Design of Bulk Stable Nanocrystalline Alloys: A New Approach to Lightweighting

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Canada is committed to the United Nation's sustainable development goals to combat global climate change via the use of sustainable structures that minimize energy consumption and poisonous greenhouse gas (GHG) emissions. The approach adopted by major contributors-energy/transportation sectors-to tackling the climate change menace is lightweighting i.e., the replacement of heavy materials, say "stronger" steel, with lighter one like "softer" aluminum alloys; but this results in the loss of load-carrying capacity. Rather than trade-off strength upon lightweighting, the research here is proposing the use of the (i) same stronger "heavy" material but using less of it, and (ii) same softer "light" material but with improved strength, both offering even much better properties than what is commercially available. Because strength of materials increases with reduction in grainsize, the proposed lightweighting concept with improved strength is achieved by developing nanocrystalline (NC) materials-materials with ~1 to 100 nm in grain size (i.e., grain size is ~900 times smaller than the thickness of human hair). However, there is currently a global drawback in the use of NC materials-(a) nanograins grow even at room temperature, resulting in loss of strength and other functional properties, and (b) even if the nanograins are well stabilized against growth, they are limited to thin nanostructured coatings of ~1-100 microns thick. For lightweight applications to minimize energy consumption in the energy and transportation sectors, "bulk" NC materials with stabilized grain boundaries and with sizes/thicknesses beyond the current thin-film-scale are required. For clarity, "bulk" in this proposal refers to sample size/thickness from hundreds of centimeter- to meter- scale. A cost-effective and scalable approach to synthesize NC metals is through a powder processing route, but the success of producing bulk NC parts by powder processing rests on the consolidation process. This research program proposes a new approach that involves the union of two powder processes-solid-state mechanical milling to obtain nanograins, and cold spray additive manufacturing process for solid-state layer-by-layer deposition of the stabilized powders to developing bulk stable NC components; the nanograins will be stabilized by carefully selecting a solute that prefers the grain boundary sites of the solvent, thereby stabilizing it against grain growth. The success of this research will save Canada billions of dollars by reducing the cost of energy, emissions, and environmental impacts. The research program will also contribute to the (i) training of highly qualified personnel, (ii) UCalgary's mission to drive innovation, and (iii) Alberta's nanotechnology and manufacturing programs and research priorities-energy and GHG mitigation, and environment and climate adaptation target areas.