Antiphase disorder

Fig. 5 illustrates a peculiar kinetic phenomenon which occurs when an initially disordered alloy is first annealed at temperature T corresponding to area b in Fig. 1 and then quenched to the final temperature T into the spinodal instability area d antiphase boundaries "replicate , generating approximately periodic patterns. This phenomenon reflects the presence of critical, fastest growing concentration waves under the spinodal instability (the Calm waves ). Lowering of the temperature to T < T results in lowering of the minority concentration minimum ("c-well ) within APB, while the expelled solute atoms build the c-bank adjacent to the well . Due to the... 

R.W Cahn, Antiphase domains, disordered films and the ductility of ordered alloys based on Ni3 Al, Mai. Res. Soc. Symp. Proc. 81 27 (1987)... 

The development of the miscibility gap for W < 0 and the antiphases ( Tjeq) for W > 0 have entirely different kinetic implications. For decomposition, mass flux is necessary for the evolution of two phases with differing compositions. Furthermore, interfaces between these two phases necessarily develop. The evolution of ordered phases from disordered phases (i.e., the onset of nonzero structural order parameters) can occur with no mass flux macroscopic diffusion is not necessary. Because the 77+q-phase is thermodynamically equivalent to the 7/iq-phase, the development of 77+q-phase in one material location is simultaneous with the evolution of r lq-phase at another location. The impingement of these two phases creates an antiphase domain boundary. These interfaces are regions of local heterogeneity and increase the free energy above the homogeneous value given by Eq. 17.14. The kinetic implications of macroscopic diffusion and of the development of interfaces are treated in Chapter 18. 

Keywords interstitial solid solutions, crystal structure, phase transformation, order-disorder, isotopic effect, antiphase domains, neutron diffraction, TiN026Hoi5, TiN026Doi5, TiN0.MH0.075D0.075 ... 

Virtually all minerals contain defects. In addition to point defects (e.g., vacancies that exist in a thermodynamically determined equilibrium number, impurities etc ), macroscopic minerals contain line defects (dislocations), and planar defects such as stacking foults, antiphase boundaries and twins. Intergrown layers of different structure or composition, and polytypic disorder also may be present. 

Fig. 12.4. Schematic illustration of conserved and nonconserved order parameters for characterizing the internal state of a binary alloy (adapted from Chen and Wang (1996)) (a) disordered phase (rj = 0) with uniform composition cq, (b) two-phase mixture consisting of disordered phases (rj = 0) with composition Cq, and c, (c) ordered single phase (ri = 1) of single composition ci with an antiphase domain boundary.

The Portevin-Le Chatelier effect with serrated yielding was observed for both the ordered and disordered state (Mohamed etal., 1974). Recovery and recrystallization have been analyzed in detail (Vidoz etal., 1963 Cahn, 1990, 1991). Experimental and theoretical studies have been directed at dislocations and antiphase domain boundaries (see, e.g. Tichelaar and Schapink, 1991 D. G. Morris, 1992 Veyssiere, 1992), grain boundaries (Yan etal., 1992), and the electronic structure (Bose et al., 1991). It is noted that disordered layers are formed in ordered CujAu on antiphase boundaries and twin boundaries just below the order-disorder transition temperature (Tichelaar et al., 1992). This may be expected in other phases, too, and may improve the ductility of less ductile phases, as is discussed for NijAl (see Sec. 4.1.2). 

Mechanical properties of disordered alloys are also different to those of ordered alloys. This can have a bearing on the techniques used to prepare the alloys. Ordered structures are usually harder than disordered ones. In the former, dislocations have higher energy the Burgers vector is larger because it is defined on the basis of the superlattice. Ako, dislocation movement is hindered by antiphase domain boundaries which may be present in the ordered state. 

---