Synthesis, characterization, and properties of carbide dispersion-strengthened tungsten alloys for use as a plasma-facing material in nuclear fusion reactors

Lang, Eric Joseph

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Description

Title

Synthesis, characterization, and properties of carbide dispersion-strengthened tungsten alloys for use as a plasma-facing material in nuclear fusion reactors

Author(s)

Lang, Eric Joseph

Issue Date

2017-12-13

Director of Research (if dissertation) or Advisor (if thesis)

Allain, Jean Paul

Committee Member(s)

Krogstad, Jessica A.

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Tungsten
• Spark Plasma Sintering

Abstract

Tungsten is the material of choice for plasma-facing components in future plasma-burning fusion reactors because of its high melting point, high sputter threshold, and low hydrogenic species retention. However, tungsten is an intrinsically brittle material, displaying no room temperature ductility and only exhibiting non-brittle failure at temperatures above 400 C. In addition to its limited ductility, tungsten’s high melting point and low recrystallization temperature pose complications during fabrication. Traditional synthesis routes tend to result in non-fully dense samples with coarse-grained microstructures. As a consequence, there is a desire for a fine-grained, fully-dense tungsten material that exhibits enhanced ductility. Tungsten is embrittled by impurity oxygen atoms residing at grain boundaries. It is theorized that by microalloying tungsten with transition metal carbides that capture the oxygen atoms, the impurity distribution can be altered to beneficially impact the mechanical properties. Through the advent of advanced powder processing techniques such as Spark plasma sintering, dense, fine-grained tungsten samples can be developed with these microalloyed microstructures. Spark plasma sintering is a powder compaction technique that provides high pressure and heating rates, allowing for a lower final temperature and hold time for compaction. In SPS, a strong electrical current is fed through the powder and die to heat the powder. An applied uniaxial force combines to compress the powder to a solid compact, leading to fully dense materials at lower temperatures as compared to conventional sintering. In this work, spark plasma sintering is employed to develop tungsten materials alloyed with tantalum carbide, titanium carbide, or zirconium carbide. Samples are fabricated with varying compositions of added carbides (from 0.5-10 wt.%), and the sintering process results in >90% dense samples with grains <10 μm in size. Compositional studies indicate the formation of tungsten carbide and transition-metal-oxide phases after sintering. As the amount of added second phase powder increased, the hardness increased and grain size decreased. Finally, samples alloyed with 1.0 wt.% zirconium carbide may be able to resist recrystallization. Exposures to low fluence deuterium ion irradiation showed possible re-organization of the surface bonding, but limited surface structuring. Under high fluence helium and hydrogen irradiation, significant nanostructuring was observed in cracks and non-fully formed grains, an unexpected result given the temperature and fluence regimes studied, which may be attributed to slight surface chemistry changes. Finally, initial investigations indicate increased deuterium retention in alloyed samples as compared to pure tungsten. These results offer a first study of the behavior of various dispersion-strengthened tungsten alloys under ion irradiation that sufficiently motivate further investigation of these materials for plasma-facing environments.

Graduation Semester

2017-12

Type of Resource

text

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Copyright 2017 Eric Lang

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