Abstract
Helium bubbles form in metal tritide films as tritium decays into H3 e, influencing mechanical properties and long-term film stability. The bubble nucleation and growth mechanisms comprise an active research area, but there has been only one previous systematic experimental study of helium bubble growth in metal tritides, on zirconium tritides. There have been no such studies on tritides such as ErT2 that form platelike bubbles and lack a secondary bubble population on a network of line dislocations, and yet such a study is needed to inform the modeling of helium bubble microstructure development in a broader range of metal tritides. Transmission electron microscopy has been used to study the growth and evolution of helium bubbles in ErT2 films over a four-year period. The results have been used to test the present models of helium bubble nucleation and growth in metal tritides, particularly those forming platelike bubbles. The results support the models of Trinkaus and Cowgill. The observations of nonuniform bubble thicknesses and the pattern of grain-boundary bubble formation, however, indicate that these models could be strengthened by closer attention to details of interfacial energy. It is strongly recommended that efforts be made (either experimentally or by calculation) to determine anisotropy of tritide/helium interfacial energy, both for clean, stoichiometric interfaces, and also allowing for such factors as nonstoichiometry and segregation.
| Original language | English |
|---|---|
| Article number | 083514 |
| Journal | Journal of Applied Physics |
| Volume | 107 |
| Issue number | 8 |
| DOIs | |
| State | Published - Apr 15 2010 |
Funding
Discussions with D.F. Cowgill, L. Brewer, J. Knapp, and P. Kotula (Sandia National Laboratories), tritium loading by T. Venhaus (Los Alamos National Laboratory), specimen preparation by L.I. Espada, G. Bryant, and B. Ritchey (Sandia National Laboratories), and critical reading of the manuscript by L. Brewer and J. Knapp are gratefully acknowledged. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Co., for the United States Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. Table I. Hydrogen-to-metal ratios measured for two samples from each of the three load runs. Load run D:Er T:Er ( D + T ) : Er 1 0.034 1.844 1.88 1 0.036 1.927 1.96 2 0.037 1.842 1.88 2 0.036 1.987 2.02 3 0.036 1.851 1.89 3 0.039 1.909 1.95 Average 0.036 1.893 1.93 Standard deviation 0.002 0.058 0.06 Table II. Fitting parameters for log-normal curve fits. CS: cross section; PV: plan view. Equation for log-normal curve fit, y = y 0 + A [ ( √ ( 2 π ) wx ) ] − 1 exp { − [ ln ( x / x c ) ] 2 / ( 2 w 2 ) } . He:Er CS/PV y 0 x c w A 0.018 CS 0.07 ± 0.47 6.66 ± 0.44 0.52 ± 0.06 111.00 ± 10.40 0.031 CS 0.28 ± 0.60 7.78 ± 0.23 0.38 ± 0.30 203.62 ± 12.80 0.055 CS 0.66 ± 0.29 8.07 ± 0.21 0.22 ± 0.03 53.14 ± 5.38 0.060 CS 0.40 ± 0.74 9.20 ± 0.48 0.47 ± 0.05 192.51 ± 16.40 0.066 CS 0.33 ± 0.54 10.13 ± 0.41 0.42 ± 0.04 156.33 ± 11.59 0.066 PV − 0.15 ± 0.65 9.93 ± 0.49 0.50 ± 0.05 190.56 ± 13.88 0.070 CS − 0.25 ± 1.10 11.15 ± 1.11 0.50 ± 0.10 167.50 ± 24.56 0.074 PV − 0.28 ± 0.71 10.31 ± 0.40 0.51 ± 0.04 290.81 ± 15.96 0.079 CS 0.05 ± 0.47 13.10 ± 0.30 0.39 ± 0.02 223.90 ± 10.59 0.079 PV 0.33 ± 1.10 10.47 ± 0.49 0.49 ± 0.04 351.84 ± 24.69 0.098 PV − 0.07 ± 0.49 10.70 ± 0.39 0.43 ± 0.04 171.41 ± 10.96 0.108 CS − 0.33 ± 0.38 12.21 ± 0.31 0.39 ± 0.03 166.69 ± 8.56 0.112 CS − 0.34 ± 0.52 14.06 ± 0.65 0.46 ± 0.05 156.00 ± 11.85 0.143 PV 0.42 ± 0.49 17.68 ± 0.71 0.34 ± 0.05 111.85 ± 11.19 0.370 CS 0.85 ± 0.96 14.81 ± 0.34 0.33 ± 0.02 388.80 ± 21.99 0.370 PV − 0.71 ± 1.34 17.85 ± 0.57 0.37 ± 0.04 430.31 ± 30.93 Table III. Table of means [E(X)] and standard deviations [ σ X ] for bubble diameter based on log-normal distribution, together with R 2 values for the log-normal curve fits. CS: cross section; PV: plan view; and n: number of datum points. He:Er Days CS/PV n R 2 E(X) σ X 0.018 63 CS 57 0.89 6.8 4.9 0.031 106 CS 108 0.95 8.2 4.1 0.055 190 CS 41 0.88 11.8 5.6 0.060 206 CS 105 0.90 11.6 6.3 0.066 228 CS 85 0.93 11.7 5.8 0.066 228 PV 92 0.93 11.2 5.2 0.070 242 CS 78 0.76 12.3 5.6 0.074 256 PV 139 0.96 11.3 5.8 0.079 272 CS 113 0.97 13.8 5.5 0.079 272 PV 183 0.93 12.3 6.1 0.098 342 PV 84 0.94 11.6 4.8 0.108 377 CS 76 0.96 12.3 4.1 0.112 391 CS 70 0.92 14.6 6.2 0.143 504 PV 65 0.86 19.9 8.1 0.370 1313 CS 213 0.95 16.0 6.2 0.370 1312 PV 198 0.93 18.3 6.2 FIG. 1. CS at He:Er 0.060 (206 days). (a) Bright-field transmission electron micrograph showing two sets of platelike helium bubbles (at an angle of ∼ 72 ° ) and (b) corresponding selected-area diffraction pattern (close to [ 0 1 ¯ 1 ] zone axis). FIG. 2. Bright-field images from samples at He:Er ratios of (a) 0.060 (206 days), b) 0.098 (342 days), and (c) 0.37 (1302 days). [Note that images (a) through (c) are at progressively lower magnifications.] FIG. 3. He:Er 0.098 (342 days), PV, bright-field, under/over-focus pairs with arrows indicating bubble linkage in (a) and (b), and bubbles of nonuniform thickness in (c) and (d). FIG. 4. He:Er 0.416 (1,468 days). (a) Low-magnification image showing bubble microstructure across entire width of film. (b) and (c) Higher-magnification images showing extensive bubble linkage and cracking. FIG. 5. He:Er 0.37 (1302 days). (a) and (b) Views of bubble microstructure across entire width of film including ribbonlike features and showing development of paths for helium migration across film through linked bubbles and cracks. FIG. 6. He:Er 0.377 (1328 days), showing linked bubbles (l) and cracks (c), including ribbonlike features (R). FIG. 7. He:Er 0.37 (1305 days), showing preferential bubble formation along regions of grain boundary approximately parallel to a set of {111} planes in the grain to the left. FIG. 8. Histograms of bubble diameter with log-normal curve fits, for He:Er ratios of: (a) 0.018 (CS), (b) 0.031 (CS), (c) 0.055 (CS), (d) 0.060 (CS), (e) 0.066 (CS), (f) 0.066 (PV), (g) 0.070 (CS), (h) 0.074 (PV), (i) 0.079 (CS), (j) 0.079 (PV), (k) 0.098 (PV), (l) 0.108 (CS), (m) 0.112 (CS), (n) 0.143 (PV), (o) 0.370 (CS), and (p) 0.370 (PV). CS: cross section; PV: plan view; bin size 2 nm. FIG. 9. Mean bubble diameter (calculated from log-normal distributions) vs He:Er ratio. CS: cross section; PV: plan view.