CAREER: New Solid State Metal Foams Using Oxide Reduction and Intraparticle Expansion

Millersville University, Millersville PA

Investigators

Abstract

Nontechnical Description Metals are useful materials, as they are durable and easy to manufacture, but they have characteristically high densities which make them inefficient in many weight-sensitive applications. Their density can be reduced by introducing porosity, but porosity is also useful for increasing surface area, enhancing energy absorption, assisting bone ingrowth and more. Methods used to "foam" metals often require high temperatures, high pressures and/or complex processes, which limit the availability and cost-effectiveness of these materials. A new method has been developed to enhance porosity in metals at modest temperatures using simple, well-established powder metallurgy processes. The fundamental difference is that pores are developed within individual powder particles in the solid state, and this expandable metal powder can be incorporated into current, state-of-the-art foaming processes to enhance the level of porosity, or it can be used as a stand-alone process. This unique approach to creating porous metals may allow for cost reduction, the development of new metals and alloys for solid state foaming and the overall improvement of solid state foam processing. Both the fundamental mechanisms and commercial potential of this technology will be investigated to create a holistic understanding of the process and resulting materials. This technology can result in more fuel-efficient transit, safer vehicles, reduced emissions and much more. Through this work, educational awareness and career opportunities will be developed in partnership with the local community. This will involve establishing, promoting and expanding undergraduate research initiatives and promoting diversity in materials research and nanoscale technologies. Technical Description A new method in the solid state foaming of metals has been developed using standard powder metallurgy processing. The technique involves two steps: (1) disperse oxides in a metal powder and (2) reduce oxides at a temperature sufficient to allow expansion. In general, mechanical milling can be used to create an oxide dispersion-strengthened (ODS) metal which is later foamed under hydrogen at elevated temperature via the formation of steam at oxide sites. The conditions under which this process is achieved depend on the oxide and matrix composition but are expected to be modest when compared to current foaming processes. This fundamentally different approach will allow for advances on three major fronts: (1) the ability to make metal foams with new compositions, (2) enhanced ability to control the structure and properties of the metal foam, and (3) the ability to combine this process with established powder metallurgy foaming methods to significantly increase the resulting porosity. This work will help to elucidate the fundamental mechanisms of this new method and has the potential to alleviate some of the critical issues plaguing solid state foaming. These issues include the complexity of processing, the lack of diversity in foamed metals and alloys and the modest porosities and/or mechanical properties achieved using current solid-state foaming methods. A number of fundamental research questions will be addressed, including: What is the influence of process variables, including reduction temperature, oxide chemistry, content and character, the microstructural properties of the matrix, and quantity and chemistry of process gas(es) on the pore formation process? What is the reduction and pore expansion behavior of multi-element systems, especially in which the elements possess greatly differing oxidation potentials? And what is the fundamental behavior of the ODS powder feedstock during bulk processing? A process map will be produced for creating metallic foams of a given porosity while independently controlling pore size, morphology and interconnectedness. This work will be conducted in a multi-scale manner, and an understanding of the nano- and micro-scale phenomena will be leveraged to inform processing decisions and methods by which to incorporate this feedstock into current, state-of-the-art methodologies. Educational initiatives will impact the university and community through increased collaboration internally and engaging local organizations to provide new opportunities for students to develop professionally. Expanded programs and opportunities in undergraduate research will be developed through this work, and outreach in the community will be conducted to help inform others of advanced materials and manufacturing as well as to inspire the future generation toward science and engineering.