Grain boundaries in nanophase palladium: High resolution electron microscopy and image simulation

Scripta HETALLURGICA et MATERIALIA

Vol. 24, pp. 201-206, 1990 Printed in the U.S.A.

Pergamon

Press plc

GRAIN BOUNDARIES IN NANOPHASE PALLADIUM: HIGH RESOLUTION ELECTRON MICROSCOPY AND IMAGE SIMULATION G. J. THOMASt, R. W. SIEGELI"t and J. A. EASTMANtl" "['Division 8341, Sandia National Laboratories, Livermore, CA. 94551-0969 "M'Matedals Science Division, Argonne National Laboratory, Argonne, IL. 60439-4838 ( R e c e i v e d O c t o b e r 20, 1989) (Revised November 14, 1989) Introduction Nanophase materials are comprised of small grains (typically 2 to 20 nm in diameter), which results in a significant fraction of their atoms being located within a few atomic spacings of grain boundaries. The grain boundaries can thercfore be expected to affect the macroscopic properties of nanophase materials to a far greater extent than in conventional coarser-grained materials, Moreover, if the structures of nanophase boundaries were also radically different from those found in conventional pulycrystals, such macroscopic properties could be even more strongly affected. Indeed, suggestionsby Gleiterand coworkers of random grainboundary slzucturesin nanocrystalIInemetals have been made, based upon the interpretation of x-ray scattering from nanocrystalline a-Fe [1,2] and subsequently supported by the results of a variety of other investigations of nanocrystalline metals [3]. Most recendy, EXAFS measurements on nanocrystalline Cu and I'd [4] have been carried out and interpreted to indicate "a new solid state structure with randomly arranged atoms." In addition, electrochemical measurements [5] by Mtitschele and Kirchheim have indicated different solubility limitsfor hydrogen in nanocrystalline I'd compared to a coarse-grained sample. These d~tA were interpreted to indicate that a local structural distortion exists at grain boundaries in nanophase Pd that prevented hydride formation. They estimated that the volume of this distorted region was between 0.7 and 1.1 nm wide, corresponding to 3 to 5 nearest-neighbor distances, and recent small angle neutron scattering (SANS) measurements [6] on nanophase I'd appear to support such an estimate. In conlrast to these investigations, studies of nanophaseTiO2 (ruffle) using Raman spectroscopy [7] and SANS [8] have indicated that the grain boundaries in this ultrafme-gralned ceramic exhibit the local structural order of ruffle and are about 0.5 nm wide; they are thus rather similar to those in conventional polycrystalline ruffle. Also, a more detailed model analysis of x-ray scattering from nanocrystallin¢ Pd [9] has recently indicated that the observed scattering, with its strong diffuse component, can be explained without invoking random grain boundary structures. Clearly, direct observations of the grain boundaries in nanophase materials could help elucidate their structure. In the present work we report such direct lattice resolution electron microscopy observations of grain boundary structures in nanophase Pd along with complementary image simulations to compare with experiment. A preliminary report of these results has been given elsewhere [I0];a complete reportof thiswork willbe publishedlater. Exnerimental Procedures The nanophase Pd sample investigated in the present work was synthesized by the consolidation at 1.4 OPa in vacuo of ultrafine powders formed by the gas-condensation method [I 1]. This produced a sample of 30 mg mass, 9 nun diameter, and about 5 run average grain size determined by conventional eleclron microscopy. The sample was first used for measurements of hydrogen solubility from the gas phase using pressure-volume techniques prior to the high resolution microscopy (HREM) investigation. After a single hydriding-dehydriding cycle, during which the sample temperature was maintained at 310K, all of the hydrogen was pumped from the sample, which was subsequently removed from the gas manifold. A maximum hydrogen pressure of 100 kPa at 310K yielded the [3-hydride phase with a hydrogen stoichiometry of less than 0.7 H/Pd. The results of the hydrogen solubility measurements will be published elsewhere. This work was supported by the U.S. Department of Energy, BES-Materials Sciences, under Contracts DE-ACO4-76DP0078 at Sandia National Laboratories and W-31-109-Eng-38 at Argonne National Laboratory.

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The lattice resolution electron microscopy observations were made using the Atomic Resolution Microscope (ARM) at the National Center for Electron Microscopy, Lawrence Berkeley Laboratory. An operating voltage of 800 kV was used in all cases. Owing to the ultrafine grain size and randomness of the crystailite orientations, a significant fraction of the grains in a given region exhibited lattice contrast without specimen tilting. Image simulations, which are a necessary complement to high resolution work, were made using the EMS [12] suite of programs. Electron transparent foils were prepared from the nanophase Pd sample using conventional electrochemical jet-thiuning methods. The conditions for polishing (15 V. at 0 ° C in 42% butyl cellusol/42% acetic acid/16% perchloric acid electrolyte) are essentially the same as those used for thinning coarse-grained Pd.

Hil~h-Re~olutionElectron Microscot)v Results In order to obtain a representative sampling of grain boundary structures, through-focus series of micrographs were taken in about 15 different areas from three separate foils. The micrographs were then examined for cases where adjacent grains exhibited lattice contrast. In these regions, the foil thickness was of the order of, or less than, the grain size, since overlapping, misoriented grains would not yield satisfactory images. An example of an area containing a number of grains is shown in Figure 1. Note that most of the grains exhibit fringe contrast. Typically, one finds an abrupt stop to the lattice fringes in each grain at the grain boundary, indicating that there is little or no atomic disorder perpendicular to the imaged planes. The two small regions near the center of the figure where the lattice fringes are smeared out are not associated with a grain boundary, and may be due to surface contamination. Figure 2 shows a grain boundary (from another area) at a higher magnification. The two grains are oriented close to a common <110> zone axis and the lauice fringes in both grains are from {111 } planes. At the boundary, which appears to be closely aligned with a {111 } plane of the upper grain, the lattice fringes show an abrupt change in direction. Steps in the boundary plane occur about every 10 {111 } layers, bringing the planes from both grains into registry. The measured lattice fringe spacing near the boundary was not found to differ from the spacing near the grain center. Contrast features at the boundary which differ from the straight fringe contrast do not appear to be wider than 0.4 nm on this micrograph. Hence, if these features were associated with disorder, the extent of this disorder is confined to the boundary plane and immediately adjacent planes. Of course, lattice relaxations which alter the periodic image structure are often found at grain boundaries in coarse-grained materials and the observed features may simply be associated with such relaxations. Imaee Simulation Results Image simulations play an important role in interpreting HREM lattice images. In the present study, where we are specifically interested in the possibility of lauice disorder at the grain boundaries, simulations can be used to indicate the sensitivity of the HREM technique to the degree of lattice disorder, as well as to show the effect of random disorder on the image. Many parameters affect lattice images and a detailed description will be published elsewhere. Here we only consider, as a representative case, a ~ 5 symmetric <0131> tilt boundary normal to a 7.6 um thick foil imaged near the Scherzer defocus condition, using the ARM instrument parameters. In this tilt grain boundary, the boundary plane is of type (210} and the grains are rotated 36.9 ° about a direction. The foil normal in this case is also <001>. Disorder at the boundary was simulated by randomly displacing atoms on each side of the interface whose centers were initially within 0.174 nm of the interface plane. The resulting full width of the disordered region was about 0.5 nm, containing the boundary plane and the two adjacent (420) planes on each side. Atoms were randomly displaced in both magnitude and direction, up to a maximum of half a nearest neighbor distance. Hence, the average displacements were one half of the maximum allowed displacement value. An example is shown in Figure 3. The set of models used here are more conservative than in the model used by Zhu, et al. [2] to explain their xray diffraction observations. In their model the grain boundaries were represented by a single 1.0 nm wide boundary with all atoms displaced by half a nearest neighbor distance in random directions. Figure 3a shows a simulated image of the "perfect" boundary with no atomic displacements. Crossed fringe contrast occurs in both grains because they are aligned with a common zone axis, and atom columns coincide with the dark regions for the thickness and defocus conditions used. A particular disordered case is shown in Figure 3b. The size of

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the crystal used in the simulation was one-third the width of the image shown and the image repetitively printed in the figure to make the boundary more visible. Hence, the periodicity in the horizontal direction is artificial. In the case shown, tim average displacement was 12.5% of the nearest neighbor distance. It can be seen that this amount of disorder smears out the periodic image and results in a much lower contrast region. The width of the distorted image corre..s~nds closely to the width of the atomically disordered region. Larger magnitudes of atomic displacement result in a grain boundary image similar to amorphous materials and are even more apparent than the case shown here. Conclusions and Discussion

Several conclusions can be drawn l'rom the present investigation of nanophase Pd: (1) Any observed contrast changes (e.g., deviations from planarity) with respect to the contrast observed far from the interfaces were observed to be localized to a width of about 0.4 run (i.e. about +_0.2 nm from the interface plane) at grain boundaries. (2) Local changes in image contrast at grain boundaries can be caused by several phenomena, including disorder, none of which can be ruled out at the present time,. These include vacancies or ultrafine porosity, atomic relaxations, and impurity segregation. However, similar localized changes in contrast are frequently observed in studies of grain boundary structure in bicrystals or coarse-grained polycrystals [13] as well. (3) No manifestations of grain boundary structures with random displacements of the type and extent suggested by x-ray studies [2,4] of nanocrystallinc metals were observed. Image simulations indicate that random atomic displacements of average magnitude greater than about 12% of a nearest neighbor distance should be readily observable by HREM for the assumed contrast conditions. (4) The localized nature of the observed contrast changes argues against the possibility that the interface atomic structure in nanophase materials can be fundamentally different from that observed in coarser-grained polyerystals. Such a fundamental difference could only be caused if atom positions were determined by their interactions with more than one boundary at a time. The present results suggest that displacements fall off rapidly for distances much smaller than even the small grain diameters of these materials, indicating that atomic relaxations must be dominated by the influence of only the closest boundaries, as they are in conventional polycrystals. This also implies that the action of thinning the HREM foil, and hence removing the grains above and below the volume under observation, does not in itself signLficantly affect the structures observed. It should also be noted, however, that in the future as nanophase grain sizes are reduced into the 1 nm range, or as nanophase semiconductors with longer healing distances are synthesized, departures from conventional interface structures may become important. The model used here to investigate the suitability of HREM for the elucidation of random atomic displacements in grain boundaries is, of course, rather simplistic and was chosen only as a convenient means of simulation. Other defects in the lattice structure can occur near grain boundaries, such as vacancies, relaxations in int~ratomic spacings, and the presence of impurities, for example. Any of these can alter the local atomic density. Small perturbations in density or atom positions could change hydrogen solubility, as observed by M0tschele and Kirchheim [5], but would not be expected to completely prevent formation of a hydride phase. Rather than random disorder, changes in local symmetry might also explain their findings; however, these would be expected to be more readily discernible by the microscopy techniques applied here, or by the x-ray diffraction studies previously reported. Further work is needed to answer these questions. The results presented here should be representative of nanophase Pd, even though only a limited number of grain boundaries were studied in detail. In any given region, lattice fringe contrast was determined by the crystallographic orientations of individual grains, and selected area electron diffraction patterns gave no evidence of textures or preferred orientations. The grain boundary sampling used in this study should therefore have been random. Furthermore, in the previous x-ray [2,4] and hydrogen solubility [5] studies, the macroscopic material was probed and the results were interpreted in terms of a total volume fraction of disordered grain boundary material. If some fraction of the grain boundaries were not disordered, as our results clearly show, then the remaining boundaries would have to exhibit correspondingly larger disordered regions to account for the total disordered volume. Such regions would be readily discernible by electron microscopy, but were not found in the present investigation. They apparently do not exist.

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It should be pointed out in closing that the presence of ultrafine-scale porosity could, however, give nanophase interfaces a unique character beyond such atomic-structure considerations as discussed here. Such porosity would influence the atomic diffusion in these materials, which has been shown to be enhanced over that in conventional polycrystals [14]. In any event, the large number of grain boundaries in nanophase materials provides a fertile ground for their study, and such studies will certainly help to better understand interfaces in general.

~nmzt~zme,~ We gratefully acknowledge the assistance of the slaff at the National Center for Elecu'on Microscopy, Lawrence Berkeley Laboratory, and wish to thank Glenda Genlry for technical assistance.

Reference.4 1. R. Birringer, H. Gleiter, H. P. Klein, and P. Marquardt, Phys. Left. 102A, 365 (1984). 2. X. Zhu, R. Birringer, U. Herr, and H. Gleiter, Phys. Rev. B 35, 9085 (1987). 3. R. Birringer and H. Gleiter, in Encvclooedia of Materials Science and En~inecrin ~ Suppl. Vol. 1, R. W. Calm, ed. (Pergamon Press, Oxford, 1988) p. 339. 4. T. Haubold, R. Birringer, B. Lengeler, and H. Gleiter, Phys. Lett. A, 135, 461 (1989). 5. T. Mtltschele and R. Kirchheim, Scripta Metall. 21,315, 1101 (1987). 6. G. Wallner, E. Jorra, H. Franz, J. Peisl, R. Birringer, H. Gleiter, T. Haubold and W. Petty, Mater. Res. So<:. Symp. Proc. 132, 149 (1989). 7. C.A. Melendres, A. Namyanasamy, V. A. Maroni, and R. W. Siegel, J. Mater. Res. ~:, 1246 (1989). 8. J.E. Epperson, R. W. Siegel, J. W. White, T. E. Klippert, A Narayanasamy, J. A. Eastman and F. Trouw, Mater. Res. Soc. Symp. Proc. 132. 15 (1989). 9. J.A. Eastman and L. J. Thompson, Mater. Res. So<:.Proc. 153.27 (1989). 10. G. J. Thomas, R. W. Siegel, and J. A. Eastman, Mater. Res. Soc. Proc. 153, 12 (1989). 11. R. W. Siegel and H. Hahn, in Current Trends in the Physics of Materials, M. Yussouff, ed. (World Scientific Publ., Singapore, 1987) p. 403; R. W. Siegel and J. A. Eastman, Mater. Res. Soc. Symp. Proc. 132, 3 (1989). 12. P. Stadelmann, Ullramicroscopy 21, 131 (1987). 13. R. Raj and S. L. Sass, eds., Interface Science and En~ineerin[., '87, J. de Physique 49, Colloque C5 (1988). 14. R. S. Averback, H. Hahn, H. J. HOller, J. L. Logas, and T. C. Shen, Mater. Res. Soc. Syrup. Proc. ~ 3

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FIG. 1 High resolution transmission electron micrograph of a region of nanophase Pd containing a number of grains. Note that most of the grains exhibit fringe contrast and that the fringes end abruptly at the grain boundaries.

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FIG. 2 High resolution transmission electron micrograph of a single grain boundary in nanophase pd. Details are discussed in the text.

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FIG. 3 Image simulations for a E5 symmetric <001> tilt boundary in 7.6 nm thick Pd, using instrument parameters consistent with the ARM: (a) 'perfect' structure with no atomic displacements; (b) randomly disordered sturcture near the grain boundary with a maximum displacement of 0.25 of the mcarest neighbor distance

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