Abstract
Susceptibility to environment-assisted cracking (EAC) of super-elastic TiNi alloy has been correlated with the states and their amount of hydrogen in the material. The TiNi alloy specimen was immersed in sulfate solutions at a given pH, and a giving cathodic potential was applied to it for 13 ks to absorb hydrogen into it. The specimen was subjected to X-ray diffraction (XRD) test to detect hydride, and to thermal gas desorption spectroscopy (TDS) test to determine the state and its amount of hydrogen in it. In the case that a potential of -0.5 VAg/AgCl was applied to the specimen in the solution of pH 3, the XRD detected no hydride on it. Whereas, application of -1.5 VAg/AgCl was induced formation of hydride. The TDS indicated that the specimen treated at -0.5 VAg/AgCl involved hydrogen desorbing around 810 K, and the one at -1.5 VAg/AgCl involved hydrogen desorbing around 500 as well as 810 K. Therefore, it was suggested that the hydrogen desorbing at lower and higher temperatures obtained by the TDS were of hydride state and solid-solution state, respectively. The specimens treated under various conditions were analyzed by the same way as mentioned before, and an amount of the hydrogen in each state as well as a cathodic charge density applied to the specimen were obtained to be correlated. A logarithm of the amount of the hydrogen in the each state increased almost linearly with an increase in a logarithm of a charge density. Susceptibility to the EAC of the material suddenly increased around a charge density of 0.025 MC·m-2, and the charge density was considered to induce the amounts of hydrogens in hydride and solid-solution state of 10 and 100 mass ppm, respectively.
Highlights
TiNi alloy demonstrates unique mechanical properties such as shape memory or super-elasticity
The specimens were subjected to the X-ray diffraction (XRD) test, and it was found that the specimen treated at -0.5 VAg/AgCl consisted of main TiNi and slight Ti2Ni, identical to the specimen without the treatment
In comparison of the results of the XRD with those of the thermal gas desorption spectroscopy (TDS) tests, the followings were suggested: Since the specimen treated at -0.5 VAg/AgCl had no hydride but involved hydrogen desorbing at higher temperature around 810 K, the hydrogen desorbing at the higher temperature may be hydrogen in solid-solution state in the specimen
Summary
TiNi alloy demonstrates unique mechanical properties such as shape memory or super-elasticity. The electrochemical hydrogen absorption technique has been widely employed in these studies to determine the relation between the EAC susceptibility of the alloy and the amount of hydrogen absorbing into it. The effects of the electrochemical potential and the solution pH on the EAC susceptibility of TiNi alloy with super-elastic property was investigated [9] and it was found that the EAC susceptibility was significantly enhanced for lower potentials as well as lower pH values, quite different EAC map against that of TiAl. The investigation on EAC of TiNi revealed that the EAC had possibility to be affected by hydride and/or hydrogen as solid-solution state. This research, aimed to understand the changes in the amounts of hydride and hydrogen with hydrogen absorption conditions, and to discuss the relation between the two amounts and the EAC susceptibility of TiNi
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