Environment-Assisted Cracking of Super-Elastic TiNi Alloy Depending on Solution pH and Electrochemical Potential

The behavior of environment-assisted cracking (EAC) for super-elastic TiNi alloy was investigated as functions of solution pH and electrochemical potential. The specimen was immersed in sulfate solutions at various pHs, and subjected to a potentiostatic slow strain rate tensile test. As a result, the EAC map for the TiNi alloy as functions of the solution pH and the applied potential was successfully produced, and it was revealed that larger EAC susceptibility was obtained in the region of lower potential and lower pH. The tendency was quite typical for hydrogen embrittlement, but not similar to that of TiAl. The EAC susceptibility was classified by cathodic charge density, irrespective to pH nor potential: The charge density larger than 0.025 MC.m-2 induced a maximum EAC susceptibility.