Effect of interstitials on the trapping of hydrogen in high purity iron as studied by positron annihilation and electron microscopy

This report summarizes research on the determination of: (1) the dislocation densities of both edge and screw components using two positron annihilation techniques; and (2) the density of hydrogen traps by analysis of electrolytic permeation measurements. Dislocation densities of single crystal iron specimens deformed in three different manners were in good agreement with disclocation densities obtained by ETCH-PIT and TEM methods on the same specimens as well as being bracketed by three earlier TEM investigations. The positron lifetimes were 114 psec. in the trap-free bulk, 165 in an edge and 142 in a screw dislocation. The first two of these values are in good agreement with values obtained several previous authors using poorly characterized specimens. It had been assumed that positron trapping in a screw dislocation was very unlikely - so the value for the latter type of dislocation is an important result. The binding energies of a hydrogen atom to an edge was deduced to be 37.4 and to a screw is 25.6 KJ/mole. The density of hydrogen at different strains was in good agreement with the number of dislocation traps. The occupancy sites along the dislocation was approximately 20%.