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Please use this identifier to cite or link to this item: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/29755

Title: 環境效應對雙相 α + β 鈦合金雷射銲件之疲勞裂縫成長行為
The Influence of Environment on Fatigue Crack Growth of α + β Titanium Laser Welds
Authors: Yi-Shiun Ding
丁逸勳
Contributors: NTOU:Institute of Materials Engineering
國立臺灣海洋大學:材料工程研究所
Keywords: Ti-6Al-4V;SP-700;Ti-6Al-6V-2Sn;疲勞裂縫成長試驗;雷射銲件
Ti-6Al-4V;SP-700;Ti-6Al-6V-2Sn;4Vfatigue crack growth behavior;laser welds.
Date: 2011
Issue Date: 2011-11-23T03:13:08Z
Abstract: 研究中主要探討雙相鈦合金Ti-6Al-4V、SP-700及Ti-6Al-6V-2Sn母材,銲件及其經不同銲後熱處理後,在不同環境(大氣環境、氣態氫、高溫環境)之疲勞裂縫成長行為。在不同氣氛環境中,主要探討Ti-6Al-4V及SP-700母材,在大氣環境、微真空及氣態氫環境下顯微組織尺寸對疲勞裂縫成長速率的影響。在高溫環境,探討SP-700及Ti-6Al-6V-2Sn母材及銲件在負載頻率10Hz,其高溫氧化、裂縫尖端塑性域與顯微組織尺寸交互作用與疲勞裂縫成長間的關聯。 在常溫疲勞裂縫成長試驗中,Ti-6Al-4V母材相較於SP-700及Ti-6Al-6V-2Sn而言,受塑性域與較粗大α+β組織尺寸作用的關係,在低△K範圍滑移傾向單滑移系統,破裂面上可發現α相易引發脆性破壞,而有一明顯轉折點,△KT值,當△K>△KT時,滑移趨於多重滑移系統,呈典型穿晶疲勞破壞伴隨α相脆性面的消失,而提高實驗應力比至0.5時,將降低其△KT值。在銲件數據上,Ti-6Al-4V及Ti-6Al-6V-2Sn銲件顯示較母材佳的抵抗裂縫成長能力,此現象與銲道內微細之針狀顯微組織與母材之滾軋帶狀α+β組織有關。SP-700母材,銲件及其經不同銲後時效試片在低實驗應力比(R=0.1)呈現大致相同裂縫成長速率,提高實驗應力比至0.5,受高應力比相對高應力集中的關係,裂縫尖端之缺口敏感性增加,使W-482頂時效試片有最快的疲勞裂縫成長速率,W-704顯示較慢疲勞裂縫成長速率則與粗大化的α+β相及較厚之晶界α有關 。 在氣態氫對Ti-6Al-4V及SP-700母材進行疲勞裂縫成長試驗,結果顯示,在實驗應力比為0.1時,Ti-6Al-4V在氣態氫環境下顯示較大氣環境下佳之抵抗疲勞裂縫成長能力,此現象可歸因於在α相脆性破壞造成明顯裂縫路徑曲折,而造成較慢的裂縫成長速率。然而,提高其實驗應力比至0.5時,裂縫曲折程度減緩,因此會顯示與大氣環境下有相同裂縫成長速率。相對地,SP-700不管在氣態氫或大氣環境下,其結果皆顯示疲勞裂縫成長速率對環境因子不敏感。 在高溫疲勞裂縫成長試驗中,當環境溫度達600℃時,SP-700及Ti-6Al-6V-2Sn母材及銲件其疲勞裂縫成長速率才有明顯加速的現象,主要與降伏強度降低有關。SP-700銲件隨環境溫度提升,破壞面特徵顯示較易引發次裂縫生成,此現象可歸究於高溫下氧氣與滑移帶交互作用所造成。溫度提升至600℃時,在低△K時裂縫成長速率受顯微組織影響而較曲折,柱狀晶內整體之脆性破壞使局部性位置有裂縫加速之現象。而SP-700及Ti-6Al-6V-2Sn銲件在高△K範圍,破裂模式會由穿晶轉變為沿晶破壞。
In this work, the fatigue crack growth test in distinct environment (such as gaseous hydrogen or elevated temperature) were carried out on three two phase α+β Ti-6Al-4V, SP-700 and Ti-6Al-6V-2Sn titanium alloys and laser welds to evaluate the fatigue mechanism in distinct conditions. At high temperature, the loading frequency of 10Hz was employed to determine the effect of pure fatigue (cyclic control) on SP-700 and Ti-6Al-6V-2Sn base metal and laser welds. And the fatigue crack growth rates (FCGRs) of Ti-6Al-4V and Ti-4.5Al-3V-2Mo-2Fe alloys, were determined in gaseous hydrogen, soft vacuum and laboratory air at room temperature. At room temperature, the FCGRs of the Ti-6Al-4V alloy exhibits an obvious transition △KT at low △K comparing with that of the SP-700 and Ti-6Al-6V-2Sn alloy, resulting from an obvious crack deflection occurring at α phase. It might be deduced that the fatigue mechanism tends to form a single slip when the microstructure size is smaller than plastic zone size. However, when the stress ratio arises to 0.5, the △KT would occur earlier than that of tested at stress ratio of 0.1. The Ti-6Al-4V and Ti-6Al-6V-2Sn welds with basket weave microstructure exhibit a better FCGR resistance than that of the banded base metal. In contrast, in case of the SP-700 samples, both base metal and welds present almost identical FCGRs at low stress ratio of 0.1. When arising the stress ratio to 0.5 with higher stress concentration at crack tip, the W-482 specimen with fine, homogeneous precipitates distributed in the matrix behaves the highest FCGRs. However, a comparable coarse structure of the W-704 specimen exhibits the lowest FCGRs. At elevated temperature, the FCGRs of the SP-700 and Ti-6Al-6V-2Sn base metal and welds showed minor changes when tested below 4500C, but exhibited a noticeable increase in FCGRs at 600℃, regardless of the specimens. The apparent rise in FCGRs at 600℃ could be attributed to the marked decrease in yield stress at elevated temperature. At low ΔK and 600℃, the FCGRs of the SP-700 and Ti-6Al-6V-2Sn welds showing a slight deflection, is deduced to the interaction between the enlarging plastic zone and the microstructure with high level orientation. Its fracture surface reveals an extent of cleavage at specific grains. At high ΔK, the welds presents the intergranular fracture along the column solidification, attributed to the plastic zone size exceeds the size of columnar grain. In hydrogen at stress ratio (R) of 0.1, the deflected crack path associated with the localized brittle fracture of α-phase accounted for the reduced FCGR of Ti-6Al-4V specimens. Moreover, hydrogen embrittlement enhanced cracking of the specimen at R=0.5, which alleviated the effect of crack deflection, led to a similar FCGR as those tested in air. In contrast, the FCGR of SP-700 specimen was insensitive to the environments and stress ratio.
URI: http://ethesys.lib.ntou.edu.tw/cdrfb3/record/#G0D95550002
http://ntour.ntou.edu.tw/handle/987654321/29755
Appears in Collections:[材料工程研究所] 博碩士論文

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