A study of dislocation structures in bulk-strained and in situ TEM strained nickel aluminide alloys
Subramanian, Raja
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https://hdl.handle.net/2142/19992
Description
Title
A study of dislocation structures in bulk-strained and in situ TEM strained nickel aluminide alloys
Author(s)
Subramanian, Raja
Issue Date
1992
Doctoral Committee Chair(s)
Birnbaum, Howard K.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Engineering, Materials Science
Language
eng
Abstract
In the research work presented in this thesis, the dislocation structures in bulk strained and in situ TEM strained alloys of hypo-stoichiometric Ni$\sb3$Al with and without boron, and hyper-stoichiometric Ni$\sb3$Al with boron have been studied. Bulk samples were deformed in tension to 3% and 9% strain and the resulting dislocation structure studied using transmission electron microscopy (TEM). Particular emphasis has been placed on the dislocation structure vicinal to grain boundaries and transgranular cracks. At both strain levels, the dislocation structures are similar near the grain boundaries and transgranular cracks, and are similar to those observed in in situ TEM straining studies.
Samples have been deformed in tension inside the TEM and the dynamic results have been video recorded. During in situ straining, the glide of dislocations bounding stacking faults in the slip trace ahead of a crack tip, nucleation of partial dislocations at the surface, and the glide of these nucleated partial dislocations in the fault plane have been observed. Pinning of super-Shockley partial dislocations and glide of segments of pinned partial dislocations have been observed. Similar stacking fault structures have been observed near the grain boundaries and the transgranular cracks in bulk strained samples and in in situ TEM deformed samples.
The change in the stacking fault structures at high temperature has been studied in in situ TEM heating experiments. The super-Shockley partial dislocations slowly overcome the pinning obstacles at 773 K and glide to eliminate the faulted area. The effect of planar-fault energies, line tension of the dislocation, and the elastic anisotropy has been discussed.
The Ni segregation at grain boundaries of known $\Sigma$ value in the alloys, Ni$\sb{76}$Al$\sb{24}$ with and without 100 wt ppm boron, and Ni$\sb{74}$Al$\sb{26}$ 100 wt ppm boron has been studied by x-ray microanalysis in the HB5 Scanning Transmission Electron Microscope.
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