Precipitate-free zones

Fig. 20.SS Transmission electronmicrograph showing intergranular and imragranular precipitation and the precipitate-free zone adjacent to the grain boundary in a high-strength precipitation-hardening Al-Zn-Mg alloy (x24 000, courtesy G. Lorimer)...

Precipitation-free zones at grain boundaries in an aluminum-base alloy. From D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, 2nd ed. (London Chapman Hall, 1992), p. 306. 

The width of the precipitate-free zone can be minimized by rapid quenching. ca- and ca are the equilibrium compositions at high and low temperatures, respectively. 

Fig. 9.2. Illustration of a precipitate-free zone in a Cr-3 at.% Nb alloy (courtesy of Sharvan Kumar). The grain boundary runs from left to right through the middle of the picture, and reveals the existence of Cr2Nb precipitates along the boundary itself and in the Cr matrix and the absence of such precipitates in a band near the boundary.

Fig. 12 Schematic microstructures in the case of (a) completely heterogeneous nucieation (b) mixed heterogeneous-homogeneous nucieation with a precipitate free zone (PFZ) and (c) homogeneous nucieation. P phase precipitates in a matrix are indicated.

Fig. 1.16 Electron microscopy images and EDX profiles around grain boundaries of the Al-Cu alloy aged at 433 K for 7.2 ks. (a) under-aged, (b) 90 k (peak-aged), and (c) 1728 ks (over-aged). The average widths of precipitate free zone (PFZ) and solute depletion zone (SDZ) are indicated by arrow [46].

S. Hirosawa, Y. Ogini, T. Sato, Experimental and computational investigation of formation of precipitate free zones in an Al-Cu alloy. Mater. Trans. 46 (2005) 1230-1234. 

Figure 15.8 shows two examples of precipitate-free zones (PFZ). Figure 15.8a is the classical PFZ at a GB. During cooling from the processing temperature, the Fe ions in the doped NiO have segregated to the dislocations... 

FIGURE 15.8 Precipitate-free zones in Fe-doped NiO. (a) At a low-angle GB (with precipitation along the GB) (b) at particles of spinel after oxygen diffusion has ceased. 

A precipitate-free zone exists next to the grain boundaries in Al-Zn-Mg alloys [74]. It has been observed that the susceptibility... 

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