Publication Date

5-2014

Date of Final Oral Examination (Defense)

12-10-2013

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Megan Frary, Ph.D.

Advisor

Rick Ubic, Ph.D.

Advisor

Dmitri Tenne, Ph.D.

Abstract

Amorphous and nanocrystalline W-based materials are candidate alloys where high toughness and abrasion resistance are required, such as for kinetic energy penetrators (KEPs). Using a bottom-up approach to produce a tungsten heavy alloy (WHA), W-based alloys, including W-Mo-Fe and W-Cr-Fe, have been produced using high-energy ball milling and consolidated with pulsed electric current sintering (PECS). Powder consolidation using PECS allows for fast consolidation and for high relative densities to be achieved after sintering at a fraction of the melting temperature. Rapid consolidation of the alloyed powders also reduces grain growth, resulting in improved mechanical properties. During consolidation of mechanically alloyed powders, the structure evolves with sintering conditions due to the residual stress introduced during ball milling and the addition of other component elements added to promote amorphization. Multiple nanoscale crystalline microstructures were seen in the W based alloys depending on processing conditions. Negligible porosity was achieved at nearly one third of homologous temperature, 1050 °C for W-Mo-Fe and 1200 °C for W-Cr-Fe, using nanocrystalline powders and consolidating with PECS. Specimens containing nanocrystalline microstructures showed extremely high hardness and improved mechanical properties over several baseline microcrystalline tungsten alloys. Consolidated alloys had improved hardnesses over baseline materials, up to 300%. Intermetallic formation aided sintering by filling in pores and acting as a binder phase between nanocrystalline powder particles. At temperatures above 1500 °C, liquid-phase vi sintering characteristics were observed, including the formation of spheroidal grains and rapid grain growth. PECS offers a novel processing route for tailoring the structure and properties of these advanced alloys by altering the grain size. The ballistic performance of these alloys under selected process parameters will be evaluated during subsequent studies and will determine whether or not these alloys can be used as effective KEPs.

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