本研究探討二氧化碳對Ti-6Al-4V合金於0.1 M硫酸鈉水溶液中之氫誘發破裂行為的影響。藉由在控制電位(-1.4、-1.6和-1.9 VSCE)的條件下進行慢應變速率拉伸試驗(slow strain rate testing, SSRT)，以評估Ti-6Al-4V雙相鈦合金之氫誘發破裂(hydrogen assisted cracking, HAC)敏感性。
拉伸試驗結果顯示，Ti-6Al-4V合金在0.1 M硫酸鈉水溶液中，其機械性質不會因為電位的改變而有明顯的變化，幾乎與在空氣中所測得結果相近。於0.1 M硫酸鈉水溶液中通入二氧化碳氣體後，在-1.4及-1.6 VSCE的控制電位下，其機械性質亦無顯著的差異。當電位控制在-1.9 VSCE時，則發現Ti-6Al-4V合金其降伏強度(yielding strength, YS)從1120 MPa下降至860 MPa，而總延伸率(total elongation)從15.2 %減少至12.1 %，並且於試片斷裂面的中央及邊緣區域部分，可觀察到脆性破裂特徵與二次裂痕(secondary crack)的存在，顯示二氧化碳會促進Ti-6Al-4V合金之氫誘發破裂敏感性。
Ti-6Al-4V合金在800℃持溫5小時，再以水淬方式冷卻至室溫後，除了可改善Ti-6Al-4V合金之柱狀晶結構組織，同時亦提高β相的體積比例。由拉伸試驗結果得知，經熱處理後之Ti-6Al-4V合金其延伸率有明顯增加的現象，呈現較佳的抗形變能力。另外，於0.1 M硫酸鈉水溶液中通入二氧化碳氣體後，當電位控制在-1.9 VSCE時，經熱處理後之Ti-6Al-4V合金其降伏強度從900 MPa下降至760 MPa，而總延伸率從16 %降低至13.5 %，並且在拉伸試棒的標距(gage)邊緣處可明顯發現二次裂紋的形成。
X-ray分析結果顯示，原材及經熱處理後之Ti-6Al-4V合金於含二氧化碳之0.1 M硫酸鈉水溶液中，在-1.9 VSCE陰極電位下經慢應變速率拉伸試驗後，可在兩者試片中發現到δ型氫化鈦的繞射峰。說明在-1.9 VSCE陰極電位下，二氧化碳氣體的通入有助於氫化鈦在兩者試片表面上形成。

The effects of carbon dioxide on the hydrogen assisted cracking (HAC) behavior of Ti-6Al-4V alloy in 0.1 M sodium sulfate solution were investigated. At applied potentials (-1.4, -1.6, -1.9 VSCE), slow strain rate tests (SSRT) were employed to evaluate the HAC resistance of dual phase Ti-6Al-4V alloy.
The SSRT experimental results exhibited that the mechanical behavior of Ti-6Al-4V alloy in 0.1 M sodium sulfate solution was almost the same as that in air, and independent of electrochemical potential in the range applied. At applied potentials of -1.4 and -1.6 VSCE, a carbon dioxide purging did not affect the mechanical properties of Ti-6Al-4V alloy in 0.1 M sodium sulfate solution. However, at an applied potential of -1.9 VSCE, a carbon dioxide purging caused a decrease in yielding strength from 1120 MPa to 860 MPa, and a reduction in elongation from 15.2 % to 12.1 %. And the brittle characteristics and secondary crack were observed at the central and edge areas of the fractured surface. The results indicated that the dissolved carbon dioxide in 0.1 M sodium sulfate solution would promote the susceptibility of hydrogen assisted cracking in Ti-6Al-4V alloy.
Ti-6Al-4V alloy was solution treated at 800℃ for 5 h and then water quenched to room temperature. In addition to modifying the columnar microstructure of Ti-6Al-4V alloy, the volume ratio of β phase was also increased by applying the heat treatment that mentioned above. The SSRT experimental results exhibited that the elongation had enhanced after heat treatment. However, at an applied potential of -1.9 VSCE, a carbon dioxide purging caused a decrease in yielding strength from 900 MPa to 760 MPa, and a reduction in elongation from 16 % to 13.5 %. The fractograph were also observed a large number of secondary crack at the gage section of the fractured specimens.
The X-ray analyses indicated that as-received and heat-treatment Ti-6Al-4V alloy in the dissolved carbon dioxide of 0.1 M sodium sulfate solution at an applied potential of -1.9 VSCE, the δ titanium hydride diffraction peaks were found at the both fractured specimens after slow strain rate testing. The facts explained that a carbon dioxide purging resulted in a production of titanium hydride on the surface of the specimens.

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