Steel

Endlessly fascinating. You could probably teach a whole semester just about the different variants of steels and their properties, based on their crystal structure, C%, presence of other alloying metals that can either serve as stabilizers forcing the steel to follow a certain crystal lattice or as lattice anchors forming carbides with the carbon more easily. It’s a world that’s super easy to get lost in, almost akin to trading cards due to all their similarities in structure while the minute differences make up most of the varying properties we see. Bonus points for actually genuinely being so ubiquitous in everyday life, unlike many other metallurgical interests (intermetallics, mainly).