Guinier-Preston zones
/ gee-NYAY PRESS-tun /
In the very earliest stage of age hardening, before any proper new-phase particle exists, the trapped solute atoms first huddle together into ultra-thin clusters just one or two atoms thick, still sitting on the parent lattice. These first whispers of precipitation are Guinier-Preston zones, named after the two scientists who detected them by X-ray scattering.
GP zones are tiny, coherent clusters. Coherent means their atomic planes line up continuously with the surrounding matrix, so they strain the lattice around them rather than forming a sharp boundary. That strain field is exactly what dislocations find hard to push through, so GP zones already give a big strength boost. They are the first step of a sequence: supersaturated solution, then GP zones, then transition precipitates (partially coherent), then the equilibrium precipitate (incoherent and coarse).
Peak strength in many aluminium alloys comes from GP zones or the fine transition precipitates that follow them, not from the coarse equilibrium phase, which actually corresponds to the soft over-aged state. So understanding GP zones explains why age hardening peaks early and fades late, and why natural aging at room temperature can strengthen an alloy over just a few days.
In Al-Cu, copper-rich GP zones only a few atoms thick form within hours at room temperature and are the main reason a freshly quenched rivet hardens as it sits.
The first coherent clusters: small, strained, and very effective obstacles.
GP zones are coherent clusters, not the equilibrium precipitate; ironically the coarse equilibrium phase gives less strength, because peak hardness comes from these early, fine, coherent stages.