From Pop Sci, Theodore Gray on making bizmuth crystals -
When I was a teenager melting elements in my parents’ basement, I noticed that cooling lead would sometimes form a snowflake-like pattern on its surface. Snowflakes are crystals, and I had never thought of metal as crystalline. Metals are shiny, malleable things. You can’t bend a crystal into a coat hanger!
But most metals are crystalline, at least at the microscopic level. For example, iron and its alloys consist of a mixture of microscopic crystal zones interlocked with one another. The difference in properties between cast iron and high-carbon tool steel or the nickel-iron superalloys used in turbine blades are largely the result of the various sizes and shapes of the constituent microscopic crystals and what’s in the space between them. Quenching (fast cooling) and annealing (slow cooling) are nothing more than ways of controlling the size of the metal’s crystals. In general, the slower a material is cooled, the larger its crystals can grow.