By UCLA Samueli Newsroom
Xiaochun Li, professor at UCLA, has been incorporating dense nanoparticles to make materials stronger and tougher. But those enhancing qualities take effect when the material is in its final form.
Li started to wonder, “what if nanoparticles could help make super strong metals, during the actual formation?” That is during casting, when they solidify from a hot molten liquid in a mold.
At the atomic level, atoms of metals group together to form crystal lattices. Those lattices then form units are known as crystalline “grains.”
A denser scaffolding network, makes a stronger metal. Typically, grain sizes are tens to hundreds of micrometers in diameter. If the grain sizes are at ultrafine sizes (less than one micrometer in diameter, or one-thousandth of a millimeter) or down to the nanoscale (less than 100 nanometers, or ten -thousandth of a millimeter), metals could have strength levels much stronger than their counterparts cast by the current methods.
When liquid metals are cast, these grains start forming as it cools. Atoms travels quickly to form a solid tiny clusters in the hot liquid metal – a process called nucleation.
Currently, grains can only be kept small down to ultrafine or nanoscale using specialized rapid cooling equipment that cools at a rate of tens of thousands to millions of degrees Kelvin per second. However, this method only produces thin metals of tens of micrometers thick, making it impractical for industrial manufacturing.
Typically, in manufacturing, metals and metal alloys like steel cool at rates closer to 100 degrees per second at their fastest. These slow rates make it impossible to cast bulk metal grain sizes down to ultrafine or nanoscale sizes.
Li found that the ceramic nanoparticles that he’s been working with to toughen materials could also make a fundamental change during casting and solidification – one that keeps grain sizes down to nanoscales without relying on rapid cooling. And, it could be possible to cast large pieces – steel beams or aluminum panels, for example. He and his colleagues published their study in Science Advances.
Using tungsten carbide particles, they cast ingots of copper that resulted in grain sizes at less than 200 nanometers, which is more than 1,000-times smaller than grains in conventional casting. They examined these particles using a microscope at the Foundry’s NCEM facility and found similar results in zinc and aluminum.