macrostructure

Macrostructure is the structure you can see with the naked eye or a simple magnifier, the coarsest level of the hierarchy. Think of the visible grain in a piece of oak, the swirling pattern on a Damascus knife blade, the layers in plywood, or the crack running across a broken casting. No microscope needed: these features are millimeters to centimeters in size.

More precisely, macrostructure covers features roughly a millimeter and larger: the overall grain flow in a forged part, weld beads and heat-affected zones, porosity and shrinkage cavities in a casting, fiber layout in a composite, and large-scale segregation where composition varies across a big ingot. These are examined by eye, by low-power magnifier, or after etching, and sometimes by radiography (X-ray photographs of the whole part), rather than by microscope.

Macrostructure matters because big flaws often decide whether a part survives: a single large casting pore or a badly aligned weld can start the crack that breaks a component, no matter how good the finer structure is. In wood and composites, the visible fiber direction directly controls strength along versus across the grain. Caveat: macro is the top of the same continuous ladder as micro and nano, the line at about a millimeter is a convention, and the same defect (say a pore) can exist at several scales at once.