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The above discussion of twinning and pressure dependent diffusion controlled creep indicate that either can account for the large magnitude of the SD effect in a-Pu. At present the author favors the diffusion controlled model since creep of a-Pu does appear to be diffusion controlled in this stress range and no twins were observed. However, since the mechanisms of plastic deformation in ~-Pu are not completely understood, further microstructural and phenomenological observations of creep are necessary to resolve this question. Clearly the relation of the mentioned structural changes during deformation to flow stress is not yet understood. The SD effect in ~-Pu is larger than that in any other metal and this warrants further study.The author thanks Dr. R. D. Nelson for his helpful suggestions and review of the manuscript and H. E. Kjarmo for perseverance in doing the experimental work. The comments of Professor J. P. Hirth on the diffusion creep model for the SD effect were appreciated.l. J. P. Hirth and Morris Cohen: Met. Trans., 1970, vol. l, p. 3. 2. R. Chair: Met. Trans., 1972, vol. 3, p. 365. 3. G. C. Ranch and W. C. Leslie: Met. Trans., 1972, vol. 3, p. 373, 4. D. Kalish and H. J, Rack: Met. Trans., 1972, vol. 3, p. 2289. 5. C. Bauwens-Crowet, J. C. Bauwens, and G. Homes: J. Mater. Scs 1972, vol. 7, p. 176. 6. M. D. Merz: J. Nucl. Mater., 1970, vol. 34, p. 108. 7. M. D. Merz and R. D. Nelson: Met. Trans., 1971, vol. 2, p. 1399. 8. B. Sprier: J. Nucl. Mater., 1964, vol. 12, p. 159. 9. S. E. Bronisz: J. Nucl. Mater., 1963 Thin foils were prepared from the gage sections of tensile specimens of all five types of material which had been deformed at temperatures between 77 and 540 K. Twinning was observed in types I through IV materials deformed at all temperatures studied. The twin thickness decreased with increasing temperature and oxygen content as did the twinning frequency. Selected area electron diffraction and trace analysis showed that nearly all the twins were of the {1012} type. In all cases where twins were observed, profuse slip had also occurred in both the matrix and the twin. Burgers vector determination experiments showed that the deformation had almost exclusively occurred by a vector slip. There were also appreciable numbers of c + a type dislocations present, however these dislocations were confined to regions near the twin: matrix interface.s. All materials showed a trend toward more uniform distributions of dislocations with increasing temperature or with decreasing oxygen concentration at low temperatures -< 300 K. With the exception of the types I and II material, a transition from planar to wavy glide was observed; the temperature range over which the transition occurs increases with increasing oxygen concentration. When planar lide occurs, the preferred glide planes are {1010} and 0il} with essentially no well-developed basal bands trate the slip mode transition below and above room temperature in types III and V material, respectively. The types I and II material did not show a ...

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The internal-friction spectrum of zone-refined nickel [R(273°K)/R(4.2°K)=600] has been examined at 30 kc/sec, in the temperature range 77°-350 o K. Plastic deformation produces two prominent relaxation peaks, each the sum of several simple peaks. At 30 kc/sec, one peak, which occurs at 138°±7°K, has been identified as the Niblett and Wilks peak, and the other, which has its maximum at 248°±40oK, has been shown to be the Bordoni peak. Both peaks have been studied as a function of cold work, grain size, state of annealing, and strain amplitude.One major result of these studies is a demonstration that the Bordoni peak is only large at low-to-moderate flow stresses and vanishes at high flow stress (greater than 50000 psi) with the Niblett and Wilks peak being the predominant feature of the damping spectrum at the higher-flow stresses (greater than 30 000 psi). Moreover, the present results in Ni show such similarity to results obtained on deformation-induced peaks in bcc metals that, if the definition of the Bordoni peak given by Niblett and Wilks is broadened to include Ni, then' the peaks in bcc metals can also be included under the definition. Arguments are presented to show that the Bordoni relaxation is best described by the Seeger-Pare hypothesis.