A region in which solute atoms gather in a plate one to a few atomic layers thick on particular planes of the matrix early in aging. It is coherent with the matrix and responsible for the hardening of aluminum alloys by natural aging.
Japanese name: GPゾーン
GP zone appears in 2 practice questions across 2 topics, including Nonferrous metals and alloys, Heat treatment and metallic materials.
Field: Nonferrous metals and alloys
In 1906 Wilm found that an Al-Cu-Mg alloy quenched and left at room temperature hardened over several days, but no change in the microstructure could be seen in the optical microscope, and the cause long remained a mystery. In 1938 Guinier and Preston, working independently, studied the diffuse scattering of X-rays and showed that extremely small regions in which solute atoms had gathered formed within the matrix. These regions are named GP zones after the two. They were later observed directly with the transmission electron microscope and became the starting point for understanding age hardening on the atomic scale.
GP zones in Al-Cu alloys are disks a few nanometers across, consisting of one to several layers of Cu atoms on the {100} planes of the matrix. The crystal structure remains the FCC of the matrix, and because Cu atoms are smaller than Al atoms the surrounding lattice is strained, and this strain hinders dislocation motion. Because the interface is fully coherent, its energy is small and the nucleation barrier is low, so the zones form at very high densities in the matrix. The form varies with the alloy: spherical zones form in Al-Zn-Mg, and clusters that precede needle-shaped precipitates form in Al-Mg-Si.
GP zones are the first stage of the precipitation sequence; in Al-Cu it runs GP zones → θ'' → θ' → θ, passing from coherent to semicoherent to incoherent as aging proceeds. GP zones form at room temperature because the excess vacancies retained by quenching from the solution-treatment temperature assist the diffusion of Cu atoms. GP zones have an upper temperature (solvus) above which they dissolve, and in artificial aging above it intermediate phases may precipitate directly without passing through GP zones. The reversion phenomenon, in which naturally aged material softens when heated briefly, is also due to dissolution of GP zones.
Answer: A. They are disk-shaped clusters of Cu atoms one to a few atomic layers thick on the {100} planes of the matrix, fully coherent with it and able to form even during aging at room temperature
Cu atoms made supersaturated by quenching first gather on the {100} planes of the matrix to form disk-shaped GP zones one to a few atomic layers thick and a few nanometers across. They keep the crystal structure of the matrix and are fully coherent, and with their small interface energy the nucleation barrier is low, so with the help of excess vacancies retained by quenching they form even at room temperature and are responsible for the hardening of natural aging. As aging proceeds, the alloy passes through the coherent and semicoherent intermediate phases θ'' and θ' to the incoherent, coarse equilibrium phase θ and softens on overaging. The band near grain boundaries where Cu and vacancies are depleted and no precipitates form is the precipitate-free zone (PFZ), a different thing from GP zones. Coarse θ is not a GP zone but the equilibrium phase of the final stage of aging.
GP zones are regions where solute has gathered while keeping the crystal structure of the matrix, and they are usually not regarded as separate phases. Particles with crystal structures different from the matrix, such as θ' and θ, are called precipitates to distinguish them.
Because a dislocation shearing through a GP zone must do work against its strain field and to create new interface. GP zones are extremely fine and dense, so dislocations are caught on them frequently.
In 6000-series alloys, long storage at room temperature after solution treatment can lower the strength reached in subsequent artificial aging. The room-temperature holding time is therefore controlled in the process.
Cu atoms made supersaturated by quenching first gather on the {100} planes of the matrix to form disk-shaped GP zones one to a few atomic layers thick and a few nanometers across.
When an Al-Cu alloy is solution treated, quenched and aged,…Holding it at room temperature (natural aging) or at about 100–200 °C (artificial aging) then forms fine precipitates that are coherent or semicoherent with the matrix, such as GP zones, θ″ and θ′, which obstruct dislocation motion and raise the strength.
Which is the correct sequence for precipitation hardening a…Precipitation hardening Dislocation Al-Li alloy Alclad Beryllium copper Temper designation Tough-pitch copper γ'' phase