氏名　SHAIFULAZUAR BIN ROZALI
学位の種類　博士（工学）
学位記番号　博甲第576号
学位授与の日付　平成23年3月25日
学位論文題目　Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Corrosive Environment　（マグネシウム合金 AZ61の疲労き裂伝ぱに及ぼす腐食環境の影響）
論文審査委員
　主査　教授　武藤　睦治
　副査　実務家教授　永田　晃則
　副査　教授　岡崎　正和
　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

1 Introduction
　1.1　Overview on Magnesium Alloys　p.1
　1.2　Mechanical Properties of Magnesium Alloys　p.2
　1.3　Corrosion Properties of Magnesium Alloys　p.3
　1.4　Advantages and Applications of Magnesium Alloys　p.6
　1.5　Motivation of the Present Work　p.11
　1.6　Scope and Objective of the Present Work　p.11
　1.7　Dissertation Outline　p.12
　1.8　References　p.14

2 Literature Review
　2.1　Fatigue Behavior of Magnesium Alloys　p.16
　2.1.2　Crack Free Fatigue　p.17
　2.1.3　Crack Initiation　p.18
　2.1.4　Fatigue Crack Propagation　p.19
　2.2　Factors that Affect Fatigue Behavior of Magnesium Alloys　p.21
　2.2.1　Processing, Heat Treatment and Microstructures　p.21
　2.2.2　Environmental Effects　p.26
　2.3　Effect of Corrosive Environment on Fatigue Behavior of Magnesium Alloys　p.26
　2.3.1　Fatigue Strength and Life　p.27
　2.3.2　Crack Initiation Mechanism　p.29
　2.3.3　Crack Propagation　p.30
　2.3.4　Effect of Processing　p.31
　2.4　Conclusion　p.33
　2.5　References　p.34

3 Effect of Stress Ration on Fatigue Crack Growth Behavior Magnesium Alloy AZ61 Under Sprayed 3.5 mass% NaCl Environment
　3.1　Intoroduction　p.44
　3.2　Experimental Procedures　p.44
　3.2.1　Material　p.44
　3.2.2　Specimen　p.46
　3.2.3　Fatigue Crack Growth Test Procedures　p.47
　3.3　Results and Discussion　p.51
　3.3.1　Fatigue Crack Growth Behavior Under Low Humidity Environment　p.51
　3.3.1.1　Fatigue Crack Growth Curve　p.51
　3.3.1.2　Fracture surface observations　p.54
　3.3.2　Fatigue Crack Growth Behavior Under Sprayed 3.5 mass% NaCl Environment　p.57
　3.3.2.1　Fatigue Crack Growth Curve　p.57
　3.3.2.2　Fracture surface observations　p.60
　3.4　Conclusions　p.64
　3.5　References　p.65

4 Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Sprayed and Immersed 3.5 mass% NaCl Environment
　4.1　Introduction　p.68
　4.2　Experimental Procedures　p.69
　4.2.1　Material　p.69
　4.2.2　Specimen　p.69
　4.2.3　Fatigue crack growth test procedures　p.69
　4.3　Results and Discussion　p.71
　4.3.1　Fatigue Crack Growth Behavior　p.71
　4.3.2　Fracture Surface Observations　p.74
　4.3.2.1　Under Low Humidity （55% Relative Humidity） Environment　p.74
　4.3.2.2　Under 3.5 mass% NaCl Environment　p.76
　4.4　Conclusions　p.81
　4.5　References　p.83

5 Effect of Frequency on Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Immersed 3.5 mass% NaCl Environment
　5.1　Introduction　p.87
　5.2　Experimental Procedures　p.89
　5.2.1　Material　p.89
　5.2.2　Specimen　p.89
　5.2.3　Fatigue crack growth test procedures　p.89
　5.3　Results and Discussion　p.91
　5.3.1　Fatigue crack growth behavior　p.91
　5.3.1.1　Fatigue crack growth test　p.91
　5.3.1.2　Constant ΔK tests under wide range of frequency　p.95
　5.3.2　Fracture surface observations　p.98
　5.3.2.1　Under low humidity （RH 55%） environment　p.98
　5.3.2.2　Under immersed NaCl environment　p.99
　5.4　Conclusions　p.103
　5.5　References　p.104

6 Effect of Single Overload on Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Immersed 3.5 mass% NaCl Environments
　6.1　Introduction　p.108
　6.2　Experimental Procedures　p.109
　6.2.1　Material　p.109
　6.2.2　Specimen　p.109
　6.2.3　Fatigue crack growth test procedure　p.109
　6.3　Results and Discussion　p.112
　6.3.1　Fatigue crack growth behavior following the overload　p.112
　6.3.1.1　Under low baseline stress intensity factory range,ΔKBL level （3.5Mpa.m1/2）　p.112
　6.3.1.2　Under high baseline stress intensity factory range,ΔKBL level （5.0Mpa.m1/2）　p.116
　6.3.2　Observations of fracture surface and vicinity area of overload point　p.122
　6.3.2.1　Fracture surface　p.122
　6.3.2.2　In the vicinity of overload point　p.127
　6.4　Conclusions　p.134
　6.5　References　p.135

7 Fatigue Crack Growth Mechanism and Comparison with Other Alloys
　7.1　Fatigue crack growth mechanism of magnesium alloy AZ61 under NaCl environment　p.139
　7.2　Comparison of fatigue crack growth behavior with other alloys　p.142
　7.3　References　p.149

8 Conclusions and Suggestions for Future Work
　8.1　Conclusions　p.153
　8.2　Suggestions for Future Work　p.154

List of Publications　p.156

　From global environment and energy saving viewpoints, reduction of weight becomes one of the main concerns in aerospace and transportation industries, where magnesium alloy is considered as a candidate material. Magnesium alloys have superior advantages, not only light weight but also high specific strength, machinability, recyclability, etc. As structural materials, fatigue characteristic is the main concern in design because fatigue failure is the dominant cause of accidents. Since many mechanically loaded parts in automobile, etc. are often subjected to prolong cyclic stresses under corrosive environment, it is very important to understand fatigue behavior of magnesium alloys under corrosive environment. Hence, in the present study, a systematic investigation has been performed in order to understand fatigue crack growth behavior of magnesium alloys AZ61 under the influence of corrosive environment. Several series of experiments were carried out in the sequence of study on the effects of stress ratio, exposure condition of NaCl environment, loading frequency and overload event during crack propagation.

　In Chapter 1 “Introduction”, a brief introduction about history and development, mechanical and corrosion properties, advantages and applications of magnesium alloys have been outlined. The motivations, scope and objective of the present study also have been addressed.

　In Chapter 2 “Literature Review”, literature review related to the present research works have been presented in this chapter. The previous studies on fatigue properties and factors that might affect fatigue behavior of magnesium alloys have been summarized.　The effect of corrosive environment on fatigue performance of magnesium alloys has been discussed based on the investigations reported by previous researchers.

　In Chapter 3 “Effect of Stress Ratio on Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 Under Sprayed 3.5 mass% NaCl Environment”, in the first step, fatigue crack growth tests have been carried out at various stress ratios. The results showed that fatigue crack growth behavior of magnesium alloy AZ61 was significantly influenced by stress ratio. Fatigue crack growth rates became higher with increasing stress ratio under low humidity （55% relative humidity） and sprayed 3.5% NaCl environments. However, fatigue crack propagation rates under sprayed 3.5% NaCl environment were lower compared to those in low humidity environment for all stress ratios. The difference in crack growth rate was mainly resulted from the different crack closure behavior which was characterized by ΔKeff. The fatigue crack growth curves arranged by ΔKeff merged into one curve regardless of stress ratio except in the near-threshold region. This suggests the influence of corrosive environment in accelerating crack propagation at the near-threshold region.

　In Chapter 4 “Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Sprayed and Immersed 3.5 mass% NaCl Environments”, in the next step, fatigue crack growth test was performed under immersion in 3.5mass% NaCl solution （immersed NaCl environment）. The results showed significant influence of exposure condition on fatigue crack growth behavior of magnesium alloy AZ61. The effect was also dependent on the applied stress ratio, R. However, the fatigue crack growth curves were merged into one curve after rearranged by ΔKeff except in the near-threshold region. These results suggested that fatigue crack growth rates at higher ΔK region was governed by crack closure induced by corrosion products formed on the fractured surface. While, fatigue crack propagation was accelerated due corrosion at the crack tip in the near-threshold region.

　In Chapter 5 “Effect of Frequency on Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Immersed 3.5 mass% NaCl Environment”, further study on the effect of time dependent corrosion attack on fatigue crack growth behavior of magnesium alloy AZ61 has been carried out by reducing test frequency. Effect of frequency was clearly observed in low ΔK region, where fatigue crack growth rate decreased with decreasing frequency. Crack closure would be a dominant factor for the frequency effect observed under immersed NaCl environment at frequencies higher than 0.5Hz. Fatigue crack growth rates at frequencies lower than 0.05Hz were higher than those at frequencies higher than 0.5Hz. The corrosion attack at the crack tip would be contributed to the accelerated fatigue crack growth rates at frequencies lower than 0.05Hz.　While, crack closure level was not further increase as frequency decreased below 0.05Hz. Thus, ample time was required for corrosion at the crack tip to accelerate fatigue crack growth rate.

　In Chapter 6 “Effect of Single Overload on Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Immersed 3.5 mass% NaCl Environment”, the investigation has been further extended to the effect of overload on fatigue crack growth behavior under corrosive environment. The results showed that fatigue crack growth retardation after overload event in both low humidity and immersed NaCl environments. However, crack retardation was more pronounced under immersed NaCl environment. Crack growth retardation under low humidity environment and that of low ΔKBL under immersed NaCl environment were attributed to the crack closure effect, which was characterized by ΔKeff. However, crack growth rates for higher ΔKBL under immersed NaCl environment were higher than fatigue crack growth curve without overload effect after arranged by ΔKeff. This suggested the influence of corrosive environment in assisting crack propagation after overload event. Micro-cracking at grain boundaries were observed in the vicinity area of overload event. Therefore, higher fatigue crack growth rates observed after compensation of crack closure effect might be due to formation of micro-cracks in front of the crack tip, which could enhance crack propagation rate.

　In Chapter 7 “Fatigue Crack Growth Mechanism and Comparison with Other Alloys”, based on the results obtained in this study, the embrittlement of material ahead of crack tip due to hydrogen absorption has been proposed as the mechanism in acceleration of fatigue crack propagation under corrosive environment. Hydrogen was produced on the fractured surface in the crack as a result of corrosion cathodic reaction. This hydrogen could penetrate into the material ahead of crack tip through hydrogen diffusion and hydrogen transport by mobile dislocations. In addition, fatigue crack growth rate under corrosive environment could be accelerated due to cracking at grain boundary induced by high applied stress intensity factor, K and preferential corrosion attack at grain boundary. This behavior would play a significant role for the fatigue crack growth after overloading. The role of cracking at grain boundary in accelerating fatigue is expected become more significant at low frequency.

　In Chapter 8 “Conclusions and Suggestions for Future Work”, the effect of corrosive environment on fatigue crack growth behavior of magnesium alloy AZ61has been summarized. The general conclusions of the current study and suggestions for further research have been addressed.

　本論文は、「Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Corrosion 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歴史、特性、応用の概略を示すとともに、本研究の目的と範囲を述べている。
第2章「Literature Review」では、これまでのマグネシウム合金の疲労とそれに及ぼす因子に関する研究について概括するとともに、それらに基づき、マグネシウム合金の疲労に及ぼす腐食環境の影響についてまとめている。
第3章「Effect of Stress Ratio on Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Sprayed 3.5 mass% NaCl Environment」では、NaCl水溶液噴霧環境下での疲労き裂伝ぱ特性に及ぼす応力比の影響について調べ、応力比の影響は下限界領域を除き、き裂開口挙動の相違に基づくものであることなどを明らかにしている。
第4章「Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Sprayed and Immersed 3.5 mass% NaCl Environments」では、両環境下での疲労き裂伝ぱ挙動の相違は、き裂開口挙動の相違に基づくものであり、下限界領域では、腐食反応に基づく水素脆化の影響が認められることなどを明らかにしている。
第5章「Effect of Frequency on Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under Immersed 3.5 mass% NaCl Environmnet」では、繰返し周波数が15Hzから0.5Hzまでの範囲では、繰り返し周波数の影響はほとんど認められないが、0.05Hzよりも遅くなると認められ、き裂伝ぱ速度は高くなることなどを明らかにしている。
第6章「Effect of Single Overload on Fatigue Crack Growth Behavior of Magnesium Alloy Az61 under Immersed 3.5 mass% NaCl Environment」では、低応力領域での過大荷重では、通常の遅延挙動が認められるが、高応力領域での過大荷重では、過大荷重により前方に微小な粒界割れを多数生じ、これらが連結することにより、き裂伝ぱ速度の加速を生じることなどを明らかにしている。
第7章「Fatigue Crack Growth Mechanism and Comparison with Other Alloys」では、腐食環境下でのマグネシウム合金のき裂伝ぱのメカニズムを検討し、腐食反応で生じる水素によるき裂先端近傍の脆化が、下限界領域のでき裂伝ぱの加速、下限界値の低下をもたらしていること、従来の他の金属材料の腐食疲労き裂伝ぱとの類似点、相違点などについて明らかにしている。
第8章「Conclusions and Suggestions for Future Work」では、以上の結果を総括するとともに、今後取り組むべき課題について示している。
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。