The Al-Zn system was the first studied extensively in an attempt to verify the theory for spinodal decomposition [24). The equilibrium diagram for this system, shown in Fig. 18.12, shows a monotectoid in the Al-rich portion of the diagram. The top of the miscibility gap at 40 at. % Zn is the critical consolute point of the incoherent phase diagram. In concentrated Al-Zn alloys, the kinetics of precipitation of the equilibrium B phase from a are too rapid to allow the study of spinodal decomposition. An Al-22 at. % Zn alloy, however, has decomposition temperatures low enough to permit spinodal decomposition to be studied. For Al-22 at. % Zn, the chemical spinodal temperature is 536 K and the coherent spinodal tempera- ture is 510 K. The early stages of decomposition are described by the diffusion equation ac = D [(1 + 207 ) a2c 2K 84c] at f" dx4 .62) Atomic percent zinc 0 10 20 30 40 50 60 70 80 90 100 800 700 660.452 L 600 500 Temperature (C) 419.5 400 (Al) 300 200 (Zn) - 100 0 0 10 20 30 40 50 60 70 80 90 100 Al Weight percent zinc Zn Figure 18.12: Equilibrium diagram for Al-Zn alloys. From ASM Handbook: Alloy Phase Diagrams, Vol. 3 [25].Data. Assume for Al-Zn alloys that ?Y is isotropic, the enthalpy of mixing of Al-Zn solutions is independent of temperature, and the entropy of mixing, s, is ideal; that is, s = -nk clnc + (1 - c) In(1 - c)] n = 6 x 1022 cm-3(number of atoms per unit volume) D = [c(1 - c)]f"D'e-Q/(N.KT) Q = 104 kJ mol-1 K = 1.6 x 10-12 J cm-1 D = -2.6 x 10-18 cm2 s ' (at 338 K and 22 at. % Zn) f" = -1.17 kJ cm (at 338 K and 22 at. % Zn)