| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 98 === 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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