Kinetics of dislocation annealing and the effect of trapped hydrogen, investigated with in-situ diffraction

Abstract: In-situ powder diffraction was used to study the annealing of dislocations in the archetypal hydrogen absorbers Pd and LaNi5. The relationship between dislocations and trapped hydrogen was explored using thermally induced desorption. It was found that the dislocations in Pd caused by hydrogen absorption anneal over a wide range of temperatures and that although they start to anneal below 250 $^\circ$C, temperatures well above 750 $^\circ$C are required to fully anneal the metal. It was shown that allowing further time at lower temperatures does not further anneal the metal. It is suggested that this is due to dislocation tangling and pinning, causing different temperatures to be required for different pinning defects. It was found that hydrogen trapped in LaNi5 is released in a wide range of temperatures and it was therefore concluded that hydrogen is trapped in the dislocation strain field and dislocation core as well as vacancies. The direct comparison of deuterium release and dislocation density showed no correlation, in agreement with previous indirect comparisons. Dislocations in LaNi5 were shown to anneal at temperatures as low as 150 $^\circ$C, in contrast to previous reports which suggested more than 500 $^\circ$C was required. This lower annealing temperature for dislocations at least partly explains why low temperature ageing increases the pressure hysteresis in hydrogen cycled LaNi5.