Hydrogen interaction with defects in metals.

Abstract

Numerical calculations using Fermi-Dirac form have been performed to estimate the number of hydrogen atoms trapped at dislocations and grain boundaries as a function of the bulk concentration of hydrogen and temperature in palladium which is a face centered cubic structure. The calculations performed include the trapping of hydrogen atoms at both the dislocation core and strain fields surrounding the defects. The results of these calculations agree with the SANS measurements previously carried out by Heuser et al. at a hydrogen bulk concentration of 5500 parts per million. Heuser et al.'s results indicate that 2.6 hydrogen atoms per Angstrom and 0.4 $\pm$ 0.2 hydrogen atoms per square Angstrom are trapped at the dislocations and grain boundaries, respectively. For the purpose of this research, an assumption is made that the defects are edge dislocations and low angle grain boundaries. Numerical calculations are performed at various hydrogen bulk concentrations ranging from a few hundred to a few thousand parts per million. At a bulk concentration of 5500 ppm, the total calculated amount of hydrogen trapped at dislocations is 2.3 atoms per Angstrom of dislocation line and 0.2 atoms per square Angstrom of grain boundary area. The calculations indicate that about seventy-five percent of the trapped hydrogen lies within 5 to 8 Burgers vectors of the dislocation center. Depending on the core size and number of trapping sites assumed in the calculations, up to eighty percent of the total number of hydrogen atoms are found to be trapped in the core region which can be used to determine the core binding energy of hydrogen to these metal defects. The core binding energy is calculated to be 0.3 eV. At a bulk concentration of 5500 ppm, and temperature of 77 K, the calculations show an enhancement of ten times of hydrogen concentration in the neighborhood of the dislocations. Based on the results of the present mode a set of SANS experiments is proposed which will allow an accurate determination of the core energy, number of sites and core radius in dislocations.Ph.D.Nuclear EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104248/1/9513287.pdfDescription of 9513287.pdf : Restricted to UM users only