Bicontinuous Nanocomposite Refractories

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Non-Technical SummaryThe discovery and development of new metals is critical for technical applications where high strength and ductility are necessary. In this program, we explore a new class of bulk nanostructured metals made by an advanced processing method called liquid metal dealloying, in which an alloy of a refractory metal such as tungsten and titanium is immersed into molten copper under conditions in which the titanium is dissolved away and the refractory metal re-organizes itself into a nanoscale network, a kind of truss network whose ligaments are only a few hundred atoms across. Upon cooling, the composite material is bicontinuous, i.e. comprised of two interpenetrating networks of distinct materials, a hard refractory phase and a ductile copper phase, combining the best attributes of each metal to make a novel strong and ductile material. In this program, we will explore processing methods to make these new materials and test their properties in systematic ways so as to understand the fundamental materials physics that govern their behavior. We will disseminate this understanding via student participation and the development of classroom resources.Technical SummaryThis project will examine the structure/processing/property relationship of a new class of nanostructured metals, which possess a bicontinuous ?spinodal decomposition?-like microstructure, but where one phase is a hard refractory metal such as tungsten and the other phase is a ductile metal such as copper. These materials are made using a new method we call liquid metal dealloying (LMD), in which titanium/refractory alloys are immersed in copper alloy melts at elevated temperatures. Whereas titanium dissolves out into the copper, the refractory is immiscible in the melt so during immersion it reorganizes via interface diffusion into a highly porous structure (were the copper phase to be removed), with a tunable lengthscale from approximately 50 nm to 5000 nm. The process is akin to electrochemical dealloying, such as is used to create nanoporous gold, except that dissolution in LMD is driven by thermodynamic phase behavior, and not electrochemical dissolution. Our focus here is on the kinetics of formation and the mechanical properties of non-porous composites fabricated by this method, which have potential utility in the diverse number of applications for ultra-strong materials with high toughness. More generally and fundamentally, the spontaneous formation of the bicontinuous microstructure during LMD is a platform on which to examine phase transformations, kinetics, and mechanical of metallic materials at the nanoscale. In addition to the scientific technology drivers, this program will involve students in the development and dissemination of kinetic Monte Carlo simulation code for the simulation and study of the kinetics of morphological evolution in nanostructured materials that will complement the development of a new undergraduate textbook on Kinetics and Phase Transformations for Materials Science.