................ 9

Fig.8 Schematic illustration of the principle of five parameter stereology

method ...................................................................................................... 10

Fig.9 Schematic illustrations showing the location of twist and tilt boundaries

for ∑3 boundaries .................................................................................. 11

Fig.10 Electron backscatter diffraction pattern from Si ........................................ 12

Fig.11 The orientation map of HT9 steel after heat treatment .............................. 12

Fig.12 The image quality map of HT9 steel after heat treatment ......................... 13

Fig.13 Selection of pixels for calculating KAM ................................................... 14

Fig.14 Dimension of tensile bars used for the creep test ...................................... 15

Fig.15 Various structures composing the microstructure of 9 to 12 pct Cr

Steels ........................................................................................................ 16

Fig.16 The creep test frame .................................................................................. 17

VIII

Fig.17 Empirical relationship of creep life as a function temperature and stress . 18

Fig.18 EBSD orientation map of HT9 steel .......................................................... 20

Fig.19 The grain size distribution of HT9 steel .................................................... 21

Fig.20 The misorientation distribution map for HT9 steel ................................... 22

Fig.21 SEM images of HT9 steel fracture surface ................................................ 23

Fig.22 SEM micrograph of Crack on the surface of HT9 steel ............................ 24

Fig.23 The orientation map and the image quality map of HT9 steel after creep

test ............................................................................................................ 24

Fig.24 The misorientation distribution map for HT9 steel after short term creep

test ............................................................................................................ 25

Fig.25 Image quality map and orientation map of HT9 steel after short term creep

testing ....................................................................................................... 26

Fig.26 The misorientation angle distribution of damaged boundaries ................. 27

Fig.27 Misorientation distribution function associated with the cracked grain

boundaries ................................................................................................ 28

Fig.28 Various pole figures of the crystallites along the crack ............................. 29

Fig.29 Kernel average misorientation map of HT9 .............................................. 29

Fig.30 The misorientation distribution map for HT9 steel after long term creep

test ............................................................................................................ 30

Fig.31 SEM images of the high density void area of HT9 steel after long term

creep test .................................................................................................. 31

Fig.32 SEM images and orientation map of the Cracks on the surface of HT9

IX

steel after long term creep test ................................................................. 31

Fig.33 The misorientation angle distribution of damaged boundaries along the

crack ......................................................................................................... 33

Fig.34 Axis/angle misorientation distribution plot for the crack .......................... 33

Fig.35 Various pole figures of the crystallites along the crack ............................. 34

Fig.36 The misorientation angle distribution of boundaries beside the voids ...... 35

Fig.37 Axis/angle misorientation plot of the voids in the high voids density

area ........................................................................................................... 35

Fig.38 Pole figures of the crystallites besides the void ......................................... 36

Fig.39 Distribution of boundary plane normals in HT9 steel after heat

treatment .................................................................................................. 37

Fig.40 Distribution of boundary plane normals in HT9 steel after short term creep

test ............................................................................................................ 37

Fig.41 Distribution of boundary plane normals in HT9 steel after long term creep

test ............................................................................................................ 38

Fig.42 Axis/angle misorientation plot of HT9 steel after the long term

creep ......................................................................................................... 40

Fig.43 The relation between the dimension of the function and the number of data

that give a reliable result .......................................................................... 42

1

1. INTRODUCTION

1.1 General.

The expected increasing demand for energy in the twenty-first century has boosted the

world-wide cooperation to consider ways to meet energy needs while maintaining and

improving the environment. With the supplies of fossil fuel fast running out, the

search is on for a plentiful and reliable replacement. No matter what we decide to use,

wind power, solar energy, the tide and waves, it seems that nuclear power will play a

major role. Indeed, it is already an important economical source of electricity too, but

it also poses many difficult problems. Principal amongst these are the problems of

developing materials sufficiently resistant to the extreme environments produced by

the reactor core, such as high temperatures, large stresses, chemically reactive

environments and so on.

For structural applications in reactors, high-chromium(9-12 wt%) ferritic/martensitic

steels have several advantages such as good creep strength and excellent swelling

resistance in comparison with austenitic stainless steels [1,2].

During their service life, one of the major damage types is from thermal creep, a

continuing, time-dependent plastic deformation of materials subjected to constant

stress at elevated temperature [3]. Results from experimental and theoretical study

show that the creep of crystalline solids is the result of thermal activated migration of

dislocation, grain-boundary diffusion, and the diffusion of vacancies, the mechanism

2

that contribute to the creep includes: dislocation glide, dislocation creep, diffusion

creep and grain boundary sliding [4]. The dominant creep mechanism is highly

dependent on the applied stress and temperature, as is shown in the deformation

mechanism map below [5]:

Fig.1 Schematic illustration of a deformation mechanism map [5]

Among these mechanisms, two are grain boundary related: grain boundary sliding and

grain boundary diffusion creep (coble creep), thus the grain boundary character plays

an important role in the creep damage. Actually, it is already observed that creep

cavities frequently nucleate on grain boundaries [6,7,8,9], and grain boundary sliding

is suggested as the driving force for creep void nucleation as is shown in Fig.2. Voids

nucleate at stress concentration areas such as triple points or second phase boundaries,

and the growth of these voids result from the grain boundary diffusion. Finally, the

grain boundary cavities coalesced to form intergranular cracks [10,11,12]. Grain

boundaries with certain crystallographic relationships are more favored by the cracks

3

and voids. In most cases the ∑3 boundary is more resistant to crack formation than

random boundaries [13-15, 16,17], however, in the case of void formation it did not

has such a great effect[18].

Fig.2 Wedge crack formed at the triple junction in association with grain boundary

sliding[7].

As a typical ferritic/martensitic stainless steel, HT9 steel has been in use in nuclear

reactors as pressure vessels or pressure tubes for many years [19]. Although the creep

properties of HT9 steel have already been studied [20-24], none of the literature

concerns creep damage with respect to grain boundaries of HT9 steel. It is therefore

necessary to inspect the damaged grain boundaries and determine grain boundary

character for boundaries which are susceptible to damage and those which are more

resistant. If more damage resistant boundaries are induced in HT9 steel by grain

boundary engineering, the creep resistant ability will surely be improved, thus HT9 is

still one of the best options for structure material.

4

1.2 Objective.

The main objective of this study was to determine the high temperature creep strength

and the resistance to environmental attack of certain boundary types by characterizing

the grain boundaries that are damaged under the creep test and by identifying the

boundaries that are susceptible to damage. For this purpose, samples which had been

crept at elevated temperatures are examined and quantitatively characterized for

damage and grain boundary structure.

5

2. BACKGROUND AND LITERATURE REVIEW

2.1 Review of creep phenomenon.

At high temperatures, stresses well below the material’s yield strength can result in

permanent deformation over a period of time. This time dependent progressive

deformation of the material at constant stress is called creep. The engineering creep

curve of a material can be obtained by applying a constant load to the sample at

constant temperature, and the resulting strain is determined as a function of time. A

creep curve can be divided into three stages: primary creep, secondary creep and

tertiary creep. A schematic of this type of curve is shown in Fig.3.

Fig.3 Schematic creep curves for constant tensile load [25].

Primary creep is the period with a decreasing creep rate. Dislocations are being

generated continuously in this stage and with increasing time, more and more

dislocations are present. The dislocations produce increasing interference with each

6

other’s movement, thus causing the creep rate to decrease. The secondary creep is a

period of constant creep rate. In this stage, a situation arises where the number of

dislocations being generated is exactly equal to the number of dislocations being

annealed out. This dynamic equilibrium results in the metal to creep at a constant rate,

thus this stage is also called steady-state creep. During the tertiary creep stage, the

creep rate increases and the specimen fails due to localized necking of the specimen.

This includes void and micro crack formation at high diffusivity regions such as grain

boundaries.

Fig.4 shows the effect of applied stress on the creep curve at constant temperature, it

can be observed that only for certain combination of stress and temperature, the three

creep curve with well-defined stages can be identified. The higher the stress, the

greater the observed creep rate.

Fig.4 Effect of Applied Stress on a Creep Curve at Constant Temperature [26]

2.2 Creep mechanism

As is already stated, the creep is contributed to by several mechanisms, and the

7

dominant mechanism is highly dependent on the applied stress and temperature.

For dislocation creep, it refers to deformation controlled by dislocation slip in the

grain lattice as shown in Fig.5. The dislocation motion process involves both glide on

slip planes and climb over obstacles. At low stresses, there is a linear dependence of

strain rate on the applied stresses, it is known as Harper-Dorn creep, and this is

governed by climb-controlled dislocation motion. At intermediate stresses, power-law

creep is observed and the creep can be controlled by both glide and climb mechanism.

For solid-solutions, creep is controlled by glide step and for pure metals, creep is

controlled by climb. At high stresses, the creep process is a glide controlled instead of

climb-controlled, and a simple power law is not enough to predict the creep rate.

Fig.5 Schematic illustration of dislocation creep involving both climb and glide of

dislocations[27].

Diffusion creep mechanism is significant at low stress and high temperature, and it is

grain size dependent. Under the driving force of applied stress shown in Fig.6, atoms

8

diffuse from the sides of the grains to the tops and bottoms. The grain becomes longer

as the result of applied stress, and the process will be faster at high temperature as

there are more vacancies. For diffusion paths through the grains, the atoms have a

slower jump frequency, but more paths, it is called Nabarro-Herring creep, and for the

diffusion paths along the grain boundaries, the jump frequency is higher, but fewer

paths exist, it is called Coble creep.

Fig.6 Illustration of diffusion creep[27]

Grain boundary sliding occurs during creep because grain boundaries are imperfectly

bonded and thus weaker than the ordered crystalline structure of grains. Grain

boundary sliding is prevalent at lower stresses and it helps to initiate intergranular

fracture by facilitating cracks and voids on the grain boundaries.

2.3. Grain boundary analysis

A grain boundary is the interface between two grains in a polycrystalline material, and

9

it is one of the major factors that determine the physical, mechanical, electrical, and

chemical properties of polycrystalline materials.

Normally, the macroscopic description of grain boundary geometry is derived through

mathematical means using crystal orientations determined from crystallographic

diffraction patterns. Five independent parameters are necessary to describe or

characterize a grain boundary, the misorientation between the two adjacent lattices is

described by three parameters (two parameters describe the rotation axis and one

parameter describes the rotation angle about the axis), and the orientation of the grain

boundary plane normal is described by the remaining two. Since a grain boundary

with a fixed misorientation usually consists of many differently oriented boundary

plane segments, as shown in Fig.7, it can be expressed as the grain boundary plane

distribution at a grain boundary with specific misorientation. This is done by

partitioning the boundary into small sections, each with a given plane normal

orientation (as shown in Fig.7).

Fig.7 Illustration of different oriented grain boundary plane segments[28].

The stereological procedure to extract boundary plane data from a single section is

10

summarized as follows: electron backscatter diffraction (EBSD) mapping is used to

obtain both grain orientations and to reconstruct the boundary segment traces. This

provides four out of the five independent parameters to describe the grain boundary.

Only the boundary inclination angle is unknown, however, since the boundary normal

must be perpendicular to the boundary trace on the single traction, thus these normals

can be represented as a great circle on the stereographic projection, as shown in Fig.8.

If a sufficiently large number of observations are made, the true boundary plane

normal will accumulate more than the false planes, and form a peak in the distribution.

The background which contains false planes is finally removed based on the

assumption of random sampling [28,29].

Fig.8 Schematic illustration of the principle of the five parameter stereology method

[28,29].

From the distribution of grain boundary plane plot, it is easy to identify the boundary

type: twist or tilt. Twist boundaries have their boundary normal parallel to the

11

misorientation axis while tilt boundaries are 90 degrees from the misorientation axis,

The triangle mark and the dark line in Fig.9 represent the twist ∑3 boundaries and

tile ∑3 boundaries, respectively[30].

Fig.9 Schematic illustrations showing the location of twist and tilt boundaries for ∑3

boundaries [30].

2.4 EBSD observation

2.4.1 Crystal orientation map

Electron backscatter diffraction (EBSD) is a technique used to examine the

crystallographic orientation of many materials. It is used in various fields of materials

science of crystalline or polycrystalline materials such as recrystallization, grain

boundary analysis and anything for which the orientation of a crystallite, or the

distribution of crystallite lattice orientations, must be known.

An electron backscatter diffraction pattern is formed when many different planes

diffract different electrons to form Kikuchi bands as seen in Fig.10. Each of these

bands corresponds to a different diffracting plane in the lattice. The bands present in

12

the pattern are related to the underlying crystal phase and orientation of the lattice,

such as Fig.11. For the single phase material studied, grain boundaries were defined

by an orientation difference exceeding 5 degrees.

Fig.10 Electron backscatter diffraction pattern from Si[31].

Fig.11 The orientation map of HT9 steel after heat treatment

13

2.4.2 Image quality map

Image quality (IQ) describes the quality of an electron backscatter diffraction pattern

and it dependence on the material and its condition. For the grain boundaries, when

the electron beam is situated near a grain boundary, the diffraction volume may

contain both crystal lattices separated by the boundary. Thus, the diffraction pattern

will be composed of a mix of both patterns, leading to a lower quality pattern which is

a dark line in the IQ map. The dark dots in Fig.12 represent the voids in the sample.

Fig.12 The image quality map of HT9 steel after heat treatment

2.4.3 Kernel average misorientation map

Kernel average misorientation (KAM) is the average orientation differences between

the neighboring pixels of a measurement target pixel within a grain. From the KAM

14

maps, the information of local deformation in each crystal will be obtained. Fig.13

contains a schematic illustration of how KAM is determined [32]:

Fig.13 Selection of pixels for calculating KAM[32].

15

3. EXPERIMENTAL PROCEDURES

3.1 Material and microstructure

The material used in the present investigation is HT9 steel, a nominally

Fe–12Cr–1Mo–0.5W–0.5Ni–0.25V–0.2C steel alloy. The chemical composition of

HT9 steel is shown in Table 1. Samples were machined in the form of tensile bars

with a gage length of 24 mm and a cross section of 6 mm x 4 mm as shown in Fig.14.

The samples were austenitized at 1050C for 1 hour to dissolve the precipitates and

carbides in the austenite phase, and air cooled after austenitizing. The resulting

martensitic structure was tempered at 750C for 2 hours in order to achieve high

ductility and toughness. This condition is typical for structural components used in the

reactor such as those discussed in Chapter 1.

Table 1. Nominal chemical composition of HT9 steel( wt-%)

Alloy C S Mn Cr Mo W V others

HT9 0.20 0.4 0.60 12.0 1.0 0.50 0.25 0.50

Fig. 14 - Dimension (mm’s) of tensile bars used for the creep test. Specimen thickness

is 4 mm.

16

HT9 steel is a low carbon steel and has a ferritic-martensitic structure, which consists

of tempered martensite lath forming subgrains in a ferrite matrix. The multi-grain

structure of HT9 steel is illustrated in Fig.15. The high temperature strength results

from the subgrain structure produced by the martensitic transformation and the

precipitation of carbides and carbo-nitrides [33].

Fig.15 - Various structures composing the microstructure of 9 to 12 pct Cr steels [34]

3.2 Creep test

In the experiments performed in this investigation, creep tests were conducted using a

creep testing frame as shown in Fig.16. The temperature was set to 600C and the

stress required for failure after a given length of time was determined according to the

empirical formula proposed by Amodeo [35], as is shown below:

17

Fig.17 is the corresponding stress and creep life chart. From this figure, it can be seen

that there is a linear relationship between the log scale of ruptured stress and the creep

life.

Fig.16 – The creep test frame

Sample

Furnace

Weight

18

Fig.17 Empirical relationship of creep life as a function temperature and stress

For creep tests with failure times of about 100 hours and 1000 hours, the

corresponding stresses calculated by the formula are 250 MPa and 160 MPa. The first

sample broke after 87 hours and the second sample did not break after 1000 hours

testing. The tested sample was air cooled because it was unrealistic to remove the

specimen from the grips when they were hot. The specimens were then sectioned and

polished down to 0.3 µm manually, followed by a vibratory polish with 0.05 µm

colloidal silica solution.

19

4. RESULTS

4.1 Microstructures of material before creep testing.

Fig.18 shows the orientation maps of HT9 steel at different states: before heat

treatment and after heat treatment. Fig.19 contains the corresponding grain size

distributions before and after heat treatment. As can be seen, after the heat treatment

there is an obvious increase in grain size from 1.4 to 5.7 microns, and the packet

structures are evident that are described in Ch. 3 and shown schematically in Fig.15.

(a)

20

(b)

Fig.18 EBSD orientation map of HT9 steel a) before heat treatment, b) after heat

treatment

(a)

21

(b)

Fig.19 The grain size distribution of HT9 steel a) before heat treatment and b) after

heat treatment

Fig.20 shows the misorientation angle histogram of HT9 steel before and after heat

treatment. From the plots, it can be seen that the low angle boundaries

(misorientation<10 degrees) dominate in these two states, and there is a small increase

in the fraction of low angle boundaries after heat treatment.

22

(a)

(b)

Fig.20 - The misorientation distribution map for HT9 steel a) before heat treatment, b)

after heat treatment, the fractions of low angle boundaries are a) 0.51, b) 0.63

4.2 Microstructures of materials after short term creep testing.

Fig.21 shows secondary electron images of the fracture surface of HT9 steel. There is

23

a high density of voids in the fracture surface and the growth of these voids and the

necking decrease the load bearing cross section, and finally result in failure. The

dimpled microstructure indicates void coalescence of HT9 steel at high temperature

and a rather ductile failure that is evidence for having failed in the regime of

power-law creep.

Fig.22 shows secondary electron images of a crack seen on the polished surface of the

HT9 steel specimen. The orientation map and image quality map in Fig.23 show

another crack occurs at a grain boundary. The black line in Fig.23 corresponds to the

crack on the surface. In addition, it is obvious that the subgrains have elongated in the

loading direction.

Fig.21 - SEM images of HT9 steel fracture surface (micron bars are 100 µm and

20 µm)

24

Fig.22 - SEM micrograph of Crack on the surface of HT9 steel

(a) (b)

Fig.23 a) The orientation map and b) the image quality map of HT9 steel after creep

test

Crack

25

The misorientation distribution map for HT9 steel after short term creep test is shown

in Fig.24. This histogram indicates that the fraction of low angle boundaries decreases

greatly after the creep test (as compared to before testing), but the low angle

boundaries still dominate the distribution.

Fig.24 - The misorientation distribution map for HT9 steel after short term creep test.

The fraction of low angle boundaries (<10°) is 0.47.

In Fig. 25, two cracks were observed in the image. Misorientation measurements were

conducted along the cracks in order to determine which kind of grain boundaries are

damaged in the structure. Such measurements were performed on all observed cracks

from each polished section.

26

(a) (b)

Fig.25 a) Image quality map and b) orientation map of HT9 steel after short term

creep testing

Fig. 26 shows the misorientation angle distribution along these two cracks. As can be

seen, most of the damaged boundaries are high angle boundaries, and the fraction of

damaged low angle boundaries is 0.12. Compared with the high fraction of low angle

boundaries before and after creep testing, the large number of high angle boundaries

that are damaged indicates that the crack initiation and propagation is not a random

process. It prefers the high angle boundaries, whereas the low angle boundaries are

more resistant to thermal creep damage [36-37].

Crack

Crack

27

Fig.26 The misorientation angle distribution of damaged boundaries.

The misorientation distribution function of misorientations associated with the

cracked grain boundaries is plotted in Fig.27. This figure shows the distribution in the

axis-angle space since the misorientation can be expressed by the rotation required to

bring two neighboring crystals into coincidence and the crystal axis about which to

rotate. Thus in the following plots, as the misorientation increases, the crack prefers

<111> as the misorientation axis. This is different from the FCC structures, according

to the literature [15,18], wherein some coincidence site lattice boundaries (such as ∑

3) are more resistant to creep damage regardless of the high misorientation angle. In

the present case (BCC structure), the crack tends to prefer boundaries with a

misorientation of 60 degrees about the <111> axis, which are classified as the

well-known ∑3 boundaries.

28

Fig.27 - Misorientation distribution function associated with the cracked grain

boundaries.

The corresponding pole figures of the crystals along the crack are shown in Fig. 28. In

these plots, it is apparent that cracks are likely to form near grains of certain

orientations. However, additional geometrical information, such as the grain boundary

normal vector [28], is needed to enable conclusions about the crack preference. If one

assumes that the grain boundary normal lies in the plane of the sample surface, the

intergranular crack is likely to grow along the grain boundaries which are parallel to

the <100> axes. It is unlikely that cracks will form for the grain boundary normal

directions parallel to the <101> axis.

Fig. 29 contains the kernel average misorientation map of HT9 steel after heat

treatment and after the creep testing. The creep tested material has a higher fraction of

high angle misorientations, and most of the high angle KAM positions are located at

the cracks or at the grain boundaries, which indicate the highly deformed region there.

Cracks tend to form along regions of high deformation, particularly if the neighboring

material is less deformed.

29

Fig. 28 – Various pole figures of the crystallites along the crack.

Fig.29 - Kernel average misorientation map of HT9 a）before short term creep test,

and b) after short term creep test (color scales are in degrees).

30

4.3 Microstructures of materials after long term creep testing.

From the misorientation distribution map for HT9 steel after long term creep in Fig.

30, it can be seen that the low angle boundaries still dominate a large proportion.

Fig.30 - The misorientation distribution map for HT9 steel after long term creep test.

The fractions of low angle boundaries are 0.6.

Although the steel that was tested at lower stress for longer times did not fail, some

high void density areas and cracks could still be seen on the polished surface, as

shown in Fig.31 and Fig.32.

31

Fig.31 - SEM images of the high density void area of HT9 steel after long term creep

test

Fig.32 - SEM images and orientation map of the Cracks on the surface of HT9 steel

after long term creep test

32

The same methods as those followed for analyzing the materials that failed during

short term creep tests were again followed for analysis of the long term creep

specimens.

The misorientation angle histogram and the axis/angle misorientation distribution

plots of the crack are shown in Fig.33 and Fig.34. These figures indicate that the crack

still prefers the high angle boundaries, and the crack seldom occurs at the grain

boundaries with misorientation angles less than 50 degrees. For the failed grain

boundaries with misorientations greater than 50 degrees, the axis <111> seems to be

more preferred by the crack than other plane orientations, this is similar to the results

observed for the short term creep experiments.

The pole figures of the crystallites along the crack are different between the short term

and long term creep tests. For example, in Fig. 35, making the same assumption that

the grain boundary normal lies in the plane of the sample surface, then the

intergranular crack is likely to grow along the grain boundaries which are parallel to

the <101> and <111> axes in the long term creep (in contrast to the observations of

cracks along the apparent {001} type GB planes seen for the short term creep tests).

33

Fig.33 The misorientation angle distribution of damaged boundaries along the crack

Fig.34 Axis/angle misorientation distribution plot for the crack

34

Fig. 35 – Various pole figures of the crystallites along the crack.

The analysis of grain boundaries that contained voids was carried out on the high

density voided area of each polished section. In this case, the high angle boundary is

still the place easy for the void formation as is shown in Fig. 36, which is the same as

the crack formed in the short term and long term creep tests. However, in the

axis/angle misorientation distribution plots shown in Fig. 37, it can be seen that for

the grain boundaries with misorientation between 25 degrees and 45 degrees, the

formation of the voids is a random process, which means that the probability of void

formation is independent of misorientation axis. For voids forming at the grain

boundaries with misorientations larger than 50 degrees, the <111> axis preference still

exists.

35

Fig.36 The misorientation angle distribution of boundaries beside the voids

Fig.37 Axis/angle misorientation plot of the voids in the high voids density area

Fig. 38 shows the pole figures of the crystallites beside the voids. There is a weak

preference of the voids to specific crystallite orientations, and by comparing all the

pole figures above, it could be find that the pole figure for the crystallites in different

cases are all different, since the grain boundary normal is still unknown, additional

36

information is needed in order to give a more reliable conclusion.

Fig. 38 –Pole figures of the crystallites besides the void

4.4 The distribution of grain boundary planes for ∑3 boundary

From the result above, it seems that the ∑3 boundaries are preferentially susceptible

to the damage, so it is instructive to analyze the types of ∑3 boundaries that are

present in the distribution. Fig.39, Fig.40 and Fig.41 are the distribution of grain

boundary plane normals for ∑3 boundary of HT9 steel at different state: after heat

treatment, after short term creep test and after long term creep test.

37

Fig.39 Distribution of boundary plane normals in HT9 steel after heat treatment

Fig. 40 Distribution of boundary plane normals in HT9 steel after short term creep test

38

Fig.41 Distribution of boundary plane normals in HT9 steel after long term creep test

From those figures, it can be concludes that ∑3 tilt boundary is favored after heat

treating, with only a small portion of ∑3 boundary having twist boundary character,

and after the short term creep test, the ∑3 tilt boundary is still favored after the test,

but the portion of twist boundary increases, after the long term creep test, the ∑3 tilt

boundary still dominates, but it is more dispersed than it was before creep testing.

39

5. DISCUSSION

The result above confirms the fact that creep resulted in plastic deformation and creep

damage in the metal during heating. The cavities in Fig.21 and Fig.31 revealed

evidence of creep damage, and the coalescence of the cavities form intergranular

cracks, thus the grain boundary plays a major role in the failure process.

Various papers [38,39] show that low angle boundaries, which have misorientation

angles of less than 10 degrees, display improved creep strength relative to general or

high angle boundaries. This can be also observed from the present results, since most

of the creep damage occurs at the high angle boundaries, as is shown in Fig.26, Fig.33

and Fig.36. This is because of the low diffusivity and the greater resistance of grain

boundary sliding of those low angle boundaries, making it hard for the voids to form

and grow at these low angle boundaries. Thus these kinds of boundaries are more

resistant to creep damage.

In Refs.[39,40,41,42,43,44], all ∑3 boundaries were found to be crack resistant,

because the increased structure order results in smaller diffusivity and greater

resistance to grain boundary sliding compared with that of low angle boundaries. This

is not the case in every situation and sometimes it is not a factor that affects void

formation at all. In the long term creep tests, according to the axis/angle

misorientation plot of HT9 steel after the long term creep shown in Fig.42, it could be

seen that most of the grain boundaries after long term creep seems to have rotation

axis<111> preference. If the crack occurs at the high angle boundaries randomly, the

40

axis/angle misorientation plot of the crack after the creep should be similar to the

axis/angle misorientation plot of HT9 steel after the long term creep. However,

according to Fig.34, the rotation axis of most of the damaged boundaries are not

<111> exactly. There is a deviation of these damaged boundaries from the ideal ∑3

boundaries, thus, they are still high angle boundaries which are likely to crack. To

some extent, the rotation axes <111> for those high angle boundaries are helpful to

alleviate the probability of intergranular crack. But this kind of effect did not happen

for the short term creep, since most of the damaged boundaries have misorientation

with rotation axes<111>, this may be because of the high stress, which is the only

different factor between these two sets of experiments.

Fig.42 Axis/angle misorientation plot of HT9 steel after the long term creep

Grain boundary phenomena are not only dependent on misorientation between the

grains adjacent at the grain boundary, but also depend on the grain boundary plane

orientation. The damaged ∑3 boundary type is also a factor that should be

considered, in Fig.39, Fig.40 and Fig.41, tilt ∑3 boundaries are dominant, but they

41

are dispersed after creep, so the ∑3 tilt boundaries are deformed during the creep,

they are not so stable under such a high stress.

Figure 37 shows that creep voids which form during the long term creep have no

misorientation axis preference for the voids forming at the grain boundaries with

misorientation between 25 to 45 degrees. There is a higher probability for void

formation at grain boundaries with a higher misorientation, likely because the high

angle boundaries have a higher diffusivity which is beneficial for void growth. The

rotation axis preference of the voids may come from the fact that most of the grain

boundaries in the tested sample have <111> axis preference when the misorientation

is greater than 45 degrees. Since the exact ∑3 boundaries alleviate the probability of

creep damage the boundaries with misorientation greater than 45 degrees and rotation

axes near <111> have a higher probability for void formation. The coalescence of

those voids result in micro-cracks. This is also the reason why the misorientation

distribution of micro-cracks shown in Fig.34, have the same rotation axis preference

as the misorientation distribution of voids in Fig.37.

In this study, approximate 35,000 data points were used to characterize the boundary

structure. This includes the observation of hundreds of grain boundaries and a

relatively small number of cracked boundaries. This calls into question the statistical

reliability of the observation. According to H.J. Bunge [45], more data are needed to

obtain a reliable result. Since 5 parameters are necessary to characterize the grain

boundary, almost 105 data points are needed to give a statistically reliable result. The

result obtained at this point was from observation of a relatively low population of

42

grain boundary and may not be representative, thus future work is needed to confirm

the result.

Fig.43 The relation between the dimension of the function and the number of data that

give a reliable result

0

10

20

30

40

0 4 8 12 16 20

Dimensionality of the Function

log

(no

. d

ata

)

High resolution

Low resolution

43

6. SUMMARY AND CONCLUSION

Creep samples of HT9 steel were subjected to short term and long term creep tests at

high temperatures. The samples were observed using EBSD to characterize the

damaged grain boundaries. It is found that, for both short term and long term creep

tests, the low angle boundaries are more resistant to crack formation and propagation.

For these tested HT9 steels, the ∑3 boundaries were also susceptible to preferential

creep damage under high stress. The primary difference observed is that the crack

seldom occurs at grain boundaries with misorientations less than 50 degrees in the

long term creep and ∑3 boundaries in the long term creep test were helpful to

alleviate the probability of creep damage. Most of the ∑3 boundaries in the sample

are tilt boundaries, and during the creep test, the tilt ∑3 boundaries disperse. This

demonstrates that the ∑3 tilt boundary in this material is not a stable boundary.

In the long term creep tests, the voids formed at the grain boundaries with

misorientations between 25 degrees and 45 degrees show no misorientation axis

preference, but for the voids formed at the grain boundaries with misorientation

greater than 45 degrees, the rotation axis near <111> is preferred.

At 600C and stress conditions imposed in our experiments, HT9 steel is a ductile

material and the metal goes through plastic deformation before failure. These regions

of high plastic deformation tend to crack, particularly if the neighboring regions are

less prone to the same deformation.

44

7. FUTURE WORK

Since this thesis has illustrated that there is grain boundary character influence on

intergranular creep damage of HT9 steel, logically, the next step is to apply grain

boundary design and control to engineer the overall grain boundary character

distribution of HT9 steel. Since it is already illustrated that the low angle boundary is

more resistant to the creep damage and ∑3 boundaries seem to be more resistant to

the crack under lower stress, then increasing the fraction of low angle boundaries and

decreasing the weak boundaries have a beneficial effect on the creep strength.

Although it is concluded that high angle boundaries are more susceptible to creep

damage, other kinds of investigations should be conducted. Damaged grain boundary

normals should also be focused on because it is also a very important parameter that

determines the grain boundary creep strength.

The creep test was conducted at a very high stress in this case, future creep tests are

needed under lower stress conditions closer to the actual service conditions which the

pressure vessel steels might encounter.

45

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