Researchers from the University of Michigan have employed a novel high-resolution, three-dimensional X-ray approach to shed light on the formation, location, and role of nanoparticles in the subsequent solidification of molten metal, with a study that was published in Acta Materialia.
Image Credit: Acta Materialia (2024). DOI: 10.1016/j.actamat.2024.120189
Manufacturing cars with strong, lightweight aluminum alloys rather than steel might improve fuel economy and increase electric vehicle range, but the metals’ instability at high temperatures has prevented broad use.
Adding tiny, reinforcing particles of titanium carbide (TiC) directly to molten aluminum produces a stronger, more temperature-resistant aluminum-based material known as a metal matrix nanocomposite.
Researchers' inability to comprehend the formation or interaction of these nanoparticles with other microstructure characteristics is impeding the material’s industrial-scale manufacture.
Most metals start their lifetimes in the liquid state. How they convert from liquid to solid will ultimately determine their microstructures, and hence, their properties and applications.
Ashwin Shahani, Study Co-Corresponding Author and Associate Professor, University of Michigan
Shahani added, “The study enabled us to understand exactly how the nanoparticles interact with secondary phases in casting, which has been a major challenge for the past half-century.”
The researchers employed a potent imaging method called synchrotron-based X-ray nanotomography to observe metal microstructure nondestructively in 3D, a feat not achievable with traditional imaging techniques since nanoparticles are fewer than 100 nanometers or one ten-thousandth of a millimeter.
To achieve the visualizations, the researchers developed an aluminum composite reinforced with titanium carbide (TiC). This entailed reacting an aluminum melt with a combination of carbon powder and salt containing titanium using flux assistance.
Three-dimensional reconstructions revealed an unanticipated range of titanium aluminide (Al3Ti) intermetallic complexes, including one that developed directly on TiC nanoparticles larger than 200 nm in diameter. In that situation, the Al3Ti crystals formed an interesting orthogonal plate structure.
Meanwhile, TiC nanoparticles smaller than 200 nanometers split the Al3Ti intermetallic plates during solidification, resulting in branched structures.
In addition to imaging, the researchers employed phase-field simulations to fill spatiotemporal “gaps” in the experiments and suggest a mechanism for microstructure creation.
Shahani added, “We now have evidence that the nanoparticles form well before the intermetallics, and not the other way around, which has important implications regarding the nucleation of the nanoparticles in the first place.”
With these findings in hand, industry partners can now direct the creation of TiC and Al3Ti when producing aluminum composites on a wide scale by modifying the alloy chemistries or solidification pathways to get the desired microstructure and its corresponding attributes.
We have known for a long time that nano-sized particles could improve the performance of metal matrix composites, but the materials could not be produced at scale. We now understand the formation mechanisms that will enable our industry partners to optimize the process for lightweighting applications.
Alan Taub, Study Co-Corresponding Author and Robert H. Lurie Professor, Engineering and Director, Electric Vehicle Center, University of Michigan
Journal Reference:
Gladstein, A., et. al. (2024) Direct evidence of the formation mechanisms of TiC nanoparticles and Al3Ti intermetallics during synthesis of an Al/TiC metal matrix nanocomposite. Acta Materialia. doi:10.1016/j.actamat.2024.120189