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Fatigue Crack Growth Behavior of Titanium Alloys and Welds at High Temperatures
|Contributors: ||NTOU:Institute of Materials Engineering|
|Issue Date: ||2011-06-28T07:09:31Z
|Abstract: ||摘要:鈦合金基本上分為α、nearα、α-β及β四大類，其中α-β型 鈦合金可經由熱處理來改變其顯微組織，因而其機械性能隨著產生變 化，因此高強度鈦合金經常是選用α-β型，其中代表性合金為 Ti-6Al-4V，而β型鈦合金由於其穩定之結構，且經未時效處理前有 良好之成型性，而時效後其強度則可大幅提升但伴隨延展性下降之缺 點， 因此β 型鈦合金經常使用於高溫環境下， 其中以 Ti-15V-3Cr-3Al-3Sn 為典型代表合金。但一般β型鈦合金添加大量 合金元素，因此銲接性較差。 500℃以下鈦合金仍保有優良之抗氧化性及比強度，工程材料具備 優越之高溫疲勞裂縫成長特性，亦為航太結構材料所需具備重要性 質。本研究為針對二種鈦合金，其中之一為α-β型(Ti-6Al-6V-2Sn, Ti-6-6-2)，另一為β型(Ti-15V-3Cr-3Al-3Sn, Ti-15-3)，進行板材 及銲件之疲勞裂縫成長特性試驗，評估不同顯微組織對鈦合金疲勞性 能之影響，本研究前兩年度主要進行高溫疲勞裂縫成長試驗，採用電 壓降比對法來量測裂縫長度，並於不同溫度進行疲勞裂縫成長試驗， 來測定鈦合金板材及銲件之高溫疲勞性質。粗大組織ㄧ直是有害於鈦 合金之強度及延、韌性，本研究第三年度是針對粗大α-β 鈦合金銲 道組織予以微細化，細化製程為依據本研究群體吳教授研究成果，進 行衝擊及疲勞裂縫成長試驗，以評估鈦合金微細組織對其銲件機械性 能之影響，本年度研究有助於此研究群體之橫向研究整合及設備整 合。|
abstract:Titanium alloys can be mainly divided into four groups, i.e., α, near α, α-β, and β titanium alloys. In case of α-β type of alloys, the microstructures can be altered through heat treatment hence the mechanical properties will behave great changes depending on the inherent microstructures. High strength titanium alloys used as structural materials often belong to α-β or βalloys. Ti-6Al-4V is one of the most widely used α-βtitanium alloys. For the demands of higher strength during service, or greater plastic deformability during fabrication, alternative alloys are developed to replace Ti-6-4 alloy, e.g., Ti-6Al-6V-2Sn alloy is reported to be used for critical applications. Additionally, the weld metal toughness of Ti-6Al-6V-2Sn alloy is also higher than that of Ti-6-4. The βtitanium alloy possesses excellent cold forming ability in the solution-treated condition and achieves high strength after aging treatment. The typical β titanium alloy is Ti-15V-3Cr-3Al-3Sn. The stable structure of the βalloy at high temperature makes it extensively used as the material for high temperature service. In this work, the fatigue crack growth behaviors of two different titanium alloys, i.e., Ti-6-6-2 and Ti-15-3-3-3 will be evaluated, especially pay attention to the properties of laser welds aged at various temperatures. The detail microstructures of the welds will be examined. Moreover, the variation in fatigue crack growth behaviors will be correlated with the microstructures in distinct specimens. Fatigue crack growth rate of titanium alloys at the temperatures below 500oC, is also one of important characteristics to be used as the candidate material for aerospace applications. During this research, alternate current potential drop (ACPD) technique will be applied to measure the crack length of the specimens subjected to fatigue-loading in the controlled temperatures. This technique is reported to be a reliable method to determine the length of a fatigue crack, especially for the fatigue crack growth at high temperature. The development of this technique to measure the size of a flaw precisely is an important issue for this project. The influence of service temperature on the fatigue crack growth behaviors of high strength titanium alloys and welds will be evaluated at the temperatures less than 500 oC. Coarse columnar structures are known to damage the mechanical properties of titanium alloy welds as compared with the base metal. In the third year of the investigation, a special procedure developed by Professor Wu will be used to refine the microstructure of the laser welds. The influence of solidified microstructures on the fatigue crack growth rate of various specimens will be determined as compared with the counterpart specimens with coarse graine size. Fatigue fracture appearance of various specimens were examined by scanning electron microscopy (SEM) to identify typical fracture features and correlated with fatigue crack growth characteristics.
|Appears in Collections:||[材料工程研究所] 研究計畫|
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