Metallurgists have all kinds of ways to make a chunk of metal harder. They can bend it, twist it, run it between two rollers or pound it with a hammer. These methods work by breaking up the metal's grain structure—the microscopic crystalline domains that form a bulk piece of metal. Smaller grains make for harder metals.
source https://phys.org/news/2021-01-technique-super-hard-metals-nanoparticles.html
