BCC, FCC, and HCP unit cells, atomic packing factor and its effect on density and ductility, Miller indices and slip systems, and polymorphism & allotropy — including the iron and carbon transformations the rest of this program keeps returning to.
Module 1 established that metallic bonding is nondirectional, which is why metal atoms settle into repeating, close-packed geometric arrangements rather than the bond-angle-dictated networks covalent bonding produces. This module names those arrangements precisely: the body-centered cubic, face-centered cubic, and hexagonal close-packed unit cells that account for nearly every engineering metal, how packing factor connects crystal geometry to density, how Miller indices identify the specific planes and directions slip actually occurs on, and how a single element like iron or carbon can adopt more than one crystal structure with dramatically different properties in each.
By the end of this module you should be able to explain why FCC metals resist the ductile-to-brittle transition that BCC metals are prone to, and why that single crystallographic fact drives real material choices for cryogenic and cold-climate service — reasoning Module 4's metal alloy systems and Module 9's heat treatment module both build on directly.