The interaction of hydrogen and dislocations in iron

Abstract

The effusion of hydrogen from deformed iron wires has been measured by combining low temperature internal friction measurements of the cold-work peak with a thermal cycling procedure. Diffusion constants have been calculated by a modification of the technique developed by Armstrong, for conditions where diffusion takes place over a range of temperatures at a constant hsating rate. The activation energy for effusion between 200 and 300°K is 7–8 kcal/g-mole, in good agreement with previous work based on other methods. A linear variation of the cold-work peak with the percentage of hydrogen suggests that dislocations act as the major trapping sites when the deformation level (16 per cent R.A.) is insufficient to produce internal voids, and when the hydrogen content is low (~1 ppm). A binding energy of 4–6 kcal/g-mole can be derived by taking into account current values for the basic lattice diffusion of hydrogen. This is consistent with a basic elastic interaction energy model based on a partial molar volume for hydrogen of~2 cc/g-mole, and supports the location of lattice hydrogen in octahedral interstices. The similarity between the dislocation binding energy, and the energy involved in adsorption at free surfaces, may account for the conflict which exists regarding the predominant location of hydrogen in alloys containing both internal voids and a high dislocation density.