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«Study on Harmonic Structure Design and Deformation Mechanism in SUS304L Austenitic Stainless Steel Ritsumeikan University Graduate School of Science ...»

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2013 (Heisei 25)

Doctoral Thesis

Study on Harmonic Structure Design and Deformation

Mechanism in SUS304L Austenitic Stainless Steel

Ritsumeikan University

Graduate School of Science and Engineering

Doctoral Program in Integrated Science and Engineering

ZHANG Zhe

Study on Harmonic Structure Design and Deformation

Mechanism in SUS304L Austenitic Stainless Steel

December, 2013

Doctor of Philosophy

Zhe ZHANG

Committee in charge:

Professor Kei AMEYAMA, Chair Professor Masao SAKANE Professor Akira UENO Doctoral Program in Integrated Science and Engineering Graduate School of Science and Engineering Ritsumeikan University Abstract Abstract Study on Harmonic Structure Design and Deformation Mechanism in SUS304L Austenitic Stainless Steel by Zhe ZHANG Doctoral Program in Integrated Science and Engineering Graduate School of Science and Engineering Ritsumeikan University Academic Advisor: Professor Kei AMEYAMA Owing to the excellent corrosion resistance and oxidation resistance, austenitic stainless steels have been widely used in the chemical and petrochemical industry.

However, low yield strength is the major drawback of austenitic stainless steel. Grain refinement is a well known and attractive method of strengthening structural metallic materials. However, ultrafine-grained (UFG) materials are characterized by low tensile ductility at room temperature because of plastic strain instability in the early stage of deformation. Both strength and ductility are the important mechanical properties for industrial applications. Bimodal grain size distribution is a useful strategy to enhance ductility. However, the outcome properties span a relatively wide range depending on the microstructural variations.

i Abstract Recently, professor Ameyama and co-workers proposed a powder metallurgy (PM) based approach to create a novel bimodal microstructure design called “harmonic structure”. Effectively, harmonic structure is a periodic “nano and heterogeneous bimodal structure” consisting of coarse-grained (CG) areas (or “cores”) enclosed in a three-dimensional continuously connected network of UFG structure (or “shell”). The concept of harmonic structure design has been successfully applied in a variety of pure metals and metallic alloys by mechanical milling (MM) and subsequent powder metallurgy (PM) process. All these materials demonstrate a winning combination of improved strength and ductility as compared to their coarse-grained as well as ultrafine-grained counterparts.

The present dissertation describes the work that has been carried out to obtain a better understanding of the manufacture, mechanical properties, deformation and fracture mechanism of harmonic-structured austenitic stainless steel. Plasma rotating electrode process (PREP) powders, which are characterized by low oxygen content and spherical powder shape, are applied in present work. The harmonic structure has been synthesized in SUS304L stainless steels produced by mechanical milling (MM) and subsequent spark plasma sintering (SPS) process. Martensite transformation and subsequent austenite reversion plays an important role on grain refinement in the SUS304L steels.

The volume fraction of UFG can be adjusted by controlling mechanical milling conditions.

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harmonic-structured SUS304L steels exhibit good reproducibility, higher average strength and ductility simultaneously as compared to the steels having same grain size and volume fraction but irregular CG and UFG spatial distribution.

Through comparing the tensile deformation of the specimens having different bimodal structure heterogeneity and topology, it is concluded that the three-dimensional continuous connected network of UFG structure is conducive to restraining strain localization during mechanical loading. The suppressed strain localization leads to postpone the plastic strain instability and delay the neck formation. Therefore, the ductility, in particular uniform elongation in tension of harmonic bimodal structured SUS304L steels is superior to the value of heterogeneous bimodal structured ones. The ductility of bimodal structured materials can be improved by controlling the CG and UFG spatial distribution. “Harmonic structure design” is proposed to be an effective strategy to achieve outstanding mechanical properties.

Since water-atomization is the most common technique for producing low-cost metal powders, water-atomized steel powders have been widely used in industry. Therefore, “harmonic structure design” is also attempted to synthesize in water-atomized SUS304L powder steels. However, owing to the high oxygen content in water-atomized powders, a large amount of SiO2 particles precipitate in the UFG region of sintered compacts.

SiO2 particles tend to impair the inter-particle bonding and result in poor ductility.

Therefore, in order to improve mechanical properties, it is necessary to reduce oxidized contaminations in the PM steels.

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I would like to take this opportunity to express my most sincere gratitude to Professor Kei Ameyama, who taught and gave me invaluable advice and generous help through these six years at Ritsumeikan University. Although I cannot study in the university at work time, he still helped me apply for doctoral program, gave me supervision in his spare time. I would like to remember his inculcation for clarify the critical logical relation in the research forever. His guidance and encouragement enabled me to become stronger and wiser, in both academic matters and personal growth.





I am grateful to the company, Dainippon Screen MFG. Co., Ltd., who gave me scholarship for the study of master program and provided me the work opportunity in Japan. Special thanks to Mr. Yoshiyuki Nakazawa and Mr. Kazuhiko Asada for their patient guidance for my work in the company and the convenience for my study.

Special thanks to Associate Professor Hiroshi Fujiwara (now in Doshisha University, Kyoto) for his selfless support in texture and microstructural analysis and his constant encouragement and fruitful suggestions. I also wish to thank Professor Akira Ueno (Ritsumeikan University) for his kind support and useful suggestions in fractographical analysis on fracture mechanism of harmonic-structured stainless steels.

I would like to express my deep sense of gratitude to Dr. Octav Paul Ciuca, Dr.

–  –  –

Dmitry Orlov and Dr. Sanjay Kumar Vajpai (Global Innovation Research Organization, Ritsumeikan University) for excellent advices to use EBSD and TEM equipments and useful comments and advice on my research and journal paper preparation.

I am also heartily thankful to Professor Masao Sakane (Ritsumeian University) and Professor Xu Chen (Tianjin University), who gave me this opportunity to study in Japan and encouragement during my study.

I owe my most sincere gratitude to all of members in Ameyama laboratory, such as Dr.

Eji Ota, Dr. Tatsuya Sekiguchi, Kouryu Kawadani, Masashi Nakatani, and so on, who have gave me many useful advice on my research and usual life in Japan. Special thanks to Bo Tong for his wonderful assistance.

My heartfelt thanks to all my friends, their encouragement and company kept my spirits high and joyful. Many thanks to all the other people not mentioned, but to whom I am grateful for their kind assistance in one way or another during the past six years.

Finally, I express my profound gratitude to my parents for their sacrifices, constant support, understanding and encouragement extended during my life in Japan.

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Abstract……………………………………………………………..………………….. i Acknowledgements………………………………………………………..…..…...…. iv Table of Contents…………………………………………………………………...… vi Chapter 1 Introduction

1.1 Background…………………………………………………………………….… 1

1.2 Objective of dissertation…………...………………………………………...…... 4

1.3 Outlet of dissertation…..……………………………………………………….... 5 Chapter 2 Literature Review…………………………………………………………. 7

2.1 Austenitic stainless steels…………………………………...……………….…… 7

2.2 Powder metallurgy……………………………………………………………….. 9 2.2.1 Manufacture and characteristics of metal powders…………………...…… 10 2.2.2 Limitations of powder metallurgy…………………………………….…… 18

2.3 Ultrafine-grained (UFG)/ nanocrystalline (NC) metals………………………... 19 2.3.1 Manufacture processes of UFG/NC metals………………………..……… 19 2.3.2 Grain refining mechanisms of metals by SPD…………………………….. 23 2.3.3 Mechanical properties of UFG/NC metals………………………………… 24 2.3.4 Deformation mechanisms of UFG/NC metals……………………….……. 33 2.3.5 Limitations of UFG/NC metals……………………………………………. 35 2.3.6 Approaches to improve ductility of UFG/NC metals…………………..….. 38

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2.4 Research progress of harmonic-structured metals………………………..…….. 44 References………………………..…………………………………………...……. 46 Chapter 3 Experimental Procedure…...……………………………………………. 60

3.1 Mechanical milling (MM)……………………………………………………….60

3.2 Sintering………………………………………………………………………. 63 3.2.1 Spark plasma sintering (SPS)……….……………………….…………. 63 3.2.2 Hot isostatic pressing (HIP)………………..…………………………… 65

3.3 Characterizations……………………………………………………….………. 67 3.3.1 X-ray diffraction (XRD)………………………………..………………….. 67 3.3.2 Scanning electron microscopy (SEM)…………………………….……….. 68 3.3.3 Electron backscatter diffraction (EBSD)……………..……………………. 70 3.3.4 Transmission electron microscopy (TEM)………………………………… 73

3.4 Mechanical properties…………………………………………………….…….. 74 3.4.1 Vickers hardness test……………………………………………………….. 74 3.4.2 Tensile test…………………………………………………...…………….. 75 References……………………..………………………………………...…………. 76 Chapter 4 Microstructure and Mechanical Properties of SUS304L Compacts with Harmonic Structure………………………….……………..…………… 78

4.1 Introduction…………………………………………………………………….. 78

4.2 Experimental procedure………………………………………………………… 79

4.3 Results………………………………………………………………....……….. 81 4.3.1 Microstructural characteristics of the powders…………………………..… 81

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4.3.2 Microstructural characteristics of the sintered compacts………………..… 84 4.3.3 Mechanical properties of harmonic-structured compacts……..…..……..… 88

4.4 Discussion…………………………………..……………………….....……….. 91 4.4.1 Control of microstructure in harmonic-structured SUS304L steels during fabrication ……………………………….……………...…...………….... 91

–  –  –

harmonic-structured SUS304L steels………..………..………....……..… 94 4.4.3 Benefits of harmonic structure design concept for the control of SUS304L steels performance….............……………………..……………...……..…. 96

4.5 Conclusions……………………………………………………...…….……….. 97 References……………………………………………………………..………….... 98 Chapter 5 Deformation and Fracture Mechanism of SUS304L Compacts with Harmonic Structure……………………………………………...……. 100

5.1 Introduction…………………………………………………………………… 100

5.2 Experimental procedure……………………………………….………………. 102

5.3 Results…………………………………………………………..…………….. 102 5.3.1 Mechanical properties of harmonic and heterogeneous bimodal structured compacts……………………………………………………………..….. 102 5.3.2 Deformation behavior of harmonic and heterogeneous bimodal structured compacts…………………………………………………………..…..… 107 5.3.3 Fracture behavior of harmonic and heterogeneous bimodal structured compacts……………………………………………….………………... 115

5.4 Discussion…………………………………..……………………….………… 120

–  –  –

5.4.1 Improve ductility through the control of bimodal structure heterogeneity and topology…………………………………………………………………. 120 5.4.2 Deformation mechanism of harmonic-structured SUS304L compacts…... 122 5.4.3 Fracture mechanism of harmonic-structured SUS304L compacts……….. 124

5.5 Conclusions……………………………………………………………..…….. 129 Reference…………………………………………..……………………………… 131 Chapter 6 Effects of SiO2 Particles on Deformation of SUS304L Powder Compacts with Bimodal Structure…………….……………………...……...……. 134

6.1 Introduction…………………………………………………..……………….. 134

6.2 Experimental procedure……………………….………………..………..……. 134



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