There are many methods of forming metal into the shapes required for different purposes. These include casting, machining and rolling. These processes can affect the shapes and sizes of the tiny crystal grains that make up bulk metal, such as steel, aluminum or titanium, and other commonly used metals and alloys.

Researchers from MIT now know exactly how crystal grains form during extreme deformations. This is at the smallest scale, just a few nanometers. These new findings could help to improve processing techniques to achieve better and more consistent properties like hardness or toughness.

These new findings were made possible by the detailed analysis of images taken by a suite of powerful imaging system. They are published today in the journal Nature Materials by Ahmed Tiamiyu, former MIT postdoc (now assistant professor at University of Calgary), MIT professors Christopher Schuh and Keith Nelson; former student Edward Pang; current student Xi Chen.

Schuh explains that when you make a metal, it is entrusted with a specific structure. This structure will determine its properties in service. The resulting metal will be stronger if the grain size is smaller. He says that the goal to increase strength and toughness through smaller grain sizes has been a major theme of all metallurgy over the past 80 years.

Researchers have now described the formation of the tiny crystal grains that make up the majority of solid metals for the first time. They believe that understanding this process could lead to stronger and lighter versions of commonly used metals like aluminum, steel, or titanium. Credit: Courtesy the researchers

Metalurgists have used a wide range of methods to reduce the size of grains in solid metal pieces. They do this by applying various strains and deforming them in different ways. It’s not an easy task to reduce these grains.

Recrystallization is the primary method. This involves deforming and heating the metal. This causes small defects in the piece that are scattered throughout, says Schuh, who is also the Danae and Vasilis Salpatas Professor for Metallurgy.

All those defects that are formed when the metal is heated and deformed can form nuclei in new crystals. You go from a messy mess of defects to newly nucleated crystals. Schuh explained that they are freshly nucleated and start small, which leads to smaller grains.

He says that the unique aspect of the new research is the ability to determine how the process occurs at extremely high speeds and on very small scales. Schuh states that while typical metal-forming processes, such as forging and sheet rolling, are quite fast, this new analysis examines processes that are “several order of magnitude faster.”

We use a laser to accelerate metal particles to supersonic speeds. Schuh says that it can happen in a blink of an eyes. You could do thousands of them in a blink of an eye.”

He says that such a high-speed process does not exist as a curiosity in the laboratory. There are many industrial processes that can occur at this speed, including high-speed machining and high-energy milling metal powder. Cold spray is also used to form coatings. He says that they have tried to understand the recrystallization process at extreme rates. “Because of these high rates, no one has been able to really dig in and look systematically at this process before.”

Tiamiyu used a laser-based method to shoot 10-micrometer particles on a surface. He then measured how fast the particles were moving and how hard they hit, Schuh said. He would shoot the particles at faster speeds and then open them to examine the evolution of the grain structure. This was done in collaboration with microscopy experts at the MIT.nano facility.

Schuh describes the result as a “novel pathway” that allowed grains to form down to the nanometer level. They call the new pathway nano-twinning assisted recrystallization. It is a variation on a well-known phenomenon in metals called “twinning”. This is when a part of the crystal structure flips its orientation. He describes it as a mirror symmetry flip. You end up with these stripes where the metal flips its direction and flips back, similar to a herringbone pattern. Researchers found that the greater the number of impacts, the smaller the grains would become.

The process of bombarding copper’s surface with tiny particles at high speeds could almost tenfold increase its strength in the experiments they conducted. Schuh states that this is a significant change in the metal’s properties. This is an extension of the hardening effect that ordinary forging produces. “This is a hyperforging type phenomenon.

Schuh states that they were able use a variety of imaging and measurements to exact particles and impact sites in the experiments. Different lenses are used to view the exact same material and region. When you combine all of these techniques, you get a richness in quantitative detail that is not possible with a single technique.

Tiamiyu states that the new findings can be immediately applied to metal production because they provide information about how much deformation is required, how fast it takes place and what temperatures are best to use to maximize the effect of specific metals or processing techniques. They should be able to use the graphs that they created from their experimental work. Tiamiyu states that these graphs are not hypothetical. “If you want to know if nanograins form in any metal or alloy, you can use the parameters you have. The formulas they created will show you what type of grain structure is possible at given rates of impact and temperatures.