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显微组织对TC4ELI合金损伤容限行为的影响
Alternative TitleThe effect of microstructure on damage-tolerance behaviors in TC4ELI alloy
马英杰
Subtype博士
Thesis Advisor杨锐
2009-05-29
Degree Grantor中国科学院金属研究所
Place of Conferral金属研究所
Degree Discipline材料学
KeywordTc4eli合金 显微组织 疲劳裂纹尖端塑性区 疲劳裂纹扩展 断裂韧性
Abstract本文以高损伤容限性能的TC4ELI(extra-low-interstitial)合金为实验材料,研究了两种钛合金显微组织疲劳裂纹尖端塑性区的变形特点,并以此为基础研究了影响TC4ELI合金损伤容限性能(疲劳裂纹扩展速率及断裂韧性)的因素。 首先考察了TC4ELI合金具有原始β晶粒的片层组织和短棒α组织两种显微组织中疲劳裂纹尖端塑性区的差异,分析了塑性区差异对裂纹扩展路径、裂纹扩展速率的影响。结果显示,两种显微组织中疲劳裂纹尖端塑性区尺寸及变形特征存在较大差异,片层组织中较大的塑性区及塑性区内受晶粒取向影响的大范围滑移共同造成疲劳裂纹在该组织中扩展时具有曲折的扩展路径及较高的裂纹闭合程度,并导致较低的裂纹扩展速率;短棒α组织中裂纹尖端塑性区尺寸较小,裂纹扩展路径较为平直,裂纹尖端闭合程度较低,疲劳裂纹扩展速率较高。实验还对两种显微组织中疲劳裂纹尖端塑性区的能量消耗进行了测试,也反映了两种显微组织中裂纹尖端塑性区的差异。 对片层组织疲劳裂纹扩展速率Paris区存在转折点的现象进行了分析,发现转折点的出现是由于疲劳裂纹尖端塑性区尺寸超过晶粒尺寸进而导致裂纹扩展方式发生变化所造成。 由于片层组织具有较低的疲劳裂纹扩展速率,本文中主要对片层组织不同显微组织尺寸下的疲劳裂纹扩展速率进行了测试分析,并总结得到不同ΔK阶段影响疲劳裂纹扩展速率的显微组织因素。实验发现,α片层厚度是影响近门槛值阶段疲劳裂纹扩展的主要因素,而疲劳裂纹在Paris阶段的扩展速率则受晶粒尺寸、晶团尺寸以及片层厚度的共同影响。 通过长焦距显微镜及SEM研究了疲劳短裂纹在片层组织和短棒α组织中的萌生及扩展行为,测量并计算得到短裂纹在两种显微组织不同应力比下的扩展速率,并与疲劳长裂纹扩展速率进行对比。结果显示,疲劳短裂纹在片层组织中主要在驻留滑移带处萌生,具有较低的萌生寿命;短棒α组织中短裂纹在应力集中最大处萌生,具有较高的萌生寿命。试样缺口塑性区内变形特征的差异是导致短裂纹在两种显微组织中萌生方式不同的原因。片层组织中,疲劳短裂纹扩展受显微组织尤其是晶界的影响较大,裂纹穿过晶界时经历“减速-加速-减速”的扩展过程,而短棒α组织中,短裂纹扩展受显微组织的影响较小。对比疲劳短裂纹与长裂纹扩展速率发现,片层组织中短裂纹能够在低于长裂纹扩展门槛值的范围内扩展,并在一定ΔK范围内具有高于长裂纹的扩展速率;短棒α组织中短裂纹未能在低于长裂纹扩展门槛值的范围内扩展,但在一定ΔK范围内同样具有高于长裂纹的扩展速率。 测试了TC4ELI合金不同显微组织下的断裂韧性,整体上片层组织具有较高的断裂韧性值,并且大体上断裂韧性随屈服强度的升高而降低。基于断裂韧性的本质定义,定性分析了断裂韧性与屈服强度的关系,并解释了片层组织具有较高断裂韧性的原因。
Other AbstractThe damage-tolerance behaviors including fatigue crack propagation and fracture toughness were investigated in TC4ELI (extra-low-interstitial) alloy. Crack tip plastic zone (CTPZ) in two kinds of microstructures, lamellar microstructure and short-bar microstructure, was studied to investigate the propagation behavior of fatigue crack. The effects of CTPZ on fatigue crack propagating path and crack growth rates were analyzed in the two microstructures. Results show obvious differences of CTPZ between the two microstructures. In lamellar microstructure, large CTPZ and intensive slip influenced by grain orientation lead to flexuous crack path and high crack closure level which will both reduce fatigue crack growth (FCG) rates. In short-bar microstructure, small CTPZ and flat crack path lead to high FCG rates. Meanwhile the investigations of energy dissipated in CTPZ also reveal the difference of CTPZ between the two microstructures. The turning points in Paris region of FCG rates were studied in lamellar microstructure. Result indicates that the appearance of the turning point is induced by CTPZ size growing larger than β grain size. The low FCG rates in lamellar microstructures are more suitable for the damage-tolerance design. FCG rates of lamellar microstructure with various size parameters were tested to investigate the key microstructure factors which could affect FCG rates in different ΔK stages. Analysis of crack propagating path in lamellar structure helps to understand the effect of microstructure on FCG rates. FCG rate tests reveal that the thickness of α lamella is the main factor to affect FCG rates in near ΔKth stage, however the FCG rates in Paris region are influenced by grain size, colony size and lamellar thickness. Long-focus microscopy and SEM micrography were used to investigate short fatigue crack initiation and propagation in lamellar and short-bar microstructure. Short fatigue crack growth rates in the two microstructures were tested and compared with long crack at three load ratios. In lamellar microstructure, short crack initiates at PSB position with short initiating life; In short-bar microstructure, short crack initiates at the largest stress concentration region with long initiating life. Difference of notch plastic zone in the two microstructures is considered to be the key factor to determine the different initiating styles. Short fatigue crack propagates faster than long cracks at the same ΔK in both microstructures, but only in the lamellar microstructure can short fatigue crack propagate below long fatigue crack threshold ΔKth. Fracture toughness of TC4ELI alloy with different microstructures was tested. Results show high KIC value in lamellar microstructure, and the KIC value increases with the decreasing of yield stress. The CTPZ was introduced to interpret the variations of KIC.
Pages145
Language中文
Document Type学位论文
Identifierhttp://ir.imr.ac.cn/handle/321006/17220
Collection中国科学院金属研究所
Recommended Citation
GB/T 7714
马英杰. 显微组织对TC4ELI合金损伤容限行为的影响[D]. 金属研究所. 中国科学院金属研究所,2009.
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