Atomic mechanisms of. gamma. ' precipitate plate growth in the Al-Ag system

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Energy-dispersive x-ray spectroscopy results that the precipitates have the composition Ag/sub 2/Al. High-resolution electron microscopy of ..gamma..' precipitates in both <110>//<1120> and <111>//<0001> orientations shows that all interfaces of the precipitate are largely coherent with the matrix and are faceted along low-energy (111) and (110) matrix plans, due to the influence of surface and elastic strain energies on the transformation. Further comparison between experimental and calculated high-resolution images of the precipitate/matrix interface and of Shockley partial dislocation ledges on the precipitate faces demonstrates that both thickening and lengthening of ..gamma..' precipitate plates occurs by the passage of the Shockley partial ... continued below

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Pages: 324

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Howe, J.M. March 1, 1985.

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Description

Energy-dispersive x-ray spectroscopy results that the precipitates have the composition Ag/sub 2/Al. High-resolution electron microscopy of ..gamma..' precipitates in both <110>//<1120> and <111>//<0001> orientations shows that all interfaces of the precipitate are largely coherent with the matrix and are faceted along low-energy (111) and (110) matrix plans, due to the influence of surface and elastic strain energies on the transformation. Further comparison between experimental and calculated high-resolution images of the precipitate/matrix interface and of Shockley partial dislocation ledges on the precipitate faces demonstrates that both thickening and lengthening of ..gamma..' precipitate plates occurs by the passage of the Shockley partial dislocations along alternate (111) matrix planes by a terrace-ledge-kink mechanism. These images and electron diffraction information also indicate that the ..gamma..' precipitates are ordered, where the A-planes in the precipitate contain nearly pure Ag and the B-planes have the composition Al/sub 2/Ag, and that the limiting reaction in the growth process is the substitutional diffusion of Ag cross kinks in the Shockley partial dislocations, which terminate in the Ag-rich A-planes. The terraces between ledges are atomically flat and ledges are uniformly stepped-down from the centers to the edges of isolated precipitates. Convergent-beam electron diffraction (CBED) analyses of ..gamma..' precipitates indicate that they have the space group P6/sub 3//mmc. Effect of specimen thickness on symmetry determinations by CBED was also examined for an ..cap alpha..-titanium sample. Results show that the symmetries observed in CBED patterns from thin specimens may be due to the limited thickness of the specimen, rather than to the actual space group of the material.

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Pages: 324

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NTIS, PC A14/MF A01; 1.

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  • Other Information: Portions of this document are illegible in microfiche products. Thesis

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  • Other: DE85012558
  • Report No.: LBL-19286
  • Grant Number: AC03-76SF00098
  • DOI: 10.2172/5673494 | External Link
  • Office of Scientific & Technical Information Report Number: 5673494
  • Archival Resource Key: ark:/67531/metadc1092585

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  • March 1, 1985

Added to The UNT Digital Library

  • Feb. 10, 2018, 10:06 p.m.

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  • April 24, 2018, 5:47 p.m.

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Howe, J.M. Atomic mechanisms of. gamma. ' precipitate plate growth in the Al-Ag system, report, March 1, 1985; [Berkeley,] California. (digital.library.unt.edu/ark:/67531/metadc1092585/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.