Precipitation Guinier-Preston zones

Figure 23.2 Structures and morphologies of the Cu-rich precipitates which form during precipitation in an A1 + 1.7 at. % Cu matrix, (a) a-matrix phase, (b) Guinier-Preston Zone precipitate, (c) 8" precipitate, (d) 8 precipitate, (e) Final stable 8 precipitate. From Porter and Easterling [4],...

Figure 23.8 Solvus curves for Guinier-Preston zones and 6", 6, and 6 precipitates in...

First-Principles Approach to Guinier-Preston Zones. We have already seen that the combination of first-principles calculations with Monte Carlo methods is a powerful synthesis which allows for the accurate analysis of structural questions. In chap. 6 we noted that with effective Hamiltonians deduced from a lower-level microscopic analysis it is possible to explore the systematics of phase diagrams with an accuracy that mimics that of the host microscopic model. An even more challenging set of related questions concern the emergence of microstructure in two-phase systems. An age-old question of this type hinted at in the previous chapter is the development of precipitates in alloys, with the canonical example being that of the Al-Cu system. 

In this section, the theoretical basis of SANS will be exposed as is needed in polymer physics and will be clarified with simple experimental examples. Since the end of 1939, the method of SAS was developed by Guinier and Kratky and was mainly applied for questions in metal physics. In one of his first experiments, Guinier correctly interpreted scattering from copper precipitates in aluminum, the so-called Guinier-Preston zones. Today, SANS techniques are broadly used in soft matter a main reason is the relatively simple possibilities of contrast generation and variation for polymer chains and other hydrocarbon molecular items by the exchange of hydrogen and deuterium. 

Wrought heat-treatable Al alloys contain Mg, Zn, and Cu on the level of several percent as precipitation and solid solution hardening ingredients. The precipitation process forms nanometer size Guinier Preston (GP) zones that are coherent with the matrix and produce the age-hardening effect... 

The fundamental aspects of decomposition of a supersaturated solid solution are complex [11-13]. Typically, however, Guinier-Preston (GP) zones and intermediate phases are formed as precursors to the equilibrium precipitate phase [10]. (Figure 16.1 reveals a typical micrograph showing precipitate particles.)... 

This process is used to strengthen Al-Cu alloys. The initial precipitates are thin discshaped clusters of Cu atoms called GP zones after Guinier and Preston who first identified these structures. The GP-1 zones are 1-2 atoms thick and 25 atoms in diameter that form platelets parallel to the (100) planes. These zones maintain the same lattice structure as the A1 matrix, but the smaller Cu atoms produce lattice strain, which provides some strengthening of the material. 

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