Tungsten (W) is lightweight, with the highest melting point among metals. It retains its ultrahigh strength and hardness at extreme temperatures. This allows it to be used in several high temperature applications that are important to the U.S. economy, such as, filaments for incandescent bulbs (household), heating elements for furnaces (manufacturing), magnetic fusion energy devices (alternative energy), engine and plasma-facing components (transportation), and space electric propulsion (advancement of science). Tungsten belongs to the so-called body centric cubic (BCC) class of polycrystalline metals. One of the major problems with W and other BCC metals that makes them expensive and limits their even wider use is that they are very brittle at room temperature and, therefore, are very hard to machine or form into shapes using dies and other forming processes. In contrast, face centered cubic (FCC) metals such as copper are soft and easy to machine but they do not have the high strength and hardness needed for many applications. This award explores mechanisms that may impart room temperature ductility, like that displayed by FCC metals, to tungsten and other BCC metals by understanding and controlling the movement of groups of atoms in extremely small grain sizes, which together, form the structure of these metals. This research involves several disciplines including physics, mechanical engineering, manufacturing, and materials science. Graduate students trained on this project will cross over disciplinary boundaries to learn a wide array of knowledge and skills. Undergraduate and high school students will be accommodated through year-long and summer engagements, respectively, preparing them for the challenges in the rapidly evolving technological workspace.
The study will examine i) the effect of size and temperature on the flow stress, ii) the effect of strain rate and cryogenic temperatures on the flow stress, and iii) the effect of temperature, strain rate, and size on fracture toughness in BCC metals. This study will involve Focused Ion Beam (FIB)-based fabrication of nanopillars and notched nanoscale three-point bending specimens, picoindentor based deformation at low strain rates and laser spallation based deformation at high strain rates under tension, compression and bending loads over a range of temperatures (100K,900K), and site specific FIB based electron transparent TEM specimen preparation for dislocation characterization. The results will be directly compared to the predictions of Discrete Dislocation and Crack Dynamics simulations. The study will likely result in the discovery of new dislocation nucleation and mobility mechanisms, and provide further insights into the present performance limits of BCC metals.
