中文摘要：
	本研究主旨在探討球墨鑄鐵(DI)斷面尺寸及各位置因凝固時間的不同，對沃斯回火球墨鑄鐵(ADI)在應變控制下低週疲勞行為的影響。從而了解基地組織、球墨數多寡及缺陷對低週疲勞性質與破壞機構的 影響。此外，並用SEM對破斷面及包含主裂縫的量規區縱剖面加以觀察分析，以了解其疲勞破壞機構與裂縫前進的路線。實驗結果顯示，低週疲勞性質隨鑄件斷面的尺寸增加而降低，並且在厚斷面鑄件中越遠離砂模面的位置其低週疲勞性質越差。這是因為鑄造澆鑄時凝固速率的降低，造成球墨數量減少、球墨型態惡劣、缺陷的增加及晶界間產生偏析所造成。此外，不同的熱處理條件下，其低週疲勞行為亦不相同，影響最佳延性、韌性熱處理條件的ADI低 週疲勞性質主要因素為基地的穩定性，即基地中殘留沃斯田鐵受力後變態成麻田散鐵的特性，而對最佳強度、硬度熱處理條件的ADI而言，則為基地中的缺陷。整體而言，以最小斷面尺寸，並經由最佳強 度、硬度熱處理後的ADI，有最佳的低週疲勞強度。由SEM的觀察中得知，ADI的疲勞破壞起源，是由表面缺陷或不規則的球墨等應力集中處形成裂縫，在裂縫生成與延伸的區域，為脆性的劈裂破壞模式，而在最後破斷的區域，則可發現類似延性破壞的 酒渦(dimple)狀組織。不同熱處理條件下的裂縫型態不同，經最佳延性 韌性熱處理之後，裂縫開口距離較大，而經最佳強度、硬度熱處理之後，發現在裂縫的前緣有分叉的情形，兩者行進的方向皆與施加外力的方向垂直，裂縫的延伸是朝著球墨聚集處及缺陷所在的位置。此外，從破斷面的觀察中可得到，鑄造時凝固速率較快的位置，ADI有較高的破裂韌性值，裂縫起始及慢速延伸區域佔破斷面的比例較凝固 速率慢的位置大。
Abstract：
	This study investigated the relationship between low-cycle fatigue (LCF$ strength of austempered ductile iron (ADI) and cast section size and position . Uniaxial LCF tests were conducted under strain control at several strain amplitudes in tension-compression cyclic loading. The effects of austempering temperatures, nodularity, nodule counts, casting defects, and morphology of retained austenite in the matrix of ADI on the LCF behavior are discussed. Fractography with scanning electron microscopy (SEM) was used to determined the LCF failure mechanisms and fatigue crack propagation modes. Results showed that as the cast section size increased, the fatigue strength decreased. The decrease in LCF strength is attributed to the increasing graphite nodule size, the deterioration in nodule shape, the increasing of microshrinkage pores, and possibly the associated increased cell boundary microsegregation present as the cast section size is increased. In addition to nodule morphology and defects formed during casting process, the stability of matrix after austempering heat treatment is also an important factor influencing LCF strength of ADI. ADIs with optimal strength are sensitive to defects in the matrix in terms of LCF strength while those with optimal toughness are sensitive to the stability of matrix. In general, ADIs cut from the small cast section and austempered for optimal strength showed the best LCF property. Fatigue cracks usually initiated from the surface irregular nodules and casting defects. The fracture modes are different in the crack initiation zone and final fracture zone. The brittle cleavage was obsvered in the initiation zone and dimples (ductile fracture mode) appeared in the final fracture region for ADIs tested. However, the path of crack propagation for ADIs with optimal strength and optimal toughness are different. Fatigue cracks in ADIs with optimal strength propagated in the matrix with branching around the crack tip, and fatigue cracks in those with optimal toughness propagated with larger crack opening displacement. Both types of cracks propagated toward defects and highly distributed nodules in a direction perpendicular to loading direction. Fractography results also showed that ADIs with fast solidification rate during casting process had greater toughness.