氏名　BHUIYAN MD. SHAHNEWAZ
学位の種類　博士（工学）
学位記番号　博甲第573号
学位授与の日付　平成23年3月25日
学位論文題目　Corrosion Fatigue Behavior of Extruded Magnesium Alloys under NaCl Environment　（NaCl環境下のマグネシウム合金押出し材の腐食疲労挙動）
論文審査委員
　主査　教授　武藤　睦治
　副査　教授　岡崎　正和
　副査　教授　井原　邦夫
　副査　准教授　宮下　幸雄
　副査　産学融合特任講師　大塚　雄市

• 論文目次
• 論文要旨
• 論文審査の結果の要旨

［平成22（2010）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

Table of contents

ABSTRACT　p.ix
ACKNOWLEDGEMENTS　p.vi
LIST OF PUBLICATION　p.viii
TABLE OF CONTENTS　p.xi
LIST OF TABLES　p.xvi
LIST OF FIGURES　p.xvii

1．Introduction
　1.1　Magnesium（Mg）Alloys　p.1
　1.2　Corrosion fatigue　p.2
　1.3　Corrosion Fatigue Mechanisms　p.2
　1.4　Factors Affecting Corrosion Fatigue Process　p.4
　1.4.1　Metallurgical influences　p.4
　1.4.2　Environmental influences　p.7
　1.4.3　Mechanical influences　p.9
　1.5　Managing Corrosion Fatigue of Magnesium Alloys　p.10
　1.6　Scope of the work　p.14

Part one:　BASIC CORROSION FATIGUE PROPERTIES OF EXTRUDED MAGNESIUM ALLOY　p.17

2．CORROSION FATIGUE BEHAVIOR OF EXTRUDED AZ61 MAGNESIUM ALLOY
　2.1　Introduction　p.19
　2.2　Experimental procedure
　2.2.1　The Material　p.20
　2.2.2　The choice of specimen　p.22
　2.2.3　Fatigue test　p.23
　2.3　Results and Disucussion
　2.3.1　S-N curve　p.24
　2.3.2　Fractographic observation　p.26
　2.3.3　Pit growth behavior　p.30
　2.3.4　Fatigue crack nucleation life　p.32
　2.3.5　Relationship between stress amplitude and critical pit size　p.33
　2.3.6　Comparison of corrosion fatigue strength　p.35
　2.4　Conclusions　p.37

3. CORROSION FATIGUE BEHAVIOR OF EXTRUDED AZ31 MAGNESIUM ALLOY
　3.1　Introduction　p.38
　3.2　Materials and Experimental Procedure　p.39
　3.2.1　Material and Specimen　p.40
　3.2.2　Fatigue Tests　p.40
　3.3　Results and Discussion
　3.3.1　S-N curve　p.40
　3.3.2　Fractographic observations　p.41
　3.3.3　Pit growth behavior under NaCl environment　p.44
　3.3.4　Relationship between stress amplitude and critical pit size　p.46
　3.4　Conclusions　p.49

4. CORROSION FATIGUE BEHAVIOR OF EXTRUDED AZ80-T5 MAGNESIUM ALLOY
　4.1　Introduction　p.50
　4.2　Experimental Procedures
　4.2.1　Material and Specimen　p.51
　4.2.2　Fatigue Tests　p.52
　4.3　Results and Discussion
　4.3.1　S-N curves　p.52
　4.3.2　Fractographic observations　p.54
　4.3.3　Nucleation of corrosion pit in the 5% NaCl environment　p.58
　4.3.4　Relationship between stress amplitude and critical pit size　p.58
　4.3.5　Pit nucleation and growth behavitor of extruded AZ80-T5in a 5% NaCl environment　p.60
　4.4　Conclusions　p.63

5. CORROSION FATIGUE S-N CURVE CHARACTERISTICS
　5.1　Introduction　p.64
　5.2　Characteristics of S-N curve　p.64

Part two:　IMPROVEMENT OF CORROSION FATIGUE PROPERTIES OF MAGNESIUM ALLOYS　p.69

6. CORROSION FATIGUE BEHAVIOR OF CONVERSION COATED AZ61 MAGNESIUM ALLOY
　6.1　Introduction　p.71
　6.2　Experimental Procedure
　6.2.1　Material and Specimen　p.72
　6.2.2　Fatigue Tests　p.73
　6.3　Results and Discussion
　6.3.1　Characteristics of the coating　p.73
　6.3.2　S-N curve　p.75
　6.3.3　Effect of high humidity environment　p.77
　6.3.4　Effect of 5 wt% NaCl environment　p.81
　6.4　Conclusions　p.84

7. CORROSION FATIGUE BEHAVIOR OF CONVERSION COATED AND PAINTED AZ61 MAGNESIUM ALLOY
　7.1　Introduction　p.85
　7.2　Experimental Procedure
　7.2.1　Material and Specimen　p.86
　7.2.2　Fatigue Tests　p.87
　7.3　Results and Discussion
　7.3.1　Characteristics of the coating and painting layer　p.87
　7.3.2　Corrosion fatigue strength　p.88
　7.3.3　Fracture surface observations　p.90
　7.3.4　Interpretation of the mechanism of fracture　p.95
　7.3.5　Fatigue life prediction based on fracture mechanics　p.96
　7.4　Conclusions　p.98

8.SUMMARY AND CONCLUSIONS
　8.1　Introduction　p.99
　8.2　Influence of high humidity of fatigue behavior of extruded magnesium alloys　p.99
　8.3　Influence of 5 wt% NaCl environment on fatigue behavior of extruded magnesium alloys　p.100
　8.4　Corrosion fatigue resistance of conversion coated magnesium alloy　p.101
　8.5　Corrosion resistance of conversion coated and painted magnesium alloy　p.102

APPENDIX A1:　Fatigue crack growth curves of magnnesium alloys under corrosive environments　p.104
APPENDIX A2:　Fatigue crack growth curves of magnnesium alloy under low humidity and 5 wt% NaCl environment　p.105

REFERENCES　p.106

The lightweight concept followed by the transportation industry has made magnesium alloys a highly interesting choice for many structural applications. However, their use for structural engineering applications is restricted to a few structural parts because of its low corrosion resistance. It is commonly acknowledged that mechanical structures in real service conditions are subjected to both cyclic loading and corrosive medium, which degrade their mechanical properties, and threaten their integrity, safety, and service life. Therefore, it is considerably important to thoroughly understand the corrosion fatigue behavior of magnesium alloys.

Chapter 1 “Introduction”: Chapter 1 introduces the magnesium alloys as structural material and its application mainly in automotive industry, corrosion fatigue and corrosion fatigue mechanisms and factors affecting corrosion fatigue processes. This chapter also introduces successful corrosion prevention strategies for managing corrosion fatigue of magnesium alloys and a brief literature review on fatigue properties of different coating is also presented. The scope of the work is also described.

Chapter 2 “Corrosion fatigue behavior of extruded AZ61 magnesium alloy”: in this chapter the basic corrosion fatigue behavior of extruded AZ61 magnesium alloy has been investigated under three different corrosive environments: （a） high humidity environment （80% RH）, （b） 5 wt% sprayed NaCl environment and （c） 5 wt% sprayed CaCl2 environment. The experimental results showed that the fatigue limit of the material significantly reduced under all the corrosive environments: the reduction rates of fatigue limit were 22% under high humidity, 85% under NaCl and 77% under CaCl2 environment. The remarkable reduction of fatigue limit under corrosive environments was attributed to the formation of corrosion pit, which was induced by simultaneous action of mechanical loading and corrosive environment. Moreover, the NaCl environment enhances pit formation and growth more than the CaCl2 environment, due to the high Cl- concentration and low pH value.

Chapter 3 “Corrosion fatigue behavior of extruded AZ31 magnesium alloy”: As observed from the previous chapter, the NaCl environment enhances pit formation and growth more than the CaCl2 environment, therefore, in this chapter, the corrosion fatigue behavior of extruded AZ31 magnesium alloy was carried out under （a） high humidity environment （80% RH） and （b） 5 wt% sprayed NaCl environment. It was found that under NaCl environment, the S-N curve of extruded AZ31 magnesium alloy was not a single curve but two-stage curve. Above the fatigue limit under low humidity, the crack nucleation mechanism was either due to localized slip band formation mechanism or twin mechanism. Below the fatigue limit under low humidity, the reduction in fatigue strength was attributed to the corrosion pit formation and growth to the critical size for fatigue crack nucleation under the combined effect of cyclic load and the corrosive environment. The critical size was attained when the stress intensity factor range reached the threshold value for crack growth. However, the reduction rate of fatigue limit of this material under corrosive environment was 12% under high humidity and 53% under NaCl environment.

Chapter 4 “Corrosion fatigue behavior of extruded AZ80-T5 magnesium alloy”: In this chapter, a comprehensive study was carried out to understand the corrosion fatigue behavior of extruded AZ80-T5 magnesium alloy. The corrosive environments used （a） high humidity environment and （b） 5 % sprayed NaCl environment. The results showed that in both the high humidity environment and in the 5 % NaCl environment the fatigue strength was reduced relative to the low humidity environment, especially in the NaCl environment.　The reduction of fatigue strength in the 5 % NaCl environment was attributed to the formation and growth of corrosion pits to a critical size. At stress amplitudes below 100 MPa in the 5 % NaCl environment, the critical stress intensity threshold for fatigue crack nucleation from a corrosion pit was 0.73 MPa√m.

Chapter 5 “Corrosion fatigue S-N curve characteristics”: From the previous chapters, it is clear that aqueous sodium chloride environment indeed generated deleterious effects on the fatigue resistance of smooth axial specimens of extruded magnesium alloys. Moreover, the S-N curve diagram under NaCl environment also showed different trend due to different crack nucleation mechanism. Therefore, in this chapter, the characteristics S-N curve under NaCl environment is discussed based on respective fracture mode/fatigue crack nucleation mode.

Chapter 6 “Corrosion fatigue behavior of conversion coated AZ61 magnesium alloy”: There is no question that magnesium alloys especially for view parts, require an appropriate surface treatment to protect it from corrosion. A number of preventive measures have been suggested and are being adopted for overcoming the corrosion problems. These include the use of conversion coating, anodizing, polymer coatings, physical vapour deposition and so on. Therefore, in this chapter, as the following step, the feasibility of conversion coating to improve the reduced fatigue strength under corrosive environments was studied. It was found that under low humidity environment without corrosive attack, the fatigue limit of coated material was almost the same as that of the bulk material under the same environment: the reduction rate of fatigue limit was about 3%. The results showed that, conversion coating can work to a certain degree of protection from the attack of humid environment, but no enough for perfect protection from the attack of NaCl environment.

Chapter 7 “Corrosion fatigue behavior of conversion coated and painted AZ61 magnesium alloy”: It is generally admitted that in a more severe environment, the conversion coating layer on magnesium alloy have to be sealed and/painted to give adequate corrosion resistance. Therefore, to improve the corrosion resistance further, a layer of organic painting on top of the conversion coating was applied and the effectiveness os such coating and painting layer for combating the corrosion fatigue strength of extruded AZ61 magnesium alloy is discussed in this chapter. The painting on the coated surface shows superior improvement of fatigue limit: fatigue limit does not decrease even under corrosive environments.

Chapter 8 “Summary and Conclusions”: The study of the effect of corrosive environments on fatigue behavior of extruded magnesium alloys and the role of conversion coating for preventing the extruded magnesium alloy from the attack of corrosive environments is summarized. The most significant research results and their significance are described here.

　本論文は、「Corrosion Fatigue Behavior of Extruded magnesium Alloys under NaCl Environment」と題し、8章より構成されている。
　第1章「Introduction」では、マグネシウム合金の腐食疲労挙動、腐食疲労メカニズムなどに関する従来の研究の概要を示すとともに、本研究の目的と範囲を述べている。
　第2章「Corrosion fatigue behavior of extruded AZ61 magnesium alloy」では、AZ61合金押出し材の高湿度下、NaClおよびCaCl2水溶液噴霧環境下における腐食疲労挙動を調べ、高湿度下で疲労強度が低下すること、NaCl及びCaCl2環境下ではさらに顕著な疲労強度の低下があり、それらが低応力下での腐食ピットの生成に起因することなどを明らかにしている。
　第3章「Corrosion fatigue behavior of extruded AZ31 magnesium alloy」では、AZ31合金押出し材の高湿度下とNaCl水溶液噴霧環境下での腐食疲労挙動を調べ、AZ61合金同様高湿度下、NaCl環境下において疲労強度が低下すること、S-N曲線が単一の曲線ではなく、2段のS-N曲線を示すこと、これが折れ曲がりの高応力側と低応力側でのき裂発生機構の相違に基づくことなどを明らかにしている。
　第4章「Corrosion fatigue behavior of extruded AZ80-T5 magnesium alloy」では、AZ80-T5合金押出し材の高湿度下とNaCl環境下での腐食疲労挙動を調べ、AZ80-T5合金の場合も他のマグネシウム合金と同様に、NaCl環境下で顕著な疲労強度の低下を示すこと、それが腐食ピットの形成によることなどを明らかにしている。
　第5章「Corrosion fatigue S-N curve characteristics」では、これまでの金属材料の腐食疲労に関する報告を参照しながら、腐食疲労のS-N曲線が、本質的には2段のS-N曲線を示すこと、腐食ピット形成の特性により、見掛け上単調な曲線を示す場合のあることなどを示している。
　第6章「Corrosion fatigue behavior of conversion coated AZ61 magnesium alloy」では、化成表面処理をしたAZ61合金押出し材の腐食疲労挙動を調べ、高湿度下では疲労強度の改善が認められるが、NaCl環境下では、改善は限定的であることなどを明にしている。
　第7章「Corrosion fatigue behavior of conversion coated and painted AZ61 magnesium alloy」では、化成処理表面に塗装処理を施すことにより、NaCl環境下においても十分な疲労強度の改善が達成できることなどを明らかにしている。
　第8章「Summary and Conclusion」では、以上を総括するとともに、今後の研究展望を示している。
　よって、本論文は工学上及び工業上貢献するところが大きく、博士（工学）の学位論文として十分な価値を有するものと認める。