dc.contributor.author	Zeagler, Andrew	en
dc.contributor.committeechair	Aning, Alexander O.	en
dc.contributor.committeemember	Reynolds, William T. Jr.	en
dc.contributor.committeemember	Poquette, Ben D.	en
dc.contributor.committeemember	Lu, Guo-Quan	en
dc.contributor.committeemember	Kampe, Stephen L.	en
dc.contributor.department	Materials Science and Engineering	en
dc.date.accessioned	2017-04-06T15:42:54Z	en
dc.date.adate	2011-07-25	en
dc.date.available	2017-04-06T15:42:54Z	en
dc.date.issued	2011-06-17	en
dc.date.rdate	2016-10-24	en
dc.date.sdate	2011-06-24	en
dc.description.abstract	Elemental W and Ni powders were mechanically alloyed in a SPEX mill with WC grinding media for durations ranging from 5 to 50 hours, then compacted samples were sintered in hydrogen to generate bulk tungsten heavy alloys with 2, 4 and 6 wt.% Ni. Evidence of a bimodal grain size distribution was seen in X-ray diffractograms of sintered samples and confirmed by scanning electron microscopy. Grain sizes in the small-grained regions ranged from 200–600 nm; those in the large-grained regions ranged from 1–2 µm. Furthermore, the volume fraction of the small-grained region increased linearly as milling time increased. A slice from a sintered sample was prepared for examination by TEM, in which particles 30–100 nm in diameter were regularly observed on the boundaries of the 200–600 nm grains. EDS point analysis showed that the particles are WC. Therefore it is concluded that heterogeneously distributed contamination from the grinding media is continually incorporated during mechanical alloying and, during sintering, inhibits grain growth through Zener pinning. Densities of sintered samples increased as milling time increased to a maximum of almost 96% of the theoretical value. Density increases with respect to milling time were initially great but diminished upon further milling. While the samples with 4 and 6 wt.% Ni both approached 96% of the theoretical density value by 50 hours of milling, densities in the samples with 2 wt.% Ni were considerably lower. Thus it appears that the Ni that becomes incorporated into the bcc W structure during mechanical alloying activates W diffusion during sintering, though there is a limit to the amount of Ni that the W structure can accommodate. This is evinced in W lattice parameter values from the as-milled powders; while the lattice parameter drops considerable from 2 to 4 wt.% Ni, the difference between 4 and 6 wt.% Ni is much smaller and the Ni content limit surely falls between the two values. Otherwise-equivalent samples with added WC powder were also produced, but did not increase the volume fraction of the small-grained region – probably because the particles remained large and were homogeneously distributed.	en
dc.description.degree	Ph. D.	en
dc.identifier.other	etd-06242011-135451	en
dc.identifier.sourceurl	http://scholar.lib.vt.edu/theses/available/etd-06242011-135451/	en
dc.identifier.uri	http://hdl.handle.net/10919/77119	en
dc.language.iso	en_US	en
dc.publisher	Virginia Tech	en
dc.rights	In Copyright	en
dc.rights.uri	http://rightsstatements.org/vocab/InC/1.0/	en
dc.subject	powder metallurgy	en
dc.subject	tungsten	en
dc.subject	tungsten heavy alloys	en
dc.subject	mechanical alloying	en
dc.title	On a Bimodal Distribution of Grain Size in Mechanically Alloyed Bulk Tungsten Heavy Alloys	en
dc.type	Dissertation	en
dc.type.dcmitype	Text	en
thesis.degree.discipline	Materials Science and Engineering	en
thesis.degree.grantor	Virginia Polytechnic Institute and State University	en
thesis.degree.level	doctoral	en
thesis.degree.name	Ph. D.	en

On a Bimodal Distribution of Grain Size in Mechanically Alloyed Bulk Tungsten Heavy Alloys

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