Mechanism of hydrogen-induced phase transformations in metals and alloys

Theory and experiment are presented to prove that hydrogenation of austenitic steels leads to a significant increase of concentration of the thermodynamic equilibrium host-lattice vacancies. The theory predicts the vacancy induced loss of the phase stability, which creates a basis for a new mechanism of hydrogen-induced phase transformations. It is shown that an external pressure can compensate for the effect of interstitials on vacancy formation and vacancy-induced loss of the phase stability. Along with formation of the epsilon(H) martensite in hydrogen-charged stable austenitic Cr18Ni16Mn10 steel, a high density of dislocation loops has been observed by means of TEM, which is evidence of the high concentration of vacancies which lose equilibrium and form plane vacancy discs when hydrogen leaves the sample. In accordance with the theory, it is shown that gaseous hydrogenation under high external pressure does not induce gamma --> epsilon(H) transformation, although the same hydrogen concentration has been obtained as in the case of cathodic charging of this steel, which had led to formation of epsilon(H) martensite. Copyright (C) 1997 International Association for Hydrogen Energy.