Reports: B10

48431-B10 Effects of Mechanical Stress on the Phase Stability in Metal-Hydrogen Systems

Andrew Craft, University of Hartford

During the reporting period, an article based on our previous PRF supported project dealing with the use of hydrogen absorption to manipulate the mechanical properties of metals was accepted for publication in the journal Materials Science and Engineering A. The article is currently in press and scheduled to appear in the October 2009 issue of the journal.

A comment by one of the reviewers of the above-mentioned article spurred work on one of the projects carried out during this reporting period. Though not addressed in our current PRF proposal, this project involves investigating the effects of the hydrogen-exposure temperature on the mechanical properties (strength, ductility and hardness) of palladium-silver (Pd/Ag) alloys. Palladium-silver alloys are finding applications as hydrogen purification materials but have yet to have their mechanical properties thoroughly characterized so this project is very timely and relevant.

The current project involves subjecting foil samples of annealed Pd0.75Ag0.25 alloys to hydrogen gas at 10oC increments over the temperature range from 0oC to 80oC. The alloy samples were allowed to absorb hydrogen until they reached a hydrogen content of 25 atomic percent hydrogen at each respective exposure temperature. The absorbed hydrogen in each alloy sample was then desorbed completely at the same temperature as hydrogen absorption. This hydrogen-exposure treatment is referred to as absorption/desorption cycling.

Specimens of each cycled Pd/Ag sample were subjected to stress-strain analyses to determine the tensile strength and ductility characteristics of the hydrogen-free alloy matrix. Our preliminary results show an increase in tensile strength and decrease in ductility (relative to the strength and ductility of annealed Pd/Ag that was not exposed to hydrogen) of the alloy matrix because of hydrogen exposure. The degree of strengthening and loss of ductility due to the cycling treatment has been found to be temperature dependent. In particular, the higher the cycling temperature, the less pronounced is the observed increase in tensile strength and decrease in ductility. Those specimens subjected to cycling at 0oC were found to have an increase in ultimate tensile strength of 21% while specimens cycled at 80oC were found to have an increase of only 4%. Those specimens subjected to cycling at 0oC were found to have an decrease in ductility of 5% while specimens cycled at 80oC were found to have an decrease of only 2%. There are some outliers to the general trends and we are currently reanalyzing several samples to discern the significance of the outlier values.

We are currently carrying out microhardness tests on the cycled Pd/Ag specimens to determine the hardness characteristics of each specimen and the effects of the cycling temperature on the hardness. It will be quite interesting to see if the trends in hardness mirror the trends in strength and ductility.  

The other project carried out during this reporting period deals with the influence of mechanical stress on the phase stability of the niobium-hydrogen system.

Samples of well-annealed niobium foil were exposed to hydrogen gas and allowed to absorb varying amounts of hydrogen. Each niobium-hydrogen sample was then analyzed via differential scanning calorimetry (DSC). Each niobium-hydrogen sample was heated in the DSC from 50oC to 125oC and then cooled back to 50oC. DSC allows for the determination of phase transition temperatures as well as quantifying the energetics of a phase transition. The DSC detected any phase changes that occurred during heating and cooling of the niobium-hydrogen specimens. This data will be used to construct certain phase boundaries for the niobium-hydrogen system. Following completion of this portion of the project, these niobium-hydrogen specimens will be mechanically deformed (thinned) by cold rolling through a manual rolling press. Cold-rolled specimens will then be re-analyzed via DSC using identical conditions to those used prior to cold rolling. This process will be repeated several times with progressive additional thinning of the samples to investigate how the degree of mechanical deformation affects the phase stability of the specimens.