Bulk Nanocrystalline Tungsten Alloys

Development of Bulk Nanocrystalline Tungsten Alloys for Fusion Reactor Structure

This project will develop a technology for manufacturing dense bulk nanocrystalline tungsten materials (grain size <100 nm; nano-W hereafter) for fusion reactor structural applications, aiming to improve the ductility and toughness of tungsten before and after irradiation. The project involves the development of fabrication processes for making bulk nano-W, the development of new alloys of nano-W and evaluations of mechanical properties of these specific materials.

Tungsten and its alloys have already been identified as one of the primary candidate materials for fusion applications. For the purpose of using tungsten alloys as structural materials in a fusion reactor, current available tungsten materials still have serious deficiencies including high ductile-to-brittle transition temperatures (DBTT), low fracture toughness, anisotropic microstructures, fabrication and joining issues, and radiation-induced degradation. These deficiencies in mechanical properties are attributed to microstructure issues including: wide grain size distributions, non-uniform distribution of the dispersion-strengthening particles, and high levels of impurities, such as oxygen. These microstructural drawbacks hinder further improvement of the key mechanical properties of such materials.

In order to improve the properties of tungsten materials, we intend to develop new tungsten materials that will satisfy the performance requirements of fusion reactors by:

• Using bulk tungsten materials with nanoscale grain structure, i.e. nanocrystalline W, defined as grain sizes less than 100 nm.
• Using alloying additives that improve ductility and fracture toughness of W alloys.

The goal of the fabrication processes is to produce fully dense bulk nano-W with grain sizes below 100nm, with uniform distributions of grain size and additives. To date, there is no known process that can make nano tungsten with grain sizes less than 100 nm in a fully densified state, and in a cost-acceptable fashion. The task of making nano-tungsten involves a suite of nano-particle processing and sintering techniques. This program will also develop new alloys of nano-W, e.g. nano W-(Ta,V,Ti)-TiC alloys to improve ductility and toughness before and after irradiation.

In completing this project, we will achieve the following objectives:

• Demonstrate experimentally the feasibility of producing bulk nanocrystalline tungsten alloys (100% dense, <100 nm grain size).
• Demonstrate the proposed nano-W alloys, namely, W-(Ta, V, Ti)-TiC, can indeed be made using the proposed process.
• Demonstrate that the ductility, fracture toughness, and DBTT of nano tungsten alloys meet the requirements for fusion reactor applications.