Accelerated Sintering in "Nano-Duplex" Dual Phase Nanostructured Alloys

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

Nontechnical Abstract For decades, researchers have been pursuing so-called "nanocrystalline" alloys as next-generation materials. These materials have very finely controlled internal structures at the scale of just a few dozen atoms, and show dramatic improvements in strength, wear resistance, corrosion resistance, and many other functional properties. While laboratory work has succeeded in producing small samples of nanocrystalline material, this has not yet translated to commercial-scale production of "bulk" alloys, i.e., materials big enough to make engineering componentry from. This project is developing a scientific approach to solve the problem of scalability for these advanced materials. It explores the science of alloying in nanostructured powders that can be consolidated at high temperatures into bulk components, while still retaining the nanocrystalline structure. Scientifically, the project aims to develop a deep understanding of nanocrystalline alloy powders, their structure, and their processability. The project combines experimental and simulation-based tools, and focuses on a model alloy system based on the metal tungsten, used widely in cutting tools and machining equipment. Technologically, the project will identify alloys and processes that are significantly more cost-effective and energy-efficient than any known today. The broader impact of the proposal thus combines possible commercial advances in premium structural materials, with a wide range of educational benefits in the training of undergraduate and graduate students involved with the project. Technical Abstract The proposed work will develop the fundamental physics of a new class of thermodynamically stabilized nanostructured materials, called "nano-duplex" alloys, as well as a newly discovered rapid sintering mechanism that can only be induced in this class of nanostructured materials. Because these alloys exhibit the potential for rapid powder consolidation while retaining a stable nanoscale structure, they may hold the key to cost- and energy-efficient, scalable, and broadly commercially applicable synthesis of bulk nanostructured metals. These discoveries enable nanostructured alloys with nanoscale grain sizes that are stable through a full high-temperature consolidation cycle to full density, without the need for applied pressures or fields. In terms of intelletucal merits, the project will explore this new class of nanostructured alloys and the mechanisms by which they sinter. A systematic experimental study is proposed on the W-Cr system, exploring the role of alloy composition, temperature, impurity content and other processing variables on the mechanisms of densification. Kinetic parameters such as activation energies and activation volumes will be quantified, and compared with observations of microstructure evolution. Additionally, a new kinetic Monte Carlo simulation approach will be developed, calibrated, and applied to study the mechanisms of densification and to identify the separate roles of interfaces and nano-scale second phases on sintering. In the out years of the project additional alloying systems will be explored to demonstrate the broader applicability of this sintering mechanism to other technologically relevant alloys that are amenable to powder route production. In terms of broader impacts, the proposal combines possible commercial advances in advanced structural materials, with a wide range of educational benefits in the training of undergraduate and graduate students on the new sintering method itself as well as on the different characterization and modeling methods that are used in the project.