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

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, two projects were ongoing.

The first project involves an investigation of the influence of hydrogen exposure temperature on the mechanical properties of palladium-silver (Pd/Ag) alloys.

In this context, we broadened the project mentioned in last year’s report involving foil samples of annealed Pd_0.75Ag_0.25 alloys that were exposed to hydrogen gas at various temperatures. In particular, we expanded the temperature range investigated from 10^oC – 80^oC to 10^oC – 200^oC. As before, 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 cycled alloy matrix. The microhardness of each specimen also was determined. The current results show that the Pd_0.75Ag_0.25 alloy’s mechanical properties are virtually unaffected (relative to the properties of the well-annealed alloy) by hydrogen absorption/desorption over the entire range studied. This is significant because the alloy seems to be resistant to the detrimental manifestation of hydrogen embrittlement over the entire temperature range studied. Our results are currently being incorporated into a manuscript that we anticipate submitting to a peer-reviewed journal before the end of the year.

With the Pd_0.75Ag_0.25 fully investigated, we have moved on to a similar study involving the more palladium-rich alloy Pd_0.95Ag_0.05. We have completed all of the hydrogen cycling treatments on this alloy and are currently carrying out microhardness measurements on the alloy specimens. Our preliminary results on this alloy show a different behavior as compared to Pd_0.75Ag_0.25. In particular, it does appear that the hydrogen cycling temperature does affect the microhardness of the alloy matrix, but only up to a certain temperature. The results indicate that the alloy matrix experiences a noticeable increase in microhardness (as compared to well annealed Pd_0.95Ag_0.05) for cycling temperatures up to ~125^oC but shows little change in microhardness (as compared to well annealed Pd_0.95Ag_0.05) when cycling is carried out above 125^oC. We will soon be starting the stress-strain analyses on these specimens and it will be of great interest to see if these results are supportive of the microhardness results.

The other project carried out during this reporting period dealt with the influence of mechanical stress on the phase stability of metal-hydrogen systems.

We have deferred our investigation of the niobium-hydrogen system that was discussed in last year’s report to investigate the titanium-hydrogen sample. The reason for the switch was that colleagues at the University of Vermont provided samples of hydrided titanium foil of various hydrogen contents. With these materials in hand, we were eager to work with them. Each titanium-hydrogen sample was analyzed via differential scanning calorimetry (DSC). Each titanium-hydrogen sample was heated in the DSC from 100^oC to 325^oC in an attempt to detect two phase changes that occur over this temperature range. If any phase changes had been detected, the specimens would have subsequently been subjected to mechanical stress and re-analyzed in the DSC to see if the characteristics of the phase change had been altered by the stress. Unfortunately, no signal indicative of a phase change was detected. The reason for this would seem to be that the energetics of the phase change are below the detection limits of the DSC. Earlier work with niobium-hydrogen samples has shown that the energetics of a phase change are dependent on the hydrogen content of the metal hydride. We are preparing titanium-hydrogen systems with higher hydrogen contents than the previous samples and will see if any phase changes are detected in these samples by carrying out DSC analyses.