Potential-pH region for environment-assisted cracking of gamma titanium aluminide

Abstract

Environment-assisted cracking (EAC) of γ-TiAl has been investigated from electrochemical viewpoint. The material whose potential was controlled was tested in aqueous sulfate solutions of various pHs at room temperature by using a slow strain rate tensile testing apparatus. The TiAl forged at 1373 K and annealed at 1073K for 7.2 ks was used in the EAC test. It was found in the alkaline and the natural solution that maximum stress was independent of the potential in higher potential region, and decreased with decrease in the potential in the lower potential region. In acidic solution, the maximum stress changed with the potential as follows; the maximum stress was independent of the potential above −200 mV SHE, decreased with decrease in the potential between −200 and −1000 mV SHE, and increased with decrease in the potential below −1000 mV SHE. The experimental EAC results was discussed on the basis of the potential-pH diagram of Al and Ti/water system. It was revealed that the diagram of Ti/water system can predict the EAC susceptibility of TiAl, although the diagram of Al/water system shows almost no relation to the susceptibility. Therefore, the EAC of TiAl is controlled by the stability of Ti oxide, hydride, and ion on the solution/base material interface, which affect the entry of hydrogen into the material.