Direct experimental evidence for hydrogen absorption in nickel during electropolishing is presented. Single crystal nickel disks were charged with hydrogen by thinning to electron transparency in a strong acid solution at room temperature. Thin regions of the crystals underwent plastic deformation due to hydrogen absorption during the thinning process. The deformation substructure consisted principally of planar arrays of unit slip dislocations and coplanar microtwins in the thinnest regions of the foils. Cracks at the foil edges were often associated with the microtwins. The stress system in the foil is a result of nonuniform hydrogen distribution. The formation of microtwins is consistent with the three-layer microtwin mechanism proposed by Mahajan and Chin.25 The origin of the cracks at the foil edges is most probably associated with slip on planes inclined to the foil surface near the microtwins, rather than a consequence of the microtwinning itself.
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