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Fig. 2.2. Precipitation Hardening of an Aluminum-Copper Alloy3

Fig. 2.2. Precipitation Hardening of an Aluminum-Copper Alloy3

The alloy is then artificially aged at 340° F for 10 h. During the aging process, very fine particles of aluminum-copper are precipitated and the strength and hardness increases dramatically.

Precipitation hardening strengthens the alloy due to the formation of sub-microscopic precipitates that severely strain the matrix lattice. The strengthening effect is maximized when the precipitate is coherent with the matrix. A coherent precipitate is one in which the atomic arrangement of both the precipitate and the matrix is the same with no discontinuity in the lattice; however, the atomic spacings are different enough to distort the crystal lattice in the manner shown in Fig. 2.3. This causes an increase in strength by obstructing and retarding dislocation movement. In the aluminum-copper system, these solute clusters of precipitate are called Guinier-Preston (GP) zones which are solute rich domains that are fully coherent with the matrix. The GP zones are extremely fine with sizes in the range of tens of angstroms. The exact shape, size, and distribution of the GP zones depend on the specific alloy and on the thermal and

Coherent Precipitate Structure

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