DISLOCATION MECHANISM OF HYDROGEN EMBRITTLEMENT OF METALS AND ALLOYS WITH HCP AND FCC CRYSTAL-STRUCTURE

The results of the experimental study of the mechanism of hydrogen emb rittlement were generalized on the basis of a theory of. thermally act ivated (quasi brittle) fracture, developed by the authors to predict t he nature of gradual quasi-brittle transition of Me-H solid solutions. It was established that development of hydrogen brittleness for polyc rystals appeared to be controlled at low (cryogenic) temperature by th ermally activated sliding screening dislocations overcoming segregated H atoms at the microcrack tips with activation energy of 0.05-0.07 eV at the critical (hard-activated) stage of fracture propagation. If th e concentration of excess (hydrogen- and deformation-induced) vacancie s of 2-3 orders exceeds that for thermal equilibrium vacancies, the st ructure's clusterization of Me-H solid solutions was observed within t he thermally activated area of dynamic deformational aging (in the hos t metal and at the boundaries). Hereby the formation of strong ''H ato m-excess vacancy'' clusters with a binding energy of 0.2-0.5 eV, being large in comparison with ''H atom-screening dislocation'' binding ene rgy, prevent the H atom's segregation at the structure defects. The me thods for elimination of hydrogen brittleness, restoration of plastici ty and strength properties of Me-H systems are suggested on the basis of the formation of the new barriers for propagating microcracks-the d isoriented cellular structures in clusterized solid solutions with lar ge numbers of active sliding systems of screening dislocations. Ti-H, Zr-H and Al-H or superdisperse granular structures in superplastic and plastic states of clusterized substitution alloys with a lack of acti ve sliding systems of screening dislocations (Mg-Ba-H, Be-Co-H). Copyr ight (C) 1996 Published by Elsevier Science Ltd on behalf of Internati onal Association for Hydrogen Energy.