Fabrication of Fe-TiB2 Nanocomposite with Use of High-Energy Milling Followed by In-situ Reaction Synthesis and Sintering :Doctor of Philosophy In Materials Science and Engineering

Fabrication of Fe-TiB2 Nanocomposite with use of High-Energy Milling

Followed by in-situ Reaction Synthesis and Sintering

by

Huynh Xuan Khoa

Thesis submitted to the

Graduate School, University of Ulsan

in partial fulfillment of the requirement for the degree of

Doctor of Philosophy

In

Materials Science and Engineering

APPROVED:

Prof. Byoung-Kee Kim, Chairman

Prof. Young-Soon Kwon

Prof. Jin-Chun Kim

PhD. Ji-Hoon Yoo

Prof. Ji-Soon Kim, supervisor

December, 2014

Ulsan, Republic of Korea

I

Abstract

Metal matrix composites reinforced with nano-particles are very promising

materials which is suitable for a large number of applications. Fe-based composites

reinforced by TiB2 particulates have attracted much attention due to its excellent

mechanical properties as well as low coefficient of thermal expansion. In-situ

formation results in the clean particle-matrix interfaces with higher interfacial

strength, finer reinforcement size and better particle-size distribution. Hence, the

in-situ technique is the optimal choice for the synthesis of nanocomposite.

In this study, Fe-TiB2 nanocomposite was in-situ fabricated from titanium

hydride (TiH2) and iron boride (FeB) powders by high-energy ball milling and

subsequent heat-treatment. High-energy ball milling was chosen for mechanical

activation as an effective method to achieve the desired results subsequently. The

specific energy was calculated from the measured results of electrical power

consumption during milling and used for discussion on the powder characteristics

and the subsequent reaction behavior. About 20% of the input energy were

transferred into the material at the milling speed of 500 rpm and 33% at 700 rpm.

By increasing the milling energy, distribution of starting powders was gradually

homogeneous and reduced their size to a nanoscale. Moreover, the thermal

behaviors such as decomposition of TiH2 and the formation reaction of TiB2 from

Ti and FeB were lowered. Obviously, Fe-TiB2 nanocomposite powders after

reaction synthesis showed more homogeneous microstructure for powder mixture

milled with higher specific energy. Microstructure was characterized by smaller 5

nm TiB2 particulate homogeneous distributed in Fe matrix.

II

Understanding the reaction mechanism helps controlling the affecting

factors to achieve the best results. Phase change was analyzed by X-ray diffraction

and phase distribution was observed by electron microscopy during reaction

synthesis of powder mixture milled with various milling conditions in order to

explore the formation mechanism of TiB2 particles in Fe matrix. The result

indicated that titanium reacts with boron at the interface of Ti and FeB by gradual

diffusion reaction, forming TiB2 particles, reducing the amount of boron and

induced phase transition of FeB to Fe2B. The process ended when whole the Ti

phase transfered into TiB2 phase and Fe matrix formed from the position of Fe2B

left. The reaction rate strongly depended on the size and distribution of FeB

particles. With the finer FeB, the more homogeneous microstructure of Fe-TiB2

composite powder formed, involving nano TiB2 particles distributed in the Fe

matrix.

Most refractory reinforced - metal composite are used for wear resistance

parts and cutting tools, so sintering is always next stage of the manufacturing

process of materials. A part of this study intended to examine the consolidation of

nanocomposite. The sintering process was performed by both pressureless (PLS)

and pressure (SPS) sintering techniques. The main effecting factors of sintering

time and temperature were investigated. The result showed that microstructure and

properties of the composites strongly affected by sintering time and temperature.

With the dominant advantage of low sintering temperature and short sintering time,

the SPSed-samples retained nano-size TiB2 particles and obtained very high

density and hardness. The PLSed-samples showed sub-micrometer TiB2 particles,

but the hardness obtained also high, equivalent to some WC-Co systems.

III

Acknowledgements

It is my great pleasure that I am in a position to express my deep gratitude

to some persons without their help the present research work could have not

taken final shape. Foremost, I express my cordial gratitude, thanks and regards to

my supervisor Professor Ji-Soon Kim. I am extremely grateful for his sincere help,

valuable suggestions and constant encouragement and unique guidance during

the four years of my Ph.D course in the University of Ulsan.

I would like to express my sincere gratitude to Korean students, Mr. G. P.

Ahn, B. H. Lee, Y. H. Lee, Sang-W Bae, Sun-W Bae, W. J Kim and J. Y Joe for their

many valuable suggestions, encouragement and assistance during my research

work.

I am expressing my gratitude to the Korea Institute of Ceramic Engineering

and Technology for helping in measurement of mechanical properties. A special

thanks to Mr. Sang-Ha Park of Deagu Machinery Institute of Components &

Materials for the use of their equipment and advice. I am also thankful to all staffs

of UOU Research Facilities Center for their enthusiasm in the investigation of my

specimens.

I would like to thank my committee members: Prof. Ji-Soon Kim (Advisor),

Prof. Young-Soon Kwon, Prof. Byoung-Kee Kim, Prof. Jin-Chun Kim and PhD. Ji￾Hoon Yoo for their support and advice in developing this document.

Finally, I am grateful to my wife and my daughter for their constant

inspiration to carry out the research work.

Date:

University of Ulsan,

Ulsan city, Republish of Korea Huynh Xuan Khoa

I

Table of Contents

Abstract .............................................................................................................................I

Acknowledgements ........................................................................................................... III

List of Figures ................................................................................................................... IV

List of Tables .................................................................................................................. VII

Chapter 1 Introduction ..................................................................................................... 1

Reference .......................................................................................................................... 5

Chapter 2 Theoretical Background .................................................................................. 7

2.1 Metal matrix nano-composites (MMnCs) .................................................................. 7

2.1.1 Processing techniques for metal matrix nano-composites (MMnCs) ................. 9

2.1.2 Strengthening in particulate reinforced metal matrix composites ..................... 17

2.1.3 Previous works on production process of Fe-TiB2 composite .......................... 19

2.2 Mechanical activation by high-energy ball milling .................................................. 26

2.2.1 Mechanical activation ........................................................................................ 26

2.2.2 High-energy milling equipments ....................................................................... 27

2.2.3 Planetary high-energy ball milling and processing variables ............................ 27

2.2.4 Energetic of mechanical activation process ...................................................... 30

2.2.5 Effect of mechanical activation on properties of solid ...................................... 38

2.3 Synthesis reaction mechanisms and reactions in the solid state ............................... 40

2.4 Sintering process ....................................................................................................... 46

2.4.1 Presureless sintering .......................................................................................... 46

2.4.2 Spark plasma sintering – Outstanding method for sintering MMnCs ............... 47

Reference ........................................................................................................................ 54

Chapter 3 Experimental procedure ............................................................................... 58

3.1 Materials ................................................................................................................... 58

3.2 High-energy ball milling Process ............................................................................. 59

3.3 Heat-treatment process ............................................................................................. 62

3.4 Sintering of nanocomposite powder ......................................................................... 63

3.4.1 Pressureless sintering ........................................................................................ 63

3.4.2 Spark plasma sintering ...................................................................................... 65

3.5 Characterization ........................................................................................................ 65

3.5.1 Particle size analysis .......................................................................................... 67

3.5.2 XRD analysis ..................................................................................................... 67

3.5.3 SEM and TEM analysis ..................................................................................... 67

3.5.4 Thermal analysis ................................................................................................ 68

II

3.5.5 Density and hardness measurement .................................................................. 68

3.5.6 Wear test, and transverse rupture strength test (TRS) ....................................... 69

Reference ........................................................................................................................ 71

Chapter 4 Energetics of high-energy ball milling process ........................................... 72

4.1 Indirect approach – Calculation of milling energy by collision model .................... 73

4.1.1 Calculation of milling energy by collision model ............................................. 73

4.1.2 Measuring the power consumption and comparison with measurement .......... 79

4.2 Direct measurement of total energy during high-energy ball milling of FeB

and TiH2 powder mixture ...................................................................................... 81

4.3. Summary .................................................................................................................. 82

Reference ........................................................................................................................ 84

Chapter 5 High-energy ball milling process of initial FeB-TiH2 powder mixture ..... 85

5.1 The state of the powder mixture during High-energy ball milling ........................... 85

5.2 Effect of milling energy on mixing homogeneity and size of powder ..................... 85

5.3 Effect of Milling Energy on Reaction Behavior of Powder mixture ........................ 90

5.4 Sumamry ................................................................................................................... 92

Reference ........................................................................................................................ 93

Chapter 6 Fabrication of Fe-40 wt% TiB2 nanocomposite powder from FeB and

TiH2 powders - Powder synthesis and formation behavior of TiB2

particulates in Fe-matrix during reaction synthesis .................................... 94

6.1 Reaction synthesis of milled powders by heat treatment .......................................... 94

6.1.1 Shape and Particle Size of Fe- TiB2 nanocomposite powder ............................ 94

6.1.2 Phase analysis of Fe- TiB2 nanocomposite powder .......................................... 94

6.1.3 Microstructure of Fe- TiB2 nanocomposite powder .......................................... 98

6.2 Formation behavior of TiB2 particulates in the Fe - matrix during reaction

synthesis ............................................................................................................... 101

6.2.1 Phase change during reaction synthesis .......................................................... 101

6.2.2 Composite Microstructure and Analysis ......................................................... 103

6.2.3 Discussion ....................................................................................................... 108

6.3. Summary ................................................................................................................ 112

Reference ...................................................................................................................... 113

Chapter 7 Combination of Synthesis and Sintering for Consolidation of Fe-TiB2

Nanocomposite from FeB and TiH2 ............................................................. 114

7.1 Sintering Behaviors (sintering conditions vs. shrinking) ....................................... 114

7.2 Phases change during sintering ............................................................................... 119

7.3 Microstructure evolution during sintering .............................................................. 121