Hydrogen cracking in titanium-based alloys

In this paper the phenomena of hydrogen cracking and hydrogen-induced second phases formation in titanium based alloys is discussed in detail, addressing to three different alloys; the alpha+beta Ti-6Al-4V alloy, the metastable Beta-21S and the Ti-20 wt.%Nb refractory alloy. The prior microstructure of the alloy plays a very significant role on its behavior under exposure to a hydrogen-containing environment. In the Ti-6Al-4V alloy the main mechanism of hydrogen cracking is the formation and rupture of brittle titanium hydride phases. The severity of hydrogen degradation in the Ti-6Al-4V alloy depends on the amount and distribution of the β phase in the microstructure due to a more rapid diffusion transport of hydrogen in the bcc β phase. On the other hand, the Timetal Beta-21S β alloy, exposed to the electrochemical (high fugacity) hydrogen environment at room temperature, exhibits a fair resistance to hydrogen. In the Ti-20 wt.%Nb alloy hydrogen-induced phase transitions and hydrides formation were followed by an interesting softening effect, irrespective of the charging procedure.